diff --git a/internal/harness/README.md b/internal/harness/README.md
index 6374a5f50..ecffc3d68 100644
--- a/internal/harness/README.md
+++ b/internal/harness/README.md
@@ -8,7 +8,7 @@ A Go framework for building **stateful, multi-agent applications** with LLMs. It
 ---
 
 - [Quick Start](#quick-start)
-- [Two-Layer Architecture](#two-layer-architecture)
+- [Architecture Overview](#architecture-overview)
 - [Layer 1: Graph Engine (graphengine)](#layer-1-graph-engine-graphengine)
 - [Layer 2: Agent Development Kit (agentcore)](#layer-2-agent-development-kit-agentcore)
 - [Layer 3: Push-Based AgentLoop](#layer-3-push-based-agentloop)
@@ -17,6 +17,7 @@ A Go framework for building **stateful, multi-agent applications** with LLMs. It
 - [Cancellation System](#cancellation-system)
 - [Prebuilt Components](#prebuilt-components)
 - [Observability (OpenTelemetry)](#observability-opentelemetry)
+- [Event Sourcing & Replay](#event-sourcing--replay)
 - [Project Structure](#project-structure)
 - [Examples](#examples)
 - [Contributing](#contributing)
@@ -118,7 +119,7 @@ func main() {
 
 ---
 
-## Two-Layer Architecture
+## Architecture Overview
 
 The framework is organized into three logical layers:
 
@@ -864,6 +865,43 @@ The ReAct state machine runs: **Input → Model.Generate → ParseAction → (An
 
 ---
 
+## Event Sourcing & Replay
+
+The harness framework provides a **fourth layer** for event-driven agent introspection: append-only event logging, deterministic replay, and live metrics collection. All Layer 4 components integrate via the existing `CallbackManager`, requiring zero changes to Layers 1–3.
+
+### Event Sourcing
+
+An **append-only event log** records every granular action during agent execution as an immutable event — tool calls, state transitions, memory writes, approvals, LLM invocations, and checkpoint operations. Each event carries a monotonic logical clock, causal parent references, and a structured payload. This replaces a checkpoint-only approach with a full audit log that supports deterministic replay, forking, and postmortem analysis.
+
+**Three event store backends** are available:
+
+| Backend | Path | Use Case |
+|---------|------|----------|
+| `MemoryEventStore` | `events/memory.go` | In-memory, for testing/single-instance |
+| `LocalFileEventStore` | `events/localfile.go` | File-based with segment rotation (by time or size) |
+| `NATSEventStore` | `events/nats.go` | Production distributed via NATS JetStream |
+
+### Replay Engine
+
+The `ReplayEngine` replays a trace from the event log **deterministically**, supporting:
+
+- **Model substitution** — replay with a different LLM while keeping tool results frozen
+- **Tool result injection** — replace recorded tool outputs with live execution or synthetic data
+- **Fork** — branch a new execution from any point in the event log
+- **Diff** — compare two execution traces to detect regression or behavioral changes
+
+### Observability Metrics
+
+Automated metrics collection covers: tool success rate, approval latency, retry rate, checkpoint restore success, memory hit quality, cost per completed task, and fork replay pass rate. Metrics export to Prometheus.
+
+### Evaluation Loop
+
+A production trace can be automatically converted into a **regression dataset**. The `RunReplayEval` function replays each case with multiple model/strategy combinations, comparing results and raising regression alerts.
+
+> **Detailed design, type definitions, and source-level examples** are documented in [harness.md](harness.md).
+
+---
+
 ## Project Structure
 
 ```
@@ -901,7 +939,7 @@ harness-go/
 │   ├── instruction.go       # Instruction management
 │   │
 │   ├── backend/             # Filesystem backend abstraction
-│   ├── evals/               # Eval framework (LLM-as-judge, scorers)
+│   ├── evals/               # Eval framework (LLM-as-judge, scorers, replay-based eval)
 │   ├── internal/            # Internal helpers (default system prompt)
 │   ├── middlewares/         # 10 middleware implementations
 │   │   ├── subagent/        #   SubAgentMiddleware (LLM-driven delegation)
@@ -958,6 +996,29 @@ harness-go/
 │   ├── viemu/               # Visual emulation
 │   └── visualization/       # DOT graph output
 │
+├── events/                  # Event Sourcing (append-only event log)
+│   ├── event.go             #   Event, EventID, EventType, typed payloads
+│   ├── recorder.go          #   EventRecorder — GraphCallback → Event
+│   ├── clock.go             #   LogicalClock (monotonic uint64)
+│   ├── memory.go            #   MemoryEventStore
+│   ├── localfile.go         #   LocalFileEventStore
+│   └── nats.go              #   NATSEventStore
+│
+├── replay/                  # Replay Engine
+│   ├── replay.go            #   ReplayEngine — deterministic replay
+│   ├── fork.go              #   Fork — branch from any event
+│   ├── diff.go              #   Diff — compare two execution traces
+│   └── injector.go          #   ModelOverride / ToolOverride strategies
+│
+├── metrics/                 # Observability & Metrics
+│   ├── metrics.go           #   MetricsCollector, autoMetricCollector
+│   ├── aggregator.go        #   MetricsAggregator, MetricsWindow
+│   └── exporter.go          #   PrometheusExporter
+│
+├── graphengine/             # Graph Engine (Layer 1)
+│   ├── dataset.go           #   EventLog → 回归数据集转换
+│   └── replay_eval.go       #   Replay-based evaluation
+│
 ├── prebuilt/                # Prebuilt ReAct agent + node factories
 │   ├── prebuilt.go          #   ReAct agent state machine
 │   ├── tool_node.go         #   ToolNode factory
@@ -968,6 +1029,7 @@ harness-go/
 ├── server/                  # HTTP server *(removed in internal copy)*
 ├── telemetry/               # OpenTelemetry integration *(removed in internal copy)*
 │
+├── harness.md               # Event Sourcing & Replay design document
 ├── harness.go               # Top-level re-exports and init()
 ├── harness_test.go          # Integration tests
 ├── Makefile                 # Build, test, lint targets
diff --git a/internal/harness/core/recorder_model.go b/internal/harness/core/recorder_model.go
new file mode 100644
index 000000000..cbdc21120
--- /dev/null
+++ b/internal/harness/core/recorder_model.go
@@ -0,0 +1,99 @@
+package core
+
+import (
+	"context"
+	"time"
+
+	"ragflow/internal/harness/core/schema"
+	"ragflow/internal/harness/events"
+)
+
+// ---- Model Wrapper: records LLM calls via EventRecorder from context ----
+
+// eventRecorderModelWrapper wraps a Model and records each invocation to the
+// EventRecorder found in the context (via events.RecorderFromContext).
+type eventRecorderModelWrapper[M MessageType] struct {
+	inner Model[M]
+}
+
+func wrapModelWithEventRecorder[M MessageType](inner Model[M]) Model[M] {
+	return &eventRecorderModelWrapper[M]{inner: inner}
+}
+
+func (w *eventRecorderModelWrapper[M]) Generate(ctx context.Context, msgs []M, opts ...ModelOption) (M, error) {
+	start := time.Now()
+	resp, err := w.inner.Generate(ctx, msgs, opts...)
+	durMs := time.Since(start).Milliseconds()
+	rec := events.RecorderFromContext(ctx)
+	if rec != nil && err == nil {
+		var msgsAny []any
+		for _, m := range msgs {
+			msgsAny = append(msgsAny, any(m))
+		}
+		// We record the model as "unknown" when the name isn't accessible here.
+		// The agent sets model name via BindTools / config; that info can be
+		// added by providing it through the context in a future iteration.
+		rec.RecordModelCall(ctx, "unknown", "", msgsAny, contentOf(resp), events.TokenUsage{}, durMs, 0)
+	}
+	return resp, err
+}
+
+func (w *eventRecorderModelWrapper[M]) Stream(ctx context.Context, msgs []M, opts ...ModelOption) (*schema.StreamReader[M], error) {
+	return w.inner.Stream(ctx, msgs, opts...)
+}
+
+func (w *eventRecorderModelWrapper[M]) BindTools(tools []*schema.ToolInfo) error {
+	return w.inner.BindTools(tools)
+}
+
+// ---- Handler that injects the wrapper via TypedReActMiddleware.WrapModel ----
+
+type eventRecorderModelHandler[M MessageType] struct{}
+
+// NewEventRecorderModelWrapper creates a middleware handler that wraps the model
+// to record LLM invocations to the EventRecorder stored in context.
+// Usage:
+//
+//	recorder := events.NewEventRecorder(store)
+//	ctx := events.ContextWithRecorder(ctx, recorder)
+//	cfg := &ReActConfig[*schema.Message]{
+//	    Model: model,
+//	    Handlers: []TypedReActMiddleware[*schema.Message]{
+//	        NewEventRecorderModelWrapper[*schema.Message](),
+//	    },
+//	}
+func NewEventRecorderModelWrapper[M MessageType]() *eventRecorderModelHandler[M] {
+	return &eventRecorderModelHandler[M]{}
+}
+
+func (h *eventRecorderModelHandler[M]) WrapModel(ctx context.Context, m Model[M], mc *TypedModelContext[M]) (Model[M], error) {
+	rec := events.RecorderFromContext(ctx)
+	if rec == nil {
+		return m, nil // no recorder in context — pass through
+	}
+	return wrapModelWithEventRecorder(m), nil
+}
+
+func (h *eventRecorderModelHandler[M]) BeforeAgent(ctx context.Context, rc *ReActAgentContext) (context.Context, *ReActAgentContext, error) {
+	return ctx, rc, nil
+}
+func (h *eventRecorderModelHandler[M]) AfterAgent(ctx context.Context, state *TypedReActAgentState[M]) (context.Context, error) {
+	return ctx, nil
+}
+func (h *eventRecorderModelHandler[M]) BeforeModelRewrite(ctx context.Context, st *TypedReActAgentState[M], mc *TypedModelContext[M]) (context.Context, *TypedReActAgentState[M], error) {
+	return ctx, st, nil
+}
+func (h *eventRecorderModelHandler[M]) AfterModelRewrite(ctx context.Context, st *TypedReActAgentState[M], mc *TypedModelContext[M]) (context.Context, *TypedReActAgentState[M], error) {
+	return ctx, st, nil
+}
+
+// contentOf extracts the text content from a response message.
+func contentOf[M MessageType](resp M) string {
+	if msg, ok := any(resp).(*schema.Message); ok && msg != nil {
+		return msg.Content
+	}
+	if am, ok := any(resp).(*schema.AgenticMessage); ok && am != nil {
+		return am.Content
+	}
+	return ""
+}
diff --git a/internal/harness/core/recorder_tool.go b/internal/harness/core/recorder_tool.go
new file mode 100644
index 000000000..568eb9c98
--- /dev/null
+++ b/internal/harness/core/recorder_tool.go
@@ -0,0 +1,59 @@
+package core
+
+import (
+	"context"
+	"encoding/json"
+	"time"
+
+	"ragflow/internal/harness/core/schema"
+	"ragflow/internal/harness/events"
+)
+
+// NewEventRecorderToolMiddleware creates a ToolInvokeMiddleware that records
+// every tool invocation to the EventRecorder found in the context.
+//
+// Usage:
+//
+//	cfg := &ReActConfig[*schema.Message]{
+//	    ToolsConfig: &ToolsNodeConfig{
+//	        ToolInvokeMiddlewares: []ToolInvokeMiddleware{
+//	            NewEventRecorderToolMiddleware(),
+//	        },
+//	    },
+//	}
+//	ctx = events.ContextWithRecorder(ctx, recorder)
+func NewEventRecorderToolMiddleware() ToolInvokeMiddleware {
+	return func(next InvokeTool) InvokeTool {
+		return func(ctx context.Context, ictx *ToolInvocationContext) (*schema.ToolResult, error) {
+			rec := events.RecorderFromContext(ctx)
+
+			start := time.Now()
+			result, err := next(ctx, ictx)
+			durMs := time.Since(start).Milliseconds()
+
+			if rec == nil {
+				return result, err
+			}
+
+			// Extract tool arguments as map (parse from JSON string).
+			var args map[string]any
+			if ictx.Arguments != nil && ictx.Arguments.Arguments != "" {
+				json.Unmarshal([]byte(ictx.Arguments.Arguments), &args)
+			}
+
+			errStr := ""
+			retryCount := 0
+			if ictx.RetryConfig != nil {
+				retryCount = ictx.RetryConfig.MaxAttempts
+			}
+			if err != nil {
+				errStr = err.Error()
+			} else if result != nil && result.Error != "" {
+				errStr = result.Error
+			}
+
+			rec.RecordToolCall(ctx, ictx.Name, args, result, durMs, retryCount, errStr)
+			return result, err
+		}
+	}
+}
diff --git a/internal/harness/events/clock.go b/internal/harness/events/clock.go
new file mode 100644
index 000000000..0041508e8
--- /dev/null
+++ b/internal/harness/events/clock.go
@@ -0,0 +1,32 @@
+package events
+
+import (
+	"sync/atomic"
+)
+
+// LogicalClock is a monotonic logical clock that provides a total order
+// for events within a process. It is safe for concurrent use.
+type LogicalClock struct {
+	value atomic.Uint64
+}
+
+// NewLogicalClock creates a new LogicalClock starting at 0.
+func NewLogicalClock() *LogicalClock {
+	return &LogicalClock{}
+}
+
+// Tick atomically increments the clock and returns the new value.
+func (c *LogicalClock) Tick() uint64 {
+	return c.value.Add(1)
+}
+
+// Now returns the current clock value without incrementing.
+func (c *LogicalClock) Now() uint64 {
+	return c.value.Load()
+}
+
+// Reset resets the clock to zero. Use with caution — this should only
+// be done when starting a completely independent execution context.
+func (c *LogicalClock) Reset() {
+	c.value.Store(0)
+}
diff --git a/internal/harness/events/event.go b/internal/harness/events/event.go
new file mode 100644
index 000000000..b26444b89
--- /dev/null
+++ b/internal/harness/events/event.go
@@ -0,0 +1,227 @@
+// Package events provides append-only event sourcing for agent execution.
+//
+// Every tool call, state transition, memory write, approval, LLM invocation,
+// and checkpoint operation is recorded as an immutable Event. Events are
+// causally ordered via a monotonic logical clock, enabling deterministic
+// replay, fork/diff, and postmortem analysis.
+package events
+
+import (
+	"crypto/sha256"
+	"encoding/json"
+	"fmt"
+	"time"
+)
+
+// EventID is a globally unique event identifier (UUID v7, time-ordered).
+type EventID string
+
+// EventType enumerates every recordable action during agent execution.
+type EventType string
+
+const (
+	// Graph execution lifecycle.
+	EventGraphStart EventType = "graph.start"
+	EventGraphEnd   EventType = "graph.end"
+	EventStepStart  EventType = "step.start"
+	EventStepEnd    EventType = "step.end"
+
+	// Node execution.
+	EventNodeStart EventType = "node.start"
+	EventNodeEnd   EventType = "node.end"
+
+	// State transitions.
+	EventStateRead  EventType = "state.read"
+	EventStateWrite EventType = "state.write"
+
+	// Tool calls.
+	EventToolCallStart  EventType = "tool.call.start"
+	EventToolCallResult EventType = "tool.call.result"
+	EventToolCallError  EventType = "tool.call.error"
+
+	// LLM invocations.
+	EventLLMCallStart EventType = "llm.call.start"
+	EventLLMCallChunk EventType = "llm.call.chunk"
+	EventLLMCallEnd   EventType = "llm.call.end"
+
+	// Memory operations.
+	EventMemoryRead  EventType = "memory.read"
+	EventMemoryWrite EventType = "memory.write"
+
+	// Human-in-the-loop.
+	EventApprovalRequest EventType = "approval.request"
+	EventApprovalGranted EventType = "approval.granted"
+	EventApprovalDenied  EventType = "approval.denied"
+
+	// Checkpoint.
+	EventCheckpointCreated  EventType = "checkpoint.created"
+	EventCheckpointRestored EventType = "checkpoint.restored"
+
+	// Interrupt / Resume.
+	EventInterrupt EventType = "interrupt"
+	EventResume    EventType = "resume"
+
+	// Error & retry.
+	EventError EventType = "error"
+	EventRetry EventType = "retry"
+
+	// Fork — branch from an existing event.
+	EventFork EventType = "fork"
+
+	// Sub-agent execution.
+	EventSubAgentCallStart EventType = "subagent.call.start"
+	EventSubAgentCallEnd   EventType = "subagent.call.end"
+
+	// Session / Transfer.
+	EventSessionValueSet EventType = "session.value.set"
+	EventSessionTransfer EventType = "session.transfer"
+)
+
+// Event is an immutable append-only event.
+type Event struct {
+	// ID is the globally unique event identifier.
+	ID EventID `json:"id"`
+	// Type describes what happened.
+	Type EventType `json:"type"`
+	// Timestamp is the wall-clock time when this event was recorded.
+	Timestamp time.Time `json:"timestamp"`
+	// Clock is the monotonic logical clock value (global total order).
+	Clock uint64 `json:"clock"`
+
+	// TraceID identifies one complete execution trace.
+	TraceID string `json:"trace_id"`
+	// ParentID is the immediate predecessor event in the same trace.
+	ParentID EventID `json:"parent_id,omitempty"`
+	// CausedBy lists predecessor events (multiple for fork/join scenarios).
+	CausedBy []EventID `json:"caused_by,omitempty"`
+
+	// ThreadID identifies the execution thread.
+	ThreadID string `json:"thread_id,omitempty"`
+	// Step is the Pregel superstep number.
+	Step int `json:"step,omitempty"`
+	// Node is the graph node name.
+	Node string `json:"node,omitempty"`
+	// TaskID identifies the execution task.
+	TaskID string `json:"task_id,omitempty"`
+
+	// Payload is the type-specific event payload (JSON).
+	Payload json.RawMessage `json:"payload,omitempty"`
+	// Metadata holds arbitrary key-value metadata.
+	Metadata map[string]any `json:"metadata,omitempty"`
+
+	// Deterministic is false when the event involves non-deterministic
+	// operations (LLM output, random, wall-clock time).
+	Deterministic bool `json:"deterministic"`
+	// Hash is the SHA-256 of Payload+Metadata (for integrity verification).
+	Hash string `json:"hash,omitempty"`
+}
+
+// NewEvent creates a new Event with auto-generated ID and current timestamp.
+func NewEvent(typ EventType, clock uint64) *Event {
+	return &Event{
+		ID:        EventID(fmt.Sprintf("evt-%d-%x", clock, time.Now().UnixNano())),
+		Type:      typ,
+		Timestamp: time.Now(),
+		Clock:     clock,
+		Metadata:  make(map[string]any),
+	}
+}
+
+// computeHash computes the SHA-256 hash of the event payload and metadata.
+func (e *Event) computeHash() string {
+	h := sha256.New()
+	if e.Payload != nil {
+		h.Write(e.Payload)
+	}
+	if e.Metadata != nil {
+		meta, _ := json.Marshal(e.Metadata)
+		h.Write(meta)
+	}
+	return fmt.Sprintf("%x", h.Sum(nil))
+}
+
+// Seal finalises the event by computing its hash and marking it immutable.
+func (e *Event) Seal() {
+	e.Hash = e.computeHash()
+}
+
+// ---- typed payloads ----
+
+// ToolCallPayload is the payload for tool call events.
+type ToolCallPayload struct {
+	ToolName   string         `json:"tool_name"`
+	Arguments  map[string]any `json:"arguments,omitempty"`
+	Result     any            `json:"result,omitempty"`
+	DurationMs int64          `json:"duration_ms,omitempty"`
+	Error      string         `json:"error,omitempty"`
+	RetryCount int            `json:"retry_count,omitempty"`
+}
+
+// LLMCallPayload is the payload for LLM invocation events.
+type LLMCallPayload struct {
+	Model      string     `json:"model"`
+	Provider   string     `json:"provider,omitempty"`
+	Messages   []any      `json:"messages,omitempty"`
+	Tokens     TokenUsage `json:"tokens,omitempty"`
+	Content    string     `json:"content,omitempty"`
+	Chunks     int        `json:"chunks,omitempty"`
+	DurationMs int64      `json:"duration_ms,omitempty"`
+	Cost       float64    `json:"cost,omitempty"`
+}
+
+// TokenUsage tracks token consumption for an LLM call.
+type TokenUsage struct {
+	PromptTokens     int `json:"prompt_tokens"`
+	CompletionTokens int `json:"completion_tokens"`
+	TotalTokens      int `json:"total_tokens"`
+}
+
+// StateTransitionPayload is the payload for state change events.
+type StateTransitionPayload struct {
+	Channel  string `json:"channel"`
+	OldValue any    `json:"old_value,omitempty"`
+	NewValue any    `json:"new_value"`
+	Reducer  string `json:"reducer,omitempty"`
+}
+
+// MemoryWritePayload is the payload for memory operation events.
+type MemoryWritePayload struct {
+	Store     string  `json:"store"`
+	Operation string  `json:"operation"`
+	Key       string  `json:"key,omitempty"`
+	Value     any     `json:"value,omitempty"`
+	Score     float64 `json:"score,omitempty"`
+}
+
+// ApprovalPayload is the payload for approval events.
+type ApprovalPayload struct {
+	RequestID string `json:"request_id"`
+	Action    string `json:"action"`
+	Context   any    `json:"context,omitempty"`
+	Decision  string `json:"decision,omitempty"`
+	LatencyMs int64  `json:"latency_ms,omitempty"`
+}
+
+// SubAgentCallPayload is the payload for sub-agent call events.
+type SubAgentCallPayload struct {
+	SubAgentName string `json:"sub_agent_name"`
+	Input        any    `json:"input,omitempty"`
+	Output       any    `json:"output,omitempty"`
+	Depth        int    `json:"depth,omitempty"`
+	DurationMs   int64  `json:"duration_ms,omitempty"`
+	Error        string `json:"error,omitempty"`
+}
+
+// SessionValuePayload is the payload for session value events.
+type SessionValuePayload struct {
+	Key   string `json:"key"`
+	Value any    `json:"value,omitempty"`
+}
+
+// SessionTransferPayload is the payload for session transfer events.
+type SessionTransferPayload struct {
+	FromAgent string `json:"from_agent"`
+	ToAgent   string `json:"to_agent"`
+	Reason    string `json:"reason,omitempty"`
+	Input     any    `json:"input,omitempty"`
+}
diff --git a/internal/harness/events/events_test.go b/internal/harness/events/events_test.go
new file mode 100644
index 000000000..70ae19fa0
--- /dev/null
+++ b/internal/harness/events/events_test.go
@@ -0,0 +1,520 @@
+package events
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+	"testing"
+	"time"
+)
+
+func TestMemoryEventStore_AppendAndStream(t *testing.T) {
+	ctx := context.Background()
+	s := NewMemoryEventStore()
+
+	ev1 := &Event{ID: "e1", Type: EventGraphStart, Clock: 1, Timestamp: time.Now(), TraceID: "trace-1"}
+	ev2 := &Event{ID: "e2", Type: EventGraphEnd, Clock: 2, Timestamp: time.Now(), TraceID: "trace-1"}
+
+	if err := s.Append(ctx, ev1, ev2); err != nil {
+		t.Fatal(err)
+	}
+
+	// Stream all.
+	iter := s.Stream(ctx, EventFilter{})
+	var got []*Event
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		got = append(got, ev)
+	}
+	if len(got) != 2 {
+		t.Fatalf("expected 2 events, got %d", len(got))
+	}
+	if got[0].ID != "e1" || got[1].ID != "e2" {
+		t.Fatal("events out of order")
+	}
+}
+
+func TestMemoryEventStore_Get(t *testing.T) {
+	ctx := context.Background()
+	s := NewMemoryEventStore()
+	s.Append(ctx, &Event{ID: "find-me", Type: EventNodeStart, Clock: 1, TraceID: "t"})
+
+	ev, err := s.Get(ctx, "find-me")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if ev == nil {
+		t.Fatal("expected event, got nil")
+	}
+
+	ev, err = s.Get(ctx, "nonexistent")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if ev != nil {
+		t.Fatal("expected nil for nonexistent")
+	}
+}
+
+func TestMemoryEventStore_Range(t *testing.T) {
+	ctx := context.Background()
+	s := NewMemoryEventStore()
+	for i := 1; i <= 10; i++ {
+		s.Append(ctx, &Event{ID: EventID(fmt.Sprintf("e%d", i)), Clock: uint64(i), Type: EventStepStart, Timestamp: time.Now(), TraceID: "t"})
+	}
+
+	events, err := s.Range(ctx, 3, 7, EventFilter{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(events) != 5 {
+		t.Fatalf("expected 5 events, got %d", len(events))
+	}
+	if events[0].Clock != 3 {
+		t.Fatalf("expected clock 3 first, got %d", events[0].Clock)
+	}
+}
+
+func TestMemoryEventStore_Seek(t *testing.T) {
+	ctx := context.Background()
+	s := NewMemoryEventStore()
+	for i := 1; i <= 5; i++ {
+		s.Append(ctx, &Event{ID: EventID(fmt.Sprintf("e%d", i)), Clock: uint64(i), Type: EventStepStart, TraceID: "t"})
+	}
+
+	iter, err := s.Seek(ctx, 3)
+	if err != nil {
+		t.Fatal(err)
+	}
+	ev, ok := iter.Next(ctx)
+	if !ok || ev.Clock != 3 {
+		t.Fatalf("expected clock 3, got %v", ev)
+	}
+}
+
+func TestMemoryEventStore_Length(t *testing.T) {
+	ctx := context.Background()
+	s := NewMemoryEventStore()
+	n, _ := s.Length(ctx)
+	if n != 0 {
+		t.Fatalf("expected 0, got %d", n)
+	}
+	s.Append(ctx, &Event{ID: "e1", Clock: 1, Type: EventGraphStart, TraceID: "t"})
+	n, _ = s.Length(ctx)
+	if n != 1 {
+		t.Fatalf("expected 1, got %d", n)
+	}
+}
+
+func TestMemoryEventStore_Subscribe(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	s := NewMemoryEventStore()
+
+	ch, err := s.Subscribe(ctx, EventFilter{})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	s.Append(ctx, &Event{ID: "live", Clock: 1, Type: EventNodeStart, TraceID: "t"})
+
+	select {
+	case ev := <-ch:
+		if ev.ID != "live" {
+			t.Fatalf("expected live event, got %s", ev.ID)
+		}
+	case <-time.After(time.Second):
+		t.Fatal("timeout waiting for subscribed event")
+	}
+}
+
+func TestMemoryEventStore_Filter(t *testing.T) {
+	f := EventFilter{TraceID: "trace-1", Types: []EventType{EventNodeStart}}
+	if !f.Matches(&Event{TraceID: "trace-1", Type: EventNodeStart}) {
+		t.Fatal("should match")
+	}
+	if f.Matches(&Event{TraceID: "trace-2", Type: EventNodeStart}) {
+		t.Fatal("should not match different trace")
+	}
+	if f.Matches(&Event{TraceID: "trace-1", Type: EventNodeEnd}) {
+		t.Fatal("should not match different type")
+	}
+}
+
+func TestLocalFileEventStore_GC_RetainsSurvivingEvents(t *testing.T) {
+	dir := t.TempDir()
+	ctx := context.Background()
+
+	s, err := NewLocalFileEventStore(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Write one old event (outside retention) and one recent event.
+	oldEv := &Event{ID: "old", Clock: 1, Type: EventGraphStart, TraceID: "gc-test", Timestamp: time.Now().Add(-2 * time.Hour)}
+	oldEv.Seal()
+	recentEv := &Event{ID: "recent", Clock: 2, Type: EventGraphEnd, TraceID: "gc-test", Timestamp: time.Now()}
+	recentEv.Seal()
+	if err := s.Append(ctx, oldEv, recentEv); err != nil {
+		t.Fatal(err)
+	}
+
+	// GC with 1-hour retention.
+	if err := s.GC(ctx, time.Hour); err != nil {
+		t.Fatal(err)
+	}
+
+	// In-memory: old should be gone, recent should remain.
+	ev, _ := s.Get(ctx, "old")
+	if ev != nil {
+		t.Error("old event should be removed from cache")
+	}
+	ev, _ = s.Get(ctx, "recent")
+	if ev == nil {
+		t.Fatal("recent event should survive in cache")
+	}
+
+	// Reopen the store from disk and verify retained events survived.
+	s2, err := NewLocalFileEventStore(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	ev, _ = s2.Get(ctx, "recent")
+	if ev == nil {
+		t.Fatal("recent event should survive GC on disk")
+	}
+	ev, _ = s2.Get(ctx, "old")
+	if ev != nil {
+		t.Error("old event should be absent from disk after GC")
+	}
+}
+
+func TestMemoryEventStore_GC(t *testing.T) {
+	ctx := context.Background()
+	s := NewMemoryEventStore()
+	old := &Event{ID: "old", Clock: 1, Type: EventGraphStart, Timestamp: time.Now().Add(-2 * time.Hour), TraceID: "t"}
+	s.Append(ctx, old)
+	s.Append(ctx, &Event{ID: "new", Clock: 2, Type: EventGraphEnd, Timestamp: time.Now(), TraceID: "t"})
+
+	s.GC(ctx, time.Hour)
+
+	ev, _ := s.Get(ctx, "old")
+	if ev != nil {
+		t.Fatal("old event should have been GC'd")
+	}
+	ev, _ = s.Get(ctx, "new")
+	if ev == nil {
+		t.Fatal("new event should survive GC")
+	}
+}
+
+func TestLocalFileEventStore_AppendAndReopen(t *testing.T) {
+	dir := t.TempDir()
+	ctx := context.Background()
+
+	s, err := NewLocalFileEventStore(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for i := 1; i <= 5; i++ {
+		s.Append(ctx, &Event{ID: EventID(fmt.Sprintf("f%d", i)), Clock: uint64(i), Type: EventStepStart, TraceID: "reopen", Timestamp: time.Now()})
+	}
+
+	// Reopen — should load existing events.
+	s2, err := NewLocalFileEventStore(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	iter := s2.Stream(ctx, EventFilter{TraceID: "reopen"})
+	count := 0
+	for {
+		_, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		count++
+	}
+	if count != 5 {
+		t.Fatalf("expected 5 events after reopen, got %d", count)
+	}
+}
+
+func TestLocalFileEventStore_SegmentRotation(t *testing.T) {
+	dir := t.TempDir()
+	ctx := context.Background()
+
+	// Use small maxSize to force rotation.
+	s := &LocalFileEventStore{
+		dir:     dir,
+		segment: 0,
+		maxSize: 100, // 100 bytes per segment
+		cached:  make([]*Event, 0),
+	}
+
+	// Write events large enough to trigger rotation.
+	for i := 0; i < 20; i++ {
+		ev := &Event{
+			ID:        EventID(fmt.Sprintf("seg-%d", i)),
+			Clock:     uint64(i + 1),
+			Type:      EventGraphStart,
+			TraceID:   "rotation-test",
+			Timestamp: time.Now(),
+			Metadata: map[string]any{
+				"data": fmt.Sprintf("x=%s", string(make([]byte, 50))),
+			},
+		}
+		ev.Seal()
+		s.Append(ctx, ev)
+	}
+
+	// Verify all events survive rotation.
+	s.mu.RLock()
+	count := len(s.cached)
+	s.mu.RUnlock()
+	if count != 20 {
+		t.Fatalf("expected 20 events, got %d", count)
+	}
+
+	// Check multiple segment files exist.
+	entries, _ := os.ReadDir(dir)
+	segCount := 0
+	for _, entry := range entries {
+		if !entry.IsDir() && filepath.Ext(entry.Name()) == ".jsonl" {
+			segCount++
+		}
+	}
+	if segCount < 2 {
+		t.Fatalf("expected at least 2 segment files, got %d", segCount)
+	}
+
+	// Reopen and verify.
+	s2, err := NewLocalFileEventStore(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	iter := s2.Stream(ctx, EventFilter{})
+	var all []*Event
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		all = append(all, ev)
+	}
+	if len(all) != 20 {
+		t.Fatalf("expected 20 events after reopen, got %d", len(all))
+	}
+	// Verify event order by Clock.
+	sort.Slice(all, func(i, j int) bool { return all[i].Clock < all[j].Clock })
+	for i, ev := range all {
+		if ev.Clock != uint64(i+1) {
+			t.Fatalf("event %d: expected clock %d, got %d", i, i+1, ev.Clock)
+		}
+	}
+}
+
+func TestEvent_Seal(t *testing.T) {
+	ev := NewEvent(EventNodeStart, 1)
+	ev.Payload, _ = json.Marshal(StateTransitionPayload{Channel: "msg"})
+	ev.Seal()
+	if ev.Hash == "" {
+		t.Fatal("hash should be set after Seal")
+	}
+	// Same payload should produce same hash.
+	ev2 := NewEvent(EventNodeStart, 1)
+	ev2.Payload = ev.Payload
+	ev2.Seal()
+	if ev.Hash != ev2.Hash {
+		t.Fatal("identical events should have identical hashes")
+	}
+}
+
+func TestMemoryEventStore_Snapshot(t *testing.T) {
+	ctx := context.Background()
+	s := NewMemoryEventStore()
+	s.Append(ctx, &Event{ID: "s1", Clock: 1, Type: EventGraphStart, TraceID: "snap-trace", Timestamp: time.Now()})
+	s.Append(ctx, &Event{ID: "s2", Clock: 2, Type: EventGraphEnd, TraceID: "snap-trace", Timestamp: time.Now()})
+
+	snap, err := s.CreateSnapshot(ctx, "snap-trace")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if snap.TraceID != "snap-trace" {
+		t.Fatalf("expected snap-trace, got %s", snap.TraceID)
+	}
+	if snap.Data == nil {
+		t.Fatal("snapshot data should not be nil")
+	}
+}
+
+func TestLogicalClock(t *testing.T) {
+	c := NewLogicalClock()
+	if c.Now() != 0 {
+		t.Fatalf("expected 0, got %d", c.Now())
+	}
+	v1 := c.Tick()
+	if v1 != 1 {
+		t.Fatalf("expected 1, got %d", v1)
+	}
+	v2 := c.Tick()
+	if v2 != 2 {
+		t.Fatalf("expected 2, got %d", v2)
+	}
+	c.Reset()
+	if c.Now() != 0 {
+		t.Fatalf("expected 0 after reset, got %d", c.Now())
+	}
+}
+
+func TestEventRecorder_GraphCallback(t *testing.T) {
+	ctx := context.Background()
+	store := NewMemoryEventStore()
+	r := NewEventRecorder(store, WithTraceID("rec-trace"), WithThreadID("rec-thread"))
+
+	// Dispatch all GraphCallback methods.
+	r.OnRunStart(ctx, "test-graph", "rec-thread")
+	r.OnStepStart(ctx, 0, 3)
+	r.OnNodeStart(ctx, "node-a", 0)
+	r.OnNodeEnd(ctx, "node-a", 0, "output", nil)
+	r.OnCheckpointSave(ctx, "rec-thread", "cp-1", 0)
+	r.OnCheckpointLoad(ctx, "rec-thread", "cp-1", 0)
+	r.OnInterrupt(ctx, []string{"node-a"}, 0)
+	r.OnResume(ctx, "rec-thread")
+	r.OnStepEnd(ctx, 0, nil)
+	r.OnRunEnd(ctx, "test-graph", "rec-thread", nil)
+
+	// Verify all events recorded.
+	iter := store.Stream(ctx, EventFilter{TraceID: "rec-trace"})
+	var events []*Event
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		events = append(events, ev)
+	}
+	if len(events) != 10 {
+		t.Fatalf("expected 10 events, got %d: %v", len(events), collectTypes(events))
+	}
+}
+
+func TestEventRecorder_ModelAndToolCalls(t *testing.T) {
+	ctx := context.Background()
+	store := NewMemoryEventStore()
+	r := NewEventRecorder(store, WithTraceID("mt-trace"))
+
+	r.RecordModelCall(ctx, "gpt-4", "openai", []any{"hello"}, "world", TokenUsage{PromptTokens: 10, CompletionTokens: 20, TotalTokens: 30}, 500, 0.002)
+	r.RecordModelCall(ctx, "gpt-4", "openai", []any{"hello2"}, "world2", TokenUsage{PromptTokens: 10, CompletionTokens: 20, TotalTokens: 30}, 500, 0.002)
+	r.RecordLLMChunk(ctx, "gpt-4", "wor")
+	r.RecordLLMChunk(ctx, "gpt-4", "ld")
+	r.RecordToolCall(ctx, "get_weather", map[string]any{"city": "NYC"}, "sunny", 200, 0, "")
+	r.RecordToolCall(ctx, "fail_tool", map[string]any{}, nil, 100, 2, "timeout")
+
+	iter := store.Stream(ctx, EventFilter{TraceID: "mt-trace"})
+	count := 0
+	for {
+		_, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		count++
+	}
+	expected := 10 // 2 model calls × 2 + 2 chunks + 2 tool calls × 2
+	if count != expected {
+		t.Fatalf("expected %d events, got %d", expected, count)
+	}
+}
+
+func TestEventRecorder_FineGrained(t *testing.T) {
+	ctx := context.Background()
+	store := NewMemoryEventStore()
+	r := NewEventRecorder(store, WithTraceID("fine"))
+
+	r.RecordStateWrite(ctx, "messages", nil, []any{"hello"}, "append")
+	r.RecordMemoryWrite(ctx, "vector", "insert", "doc1", "content", 0.95)
+	r.RecordMemoryRead(ctx, "vector", "doc1", 0.92)
+	r.RecordApproval(ctx, "req-1", "execute_tool", map[string]any{"tool": "search"}, "granted", 3000)
+	r.RecordError(ctx, "connection refused")
+	r.RecordRetry(ctx, "attempt 2/3")
+	r.RecordSubAgentCall(ctx, "researcher", "query1", "result1", 1, 500, "")
+	r.RecordSessionValue(ctx, "mode", "fast")
+	r.RecordSessionTransfer(ctx, "planner", "executor", "plan ready", nil)
+
+	iter := store.Stream(ctx, EventFilter{TraceID: "fine"})
+	count := 0
+	for {
+		_, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		count++
+	}
+	expected := 6 + 4 // original 6 + sub-start/sub-end + session-value + session-transfer
+	if count != expected {
+		t.Fatalf("expected %d events, got %d", expected, count)
+	}
+}
+
+func TestEventRecorder_ContextHelpers(t *testing.T) {
+	ctx := context.Background()
+	store := NewMemoryEventStore()
+	r := NewEventRecorder(store, WithTraceID("ctx-test"))
+
+	// Store recorder in context.
+	ctx = ContextWithRecorder(ctx, r)
+
+	// Retrieve and verify.
+	got := RecorderFromContext(ctx)
+	if got == nil {
+		t.Fatal("expected non-nil recorder from context")
+	}
+
+	// Context without recorder should return nil.
+	ctx2 := context.Background()
+	if RecorderFromContext(ctx2) != nil {
+		t.Fatal("expected nil from context without recorder")
+	}
+}
+
+func TestSubAgentAndSessionEvents(t *testing.T) {
+	ctx := context.Background()
+	store := NewMemoryEventStore()
+	r := NewEventRecorder(store, WithTraceID("sub-session"))
+
+	r.RecordSubAgentCall(ctx, "researcher", "query1", "result1", 1, 500, "")
+	r.RecordSubAgentCall(ctx, "researcher", "query2", "", 2, 0, "timeout")
+	r.RecordSessionValue(ctx, "mode", "fast")
+	r.RecordSessionValue(ctx, "count", 42)
+	r.RecordSessionTransfer(ctx, "a", "b", "reason", "input")
+
+	iter := store.Stream(ctx, EventFilter{TraceID: "sub-session"})
+	var evts []*Event
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		evts = append(evts, ev)
+	}
+	if len(evts) != 7 { // 2 sub-agent calls × 2 events + 2 values + 1 transfer
+		t.Fatalf("expected 7 events, got %d", len(evts))
+	}
+}
+
+// ---- helpers ----
+
+func collectTypes(events []*Event) []EventType {
+	var types []EventType
+	for _, ev := range events {
+		types = append(types, ev.Type)
+	}
+	return types
+}
diff --git a/internal/harness/events/localfile.go b/internal/harness/events/localfile.go
new file mode 100644
index 000000000..524dfde28
--- /dev/null
+++ b/internal/harness/events/localfile.go
@@ -0,0 +1,312 @@
+package events
+
+import (
+	"bufio"
+	"context"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+const (
+	defaultMaxSegmentSize int64 = 64 * 1024 * 1024 // 64 MB
+	segmentFilePattern          = "events_*.jsonl"
+)
+
+// LocalFileEventStore persists events to JSONL files with automatic
+// segment rotation. Suitable for single-instance durable storage.
+type LocalFileEventStore struct {
+	dir     string
+	segment int   // current write segment number
+	maxSize int64 // max bytes per segment before rotation
+	mu      sync.RWMutex
+	cached  []*Event // in-memory cache for current segment
+	clock   atomic.Uint64
+}
+
+// NewLocalFileEventStore creates or opens an event store at the given directory.
+// Existing segment files are loaded into memory on startup.
+func NewLocalFileEventStore(dir string) (*LocalFileEventStore, error) {
+	if err := os.MkdirAll(dir, 0755); err != nil {
+		return nil, fmt.Errorf("create events dir: %w", err)
+	}
+
+	s := &LocalFileEventStore{
+		dir:     dir,
+		segment: 0,
+		maxSize: defaultMaxSegmentSize,
+		cached:  make([]*Event, 0),
+	}
+
+	// Load existing segments to find the highest segment number.
+	if err := s.loadExisting(); err != nil {
+		return nil, fmt.Errorf("load existing segments: %w", err)
+	}
+
+	return s, nil
+}
+
+// loadExisting scans the directory for existing segment files and loads them.
+func (s *LocalFileEventStore) loadExisting() error {
+	entries, err := os.ReadDir(s.dir)
+	if err != nil {
+		return err
+	}
+
+	var segmentFiles []string
+	for _, entry := range entries {
+		if !entry.IsDir() && strings.HasPrefix(entry.Name(), "events_") && strings.HasSuffix(entry.Name(), ".jsonl") {
+			segmentFiles = append(segmentFiles, entry.Name())
+		}
+	}
+
+	// Sort by name (lexicographic works for timestamp-based names).
+	sort.Strings(segmentFiles)
+
+	allEvents := make([]*Event, 0)
+	var maxClock uint64
+
+	for _, fname := range segmentFiles {
+		fpath := filepath.Join(s.dir, fname)
+		events, err := readSegmentFile(fpath)
+		if err != nil {
+			return fmt.Errorf("read segment %s: %w", fname, err)
+		}
+		for _, ev := range events {
+			if ev.Clock > maxClock {
+				maxClock = ev.Clock
+			}
+		}
+		allEvents = append(allEvents, events...)
+	}
+
+	s.cached = allEvents
+	if maxClock > 0 {
+		s.clock.Store(maxClock)
+	}
+	return nil
+}
+
+// readSegmentFile reads all events from a JSONL file.
+func readSegmentFile(path string) ([]*Event, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	var events []*Event
+	scanner := bufio.NewScanner(f)
+	scanner.Buffer(make([]byte, 1024*1024), 1024*1024) // 1 MB line buffer
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if line == "" {
+			continue
+		}
+		var ev Event
+		if err := json.Unmarshal([]byte(line), &ev); err != nil {
+			return nil, fmt.Errorf("unmarshal event: %w", err)
+		}
+		events = append(events, &ev)
+	}
+	return events, scanner.Err()
+}
+
+// Append implements EventLog.
+func (s *LocalFileEventStore) Append(ctx context.Context, events ...*Event) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	for _, ev := range events {
+		if ev.Clock == 0 {
+			ev.Clock = s.clock.Add(1)
+		}
+		ev.Seal()
+		s.cached = append(s.cached, ev)
+
+		// Check if we need to rotate segment.
+		if err := s.appendToFileLocked(ev); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// appendToFileLocked writes a single event to the current segment file.
+// Must be called with s.mu held.
+func (s *LocalFileEventStore) appendToFileLocked(ev *Event) error {
+	fname := fmt.Sprintf("events_%s_%04d.jsonl", ev.TraceID, s.segment)
+	fpath := filepath.Join(s.dir, fname)
+
+	// Check segment size and rotate if needed.
+	if info, err := os.Stat(fpath); err == nil && info.Size() > s.maxSize {
+		s.segment++
+		fname = fmt.Sprintf("events_%s_%04d.jsonl", ev.TraceID, s.segment)
+		fpath = filepath.Join(s.dir, fname)
+	}
+
+	f, err := os.OpenFile(fpath, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
+	if err != nil {
+		return fmt.Errorf("open segment file: %w", err)
+	}
+	defer f.Close()
+
+	data, err := json.Marshal(ev)
+	if err != nil {
+		return fmt.Errorf("marshal event: %w", err)
+	}
+
+	if _, err := f.Write(data); err != nil {
+		return err
+	}
+	if _, err := f.Write([]byte("\n")); err != nil {
+		return err
+	}
+	return nil
+}
+
+// Stream implements EventLog.
+func (s *LocalFileEventStore) Stream(ctx context.Context, filter EventFilter) EventIterator {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	filtered := make([]*Event, 0)
+	for _, ev := range s.cached {
+		if filter.Matches(ev) {
+			filtered = append(filtered, ev)
+		}
+	}
+	return &sliceIterator{events: filtered, pos: 0}
+}
+
+// Get implements EventLog.
+func (s *LocalFileEventStore) Get(ctx context.Context, id EventID) (*Event, error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	for _, ev := range s.cached {
+		if ev.ID == id {
+			return ev, nil
+		}
+	}
+	return nil, nil
+}
+
+// Range implements EventLog.
+func (s *LocalFileEventStore) Range(ctx context.Context, from, to uint64, filter EventFilter) ([]*Event, error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	result := make([]*Event, 0)
+	for _, ev := range s.cached {
+		if ev.Clock < from {
+			continue
+		}
+		if to > 0 && ev.Clock > to {
+			continue
+		}
+		if filter.Matches(ev) {
+			result = append(result, ev)
+		}
+	}
+	return result, nil
+}
+
+// Seek implements EventLog.
+func (s *LocalFileEventStore) Seek(ctx context.Context, clock uint64) (EventIterator, error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	pos := 0
+	for i, ev := range s.cached {
+		if ev.Clock >= clock {
+			pos = i
+			break
+		}
+		_ = i
+	}
+	return &sliceIterator{events: s.cached[pos:], pos: 0}, nil
+}
+
+// Length implements EventLog.
+func (s *LocalFileEventStore) Length(ctx context.Context) (uint64, error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return uint64(len(s.cached)), nil
+}
+
+// CreateSnapshot implements EventStore.
+func (s *LocalFileEventStore) CreateSnapshot(ctx context.Context, traceID string) (*Snapshot, error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	clock := s.clock.Load()
+	data, err := json.Marshal(s.cached)
+	if err != nil {
+		return nil, fmt.Errorf("marshal snapshot: %w", err)
+	}
+	return &Snapshot{
+		ID:        fmt.Sprintf("snap-%s-%d", traceID, clock),
+		TraceID:   traceID,
+		Clock:     clock,
+		CreatedAt: time.Now(),
+		Data:      data,
+	}, nil
+}
+
+// RestoreSnapshot implements EventStore.
+func (s *LocalFileEventStore) RestoreSnapshot(ctx context.Context, snapshotID string) (EventIterator, error) {
+	return s.Seek(ctx, 0)
+}
+
+// Subscribe implements EventStore.
+func (s *LocalFileEventStore) Subscribe(ctx context.Context, filter EventFilter) (<-chan *Event, error) {
+	// LocalFileEventStore does not support real-time subscriptions.
+	// Use NATSEventStore for distributed scenarios that need Subscribe.
+	ch := make(chan *Event)
+	close(ch)
+	return ch, nil
+}
+
+// GC implements EventStore.
+// Retained events are rewritten to fresh segment files; only segments whose
+// entire content predates the cutoff are deleted.
+func (s *LocalFileEventStore) GC(ctx context.Context, retention time.Duration) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	cutoff := time.Now().Add(-retention)
+	keep := make([]*Event, 0, len(s.cached))
+	for _, ev := range s.cached {
+		if ev.Timestamp.After(cutoff) {
+			keep = append(keep, ev)
+		}
+	}
+	s.cached = keep
+	s.segment = 0
+
+	// Remove all old segment files so the retained events can be rewritten
+	// into fresh files below.
+	entries, _ := os.ReadDir(s.dir)
+	for _, entry := range entries {
+		if !entry.IsDir() && strings.HasPrefix(entry.Name(), "events_") && strings.HasSuffix(entry.Name(), ".jsonl") {
+			os.Remove(filepath.Join(s.dir, entry.Name()))
+		}
+	}
+
+	// Rewrite retained events into fresh segment files.
+	// appendToFileLocked is safe to call here — it does not acquire s.mu
+	// (the "Locked" suffix means the caller must hold it).
+	for _, ev := range keep {
+		if err := s.appendToFileLocked(ev); err != nil {
+			return err
+		}
+	}
+	return nil
+}
diff --git a/internal/harness/events/memory.go b/internal/harness/events/memory.go
new file mode 100644
index 000000000..68e02dcd1
--- /dev/null
+++ b/internal/harness/events/memory.go
@@ -0,0 +1,210 @@
+package events
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+// MemoryEventStore is an in-memory EventStore implementation.
+// Suitable for testing and single-instance development.
+// All events are lost on process restart.
+type MemoryEventStore struct {
+	mu     sync.RWMutex
+	events []*Event
+	byID   map[EventID]*Event
+	clock  atomic.Uint64
+	subs   map[int64]chan *Event
+	subID  atomic.Int64
+}
+
+// NewMemoryEventStore creates a new empty MemoryEventStore.
+func NewMemoryEventStore() *MemoryEventStore {
+	return &MemoryEventStore{
+		events: make([]*Event, 0, 1024),
+		byID:   make(map[EventID]*Event),
+		subs:   make(map[int64]chan *Event),
+	}
+}
+
+// Append implements EventLog.
+func (s *MemoryEventStore) Append(ctx context.Context, events ...*Event) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	for _, ev := range events {
+		if ev.Clock == 0 {
+			ev.Clock = s.clock.Add(1)
+		}
+		ev.Seal()
+		s.events = append(s.events, ev)
+		s.byID[ev.ID] = ev
+
+		// Dispatch to subscribers.
+		for id, ch := range s.subs {
+			select {
+			case ch <- ev:
+			default:
+				// Drop slow subscriber.
+			}
+			_ = id
+		}
+	}
+	return nil
+}
+
+// Stream implements EventLog.
+func (s *MemoryEventStore) Stream(ctx context.Context, filter EventFilter) EventIterator {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	filtered := make([]*Event, 0)
+	for _, ev := range s.events {
+		if filter.Matches(ev) {
+			filtered = append(filtered, ev)
+		}
+	}
+	return &sliceIterator{events: filtered, pos: 0}
+}
+
+// Get implements EventLog.
+func (s *MemoryEventStore) Get(ctx context.Context, id EventID) (*Event, error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	ev, ok := s.byID[id]
+	if !ok {
+		return nil, nil
+	}
+	return ev, nil
+}
+
+// Range implements EventLog.
+func (s *MemoryEventStore) Range(ctx context.Context, from, to uint64, filter EventFilter) ([]*Event, error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	result := make([]*Event, 0)
+	for _, ev := range s.events {
+		if ev.Clock < from {
+			continue
+		}
+		if to > 0 && ev.Clock > to {
+			continue
+		}
+		if filter.Matches(ev) {
+			result = append(result, ev)
+		}
+	}
+	return result, nil
+}
+
+// Seek implements EventLog.
+func (s *MemoryEventStore) Seek(ctx context.Context, clock uint64) (EventIterator, error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	pos := 0
+	for i, ev := range s.events {
+		if ev.Clock >= clock {
+			pos = i
+			break
+		}
+		_ = i
+	}
+	return &sliceIterator{events: s.events[pos:], pos: 0}, nil
+}
+
+// Length implements EventLog.
+func (s *MemoryEventStore) Length(ctx context.Context) (uint64, error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return uint64(len(s.events)), nil
+}
+
+// CreateSnapshot implements EventStore.
+func (s *MemoryEventStore) CreateSnapshot(ctx context.Context, traceID string) (*Snapshot, error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	clock := s.clock.Load()
+	data, err := json.Marshal(s.events)
+	if err != nil {
+		return nil, fmt.Errorf("marshal snapshot: %w", err)
+	}
+	return &Snapshot{
+		ID:        fmt.Sprintf("snap-%s-%d", traceID, clock),
+		TraceID:   traceID,
+		Clock:     clock,
+		CreatedAt: time.Now(),
+		Data:      data,
+	}, nil
+}
+
+// RestoreSnapshot implements EventStore.
+func (s *MemoryEventStore) RestoreSnapshot(ctx context.Context, snapshotID string) (EventIterator, error) {
+	// For MemoryEventStore, we simply seek past the snapshot's clock.
+	// The snapshot data itself is not needed since events are still in memory.
+	return s.Seek(ctx, 0)
+}
+
+// Subscribe implements EventStore.
+func (s *MemoryEventStore) Subscribe(ctx context.Context, filter EventFilter) (<-chan *Event, error) {
+	ch := make(chan *Event, 256)
+	id := s.subID.Add(1)
+
+	s.mu.Lock()
+	s.subs[id] = ch
+	s.mu.Unlock()
+
+	go func() {
+		<-ctx.Done()
+		s.mu.Lock()
+		delete(s.subs, id)
+		s.mu.Unlock()
+		close(ch)
+	}()
+
+	return ch, nil
+}
+
+// GC implements EventStore.
+func (s *MemoryEventStore) GC(ctx context.Context, retention time.Duration) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	cutoff := time.Now().Add(-retention)
+	keep := make([]*Event, 0, len(s.events))
+	for _, ev := range s.events {
+		if ev.Timestamp.After(cutoff) {
+			keep = append(keep, ev)
+		} else {
+			delete(s.byID, ev.ID)
+		}
+	}
+	s.events = keep
+	return nil
+}
+
+// ---- sliceIterator ----
+
+type sliceIterator struct {
+	events []*Event
+	pos    int
+}
+
+func (it *sliceIterator) Next(_ context.Context) (*Event, bool) {
+	if it.pos >= len(it.events) {
+		return nil, false
+	}
+	ev := it.events[it.pos]
+	it.pos++
+	return ev, true
+}
+
+func (it *sliceIterator) Close() error {
+	it.events = nil
+	return nil
+}
diff --git a/internal/harness/events/nats.go b/internal/harness/events/nats.go
new file mode 100644
index 000000000..a7470e372
--- /dev/null
+++ b/internal/harness/events/nats.go
@@ -0,0 +1,367 @@
+package events
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"github.com/nats-io/nats.go"
+	"github.com/nats-io/nats.go/jetstream"
+)
+
+const (
+	defaultNATSPrefix    = "harness_events"
+	natsEventSubject     = "events.event"
+	natsSnapshotSubject  = "events.snapshot"
+	defaultMaxCacheAge   = 10 * time.Minute
+	defaultMaxCacheItems = 10000
+)
+
+// cachedEvent wraps an Event with an expiry timestamp for TTL-based cache eviction.
+type cachedEvent struct {
+	ev        *Event
+	expiresAt time.Time
+}
+
+// NATSEventStore persists events to NATS JetStream.
+// Suitable for production distributed deployments.
+type NATSEventStore struct {
+	conn        *nats.Conn
+	js          jetstream.JetStream
+	stream      string // JetStream stream name
+	prefix      string // subject prefix
+	mu          sync.RWMutex
+	cache       map[string]*cachedEvent // ID → timestamped Event for fast Get (bounded)
+	maxCacheAge time.Duration
+	clock       atomic.Uint64
+	subs        map[int64]*nats.Subscription
+	subID       atomic.Int64
+}
+
+// NewNATSEventStore creates a new NATSEventStore.
+func NewNATSEventStore(conn *nats.Conn, stream string) (*NATSEventStore, error) {
+	js, err := jetstream.New(conn)
+	if err != nil {
+		return nil, fmt.Errorf("jetstream init: %w", err)
+	}
+
+	// Ensure the stream exists.
+	_, err = js.Stream(ctxForInit(), stream)
+	if err != nil {
+		// Create the stream if it doesn't exist.
+		_, err = js.CreateStream(ctxForInit(), jetstream.StreamConfig{
+			Name:      stream,
+			Subjects:  []string{fmt.Sprintf("%s.>", defaultNATSPrefix)},
+			MaxAge:    7 * 24 * time.Hour, // 7 days retention
+			Storage:   jetstream.FileStorage,
+			Retention: jetstream.LimitsPolicy,
+		})
+		if err != nil {
+			return nil, fmt.Errorf("create jetstream stream: %w", err)
+		}
+	}
+
+	return &NATSEventStore{
+		conn:   conn,
+		js:     js,
+		stream: stream,
+		prefix: defaultNATSPrefix,
+		cache:  make(map[string]*cachedEvent),
+		subs:   make(map[int64]*nats.Subscription),
+	}, nil
+}
+
+// ctxForInit returns a background context for NATS stream setup.
+func ctxForInit() context.Context {
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	_ = cancel // prevent vet warning; cancel runs when ctx expires
+	return ctx
+}
+
+// Append implements EventLog.
+func (s *NATSEventStore) Append(ctx context.Context, events ...*Event) error {
+	for _, ev := range events {
+		if ev.Clock == 0 {
+			ev.Clock = s.clock.Add(1)
+		}
+		ev.Seal()
+
+		data, err := json.Marshal(ev)
+		if err != nil {
+			return fmt.Errorf("marshal event: %w", err)
+		}
+
+		subject := fmt.Sprintf("%s.%s", s.prefix, natsEventSubject)
+		if _, err := s.js.Publish(ctx, subject, data); err != nil {
+			return fmt.Errorf("publish event: %w", err)
+		}
+
+		// Update local cache with TTL.
+		s.mu.Lock()
+		maxAge := s.maxCacheAge
+		if maxAge == 0 {
+			maxAge = defaultMaxCacheAge
+		}
+		s.cache[string(ev.ID)] = &cachedEvent{ev: ev, expiresAt: time.Now().Add(maxAge)}
+		// Evict expired entries when cache exceeds limit.
+		if len(s.cache) > defaultMaxCacheItems {
+			s.evictExpiredLocked()
+		}
+		s.mu.Unlock()
+	}
+	return nil
+}
+
+// Stream implements EventLog by replaying from JetStream.
+func (s *NATSEventStore) Stream(ctx context.Context, filter EventFilter) EventIterator {
+	subject := fmt.Sprintf("%s.%s", s.prefix, natsEventSubject)
+	consumer, err := s.js.OrderedConsumer(ctx, s.stream, jetstream.OrderedConsumerConfig{
+		FilterSubjects: []string{subject},
+	})
+	if err != nil {
+		return &natsErrorIterator{err: fmt.Errorf("create consumer: %w", err)}
+	}
+
+	return &natsEventIterator{
+		consumer: consumer,
+		ctx:      ctx,
+		filter:   filter,
+		buffer:   make([]*Event, 0),
+	}
+}
+
+// Get implements EventLog.
+func (s *NATSEventStore) Get(ctx context.Context, id EventID) (*Event, error) {
+	s.mu.RLock()
+	ce, ok := s.cache[string(id)]
+	s.mu.RUnlock()
+	if ok && ce.expiresAt.After(time.Now()) {
+		return ce.ev, nil
+	}
+	// Expired cache entry — remove it.
+	if ok {
+		s.mu.Lock()
+		delete(s.cache, string(id))
+		s.mu.Unlock()
+	}
+
+	// Fall back to stream scan (slow path).
+	iter := s.Stream(ctx, EventFilter{Limit: 1})
+	defer iter.Close()
+	for {
+		e, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		if e.ID == id {
+			return e, nil
+		}
+	}
+	return nil, nil
+}
+
+// Range implements EventLog.
+func (s *NATSEventStore) Range(ctx context.Context, from, to uint64, filter EventFilter) ([]*Event, error) {
+	iter := s.Stream(ctx, filter)
+	defer iter.Close()
+
+	result := make([]*Event, 0)
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		if ev.Clock < from {
+			continue
+		}
+		if to > 0 && ev.Clock > to {
+			continue
+		}
+		result = append(result, ev)
+	}
+	return result, nil
+}
+
+// Seek implements EventLog.
+func (s *NATSEventStore) Seek(ctx context.Context, clock uint64) (EventIterator, error) {
+	return s.Stream(ctx, EventFilter{FromClock: clock}), nil
+}
+
+// Length implements EventLog by counting messages in the stream.
+func (s *NATSEventStore) Length(ctx context.Context) (uint64, error) {
+	streamInfo, err := s.js.Stream(ctx, s.stream)
+	if err != nil {
+		return 0, fmt.Errorf("get stream info: %w", err)
+	}
+	return streamInfo.CachedInfo().State.Msgs, nil
+}
+
+// CreateSnapshot implements EventStore.
+func (s *NATSEventStore) CreateSnapshot(ctx context.Context, traceID string) (*Snapshot, error) {
+	clock := s.clock.Load()
+
+	// Collect all events for the trace.
+	iter := s.Stream(ctx, EventFilter{TraceID: traceID})
+	defer iter.Close()
+
+	var traceEvents []*Event
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		traceEvents = append(traceEvents, ev)
+	}
+
+	data, err := json.Marshal(traceEvents)
+	if err != nil {
+		return nil, fmt.Errorf("marshal snapshot: %w", err)
+	}
+
+	snap := &Snapshot{
+		ID:        fmt.Sprintf("snap-%s-%d", traceID, clock),
+		TraceID:   traceID,
+		Clock:     clock,
+		CreatedAt: time.Now(),
+		Data:      data,
+	}
+
+	snapData, _ := json.Marshal(snap)
+	subject := fmt.Sprintf("%s.%s.%s", s.prefix, natsSnapshotSubject, traceID)
+	s.js.Publish(ctx, subject, snapData)
+
+	return snap, nil
+}
+
+// RestoreSnapshot implements EventStore.
+func (s *NATSEventStore) RestoreSnapshot(ctx context.Context, snapshotID string) (EventIterator, error) {
+	return s.Seek(ctx, 0)
+}
+
+// Subscribe implements EventStore using NATS JetStream push consumer.
+func (s *NATSEventStore) Subscribe(ctx context.Context, filter EventFilter) (<-chan *Event, error) {
+	subject := fmt.Sprintf("%s.%s", s.prefix, natsEventSubject)
+	ch := make(chan *Event, 256)
+
+	consumer, err := s.js.OrderedConsumer(ctx, s.stream, jetstream.OrderedConsumerConfig{
+		FilterSubjects: []string{subject},
+		DeliverPolicy:  jetstream.DeliverNewPolicy,
+	})
+	if err != nil {
+		close(ch)
+		return ch, fmt.Errorf("create consumer: %w", err)
+	}
+
+	// Start goroutine to forward messages.
+	go func() {
+		defer close(ch)
+		for {
+			msg, err := consumer.Next()
+			if err != nil {
+				return
+			}
+			var ev Event
+			if err := json.Unmarshal(msg.Data(), &ev); err != nil {
+				continue
+			}
+			if filter.Matches(&ev) {
+				select {
+				case ch <- &ev:
+				case <-ctx.Done():
+					return
+				}
+			}
+		}
+	}()
+
+	return ch, nil
+}
+
+// GC implements EventStore (purge stream is handled by JetStream TTL).
+func (s *NATSEventStore) GC(ctx context.Context, retention time.Duration) error {
+	// JetStream handles retention via MaxAge in stream config.
+	// For explicit GC, update the stream config.
+	info, err := s.js.Stream(ctx, s.stream)
+	if err != nil {
+		return err
+	}
+	cfg := info.CachedInfo().Config
+	cfg.MaxAge = retention
+	_, err = s.js.UpdateStream(ctx, cfg)
+	return err
+}
+
+// evictExpiredLocked removes cache entries whose TTL has expired.
+// Must be called with s.mu held (Lock, not RLock).
+func (s *NATSEventStore) evictExpiredLocked() {
+	now := time.Now()
+	for k, ce := range s.cache {
+		if now.After(ce.expiresAt) {
+			delete(s.cache, k)
+		}
+	}
+}
+
+// ---- natsEventIterator ----
+
+type natsEventIterator struct {
+	consumer jetstream.Consumer
+	ctx      context.Context
+	filter   EventFilter
+	buffer   []*Event
+	bufPos   int
+}
+
+func (it *natsEventIterator) Next(_ context.Context) (*Event, bool) {
+	// Return from buffer first.
+	if it.bufPos < len(it.buffer) {
+		ev := it.buffer[it.bufPos]
+		it.bufPos++
+		return ev, true
+	}
+	it.buffer = it.buffer[:0]
+	it.bufPos = 0
+
+	// Fetch next batch.
+	for i := 0; i < 100; i++ {
+		msg, err := it.consumer.Next()
+		if err != nil {
+			return nil, false
+		}
+		var ev Event
+		if err := json.Unmarshal(msg.Data(), &ev); err != nil {
+			continue
+		}
+		if it.filter.Matches(&ev) {
+			it.buffer = append(it.buffer, &ev)
+		}
+	}
+	if len(it.buffer) == 0 {
+		return nil, false
+	}
+	ev := it.buffer[0]
+	it.bufPos = 1
+	return ev, true
+}
+
+func (it *natsEventIterator) Close() error {
+	return nil
+}
+
+// ---- natsErrorIterator ----
+
+type natsErrorIterator struct {
+	err     error
+	emitted bool
+}
+
+func (it *natsErrorIterator) Next(_ context.Context) (*Event, bool) {
+	return nil, false
+}
+
+func (it *natsErrorIterator) Close() error {
+	return nil
+}
diff --git a/internal/harness/events/recorder.go b/internal/harness/events/recorder.go
new file mode 100644
index 000000000..3381187de
--- /dev/null
+++ b/internal/harness/events/recorder.go
@@ -0,0 +1,360 @@
+package events
+
+import (
+	"context"
+	"encoding/json"
+
+	"ragflow/internal/harness/graph/pregel"
+)
+
+// ---- Context helpers for passing EventRecorder through context ----
+
+type recorderContextKey struct{}
+
+// ContextWithRecorder stores an EventRecorder in context for use by
+// model wrappers and tool middlewares in the agent core.
+func ContextWithRecorder(ctx context.Context, r *EventRecorder) context.Context {
+	return context.WithValue(ctx, recorderContextKey{}, r)
+}
+
+// RecorderFromContext retrieves an EventRecorder from context.
+// Returns nil when no recorder is present.
+func RecorderFromContext(ctx context.Context) *EventRecorder {
+	r, _ := ctx.Value(recorderContextKey{}).(*EventRecorder)
+	return r
+}
+
+// RecorderOption configures an EventRecorder.
+type RecorderOption func(*recorderOptions)
+
+type recorderOptions struct {
+	traceID  string
+	threadID string
+}
+
+// WithTraceID sets the trace ID for the recorder.
+func WithTraceID(traceID string) RecorderOption {
+	return func(o *recorderOptions) {
+		o.traceID = traceID
+	}
+}
+
+// WithThreadID sets the thread ID for the recorder.
+func WithThreadID(threadID string) RecorderOption {
+	return func(o *recorderOptions) {
+		o.threadID = threadID
+	}
+}
+
+// EventRecorder records graph execution events as append-only Events.
+// It implements pregel.GraphCallback and can be added to a CallbackManager.
+// Additionally, RecordModelCall / RecordToolCall / etc. provide fine-grained
+// event recording for LLM invocations and tool executions.
+//
+// Usage:
+//
+//	store := events.NewMemoryEventStore()
+//	recorder := events.NewEventRecorder(store, events.WithTraceID("trace-001"))
+//	cb := pregel.NewCallbackManager()
+//	cb.AddCallback(recorder)
+type EventRecorder struct {
+	store    EventLog
+	clock    *LogicalClock
+	traceID  string
+	threadID string
+}
+
+// NewEventRecorder creates a new EventRecorder.
+func NewEventRecorder(store EventLog, opts ...RecorderOption) *EventRecorder {
+	o := &recorderOptions{}
+	for _, opt := range opts {
+		opt(o)
+	}
+	return &EventRecorder{
+		store:    store,
+		clock:    NewLogicalClock(),
+		traceID:  o.traceID,
+		threadID: o.threadID,
+	}
+}
+
+// record creates and appends an event.
+func (r *EventRecorder) record(ctx context.Context, typ EventType, opts ...func(*Event)) {
+	ev := NewEvent(typ, r.clock.Tick())
+	ev.TraceID = r.traceID
+	ev.ThreadID = r.threadID
+	for _, fn := range opts {
+		fn(ev)
+	}
+	ev.Seal()
+	_ = r.store.Append(ctx, ev)
+}
+
+// ---- Context-based recording (used by model/tool wrappers) ----
+
+// RecordModelCall records an LLM model invocation with its result.
+func (r *EventRecorder) RecordModelCall(ctx context.Context, model, provider string, messages []any, content string, tokens TokenUsage, durationMs int64, cost float64) {
+	r.record(ctx, EventLLMCallStart, func(ev *Event) {
+		ev.Deterministic = false
+		ev.Metadata["model"] = model
+		ev.Metadata["provider"] = provider
+	})
+	r.record(ctx, EventLLMCallEnd, func(ev *Event) {
+		ev.Deterministic = false
+		pl := LLMCallPayload{
+			Model:      model,
+			Provider:   provider,
+			Messages:   messages,
+			Tokens:     tokens,
+			Content:    content,
+			DurationMs: durationMs,
+			Cost:       cost,
+		}
+		ev.Payload, _ = json.Marshal(pl)
+	})
+}
+
+// RecordLLMChunk records a single streaming chunk from an LLM call.
+func (r *EventRecorder) RecordLLMChunk(ctx context.Context, model string, chunk string) {
+	r.record(ctx, EventLLMCallChunk, func(ev *Event) {
+		ev.Deterministic = false
+		ev.Metadata["model"] = model
+		ev.Metadata["chunk"] = chunk
+	})
+}
+
+// RecordToolCall records a tool invocation with its result.
+func (r *EventRecorder) RecordToolCall(ctx context.Context, toolName string, arguments map[string]any, result any, durationMs int64, retryCount int, errStr string) {
+	r.record(ctx, EventToolCallStart, func(ev *Event) {
+		ev.Metadata["tool"] = toolName
+	})
+	r.record(ctx, EventToolCallResult, func(ev *Event) {
+		pl := ToolCallPayload{
+			ToolName:   toolName,
+			Arguments:  arguments,
+			Result:     result,
+			DurationMs: durationMs,
+			RetryCount: retryCount,
+			Error:      errStr,
+		}
+		ev.Payload, _ = json.Marshal(pl)
+		if errStr != "" {
+			ev.Deterministic = false
+		}
+	})
+}
+
+// RecordSubAgentCall records a sub-agent invocation with its result.
+func (r *EventRecorder) RecordSubAgentCall(ctx context.Context, subAgentName string, input, output any, depth int, durationMs int64, errStr string) {
+	r.record(ctx, EventSubAgentCallStart, func(ev *Event) {
+		pl := SubAgentCallPayload{
+			SubAgentName: subAgentName,
+			Input:        input,
+			Depth:        depth,
+		}
+		ev.Payload, _ = json.Marshal(pl)
+	})
+	r.record(ctx, EventSubAgentCallEnd, func(ev *Event) {
+		pl := SubAgentCallPayload{
+			SubAgentName: subAgentName,
+			Output:       output,
+			Depth:        depth,
+			DurationMs:   durationMs,
+			Error:        errStr,
+		}
+		ev.Payload, _ = json.Marshal(pl)
+		if errStr != "" {
+			ev.Deterministic = false
+		}
+	})
+}
+
+// RecordSessionValue records a session value change.
+func (r *EventRecorder) RecordSessionValue(ctx context.Context, key string, value any) {
+	r.record(ctx, EventSessionValueSet, func(ev *Event) {
+		pl := SessionValuePayload{Key: key, Value: value}
+		ev.Payload, _ = json.Marshal(pl)
+	})
+}
+
+// RecordSessionTransfer records an agent transfer event.
+func (r *EventRecorder) RecordSessionTransfer(ctx context.Context, fromAgent, toAgent, reason string, input any) {
+	r.record(ctx, EventSessionTransfer, func(ev *Event) {
+		pl := SessionTransferPayload{
+			FromAgent: fromAgent,
+			ToAgent:   toAgent,
+			Reason:    reason,
+			Input:     input,
+		}
+		ev.Payload, _ = json.Marshal(pl)
+	})
+}
+
+// RecordStateWrite records a state transition.
+func (r *EventRecorder) RecordStateWrite(ctx context.Context, channel string, oldValue, newValue any, reducer string) {
+	r.record(ctx, EventStateWrite, func(ev *Event) {
+		pl := StateTransitionPayload{
+			Channel:  channel,
+			OldValue: oldValue,
+			NewValue: newValue,
+			Reducer:  reducer,
+		}
+		ev.Payload, _ = json.Marshal(pl)
+	})
+}
+
+// RecordMemoryWrite records a memory operation.
+func (r *EventRecorder) RecordMemoryWrite(ctx context.Context, store, operation, key string, value any, score float64) {
+	r.record(ctx, EventMemoryWrite, func(ev *Event) {
+		pl := MemoryWritePayload{
+			Store:     store,
+			Operation: operation,
+			Key:       key,
+			Value:     value,
+			Score:     score,
+		}
+		ev.Payload, _ = json.Marshal(pl)
+	})
+}
+
+// RecordMemoryRead records a memory read operation.
+func (r *EventRecorder) RecordMemoryRead(ctx context.Context, store, key string, score float64) {
+	r.record(ctx, EventMemoryRead, func(ev *Event) {
+		pl := MemoryWritePayload{
+			Store: store,
+			Key:   key,
+			Score: score,
+		}
+		ev.Payload, _ = json.Marshal(pl)
+	})
+}
+
+// RecordApproval records a human-in-the-loop approval event.
+func (r *EventRecorder) RecordApproval(ctx context.Context, requestID, action string, context any, decision string, latencyMs int64) {
+	r.record(ctx, EventApprovalRequest, func(ev *Event) {
+		pl := ApprovalPayload{
+			RequestID: requestID,
+			Action:    action,
+			Context:   context,
+			Decision:  decision,
+			LatencyMs: latencyMs,
+		}
+		ev.Payload, _ = json.Marshal(pl)
+	})
+}
+
+// RecordError records an execution error.
+func (r *EventRecorder) RecordError(ctx context.Context, errMsg string) {
+	r.record(ctx, EventError, func(ev *Event) {
+		ev.Metadata["error"] = errMsg
+	})
+}
+
+// RecordRetry records a retry event.
+func (r *EventRecorder) RecordRetry(ctx context.Context, detail string) {
+	r.record(ctx, EventRetry, func(ev *Event) {
+		ev.Metadata["detail"] = detail
+	})
+}
+
+// ---- GraphCallback implementation ----
+
+// OnRunStart implements pregel.RunCallback.
+func (r *EventRecorder) OnRunStart(ctx context.Context, graphName, threadID string) {
+	r.record(ctx, EventGraphStart, func(ev *Event) {
+		ev.Metadata["graph_name"] = graphName
+	})
+}
+
+// OnRunEnd implements pregel.RunCallback.
+func (r *EventRecorder) OnRunEnd(ctx context.Context, graphName, threadID string, err error) {
+	r.record(ctx, EventGraphEnd, func(ev *Event) {
+		ev.Metadata["graph_name"] = graphName
+		if err != nil {
+			ev.Metadata["error"] = err.Error()
+		}
+	})
+}
+
+// OnStepStart implements pregel.StepCallback.
+func (r *EventRecorder) OnStepStart(ctx context.Context, step, taskCount int) {
+	r.record(ctx, EventStepStart, func(ev *Event) {
+		ev.Step = step
+		ev.Metadata["task_count"] = taskCount
+	})
+}
+
+// OnStepEnd implements pregel.StepCallback.
+func (r *EventRecorder) OnStepEnd(ctx context.Context, step int, err error) {
+	r.record(ctx, EventStepEnd, func(ev *Event) {
+		ev.Step = step
+		if err != nil {
+			ev.Metadata["error"] = err.Error()
+		}
+	})
+}
+
+// OnNodeStart implements pregel.NodeCallback.
+func (r *EventRecorder) OnNodeStart(ctx context.Context, nodeName string, step int) {
+	r.record(ctx, EventNodeStart, func(ev *Event) {
+		ev.Node = nodeName
+		ev.Step = step
+	})
+}
+
+// OnNodeEnd implements pregel.NodeCallback.
+func (r *EventRecorder) OnNodeEnd(ctx context.Context, nodeName string, step int, output interface{}, err error) {
+	r.record(ctx, EventNodeEnd, func(ev *Event) {
+		ev.Node = nodeName
+		ev.Step = step
+		if err != nil {
+			ev.Metadata["error"] = err.Error()
+		}
+	})
+}
+
+// OnCheckpointSave implements pregel.CheckpointCallback.
+func (r *EventRecorder) OnCheckpointSave(ctx context.Context, threadID, checkpointID string, step int) {
+	r.record(ctx, EventCheckpointCreated, func(ev *Event) {
+		ev.ThreadID = threadID
+		ev.Step = step
+		ev.Metadata["checkpoint_id"] = checkpointID
+	})
+}
+
+// OnCheckpointLoad implements pregel.CheckpointCallback.
+func (r *EventRecorder) OnCheckpointLoad(ctx context.Context, threadID, checkpointID string, step int) {
+	r.record(ctx, EventCheckpointRestored, func(ev *Event) {
+		ev.ThreadID = threadID
+		ev.Step = step
+		ev.Metadata["checkpoint_id"] = checkpointID
+	})
+}
+
+// OnCheckpointUpdate implements pregel.CheckpointCallback.
+func (r *EventRecorder) OnCheckpointUpdate(ctx context.Context, threadID, asNode string) {
+	r.record(ctx, EventStateWrite, func(ev *Event) {
+		ev.ThreadID = threadID
+		ev.Node = asNode
+	})
+}
+
+// OnInterrupt implements pregel.InterruptCallback.
+func (r *EventRecorder) OnInterrupt(ctx context.Context, nodeNames []string, step int) {
+	r.record(ctx, EventInterrupt, func(ev *Event) {
+		ev.Step = step
+		ev.Metadata["interrupt_nodes"] = nodeNames
+	})
+}
+
+// OnResume implements pregel.InterruptCallback.
+func (r *EventRecorder) OnResume(ctx context.Context, threadID string) {
+	r.record(ctx, EventResume, func(ev *Event) {
+		ev.ThreadID = threadID
+	})
+}
+
+// compile-time interface checks
+var (
+	_ pregel.GraphCallback = (*EventRecorder)(nil)
+)
diff --git a/internal/harness/events/store.go b/internal/harness/events/store.go
new file mode 100644
index 000000000..0eba5c279
--- /dev/null
+++ b/internal/harness/events/store.go
@@ -0,0 +1,127 @@
+package events
+
+import (
+	"context"
+	"time"
+)
+
+// EventLog is the append-only event log interface.
+// All implementations must be safe for concurrent use.
+type EventLog interface {
+	// Append appends one or more events to the log. Events are immutable
+	// once appended.
+	Append(ctx context.Context, events ...*Event) error
+
+	// Stream returns an iterator over events matching the filter,
+	// ordered by logical clock.
+	Stream(ctx context.Context, filter EventFilter) EventIterator
+
+	// Get retrieves a single event by ID. Returns nil, nil if not found.
+	Get(ctx context.Context, id EventID) (*Event, error)
+
+	// Range returns events with logical clock in [from, to] matching the filter.
+	Range(ctx context.Context, from, to uint64, filter EventFilter) ([]*Event, error)
+
+	// Seek returns an iterator starting from the given logical clock.
+	Seek(ctx context.Context, clock uint64) (EventIterator, error)
+
+	// Length returns the total number of events in the log.
+	Length(ctx context.Context) (uint64, error)
+}
+
+// EventFilter specifies criteria for filtering events.
+type EventFilter struct {
+	// TraceID filters by trace.
+	TraceID string
+	// ThreadID filters by thread.
+	ThreadID string
+	// Types restricts to specific event types. Empty means all types.
+	Types []EventType
+	// Node restricts to a specific graph node.
+	Node string
+	// FromClock is the minimum logical clock (inclusive).
+	FromClock uint64
+	// ToClock is the maximum logical clock (inclusive). 0 means no upper bound.
+	ToClock uint64
+	// FromTime is the minimum wall-clock time.
+	FromTime time.Time
+	// ToTime is the maximum wall-clock time.
+	ToTime time.Time
+	// Limit caps the number of events returned. 0 means no limit.
+	Limit int
+}
+
+// Matches checks whether an event matches this filter.
+func (f EventFilter) Matches(e *Event) bool {
+	if f.TraceID != "" && e.TraceID != f.TraceID {
+		return false
+	}
+	if f.ThreadID != "" && e.ThreadID != f.ThreadID {
+		return false
+	}
+	if len(f.Types) > 0 {
+		matched := false
+		for _, t := range f.Types {
+			if e.Type == t {
+				matched = true
+				break
+			}
+		}
+		if !matched {
+			return false
+		}
+	}
+	if f.Node != "" && e.Node != f.Node {
+		return false
+	}
+	if f.FromClock > 0 && e.Clock < f.FromClock {
+		return false
+	}
+	if f.ToClock > 0 && e.Clock > f.ToClock {
+		return false
+	}
+	if !f.FromTime.IsZero() && e.Timestamp.Before(f.FromTime) {
+		return false
+	}
+	if !f.ToTime.IsZero() && e.Timestamp.After(f.ToTime) {
+		return false
+	}
+	return true
+}
+
+// EventIterator allows iterating over events in order.
+type EventIterator interface {
+	// Next returns the next event. Returns nil, false when exhausted.
+	Next(ctx context.Context) (*Event, bool)
+	// Close releases resources held by the iterator.
+	Close() error
+}
+
+// Snapshot represents a point-in-time snapshot of event state,
+// used to accelerate replay (avoids replaying from event 0).
+type Snapshot struct {
+	ID        string    `json:"id"`
+	TraceID   string    `json:"trace_id"`
+	Clock     uint64    `json:"clock"`
+	CreatedAt time.Time `json:"created_at"`
+	Data      []byte    `json:"data,omitempty"`
+}
+
+// EventStore extends EventLog with lifecycle management.
+type EventStore interface {
+	EventLog
+
+	// CreateSnapshot creates a snapshot for the given trace.
+	CreateSnapshot(ctx context.Context, traceID string) (*Snapshot, error)
+
+	// RestoreSnapshot loads a snapshot and returns an iterator positioned
+	// after the snapshot's clock position.
+	RestoreSnapshot(ctx context.Context, snapshotID string) (EventIterator, error)
+
+	// Subscribe returns a channel that receives new events matching the filter
+	// as they are appended. The channel is closed when the context is cancelled.
+	Subscribe(ctx context.Context, filter EventFilter) (<-chan *Event, error)
+
+	// GC removes events older than the given retention period.
+	GC(ctx context.Context, retention time.Duration) error
+}
diff --git a/internal/harness/graph/pregel/engine.go b/internal/harness/graph/pregel/engine.go
index 94d4ed4d8..c1c788f7c 100644
--- a/internal/harness/graph/pregel/engine.go
+++ b/internal/harness/graph/pregel/engine.go
@@ -48,6 +48,7 @@ type Engine struct {
 	versionsSeen        map[string]map[string]int
 	cache               Cache
 	backgroundExec      *BackgroundExecutor
+	callbacks           *CallbackManager   // lifecycle callbacks (event recording, metrics)
 	deferredCheckpoints []deferredCheckpoint // for DurabilityExit mode
 }
 
@@ -173,6 +174,15 @@ func WithBackgroundExecutor(exec *BackgroundExecutor) EngineOption {
 	}
 }
 
+// WithCallbacks sets the callback manager for the engine.
+// Callbacks are dispatched during graph execution (run start/end, step start/end,
+// node start/end, checkpoint save/load, interrupt/resume).
+func WithCallbacks(cb *CallbackManager) EngineOption {
+	return func(e *Engine) {
+		e.callbacks = cb
+	}
+}
+
 // ExecuteResult represents the result of graph execution.
 type ExecuteResult struct {
 	// Final state of the graph.
@@ -215,9 +225,39 @@ func (e *Engine) Run(ctx context.Context, input any, mode types.StreamMode) (<-c
 			}
 		})
 
-		// Deferred cleanup: close streamManager first (unblocks forward goroutine),
-		// then wait for forward goroutine to exit, then close outputCh.
+		// Resolve thread ID early (before deferred cleanup uses it).
+		threadID := e.getThreadID()
+
+		// reportRunEnd is defined before the deferred cleanup block so the
+		// defer can capture it by closure.
+		reportRunEnd := func(err error) {
+			if e.callbacks == nil {
+				return
+			}
+			gName := "state_graph"
+			if e.graph != nil {
+				nodes := e.graph.GetNodes()
+				for name := range nodes {
+					gName = name
+					break
+				}
+			}
+			e.callbacks.RunEnd(context.Background(), gName, threadID, err)
+		}
+
+		// Deferred cleanup: dispatch RunEnd, close streamManager,
+		// wait for forward goroutine, then close outputCh.
+		var exitErr error // captured for RunEnd callback dispatch
 		defer func() {
+			// Read from errCh to get the exit error for RunEnd dispatch.
+			// errCh is still open here (close(errCh) runs after this defer).
+			select {
+			case exitErr = <-errCh:
+				reportRunEnd(exitErr)
+				errCh <- exitErr // put back for the caller
+			default:
+				reportRunEnd(nil)
+			}
 			streamManager.Close()
 			fwWg.Wait()
 			close(outputCh)
@@ -256,7 +296,7 @@ func (e *Engine) Run(ctx context.Context, input any, mode types.StreamMode) (<-c
 		}
 
 		// Get thread ID for checkpointing
-		threadID := e.getThreadID()
+		// threadID already resolved above (before deferred cleanup).
 
 		// Load checkpoint when one exists for this thread_id, even when
 		// input is non-nil (resume from a previous run).  The canvas
@@ -341,6 +381,14 @@ func (e *Engine) Run(ctx context.Context, input any, mode types.StreamMode) (<-c
 				if cp, err := checkpoint.FromMap(cpData); err == nil {
 					e.currentCheckpoint = cp
 				}
+				// Dispatch CheckpointLoad callback.
+				if e.callbacks != nil {
+					cpID := ""
+					if cpid, _ := cpData["checkpoint_id"].(string); cpid != "" {
+						cpID = cpid
+					}
+					e.callbacks.CheckpointLoad(ctx, threadID, cpID, 0)
+				}
 			}
 		}
 		// Apply input only when no checkpoint was loaded.
@@ -393,6 +441,18 @@ func (e *Engine) Run(ctx context.Context, input any, mode types.StreamMode) (<-c
 			lastState = input
 		}
 
+		// Dispatch RunStart callback.
+		if e.callbacks != nil {
+			gName := "state_graph"
+			if e.graph != nil {
+				nodes := e.graph.GetNodes()
+				for name := range nodes {
+					gName = name
+					break
+				}
+			}
+			e.callbacks.RunStart(ctx, gName, threadID)
+		}
 		for {
 			// Check context cancellation at each superstep.
 			select {
@@ -408,6 +468,11 @@ func (e *Engine) Run(ctx context.Context, input any, mode types.StreamMode) (<-c
 				return
 			}
 
+			// Dispatch StepStart callback.
+			if e.callbacks != nil {
+				e.callbacks.StepStart(ctx, step, 0) // taskCount filled after prepareNextTasks
+			}
+
 			// Emit checkpoint event via stream manager
 			streamManager.EmitCheckpoint(step, channelRegistry.CreateCheckpoint())
 
@@ -449,6 +514,11 @@ func (e *Engine) Run(ctx context.Context, input any, mode types.StreamMode) (<-c
 				}
 				streamManager.EmitInterrupt(step, interruptNames)
 
+				// Dispatch Interrupt callback.
+				if e.callbacks != nil {
+					e.callbacks.Interrupt(ctx, interruptNames, step)
+				}
+
 				errCh <- &errors.GraphInterrupt{}
 				return
 			}
@@ -519,6 +589,10 @@ func (e *Engine) Run(ctx context.Context, input any, mode types.StreamMode) (<-c
 					}
 				}
 				streamManager.EmitInterrupt(step, interruptTaskNames)
+				// Dispatch Interrupt callback.
+				if e.callbacks != nil {
+					e.callbacks.Interrupt(ctx, interruptTaskNames, step)
+				}
 				// Preserve the first interrupted task's GraphInterrupt value
 				// (with Interrupts populated) instead of creating a bare one,
 				// so MustExtractInterruptContexts can extract the original
@@ -571,6 +645,10 @@ func (e *Engine) Run(ctx context.Context, input any, mode types.StreamMode) (<-c
 						errCh <- fmt.Errorf("failed to save checkpoint: %w", err)
 						return
 					}
+					// Dispatch CheckpointSave callback.
+					if e.callbacks != nil {
+						e.callbacks.CheckpointSave(ctx, threadID, checkpointID, step)
+					}
 				case types.DurabilityAsync:
 					// Asynchronous save - don't block next step
 					go func(cp map[string]any, cpID string, s int) {
@@ -583,6 +661,10 @@ func (e *Engine) Run(ctx context.Context, input any, mode types.StreamMode) (<-c
 					// Defer save until exit - accumulate checkpoints in memory
 					// Will be saved in final state
 					e.deferCheckpoint(threadID, checkpointID, step, checkpoint)
+					// Dispatch CheckpointSave callback (deferred save still counts as saved).
+					if e.callbacks != nil {
+						e.callbacks.CheckpointSave(ctx, threadID, checkpointID, step)
+					}
 				default:
 					// Default to sync behavior
 					if err := e.saveCheckpoint(ctx, threadID, checkpointID, step, checkpoint); err != nil {
@@ -595,10 +677,18 @@ func (e *Engine) Run(ctx context.Context, input any, mode types.StreamMode) (<-c
 			// Check for after-node interrupts. The checkpoint above already
 			// captures this step's output.
 			if e.shouldInterruptAfter(results) {
+				if e.callbacks != nil {
+					e.callbacks.Interrupt(ctx, []string{"after_node"}, step)
+				}
 				errCh <- &errors.GraphInterrupt{}
 				return
 			}
 
+			// Dispatch StepEnd callback.
+			if e.callbacks != nil {
+				e.callbacks.StepEnd(ctx, step, nil)
+			}
+
 			step++
 		}
 
diff --git a/internal/harness/metrics/aggregator.go b/internal/harness/metrics/aggregator.go
new file mode 100644
index 000000000..90b256b74
--- /dev/null
+++ b/internal/harness/metrics/aggregator.go
@@ -0,0 +1,192 @@
+package metrics
+
+import (
+	"math"
+	"sort"
+	"sync"
+	"time"
+)
+
+// MetricsAggregator aggregates metrics across multiple execution traces.
+type MetricsAggregator struct {
+	mu      sync.Mutex
+	metrics []*AgentMetrics
+}
+
+// NewMetricsAggregator creates a new MetricsAggregator.
+func NewMetricsAggregator() *MetricsAggregator {
+	return &MetricsAggregator{}
+}
+
+// Add adds a metrics snapshot to the aggregator.
+func (a *MetricsAggregator) Add(m *AgentMetrics) {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	a.metrics = append(a.metrics, m)
+}
+
+// AggregatedMetrics contains summary statistics across multiple traces.
+type AggregatedMetrics struct {
+	TotalTraces   int
+	TotalDuration time.Duration
+
+	// Tool metrics (averages).
+	AvgToolCalls       float64
+	AvgToolSuccessRate float64
+	AvgToolRetryRate   float64
+	P50ToolLatencyMs   float64
+	P95ToolLatencyMs   float64
+	P99ToolLatencyMs   float64
+
+	// Checkpoint metrics.
+	AvgCheckpointSaves          float64
+	AvgCheckpointRestores       float64
+	AvgCheckpointRestoreSuccess float64
+
+	// Execution metrics.
+	AvgSteps           float64
+	AvgNodesExecuted   float64
+	AvgRecoveredErrors float64
+	AvgInterrupts      float64
+
+	// Cost metrics.
+	AvgCostPerTask        float64
+	AvgForkReplayPassRate float64
+}
+
+// Aggregate computes summary statistics across all collected metrics.
+func (a *MetricsAggregator) Aggregate() *AggregatedMetrics {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+
+	result := &AggregatedMetrics{
+		TotalTraces: len(a.metrics),
+	}
+
+	if len(a.metrics) == 0 {
+		return result
+	}
+
+	var allLatencies []float64
+
+	for _, m := range a.metrics {
+		result.TotalDuration += m.Duration
+		result.AvgToolCalls += float64(m.ToolCalls)
+		result.AvgToolSuccessRate += m.ToolSuccessRate
+		result.AvgToolRetryRate += m.ToolRetryRate
+		result.AvgCheckpointSaves += float64(m.CheckpointSaves)
+		result.AvgCheckpointRestores += float64(m.CheckpointRestores)
+		result.AvgCheckpointRestoreSuccess += m.CheckpointRestoreSuccess
+		result.AvgSteps += float64(m.Steps)
+		result.AvgNodesExecuted += float64(m.NodesExecuted)
+		result.AvgRecoveredErrors += float64(m.RecoveredErrors)
+		result.AvgInterrupts += float64(m.InterruptCount)
+		result.AvgCostPerTask += m.CostPerTask
+		result.AvgForkReplayPassRate += m.ForkReplayPassRate
+
+		for _, latencies := range m.ToolLatencyMs {
+			for _, l := range latencies {
+				allLatencies = append(allLatencies, float64(l))
+			}
+		}
+	}
+
+	n := float64(len(a.metrics))
+	result.AvgToolCalls /= n
+	result.AvgToolSuccessRate /= n
+	result.AvgToolRetryRate /= n
+	result.AvgCheckpointSaves /= n
+	result.AvgCheckpointRestores /= n
+	result.AvgCheckpointRestoreSuccess /= n
+	result.AvgSteps /= n
+	result.AvgNodesExecuted /= n
+	result.AvgRecoveredErrors /= n
+	result.AvgInterrupts /= n
+	result.AvgCostPerTask /= n
+	result.AvgForkReplayPassRate /= n
+
+	// Compute latency percentiles.
+	if len(allLatencies) > 0 {
+		sort.Float64s(allLatencies)
+		result.P50ToolLatencyMs = percentile(allLatencies, 50)
+		result.P95ToolLatencyMs = percentile(allLatencies, 95)
+		result.P99ToolLatencyMs = percentile(allLatencies, 99)
+	}
+
+	return result
+}
+
+// Reset clears all collected metrics.
+func (a *MetricsAggregator) Reset() {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	a.metrics = nil
+}
+
+// MetricsWindow tracks metrics over a sliding time window.
+type MetricsWindow struct {
+	mu      sync.Mutex
+	window  time.Duration
+	entries []windowEntry
+}
+
+type windowEntry struct {
+	timestamp time.Time
+	metrics   *AgentMetrics
+}
+
+// NewMetricsWindow creates a metrics window with the given duration.
+func NewMetricsWindow(window time.Duration) *MetricsWindow {
+	return &MetricsWindow{window: window}
+}
+
+// Add adds metrics at the current time.
+func (w *MetricsWindow) Add(m *AgentMetrics) {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	w.entries = append(w.entries, windowEntry{
+		timestamp: time.Now(),
+		metrics:   m,
+	})
+	w.prune()
+}
+
+// Aggregate returns aggregated metrics for the current window.
+func (w *MetricsWindow) Aggregate() *AggregatedMetrics {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	w.prune()
+
+	agg := NewMetricsAggregator()
+	for _, entry := range w.entries {
+		agg.Add(entry.metrics)
+	}
+	return agg.Aggregate()
+}
+
+// prune removes entries outside the window.
+func (w *MetricsWindow) prune() {
+	cutoff := time.Now().Add(-w.window)
+	keep := make([]windowEntry, 0, len(w.entries))
+	for _, e := range w.entries {
+		if e.timestamp.After(cutoff) {
+			keep = append(keep, e)
+		}
+	}
+	w.entries = keep
+}
+
+// percentile computes the p-th percentile from a sorted slice.
+func percentile(sorted []float64, p int) float64 {
+	if len(sorted) == 0 {
+		return 0
+	}
+	idx := int(math.Ceil(float64(p)/100.0*float64(len(sorted))) - 1)
+	if idx < 0 {
+		idx = 0
+	}
+	if idx >= len(sorted) {
+		idx = len(sorted) - 1
+	}
+	return sorted[idx]
+}
diff --git a/internal/harness/metrics/exporter.go b/internal/harness/metrics/exporter.go
new file mode 100644
index 000000000..3f269250f
--- /dev/null
+++ b/internal/harness/metrics/exporter.go
@@ -0,0 +1,78 @@
+package metrics
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Exporter formats agent metrics for output. This is a lightweight
+// alternative to a full Prometheus client, avoiding the dependency.
+type Exporter struct {
+	namespace string
+}
+
+// NewExporter creates a new metrics exporter.
+func NewExporter(namespace string) *Exporter {
+	return &Exporter{namespace: namespace}
+}
+
+// ExportText formats metrics as Prometheus-style text output.
+func (e *Exporter) ExportText(m *AgentMetrics) string {
+	snap := m.Snapshot()
+	var b strings.Builder
+
+	ns := e.namespace
+	if ns != "" {
+		ns += "_"
+	}
+
+	// Tool metrics.
+	fmt.Fprintf(&b, "# HELP %stool_calls_total Total tool invocations\n", ns)
+	fmt.Fprintf(&b, "# TYPE %stool_calls_total counter\n", ns)
+	fmt.Fprintf(&b, "%stool_calls_total %d\n", ns, snap.ToolCalls)
+
+	fmt.Fprintf(&b, "# HELP %stool_success_rate Tool success rate\n", ns)
+	fmt.Fprintf(&b, "# TYPE %stool_success_rate gauge\n", ns)
+	fmt.Fprintf(&b, "%stool_success_rate %.4f\n", ns, snap.ToolSuccessRate)
+
+	fmt.Fprintf(&b, "# HELP %stool_retry_rate Tool retry rate\n", ns)
+	fmt.Fprintf(&b, "# TYPE %stool_retry_rate gauge\n", ns)
+	fmt.Fprintf(&b, "%stool_retry_rate %.4f\n", ns, snap.ToolRetryRate)
+
+	// Checkpoint metrics.
+	fmt.Fprintf(&b, "# HELP %scheckpoint_saves_total Total checkpoint saves\n", ns)
+	fmt.Fprintf(&b, "# TYPE %scheckpoint_saves_total counter\n", ns)
+	fmt.Fprintf(&b, "%scheckpoint_saves_total %d\n", ns, snap.CheckpointSaves)
+
+	fmt.Fprintf(&b, "# HELP %scheckpoint_restore_success Checkpoint restore success rate\n", ns)
+	fmt.Fprintf(&b, "# TYPE %scheckpoint_restore_success gauge\n", ns)
+	fmt.Fprintf(&b, "%scheckpoint_restore_success %.4f\n", ns, snap.CheckpointRestoreSuccess)
+
+	// Execution metrics.
+	fmt.Fprintf(&b, "# HELP %ssteps_total Total supersteps executed\n", ns)
+	fmt.Fprintf(&b, "# TYPE %ssteps_total counter\n", ns)
+	fmt.Fprintf(&b, "%ssteps_total %d\n", ns, snap.Steps)
+
+	fmt.Fprintf(&b, "# HELP %snodes_executed_total Total nodes executed\n", ns)
+	fmt.Fprintf(&b, "# TYPE %snodes_executed_total counter\n", ns)
+	fmt.Fprintf(&b, "%snodes_executed_total %d\n", ns, snap.NodesExecuted)
+
+	fmt.Fprintf(&b, "# HELP %sinterrupts_total Total interrupts\n", ns)
+	fmt.Fprintf(&b, "# TYPE %sinterrupts_total counter\n", ns)
+	fmt.Fprintf(&b, "%sinterrupts_total %d\n", ns, snap.InterruptCount)
+
+	return b.String()
+}
+
+// ExportCSV formats metrics as a single CSV row.
+func (e *Exporter) ExportCSV(m *AgentMetrics) string {
+	snap := m.Snapshot()
+	return fmt.Sprintf("%s,%d,%d,%d,%.4f,%.4f,%d,%d,%.4f,%d,%d,%d,%.6f",
+		snap.TraceID,
+		snap.ToolCalls, snap.ToolSuccesses, snap.ToolFailures,
+		snap.ToolSuccessRate, snap.ToolRetryRate,
+		snap.CheckpointSaves, snap.CheckpointRestores,
+		snap.CheckpointRestoreSuccess,
+		snap.Steps, snap.NodesExecuted, snap.InterruptCount,
+		snap.CostPerTask)
+}
diff --git a/internal/harness/metrics/metrics.go b/internal/harness/metrics/metrics.go
new file mode 100644
index 000000000..a29e38181
--- /dev/null
+++ b/internal/harness/metrics/metrics.go
@@ -0,0 +1,303 @@
+// Package metrics provides observability metrics collection for agent execution.
+//
+// The autoMetricCollector implements pregel.GraphCallback and tracks key
+// metrics during graph execution: tool success rate, checkpoint operations,
+// step/node counts, and error recovery. Metrics are exposed via the
+// MetricsCollector interface and can be exported to Prometheus.
+package metrics
+
+import (
+	"context"
+	"sync"
+	"time"
+)
+
+// AgentMetrics is a snapshot of all metrics for one agent execution trace.
+type AgentMetrics struct {
+	// TraceID identifies the execution trace.
+	TraceID string
+	// ThreadID identifies the execution thread.
+	ThreadID string
+	// Duration is the wall-clock duration of the execution.
+	Duration time.Duration
+
+	// ---- Tool metrics ----
+
+	// ToolCalls is the total number of tool invocations.
+	ToolCalls int
+	// ToolSuccesses is the number of successful tool invocations.
+	ToolSuccesses int
+	// ToolFailures is the number of failed tool invocations.
+	ToolFailures int
+	// ToolRetries is the number of times tools were retried.
+	ToolRetries int
+	// ToolLatencyMs holds per-tool latency histograms (toolName → durations).
+	ToolLatencyMs map[string][]int64
+	// ToolSuccessRate is the ratio of successes to total calls (0–1).
+	ToolSuccessRate float64
+	// ToolRetryRate is the ratio of retries to (calls + retries).
+	ToolRetryRate float64
+
+	// ---- Checkpoint metrics ----
+
+	// CheckpointSaves is the number of checkpoint saves.
+	CheckpointSaves int
+	// CheckpointRestores is the number of checkpoint restores.
+	CheckpointRestores int
+	// CheckpointRestoreSuccess is the ratio of successful restores (0–1).
+	CheckpointRestoreSuccess float64
+
+	// ---- Execution metrics ----
+
+	// Steps is the number of Pregel supersteps executed.
+	Steps int
+	// NodesExecuted is the number of graph nodes executed.
+	NodesExecuted int
+	// RecoveredErrors is the number of errors that were recovered.
+	RecoveredErrors int
+	// InterruptCount is the number of times execution was interrupted.
+	InterruptCount int
+
+	// ---- Computed metrics ----
+
+	// CostPerTask is an estimated metric (requires LLM cost tracking).
+	CostPerTask float64
+	// ForkReplayPassRate is the ratio of fork replays that pass assertions.
+	ForkReplayPassRate float64
+	// ApprovalLatencyMs tracks human-in-the-loop approval wait times.
+	ApprovalLatencyMs []int64
+	// ApprovalRate is the ratio of approvals to total approval requests.
+	ApprovalRate float64
+	// MemoryAvgHitScore is the average retrieval score for memory operations.
+	MemoryAvgHitScore float64
+}
+
+// NewAgentMetrics creates a new AgentMetrics with initialised maps.
+func NewAgentMetrics() *AgentMetrics {
+	return &AgentMetrics{
+		ToolLatencyMs:     make(map[string][]int64),
+		ApprovalLatencyMs: make([]int64, 0),
+	}
+}
+
+// Snapshot captures a point-in-time copy of the metrics.
+func (m *AgentMetrics) Snapshot() *AgentMetrics {
+	cp := *m
+	cp.ToolLatencyMs = make(map[string][]int64, len(m.ToolLatencyMs))
+	for k, v := range m.ToolLatencyMs {
+		durations := make([]int64, len(v))
+		copy(durations, v)
+		cp.ToolLatencyMs[k] = durations
+	}
+	cp.ApprovalLatencyMs = make([]int64, len(m.ApprovalLatencyMs))
+	copy(cp.ApprovalLatencyMs, m.ApprovalLatencyMs)
+
+	// Compute derived rates.
+	if cp.ToolCalls > 0 {
+		cp.ToolSuccessRate = float64(cp.ToolSuccesses) / float64(cp.ToolCalls)
+		cp.ToolRetryRate = float64(cp.ToolRetries) / float64(cp.ToolCalls+cp.ToolRetries)
+	}
+	if cp.CheckpointSaves+cp.CheckpointRestores > 0 {
+		cp.CheckpointRestoreSuccess = float64(cp.CheckpointRestores) / float64(cp.CheckpointSaves+cp.CheckpointRestores)
+	}
+	return &cp
+}
+
+// MetricsCollector collects and aggregates metrics for agent execution.
+type MetricsCollector interface {
+	// RecordToolCall records a tool invocation outcome.
+	RecordToolCall(toolName string, success bool, durationMs int64)
+	// RecordToolRetry records a tool retry.
+	RecordToolRetry(toolName string)
+	// RecordCheckpointSave records a checkpoint save.
+	RecordCheckpointSave()
+	// RecordCheckpointRestore records a checkpoint restore (success or failure).
+	RecordCheckpointRestore(success bool)
+	// RecordStep records a completed Pregel superstep.
+	RecordStep()
+	// RecordNode records a completed node execution.
+	RecordNode(nodeName string)
+	// RecordRecoveredError records a recovered error.
+	RecordRecoveredError()
+	// RecordInterrupt records an execution interrupt.
+	RecordInterrupt()
+	// RecordApproval records an approval outcome.
+	RecordApproval(latencyMs int64, granted bool)
+	// RecordMemoryHit records a memory retrieval score.
+	RecordMemoryHit(score float64)
+	// RecordLLMCost records an LLM invocation cost.
+	RecordLLMCost(cost float64)
+
+	// Snapshot returns the current metrics snapshot.
+	Snapshot() *AgentMetrics
+	// Reset clears all metrics.
+	Reset()
+}
+
+// ---- AutoCollector: implements GraphCallback + MetricsCollector ----
+
+// AutoCollector automatically collects metrics from graph execution callbacks.
+// It implements both pregel.GraphCallback and MetricsCollector.
+type AutoCollector struct {
+	mu sync.Mutex
+	m  *AgentMetrics
+}
+
+// NewAutoCollector creates a new AutoCollector.
+func NewAutoCollector() *AutoCollector {
+	return &AutoCollector{m: NewAgentMetrics()}
+}
+
+// ---- MetricsCollector implementation ----
+
+func (c *AutoCollector) RecordToolCall(toolName string, success bool, durationMs int64) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.m.ToolCalls++
+	if success {
+		c.m.ToolSuccesses++
+	} else {
+		c.m.ToolFailures++
+	}
+	c.m.ToolLatencyMs[toolName] = append(c.m.ToolLatencyMs[toolName], durationMs)
+}
+
+func (c *AutoCollector) RecordToolRetry(toolName string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.m.ToolRetries++
+}
+
+func (c *AutoCollector) RecordCheckpointSave() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.m.CheckpointSaves++
+}
+
+func (c *AutoCollector) RecordCheckpointRestore(success bool) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.m.CheckpointRestores++
+}
+
+func (c *AutoCollector) RecordStep() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.m.Steps++
+}
+
+func (c *AutoCollector) RecordNode(nodeName string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.m.NodesExecuted++
+}
+
+func (c *AutoCollector) RecordRecoveredError() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.m.RecoveredErrors++
+}
+
+func (c *AutoCollector) RecordInterrupt() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.m.InterruptCount++
+}
+
+func (c *AutoCollector) RecordApproval(latencyMs int64, granted bool) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.m.ApprovalLatencyMs = append(c.m.ApprovalLatencyMs, latencyMs)
+	if granted {
+		// Track approval rate via approvals/total.
+	}
+}
+
+func (c *AutoCollector) RecordMemoryHit(score float64) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.m.MemoryAvgHitScore == 0 {
+		c.m.MemoryAvgHitScore = score
+	} else {
+		c.m.MemoryAvgHitScore = (c.m.MemoryAvgHitScore + score) / 2
+	}
+}
+
+func (c *AutoCollector) RecordLLMCost(cost float64) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.m.CostPerTask += cost
+}
+
+func (c *AutoCollector) Snapshot() *AgentMetrics {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	cp := c.m.Snapshot()
+	cp.Duration = time.Since(c.startTime())
+	return cp
+}
+
+func (c *AutoCollector) Reset() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.m = NewAgentMetrics()
+}
+
+// startTime is a placeholder for tracking execution duration.
+// In practice, the collector is initialised at Run start.
+func (c *AutoCollector) startTime() time.Time {
+	return time.Now()
+}
+
+// ---- GraphCallback implementation ----
+
+// OnRunStart implements pregel.RunCallback.
+func (c *AutoCollector) OnRunStart(ctx context.Context, graphName, threadID string) {
+	c.Reset()
+	c.mu.Lock()
+	c.m.ThreadID = threadID
+	c.mu.Unlock()
+}
+
+// OnRunEnd implements pregel.RunCallback.
+func (c *AutoCollector) OnRunEnd(ctx context.Context, graphName, threadID string, err error) {}
+
+// OnStepStart implements pregel.StepCallback.
+func (c *AutoCollector) OnStepStart(ctx context.Context, step, taskCount int) {}
+
+// OnStepEnd implements pregel.StepCallback.
+func (c *AutoCollector) OnStepEnd(ctx context.Context, step int, err error) {
+	c.RecordStep()
+}
+
+// OnNodeStart implements pregel.NodeCallback.
+func (c *AutoCollector) OnNodeStart(ctx context.Context, nodeName string, step int) {}
+
+// OnNodeEnd implements pregel.NodeCallback.
+func (c *AutoCollector) OnNodeEnd(ctx context.Context, nodeName string, step int, output interface{}, err error) {
+	c.RecordNode(nodeName)
+	if err != nil {
+		c.RecordRecoveredError()
+	}
+}
+
+// OnCheckpointSave implements pregel.CheckpointCallback.
+func (c *AutoCollector) OnCheckpointSave(ctx context.Context, threadID, checkpointID string, step int) {
+	c.RecordCheckpointSave()
+}
+
+// OnCheckpointLoad implements pregel.CheckpointCallback.
+func (c *AutoCollector) OnCheckpointLoad(ctx context.Context, threadID, checkpointID string, step int) {
+	c.RecordCheckpointRestore(true)
+}
+
+// OnCheckpointUpdate implements pregel.CheckpointCallback.
+func (c *AutoCollector) OnCheckpointUpdate(ctx context.Context, threadID, asNode string) {}
+
+// OnInterrupt implements pregel.InterruptCallback.
+func (c *AutoCollector) OnInterrupt(ctx context.Context, nodeNames []string, step int) {
+	c.RecordInterrupt()
+}
+
+// OnResume implements pregel.InterruptCallback.
+func (c *AutoCollector) OnResume(ctx context.Context, threadID string) {}
diff --git a/internal/harness/metrics/metrics_test.go b/internal/harness/metrics/metrics_test.go
new file mode 100644
index 000000000..d09b71cdf
--- /dev/null
+++ b/internal/harness/metrics/metrics_test.go
@@ -0,0 +1,270 @@
+package metrics
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestNewAgentMetrics(t *testing.T) {
+	m := NewAgentMetrics()
+	if m.ToolLatencyMs == nil {
+		t.Fatal("ToolLatencyMs should be initialised")
+	}
+	if m.ApprovalLatencyMs == nil {
+		t.Fatal("ApprovalLatencyMs should be initialised")
+	}
+}
+
+func TestAutoCollector_ToolMetrics(t *testing.T) {
+	c := NewAutoCollector()
+
+	c.RecordToolCall("search", true, 100)
+	c.RecordToolCall("search", true, 200)
+	c.RecordToolCall("calc", false, 50)
+	c.RecordToolRetry("search")
+
+	snap := c.Snapshot()
+	if snap.ToolCalls != 3 {
+		t.Fatalf("expected 3 calls, got %d", snap.ToolCalls)
+	}
+	if snap.ToolSuccesses != 2 {
+		t.Fatalf("expected 2 successes, got %d", snap.ToolSuccesses)
+	}
+	if snap.ToolFailures != 1 {
+		t.Fatalf("expected 1 failure, got %d", snap.ToolFailures)
+	}
+	if snap.ToolRetries != 1 {
+		t.Fatalf("expected 1 retry, got %d", snap.ToolRetries)
+	}
+	if snap.ToolSuccessRate != 2.0/3.0 {
+		t.Fatalf("expected success rate %.4f, got %.4f", 2.0/3.0, snap.ToolSuccessRate)
+	}
+
+	// Check latency tracking.
+	if len(snap.ToolLatencyMs["search"]) != 2 {
+		t.Fatalf("expected 2 search latencies, got %d", len(snap.ToolLatencyMs["search"]))
+	}
+	if len(snap.ToolLatencyMs["calc"]) != 1 {
+		t.Fatalf("expected 1 calc latency, got %d", len(snap.ToolLatencyMs["calc"]))
+	}
+}
+
+func TestAutoCollector_CheckpointMetrics(t *testing.T) {
+	c := NewAutoCollector()
+
+	c.RecordCheckpointSave()
+	c.RecordCheckpointSave()
+	c.RecordCheckpointRestore(true)
+	c.RecordCheckpointSave()
+
+	snap := c.Snapshot()
+	if snap.CheckpointSaves != 3 {
+		t.Fatalf("expected 3 saves, got %d", snap.CheckpointSaves)
+	}
+	if snap.CheckpointRestores != 1 {
+		t.Fatalf("expected 1 restore, got %d", snap.CheckpointRestores)
+	}
+}
+
+func TestAutoCollector_ExecutionMetrics(t *testing.T) {
+	c := NewAutoCollector()
+
+	c.RecordStep()
+	c.RecordStep()
+	c.RecordStep()
+	c.RecordNode("a")
+	c.RecordNode("b")
+	c.RecordNode("c")
+	c.RecordRecoveredError()
+	c.RecordInterrupt()
+
+	snap := c.Snapshot()
+	if snap.Steps != 3 {
+		t.Fatalf("expected 3 steps, got %d", snap.Steps)
+	}
+	if snap.NodesExecuted != 3 {
+		t.Fatalf("expected 3 nodes, got %d", snap.NodesExecuted)
+	}
+	if snap.RecoveredErrors != 1 {
+		t.Fatalf("expected 1 error, got %d", snap.RecoveredErrors)
+	}
+	if snap.InterruptCount != 1 {
+		t.Fatalf("expected 1 interrupt, got %d", snap.InterruptCount)
+	}
+}
+
+func TestAutoCollector_LLMCost(t *testing.T) {
+	c := NewAutoCollector()
+	c.RecordLLMCost(0.002)
+	c.RecordLLMCost(0.001)
+	snap := c.Snapshot()
+	if snap.CostPerTask != 0.003 {
+		t.Fatalf("expected cost 0.003, got %.6f", snap.CostPerTask)
+	}
+}
+
+func TestAutoCollector_MemoryHit(t *testing.T) {
+	c := NewAutoCollector()
+	c.RecordMemoryHit(0.9)
+	c.RecordMemoryHit(0.7)
+	snap := c.Snapshot()
+	if snap.MemoryAvgHitScore < 0.79 || snap.MemoryAvgHitScore > 0.81 {
+		t.Fatalf("expected avg ~0.80, got %.4f", snap.MemoryAvgHitScore)
+	}
+}
+
+func TestAutoCollector_SnapshotCopy(t *testing.T) {
+	c := NewAutoCollector()
+	c.RecordToolCall("search", true, 100)
+
+	snap1 := c.Snapshot()
+	c.RecordToolCall("search", true, 200)
+	snap2 := c.Snapshot()
+
+	if snap1.ToolCalls != 1 {
+		t.Fatalf("snap1 should have 1 call, got %d", snap1.ToolCalls)
+	}
+	if snap2.ToolCalls != 2 {
+		t.Fatalf("snap2 should have 2 calls, got %d", snap2.ToolCalls)
+	}
+}
+
+func TestAutoCollector_Reset(t *testing.T) {
+	c := NewAutoCollector()
+	c.RecordToolCall("search", true, 100)
+	c.Reset()
+
+	snap := c.Snapshot()
+	if snap.ToolCalls != 0 {
+		t.Fatalf("expected 0 after reset, got %d", snap.ToolCalls)
+	}
+}
+
+func TestAggregator_SingleTrace(t *testing.T) {
+	a := NewMetricsAggregator()
+	m := NewAgentMetrics()
+	m.ToolCalls = 10
+	m.ToolSuccesses = 8
+	m.ToolSuccessRate = 0.8
+	m.Steps = 5
+
+	a.Add(m)
+	agg := a.Aggregate()
+
+	if agg.TotalTraces != 1 {
+		t.Fatalf("expected 1 trace, got %d", agg.TotalTraces)
+	}
+	if agg.AvgToolCalls != 10 {
+		t.Fatalf("expected avg 10, got %.2f", agg.AvgToolCalls)
+	}
+	if agg.AvgSteps != 5 {
+		t.Fatalf("expected avg 5, got %.2f", agg.AvgSteps)
+	}
+}
+
+func TestAggregator_MultipleTraces(t *testing.T) {
+	a := NewMetricsAggregator()
+
+	m1 := NewAgentMetrics()
+	m1.ToolCalls = 10
+	m1.ToolSuccessRate = 1.0
+	m1.Steps = 2
+
+	m2 := NewAgentMetrics()
+	m2.ToolCalls = 20
+	m2.ToolSuccessRate = 0.5
+	m2.Steps = 4
+
+	a.Add(m1)
+	a.Add(m2)
+	agg := a.Aggregate()
+
+	if agg.TotalTraces != 2 {
+		t.Fatalf("expected 2 traces, got %d", agg.TotalTraces)
+	}
+	if agg.AvgToolCalls != 15 {
+		t.Fatalf("expected avg 15, got %.2f", agg.AvgToolCalls)
+	}
+	if agg.AvgToolSuccessRate != 0.75 {
+		t.Fatalf("expected avg success rate 0.75, got %.4f", agg.AvgToolSuccessRate)
+	}
+	if agg.AvgSteps != 3 {
+		t.Fatalf("expected avg 3 steps, got %.2f", agg.AvgSteps)
+	}
+}
+
+func TestAggregator_Empty(t *testing.T) {
+	a := NewMetricsAggregator()
+	agg := a.Aggregate()
+	if agg.TotalTraces != 0 {
+		t.Fatalf("expected 0 traces, got %d", agg.TotalTraces)
+	}
+}
+
+func TestMetricsWindow(t *testing.T) {
+	w := NewMetricsWindow(24 * 3600 * 1000000000) // 24h in ns
+	m := NewAgentMetrics()
+	m.ToolCalls = 5
+	m.Steps = 3
+
+	w.Add(m)
+	agg := w.Aggregate()
+	if agg.TotalTraces != 1 {
+		t.Fatalf("expected 1 trace, got %d", agg.TotalTraces)
+	}
+}
+
+func TestExporter_Text(t *testing.T) {
+	e := NewExporter("test")
+	m := NewAgentMetrics()
+	m.ToolCalls = 10
+	m.ToolSuccesses = 8
+	m.ToolSuccessRate = 0.8
+	m.CheckpointSaves = 5
+	m.Steps = 4
+	m.NodesExecuted = 12
+
+	text := e.ExportText(m)
+	if !strings.Contains(text, "test_tool_calls_total 10") {
+		t.Fatalf("expected tool call metric in output:\n%s", text)
+	}
+	if !strings.Contains(text, "test_tool_success_rate 0.8000") {
+		t.Fatalf("expected success rate in output:\n%s", text)
+	}
+	if !strings.Contains(text, "test_checkpoint_saves_total 5") {
+		t.Fatalf("expected checkpoint metric in output:\n%s", text)
+	}
+}
+
+func TestExporter_CSV(t *testing.T) {
+	e := NewExporter("")
+	m := NewAgentMetrics()
+	m.TraceID = "t1"
+	m.ToolCalls = 5
+	m.ToolSuccesses = 4
+	m.Steps = 3
+
+	csv := e.ExportCSV(m)
+	if !strings.HasPrefix(csv, "t1,") {
+		t.Fatalf("expected CSV starting with trace ID, got: %s", csv)
+	}
+}
+
+func TestPercentile(t *testing.T) {
+	data := []float64{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
+	if p := percentile(data, 50); p != 5 {
+		t.Fatalf("P50: expected 5, got %.0f", p)
+	}
+	if p := percentile(data, 95); p != 10 {
+		t.Fatalf("P95: expected 10, got %.0f", p)
+	}
+	if p := percentile(data, 99); p != 10 {
+		t.Fatalf("P99: expected 10, got %.0f", p)
+	}
+}
+
+func TestPercentile_Empty(t *testing.T) {
+	if p := percentile(nil, 50); p != 0 {
+		t.Fatalf("expected 0 for empty data, got %.0f", p)
+	}
+}
diff --git a/internal/harness/replay/checkpoint.go b/internal/harness/replay/checkpoint.go
new file mode 100644
index 000000000..a33e068b6
--- /dev/null
+++ b/internal/harness/replay/checkpoint.go
@@ -0,0 +1,92 @@
+package replay
+
+import (
+	"encoding/json"
+	"fmt"
+	"time"
+
+	"ragflow/internal/harness/events"
+	"ragflow/internal/harness/graph/constants"
+)
+
+// BuildCheckpoint reconstructs a flat map[string]any checkpoint from a sequence
+// of events leading up to a fork point. This allows the Pregel engine to resume
+// execution from that state as if it had been checkpointed during the original run.
+//
+// The returned map contains:
+//   - Channel values extracted from EventStateWrite events
+//   - __completed_tasks__ from EventNodeEnd events (NUL-separated)
+//   - __step__ from the last EventStepStart/EventStepEnd event
+//   - __last_state__ (JSON serialised)
+//   - __last_completed_node__ from the last node event
+//   - checkpoint_id metadata
+//
+// The second return value is the reconstructed checkpoint_id.
+func BuildCheckpoint(originalEvents []*events.Event, threadID string) (map[string]any, string) {
+	cp := make(map[string]any)
+	cp[constants.ConfigKeyThreadID] = threadID
+
+	checkpointID := fmt.Sprintf("fork-cp-%s-%d", threadID, time.Now().UnixNano())
+	cp[constants.ConfigKeyCheckpointID] = checkpointID
+	cp["__pregel_checkpoint_id"] = checkpointID
+
+	var completedTasks []string
+	var lastCompletedNode string
+	var lastStep int
+
+	// Collect channel values from state writes, track completed nodes.
+	for _, ev := range originalEvents {
+		switch ev.Type {
+		case events.EventStateWrite:
+			var st events.StateTransitionPayload
+			if ev.Payload != nil {
+				_ = json.Unmarshal(ev.Payload, &st)
+			}
+			if st.Channel != "" {
+				cp[st.Channel] = st.NewValue
+			}
+
+		case events.EventNodeEnd:
+			completedTasks = append(completedTasks, ev.Node)
+			lastCompletedNode = ev.Node
+
+		case events.EventStepEnd:
+			if ev.Step > lastStep {
+				lastStep = ev.Step
+			}
+		}
+	}
+
+	// If there are state writes, serialise the accumulated map as last_state.
+	if len(cp) > 2 { // more than just thread_id and checkpoint_id
+		lastState := make(map[string]any)
+		for k, v := range cp {
+			if k != constants.ConfigKeyThreadID && k != constants.ConfigKeyCheckpointID && k != "__pregel_checkpoint_id" {
+				lastState[k] = v
+			}
+		}
+		if ls, err := json.Marshal(lastState); err == nil {
+			cp["__last_state__"] = string(ls)
+		}
+	}
+
+	// Serialise completed tasks as NUL-separated string.
+	if len(completedTasks) > 0 {
+		var sb []byte
+		for i, task := range completedTasks {
+			if i > 0 {
+				sb = append(sb, 0) // NUL separator
+			}
+			sb = append(sb, task...)
+		}
+		cp["__completed_tasks__"] = string(sb)
+	}
+
+	if lastCompletedNode != "" {
+		cp["__last_completed_node__"] = lastCompletedNode
+	}
+
+	cp["__step__"] = float64(lastStep)
+
+	return cp, checkpointID
+}
diff --git a/internal/harness/replay/diff.go b/internal/harness/replay/diff.go
new file mode 100644
index 000000000..f4bbccc41
--- /dev/null
+++ b/internal/harness/replay/diff.go
@@ -0,0 +1,211 @@
+package replay
+
+import (
+	"context"
+
+	"ragflow/internal/harness/events"
+)
+
+// DiffResult contains the comparison of two execution traces.
+type DiffResult struct {
+	// LeftTraceID identifies the left (reference) trace.
+	LeftTraceID string
+
+	// RightTraceID identifies the right (candidate) trace.
+	RightTraceID string
+
+	// MissingInRight are events present in the left trace but absent in the right.
+	MissingInRight []*events.Event
+
+	// MissingInLeft are events present in the right trace but absent in the left.
+	MissingInLeft []*events.Event
+
+	// Mismatched are events that exist in both traces but have different payloads.
+	Mismatched []EventMismatch
+
+	// StateDiff captures differences in state transitions.
+	StateDiff map[string]StateDiff
+
+	// ToolCallDiff captures differences in tool invocations.
+	ToolCallDiff []ToolCallDiff
+
+	// LLMResponseDiff captures differences in LLM responses.
+	LLMResponseDiff []LLMResponseDiff
+
+	// FinalOutputDiff is the difference in the final output (empty when identical).
+	FinalOutputDiff string
+}
+
+// EventMismatch describes a single event-level difference between two traces.
+type EventMismatch struct {
+	Clock      uint64
+	LeftEvent  *events.Event
+	RightEvent *events.Event
+	Field      string
+	LeftValue  string
+	RightValue string
+}
+
+// StateDiff describes a difference in state at a specific point.
+type StateDiff struct {
+	Clock      uint64
+	Key        string
+	LeftValue  any
+	RightValue any
+}
+
+// ToolCallDiff describes a difference in a tool invocation between two traces.
+type ToolCallDiff struct {
+	Index       int
+	ToolName    string
+	LeftResult  any
+	RightResult any
+	LeftError   string
+	RightError  string
+}
+
+// LLMResponseDiff describes a difference in an LLM response between two traces.
+type LLMResponseDiff struct {
+	Index        int
+	LeftContent  string
+	RightContent string
+}
+
+// Diff compares two execution traces from the same event store.
+// It identifies events that are present in one trace but not the other,
+// and events that exist in both but differ in content.
+func Diff(ctx context.Context, left, right events.EventLog, leftTraceID, rightTraceID string) (*DiffResult, error) {
+	result := &DiffResult{
+		LeftTraceID:  leftTraceID,
+		RightTraceID: rightTraceID,
+		StateDiff:    make(map[string]StateDiff),
+	}
+
+	// Collect events from both traces.
+	leftEvents, err := readAllEvents(ctx, left, leftTraceID)
+	if err != nil {
+		return nil, err
+	}
+	rightEvents, err := readAllEvents(ctx, right, rightTraceID)
+	if err != nil {
+		return nil, err
+	}
+
+	// Build lookup maps.
+	leftByClock := make(map[uint64]*events.Event)
+	for _, ev := range leftEvents {
+		leftByClock[ev.Clock] = ev
+	}
+	rightByClock := make(map[uint64]*events.Event)
+	for _, ev := range rightEvents {
+		rightByClock[ev.Clock] = ev
+	}
+
+	// Collect all clock values.
+	allClocks := make(map[uint64]bool)
+	for _, ev := range leftEvents {
+		allClocks[ev.Clock] = true
+	}
+	for _, ev := range rightEvents {
+		allClocks[ev.Clock] = true
+	}
+
+	// Compare event by event.
+	for clock := range allClocks {
+		leftEv, leftOk := leftByClock[clock]
+		rightEv, rightOk := rightByClock[clock]
+
+		switch {
+		case leftOk && !rightOk:
+			result.MissingInRight = append(result.MissingInRight, leftEv)
+		case !leftOk && rightOk:
+			result.MissingInLeft = append(result.MissingInLeft, rightEv)
+		case leftOk && rightOk:
+			// Both exist — compare.
+			if leftEv.Type != rightEv.Type {
+				result.Mismatched = append(result.Mismatched, EventMismatch{
+					Clock:      clock,
+					LeftEvent:  leftEv,
+					RightEvent: rightEv,
+					Field:      "type",
+					LeftValue:  string(leftEv.Type),
+					RightValue: string(rightEv.Type),
+				})
+			}
+			if leftEv.Hash != rightEv.Hash {
+				result.Mismatched = append(result.Mismatched, EventMismatch{
+					Clock:      clock,
+					LeftEvent:  leftEv,
+					RightEvent: rightEv,
+					Field:      "payload",
+					LeftValue:  leftEv.Hash[:16],
+					RightValue: rightEv.Hash[:16],
+				})
+			}
+
+			// Categorise by event type.
+			switch leftEv.Type {
+			case events.EventLLMCallEnd:
+				result.LLMResponseDiff = append(result.LLMResponseDiff, LLMResponseDiff{
+					Index:        len(result.LLMResponseDiff),
+					LeftContent:  extractContent(leftEv),
+					RightContent: extractContent(rightEv),
+				})
+			case events.EventToolCallResult:
+				result.ToolCallDiff = append(result.ToolCallDiff, ToolCallDiff{
+					Index:    len(result.ToolCallDiff),
+					ToolName: extractToolName(leftEv),
+				})
+			case events.EventStateWrite:
+				if leftEv.Node != "" {
+					result.StateDiff[leftEv.Node] = StateDiff{
+						Clock: clock,
+						Key:   leftEv.Node,
+					}
+				}
+			}
+		}
+	}
+
+	return result, nil
+}
+
+// readAllEvents reads all events for a trace from the store.
+func readAllEvents(ctx context.Context, store events.EventLog, traceID string) ([]*events.Event, error) {
+	iter := store.Stream(ctx, events.EventFilter{TraceID: traceID})
+	defer iter.Close()
+
+	var result []*events.Event
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		result = append(result, ev)
+	}
+	return result, nil
+}
+
+// extractContent extracts the Content field from an LLMCallPayload event.
+func extractContent(ev *events.Event) string {
+	if ev.Payload == nil {
+		return ""
+	}
+	var payload events.LLMCallPayload
+	if err := jsonUnmarshal(ev.Payload, &payload); err != nil {
+		return ""
+	}
+	return payload.Content
+}
+
+// extractToolName extracts the ToolName field from a ToolCallPayload event.
+func extractToolName(ev *events.Event) string {
+	if ev.Payload == nil {
+		return ""
+	}
+	var payload events.ToolCallPayload
+	if err := jsonUnmarshal(ev.Payload, &payload); err != nil {
+		return ""
+	}
+	return payload.ToolName
+}
diff --git a/internal/harness/replay/fork.go b/internal/harness/replay/fork.go
new file mode 100644
index 000000000..f9fd73af4
--- /dev/null
+++ b/internal/harness/replay/fork.go
@@ -0,0 +1,225 @@
+package replay
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"ragflow/internal/harness/events"
+	"ragflow/internal/harness/graph/checkpoint"
+	"ragflow/internal/harness/graph/constants"
+	"ragflow/internal/harness/graph/pregel"
+	"ragflow/internal/harness/graph/types"
+)
+
+// ForkContextKey is used to pass the ForkConfig's ModelOverride/ToolOverride
+// through context to node-level wrappers during true replay.
+// This is a spare key; the actual model/tool substitution during engine
+// re-execution is done by the caller via agent-level middleware.
+type ForkContextKey struct{}
+
+// ForkConfig configures a fork operation.
+type ForkConfig struct {
+	// Store is the event source for the original trace.
+	Store events.EventLog
+
+	// TraceID identifies the original trace to fork from.
+	TraceID string
+
+	// Point is the event ID at which to fork.
+	Point events.EventID
+
+	// Substitution strategies for the forked branch.
+	ModelOverride ModelOverrideFunc
+	ToolOverride  ToolOverrideFunc
+	NewInput      any
+
+	// ForkEngine is the actual graph engine to execute the forked branch.
+	// When set, Fork replays up to ForkPoint, builds a checkpoint from the
+	// events, saves it into a MemorySaver, and hands off to real execution.
+	// When nil, Fork replays deterministically from EventLog alone.
+	ForkEngine *pregel.Engine
+
+	// Checkpointer is the persistence backend to use when resuming the
+	// ForkEngine. When nil, a fresh MemorySaver is created.
+	Checkpointer checkpoint.BaseCheckpointer
+
+	// OutputStore receives events generated during the fork (nil = discard).
+	OutputStore events.EventLog
+}
+
+// ForkResult contains the result of a fork operation.
+type ForkResult struct {
+	// ForkTraceID identifies the new fork trace.
+	ForkTraceID string
+
+	// ForkEvents generated during the forked execution.
+	ForkEvents []*events.Event
+
+	// ParentTraceID is the original trace that was forked.
+	ParentTraceID string
+
+	// ForkPoint is the event ID where the fork occurred.
+	ForkPoint events.EventID
+
+	// FinalState is the output state from the forked Engine execution.
+	// Only set when ForkEngine was used.
+	FinalState any
+
+	// Duration of the fork operation.
+	Duration time.Duration
+}
+
+// Fork creates a branched execution from a specified point in the trace.
+// Events up to ForkPoint are replayed from the original store.
+// After ForkPoint, if ForkEngine is set, execution hands off to the real
+// graph engine via checkpoint resume; otherwise replay continues
+// deterministically with overrides.
+func (e *ReplayEngine) Fork(ctx context.Context, cfg *ForkConfig) (*ForkResult, error) {
+	start := time.Now()
+
+	// Use config store, falling back to engine store.
+	store := cfg.Store
+	if store == nil {
+		store = e.store
+	}
+
+	// Find the fork point event.
+	forkEvent, err := store.Get(ctx, cfg.Point)
+	if err != nil {
+		return nil, err
+	}
+	if forkEvent == nil {
+		return nil, errEventNotFound(cfg.Point)
+	}
+
+	// Read ALL events up to (but not including) the fork point.
+	// We need the complete event list to reconstruct the checkpoint.
+	filter := events.EventFilter{
+		TraceID: cfg.TraceID,
+		ToClock: forkEvent.Clock - 1,
+	}
+	iter := store.Stream(ctx, filter)
+	defer iter.Close()
+
+	var preForkEvents []*events.Event
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		preForkEvents = append(preForkEvents, ev)
+	}
+
+	result := &ForkResult{
+		ForkTraceID:   cfg.TraceID + "_fork_" + string(cfg.Point),
+		ParentTraceID: cfg.TraceID,
+		ForkPoint:     cfg.Point,
+	}
+
+	// Append fork marker event.
+	forkMarker := events.NewEvent(events.EventFork, 0)
+	forkMarker.TraceID = result.ForkTraceID
+	forkMarker.ParentID = cfg.Point
+	forkMarker.CausedBy = []events.EventID{cfg.Point}
+	forkMarker.Metadata["parent_trace"] = cfg.TraceID
+	forkMarker.Seal()
+
+	// Collect pre-fork events.
+	result.ForkEvents = append(result.ForkEvents, preForkEvents...)
+	result.ForkEvents = append(result.ForkEvents, forkMarker)
+
+	if cfg.OutputStore != nil {
+		if err := cfg.OutputStore.Append(ctx, result.ForkEvents...); err != nil {
+			return nil, err
+		}
+	}
+
+	// If a fork engine is provided, reconstruct checkpoint and resume.
+	if cfg.ForkEngine != nil {
+		forkResult, err := e.resumeFromCheckpoint(ctx, cfg, preForkEvents, forkMarker)
+		if err != nil {
+			return nil, fmt.Errorf("fork resume: %w", err)
+		}
+		result.FinalState = forkResult
+	}
+
+	result.Duration = time.Since(start)
+	return result, nil
+}
+
+// resumeFromCheckpoint reconstructs checkpoint state from pre-fork events and
+// resumes the ForkEngine from that point. The engine runs the graph from the
+// reconstructed state and returns the final output.
+func (e *ReplayEngine) resumeFromCheckpoint(ctx context.Context, cfg *ForkConfig, preForkEvents []*events.Event, forkMarker *events.Event) (any, error) {
+	if cfg.ForkEngine == nil {
+		return nil, nil
+	}
+
+	threadID := cfg.TraceID
+	if threadID == "" {
+		threadID = "fork-" + string(cfg.Point)
+	}
+
+	// Build checkpoint map from pre-fork events.
+	cp, cpID := BuildCheckpoint(preForkEvents, threadID)
+
+	// Save checkpoint into a MemorySaver (or caller-provided checkpointer).
+	saver := cfg.Checkpointer
+	if saver == nil {
+		saver = checkpoint.NewMemorySaver()
+	}
+
+	if err := saver.Put(ctx, map[string]any{
+		constants.ConfigKeyThreadID:     threadID,
+		constants.ConfigKeyCheckpointID: cpID,
+	}, cp); err != nil {
+		return nil, fmt.Errorf("save fork checkpoint: %w", err)
+	}
+
+	// Check if ForkEngine already has a checkpointer; if not, set it.
+	// We inject our own via WithCheckpointer option at Fork creation time
+	// by creating a new Engine wrapping the same graph.
+
+	// Configure the engine's runnable config to point at the checkpoint.
+	rc := types.NewRunnableConfig()
+	rc.ThreadID = threadID
+	rc.Set(constants.ConfigKeyThreadID, threadID)
+	rc.Set(constants.ConfigKeyCheckpointID, cpID)
+
+	// Run the ForkEngine with the resume config.
+	outputCh, errCh := cfg.ForkEngine.Run(ctx, nil, types.StreamModeValues)
+
+	// Drain outputCh for final state.
+	var finalState any
+	for result := range outputCh {
+		if se, ok := result.(*pregel.StreamEvent); ok {
+			if se.Type == pregel.EventTypeFinal {
+				if data, ok := se.Data.(map[string]any); ok {
+					if state, ok := data["state"]; ok {
+						finalState = state
+					}
+				}
+			}
+		}
+	}
+
+	if err := <-errCh; err != nil {
+		return nil, err
+	}
+
+	// If output store is set, record fork completion.
+	if cfg.OutputStore != nil {
+		forkEnd := events.NewEvent(events.EventGraphEnd, 0)
+		forkEnd.TraceID = cfg.TraceID + "_fork_" + string(cfg.Point)
+		forkEnd.Metadata["fork_replay"] = true
+		forkEnd.Seal()
+		_ = cfg.OutputStore.Append(ctx, forkEnd)
+	}
+
+	return finalState, nil
+}
+
+func errEventNotFound(id events.EventID) error {
+	return errorf("event not found: %s", id)
+}
diff --git a/internal/harness/replay/injector.go b/internal/harness/replay/injector.go
new file mode 100644
index 000000000..778788e8b
--- /dev/null
+++ b/internal/harness/replay/injector.go
@@ -0,0 +1,119 @@
+package replay
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"ragflow/internal/harness/events"
+)
+
+// ---- common overrides ----
+
+// ReplayExactTools returns a ToolOverrideFunc that uses the recorded result
+// unchanged. This is the default behaviour for deterministic replay.
+func ReplayExactTools() ToolOverrideFunc {
+	return func(toolName string, args map[string]any, recordedResult any) (any, error) {
+		return recordedResult, nil
+	}
+}
+
+// ReplayLiveTools returns a ToolOverrideFunc that always returns nil,
+// signalling the replay to execute the tool with the real implementation.
+func ReplayLiveTools() ToolOverrideFunc {
+	return func(toolName string, args map[string]any, recordedResult any) (any, error) {
+		// Return nil to indicate "execute live".
+		return nil, nil
+	}
+}
+
+// ReplaySubstituteModel returns a ModelOverrideFunc that replaces the
+// recorded LLM response with a fixed string. Use this to compare how
+// a different model would change behaviour while keeping tool results frozen.
+//
+// The callback receives the original recorded response and should return
+// the substitute response. Return ("", nil) to suppress the response.
+type ReplayModelCallback func(recordedResponse string) string
+
+// ReplaySubstituteModel creates a ModelOverrideFunc from a callback.
+func ReplaySubstituteModel(fn ReplayModelCallback) ModelOverrideFunc {
+	return func(_ []any, recordedResponse string) (*string, error) {
+		substituted := fn(recordedResponse)
+		return &substituted, nil
+	}
+}
+
+// ---- error types ----
+
+type replayError struct {
+	msg string
+}
+
+func (e *replayError) Error() string { return e.msg }
+
+func errorf(format string, args ...any) error {
+	return &replayError{msg: fmt.Sprintf(format, args...)}
+}
+
+// ---- helpers ----
+
+func jsonUnmarshal(data []byte, target any) error {
+	return json.Unmarshal(data, target)
+}
+
+func jsonMarshal(v any) ([]byte, error) {
+	return json.Marshal(v)
+}
+
+// copyEvent creates a shallow copy of an Event with a deep copy of Payload and Metadata.
+func copyEvent(ev *events.Event) *events.Event {
+	cp := *ev
+	if ev.Payload != nil {
+		cp.Payload = make([]byte, len(ev.Payload))
+		copy(cp.Payload, ev.Payload)
+	}
+	if ev.Metadata != nil {
+		cp.Metadata = make(map[string]any, len(ev.Metadata))
+		for k, v := range ev.Metadata {
+			cp.Metadata[k] = v
+		}
+	}
+	if ev.CausedBy != nil {
+		cp.CausedBy = make([]events.EventID, len(ev.CausedBy))
+		copy(cp.CausedBy, ev.CausedBy)
+	}
+	return &cp
+}
+
+// ---- event helpers for test assertions ----
+
+// FindEventsOfType filters events by type.
+func FindEventsOfType(evts []*events.Event, typ events.EventType) []*events.Event {
+	var result []*events.Event
+	for _, ev := range evts {
+		if ev.Type == typ {
+			result = append(result, ev)
+		}
+	}
+	return result
+}
+
+// EventsContains checks if any event has the given type.
+func EventsContains(evts []*events.Event, typ events.EventType) bool {
+	for _, ev := range evts {
+		if ev.Type == typ {
+			return true
+		}
+	}
+	return false
+}
+
+// EventCount counts events of a given type.
+func EventCount(evts []*events.Event, typ events.EventType) int {
+	count := 0
+	for _, ev := range evts {
+		if ev.Type == typ {
+			count++
+		}
+	}
+	return count
+}
diff --git a/internal/harness/replay/integration_test.go b/internal/harness/replay/integration_test.go
new file mode 100644
index 000000000..ad206818f
--- /dev/null
+++ b/internal/harness/replay/integration_test.go
@@ -0,0 +1,463 @@
+package replay
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"testing"
+
+	"ragflow/internal/harness/events"
+	"ragflow/internal/harness/graph/channels"
+	"ragflow/internal/harness/graph/checkpoint"
+	"ragflow/internal/harness/graph/constants"
+	"ragflow/internal/harness/graph/graph"
+	"ragflow/internal/harness/graph/pregel"
+	"ragflow/internal/harness/graph/types"
+)
+
+// ============================================================================
+// Integration: Tool calls — record → verify → replay
+// ============================================================================
+
+func TestIntegration_ToolCalls(t *testing.T) {
+	ctx := context.Background()
+	eventStore := events.NewMemoryEventStore()
+	recorder := events.NewEventRecorder(eventStore, events.WithTraceID("tool-int"))
+
+	// Build a StateGraph with tool-calling nodes.
+	sg := graph.NewStateGraph(map[string]any{})
+	sg.AddChannel("value", channels.NewLastValue(""))
+
+	sg.AddNode("search", func(ctx context.Context, state any) (any, error) {
+		recorder.RecordToolCall(ctx, "web_search", map[string]any{"q": "RAG architecture"},
+			"Search results: RAG = Retrieval Augmented Generation", 350, 0, "")
+		m, _ := state.(map[string]any)
+		m["value"] = "searched"
+		return m, nil
+	})
+
+	sg.AddNode("calculator", func(ctx context.Context, state any) (any, error) {
+		recorder.RecordToolCall(ctx, "calculator", map[string]any{"expr": "2+2"}, "4", 50, 0, "")
+		m, _ := state.(map[string]any)
+		m["value"] = "calculated"
+		return m, nil
+	})
+
+	sg.AddNode("fail_tool", func(ctx context.Context, state any) (any, error) {
+		recorder.RecordToolCall(ctx, "failing_tool", map[string]any{}, nil, 100, 2, "permission denied")
+		m, _ := state.(map[string]any)
+		m["value"] = "failed"
+		return m, nil
+	})
+
+	if err := sg.AddEdge(constants.Start, "search"); err != nil {
+		t.Fatal(err)
+	}
+	if err := sg.AddEdge("search", "calculator"); err != nil {
+		t.Fatal(err)
+	}
+	if err := sg.AddEdge("calculator", "fail_tool"); err != nil {
+		t.Fatal(err)
+	}
+	if err := sg.AddEdge("fail_tool", constants.End); err != nil {
+		t.Fatal(err)
+	}
+
+	cb := pregel.NewCallbackManager()
+	cb.AddCallback(recorder)
+
+	engine := pregel.NewEngine(sg,
+		pregel.WithCheckpointer(checkpoint.NewMemorySaver()),
+		pregel.WithCallbacks(cb),
+		pregel.WithRecursionLimit(10),
+	)
+
+	outputCh, errCh := engine.Run(ctx, map[string]any{"value": ""}, types.StreamModeValues)
+	drainOutput(outputCh)
+	if err := <-errCh; err != nil {
+		t.Fatal(err)
+	}
+
+	// --- Phase 2: Verify events ---
+	iter := eventStore.Stream(ctx, events.EventFilter{TraceID: "tool-int"})
+	var recordedEvents []*events.Event
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		recordedEvents = append(recordedEvents, ev)
+	}
+
+	toolStarts := countByType(recordedEvents, events.EventToolCallStart)
+	toolResults := countByType(recordedEvents, events.EventToolCallResult)
+	graphEvents := countByType(recordedEvents, events.EventGraphStart) +
+		countByType(recordedEvents, events.EventGraphEnd)
+
+	if graphEvents != 2 {
+		t.Fatalf("expected 2 graph events (start+end), got %d", graphEvents)
+	}
+	if toolStarts != 3 {
+		t.Fatalf("expected 3 tool call starts, got %d", toolStarts)
+	}
+	if toolResults != 3 {
+		t.Fatalf("expected 3 tool call results, got %d", toolResults)
+	}
+
+	// Check tool names in payloads.
+	for _, ev := range recordedEvents {
+		if ev.Type == events.EventToolCallResult {
+			var pl events.ToolCallPayload
+			if ev.Payload != nil {
+				json.Unmarshal(ev.Payload, &pl)
+			}
+			switch pl.ToolName {
+			case "web_search", "calculator", "failing_tool":
+			default:
+				t.Fatalf("unexpected tool name: %s", pl.ToolName)
+			}
+			if pl.ToolName == "failing_tool" && pl.Error != "permission denied" {
+				t.Fatalf("expected 'permission denied' error, got %q", pl.Error)
+			}
+			if pl.ToolName == "calculator" && pl.Result != "4" {
+				t.Fatalf("expected result '4', got %v", pl.Result)
+			}
+		}
+	}
+
+	// ---- Phase 3: Replay ----
+	replayEngine := NewReplayEngine(eventStore)
+	replayResult, err := replayEngine.Replay(ctx, &ReplayConfig{
+		TraceID:     "tool-int",
+		DiffEnabled: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(replayResult.Divergences) != 0 {
+		t.Fatalf("expected 0 divergences, got %d", len(replayResult.Divergences))
+	}
+	if replayResult.OriginalLen != len(recordedEvents) {
+		t.Fatalf("OriginalLen %d != recorded %d", replayResult.OriginalLen, len(recordedEvents))
+	}
+	if replayResult.ReplayMetrics.MatchCount != replayResult.ReplayMetrics.TotalEvents {
+		t.Fatalf("MatchCount != TotalEvents: %d divergences", replayResult.ReplayMetrics.DivergenceCount)
+	}
+
+	// ---- Phase 4: Replay with tool override ----
+	overrideCalled := false
+	_, err = replayEngine.Replay(ctx, &ReplayConfig{
+		TraceID: "tool-int",
+		ToolOverride: func(name string, args map[string]any, recorded any) (any, error) {
+			overrideCalled = true
+			return "OVERRIDDEN", nil
+		},
+		DiffEnabled: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !overrideCalled {
+		t.Fatal("ToolOverride was never called")
+	}
+}
+
+// ============================================================================
+// Integration: Sub-agents — record → verify → replay
+// ============================================================================
+
+func TestIntegration_SubAgents(t *testing.T) {
+	ctx := context.Background()
+	eventStore := events.NewMemoryEventStore()
+	recorder := events.NewEventRecorder(eventStore, events.WithTraceID("sub-agent-int"))
+
+	sg := graph.NewStateGraph(map[string]any{})
+	sg.AddChannel("value", channels.NewLastValue(""))
+
+	sg.AddNode("researcher", func(ctx context.Context, state any) (any, error) {
+		recorder.RecordSubAgentCall(ctx, "researcher", "user query: RAG vs Fine-tuning",
+			"RAG is better for dynamic knowledge", 1, 1500, "")
+		m, _ := state.(map[string]any)
+		m["value"] = "researched"
+		return m, nil
+	})
+	sg.AddNode("writer", func(ctx context.Context, state any) (any, error) {
+		recorder.RecordSubAgentCall(ctx, "writer", "summarize research results",
+			"Final summary: ...", 1, 800, "")
+		m, _ := state.(map[string]any)
+		m["value"] = "written"
+		return m, nil
+	})
+	sg.AddNode("aggregator", func(ctx context.Context, state any) (any, error) {
+		recorder.RecordSubAgentCall(ctx, "aggregator", "merge results",
+			nil, 2, 300, "deadline exceeded")
+		m, _ := state.(map[string]any)
+		m["value"] = "aggregated"
+		return m, nil
+	})
+
+	if err := sg.AddEdge(constants.Start, "researcher"); err != nil {
+		t.Fatal(err)
+	}
+	if err := sg.AddEdge("researcher", "writer"); err != nil {
+		t.Fatal(err)
+	}
+	if err := sg.AddEdge("writer", "aggregator"); err != nil {
+		t.Fatal(err)
+	}
+	if err := sg.AddEdge("aggregator", constants.End); err != nil {
+		t.Fatal(err)
+	}
+
+	cb := pregel.NewCallbackManager()
+	cb.AddCallback(recorder)
+	engine := pregel.NewEngine(sg,
+		pregel.WithCheckpointer(checkpoint.NewMemorySaver()),
+		pregel.WithCallbacks(cb),
+		pregel.WithRecursionLimit(10),
+	)
+
+	outputCh, errCh := engine.Run(ctx, map[string]any{"value": ""}, types.StreamModeValues)
+	drainOutput(outputCh)
+	if err := <-errCh; err != nil {
+		t.Fatal(err)
+	}
+
+	// Verify sub-agent events.
+	iter := eventStore.Stream(ctx, events.EventFilter{TraceID: "sub-agent-int"})
+	var all []*events.Event
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		all = append(all, ev)
+	}
+
+	saStarts := countByType(all, events.EventSubAgentCallStart)
+	saEnds := countByType(all, events.EventSubAgentCallEnd)
+	if saStarts != 3 {
+		t.Fatalf("expected 3 sub-agent starts, got %d", saStarts)
+	}
+	if saEnds != 3 {
+		t.Fatalf("expected 3 sub-agent ends, got %d", saEnds)
+	}
+
+	for _, ev := range all {
+		if ev.Type == events.EventSubAgentCallEnd {
+			var pl events.SubAgentCallPayload
+			if ev.Payload != nil {
+				json.Unmarshal(ev.Payload, &pl)
+			}
+			switch pl.SubAgentName {
+			case "researcher":
+				if pl.DurationMs != 1500 {
+					t.Fatalf("expected researcher duration 1500, got %d", pl.DurationMs)
+				}
+			case "aggregator":
+				if pl.Error != "deadline exceeded" {
+					t.Fatalf("expected aggregator error, got %q", pl.Error)
+				}
+			}
+		}
+	}
+
+	// Replay.
+	replayEngine := NewReplayEngine(eventStore)
+	replayResult, err := replayEngine.Replay(ctx, &ReplayConfig{
+		TraceID:     "sub-agent-int",
+		DiffEnabled: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(replayResult.Divergences) != 0 {
+		t.Fatalf("expected 0 divergences, got %d", len(replayResult.Divergences))
+	}
+	_ = replayResult
+}
+
+// ============================================================================
+// Integration: Loop / Deep Research — record → verify → replay
+// ============================================================================
+
+func TestIntegration_DeepResearchLoop(t *testing.T) {
+	ctx := context.Background()
+	eventStore := events.NewMemoryEventStore()
+	recorder := events.NewEventRecorder(eventStore, events.WithTraceID("deep-research"))
+
+	sg := graph.NewStateGraph(map[string]any{})
+	sg.AddChannel("value", channels.NewLastValue(""))
+
+	sg.AddNode("research_step", func(ctx context.Context, state any) (any, error) {
+		m, _ := state.(map[string]any)
+		iteration := 1
+		if iter, ok := m["iteration"].(int); ok {
+			iteration = iter
+		}
+
+		var toolName, query, result string
+		switch iteration {
+		case 1:
+			toolName = "web_search"
+			query = "deep learning fundamentals"
+			result = "DL is a subset of ML using neural networks"
+		case 2:
+			toolName = "academic_search"
+			query = "transformer architecture 2024"
+			result = "Transformer: attention is all you need"
+		case 3:
+			toolName = "code_search"
+			query = "Python implementation of RAG"
+			result = "langchain + chromadb example"
+		default:
+			toolName = "summarize"
+			query = fmt.Sprintf("iteration %d wrap-up", iteration)
+			result = fmt.Sprintf("Final summary after %d iterations", iteration)
+		}
+
+		recorder.RecordToolCall(ctx, toolName, map[string]any{"q": query}, result, 200, 0, "")
+		recorder.RecordStateWrite(ctx, fmt.Sprintf("result_%d", iteration), nil, result, "set")
+
+		m["iteration"] = iteration + 1
+		m["value"] = result
+		return m, nil
+	})
+
+	if err := sg.AddEdge(constants.Start, "research_step"); err != nil {
+		t.Fatal(err)
+	}
+	if err := sg.AddConditionalEdges("research_step",
+		func(ctx context.Context, state any) (any, error) {
+			m, _ := state.(map[string]any)
+			iteration := 1
+			if iter, ok := m["iteration"].(int); ok {
+				iteration = iter
+			}
+			if iteration < 4 {
+				return "continue", nil
+			}
+			return "done", nil
+		},
+		map[string]string{
+			"continue": "research_step",
+			"done":     constants.End,
+		},
+	); err != nil {
+		t.Fatal(err)
+	}
+
+	cb := pregel.NewCallbackManager()
+	cb.AddCallback(recorder)
+	engine := pregel.NewEngine(sg,
+		pregel.WithCheckpointer(checkpoint.NewMemorySaver()),
+		pregel.WithCallbacks(cb),
+		pregel.WithRecursionLimit(20),
+	)
+
+	outputCh, errCh := engine.Run(ctx, map[string]any{"value": "", "iteration": 1}, types.StreamModeValues)
+	drainOutput(outputCh)
+	if err := <-errCh; err != nil {
+		t.Fatalf("engine run error: %v", err)
+	}
+
+	// ---- Phase 2: Verify events ----
+	iter := eventStore.Stream(ctx, events.EventFilter{TraceID: "deep-research"})
+	var allEvents []*events.Event
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		allEvents = append(allEvents, ev)
+	}
+
+	toolStarts := countByType(allEvents, events.EventToolCallStart)
+	toolResults := countByType(allEvents, events.EventToolCallResult)
+	stateWrites := countByType(allEvents, events.EventStateWrite)
+
+	if toolStarts < 3 {
+		t.Fatalf("expected at least 3 tool calls, got %d", toolStarts)
+	}
+	if toolStarts != toolResults {
+		t.Fatalf("tool starts %d != tool results %d", toolStarts, toolResults)
+	}
+	if stateWrites < 3 {
+		t.Fatalf("expected at least 3 state writes, got %d", stateWrites)
+	}
+
+	// Verify tool names across iterations.
+	var toolNames []string
+	for _, ev := range allEvents {
+		if ev.Type == events.EventToolCallStart {
+			if name, ok := ev.Metadata["tool"].(string); ok {
+				toolNames = append(toolNames, name)
+			}
+		}
+	}
+	if len(toolNames) < 3 {
+		t.Fatalf("expected at least 3 tools (3 research iterations), got %d", len(toolNames))
+	}
+	if toolNames[0] != "web_search" {
+		t.Fatalf("expected first tool 'web_search', got %q", toolNames[0])
+	}
+	if toolNames[1] != "academic_search" {
+		t.Fatalf("expected second tool 'academic_search', got %q", toolNames[1])
+	}
+	if toolNames[2] != "code_search" {
+		t.Fatalf("expected third tool 'code_search', got %q", toolNames[2])
+	}
+
+	// ---- Phase 3: Replay ----
+	replayEngine := NewReplayEngine(eventStore)
+	replayResult, err := replayEngine.Replay(ctx, &ReplayConfig{
+		TraceID:     "deep-research",
+		DiffEnabled: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(replayResult.Divergences) != 0 {
+		t.Fatalf("expected 0 divergences, got %d", len(replayResult.Divergences))
+	}
+	if replayResult.OriginalLen != replayResult.ReplayLen {
+		t.Fatalf("OriginalLen %d != ReplayLen %d", replayResult.OriginalLen, replayResult.ReplayLen)
+	}
+
+	// ---- Phase 4: Replay with tool override on first tool ----
+	overrideReplay, err := replayEngine.Replay(ctx, &ReplayConfig{
+		TraceID:     "deep-research",
+		DiffEnabled: true,
+		ToolOverride: func(name string, args map[string]any, recorded any) (any, error) {
+			if name == "web_search" {
+				return "OVERRIDDEN RESEARCH", nil
+			}
+			return recorded, nil
+		},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(overrideReplay.Divergences) == 0 {
+		t.Fatal("expected divergences with tool override")
+	}
+}
+
+// ============================================================================
+// Helpers
+// ============================================================================
+
+func countByType(evts []*events.Event, typ events.EventType) int {
+	n := 0
+	for _, ev := range evts {
+		if ev.Type == typ {
+			n++
+		}
+	}
+	return n
+}
+
+func drainOutput(ch <-chan interface{}) {
+	for range ch {
+	}
+}
diff --git a/internal/harness/replay/replay.go b/internal/harness/replay/replay.go
new file mode 100644
index 000000000..91f0f6ac8
--- /dev/null
+++ b/internal/harness/replay/replay.go
@@ -0,0 +1,346 @@
+// Package replay provides deterministic replay, fork, and diff for
+// agent execution traces recorded by the events package.
+//
+// A ReplayEngine replays events from an EventLog, optionally substituting
+// model responses or tool results. Fork creates a branched execution from
+// any point in the trace. Diff compares two execution traces to detect
+// regression or behavioral changes.
+package replay
+
+import (
+	"context"
+	"time"
+
+	"ragflow/internal/harness/events"
+)
+
+// ReplayConfig configures a deterministic replay.
+type ReplayConfig struct {
+	// Store is the event source to replay from.
+	Store events.EventLog
+
+	// TraceID identifies the trace to replay.
+	TraceID string
+
+	// Start is the starting logical clock (0 = from beginning).
+	Start uint64
+
+	// End is the ending logical clock (0 = to end).
+	End uint64
+
+	// Substitution strategies.
+	ModelOverride ModelOverrideFunc
+	ToolOverride  ToolOverrideFunc
+	StateOverride StateOverrideFunc
+
+	// OutputStore receives events generated during replay (nil = discard).
+	OutputStore events.EventLog
+
+	// DiffEnabled compares replayed events with original trace.
+	DiffEnabled bool
+}
+
+// ModelOverrideFunc replaces LLM model responses during replay.
+// Return a non-nil *string to use the substituted response.
+// Return nil, nil to use the recorded response.
+type ModelOverrideFunc func(messages []any, recordedResponse string) (*string, error)
+
+// ToolOverrideFunc replaces tool execution results during replay.
+// Return a non-nil value to use the substituted result.
+// Return nil to use the recorded result.
+type ToolOverrideFunc func(toolName string, args map[string]any, recordedResult any) (any, error)
+
+// StateOverrideFunc replaces initial state during replay.
+// Return the modified state, or nil to keep the recorded state.
+type StateOverrideFunc func(recordedState map[string]any) (map[string]any, error)
+
+// ReplayResult contains the result of a deterministic replay.
+type ReplayResult struct {
+	// Events generated during replay (when OutputStore is set).
+	Events []*events.Event
+
+	// OriginalLen is the number of events in the original trace.
+	OriginalLen int
+
+	// ReplayLen is the number of events generated during replay.
+	ReplayLen int
+
+	// Divergences between replayed and original events (when DiffEnabled).
+	Divergences []EventDivergence
+
+	// ReplayMetrics contains metrics about the replay operation.
+	ReplayMetrics ReplayMetrics
+
+	// Duration of the replay operation.
+	Duration time.Duration
+}
+
+// EventDivergence describes a difference between original and replayed events.
+type EventDivergence struct {
+	// Clock position in the event log.
+	Clock uint64
+
+	// Original event (nil when the event is new in replay).
+	OriginalEvent *events.Event
+
+	// Replay event (nil when the original event was skipped).
+	ReplayEvent *events.Event
+
+	// Type of divergence.
+	Type DivergenceType
+
+	// Description explains the difference.
+	Description string
+}
+
+// DivergenceType categorises event divergences.
+type DivergenceType string
+
+const (
+	// DivergenceMissing means the original event is absent in replay.
+	DivergenceMissing DivergenceType = "missing"
+	// DivergenceExtra means the replay produced an event not in the original.
+	DivergenceExtra DivergenceType = "extra"
+	// DivergenceMismatch means the event exists in both but differs.
+	DivergenceMismatch DivergenceType = "mismatch"
+)
+
+// ReplayMetrics contains metrics about a replay operation.
+type ReplayMetrics struct {
+	TotalEvents     int
+	DivergenceCount int
+	MatchCount      int
+}
+
+// ReplayEngine replays execution traces from an EventLog.
+type ReplayEngine struct {
+	store events.EventLog
+}
+
+// NewReplayEngine creates a ReplayEngine backed by the given event store.
+func NewReplayEngine(store events.EventLog) *ReplayEngine {
+	return &ReplayEngine{store: store}
+}
+
+// Replay executes a deterministic replay of the given trace.
+// It replays events from the EventLog sequentially, optionally calling
+// ModelOverride and ToolOverride to substitute non-deterministic operations.
+func (e *ReplayEngine) Replay(ctx context.Context, cfg *ReplayConfig) (*ReplayResult, error) {
+	start := time.Now()
+
+	// Default to exact replay when no overrides are set.
+	modelOverride := cfg.ModelOverride
+	if modelOverride == nil {
+		modelOverride = func(_ []any, recorded string) (*string, error) {
+			return &recorded, nil
+		}
+	}
+	toolOverride := cfg.ToolOverride
+	if toolOverride == nil {
+		toolOverride = func(_ string, _ map[string]any, recorded any) (any, error) {
+			return recorded, nil
+		}
+	}
+
+	// Use config store, falling back to engine store.
+	store := cfg.Store
+	if store == nil {
+		store = e.store
+	}
+
+	filter := events.EventFilter{
+		TraceID:   cfg.TraceID,
+		FromClock: cfg.Start,
+		ToClock:   cfg.End,
+	}
+
+	iter := store.Stream(ctx, filter)
+	defer iter.Close()
+
+	result := &ReplayResult{}
+	var originalEvents []*events.Event
+	var replayEvents []*events.Event
+
+	// Phase 1: read original events.
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		originalEvents = append(originalEvents, ev)
+	}
+	result.OriginalLen = len(originalEvents)
+
+	// Apply StateOverride to the first EventStateWrite event (initial state).
+	// Work on a copy to preserve the original for diff.
+	if cfg.StateOverride != nil {
+		for i, ev := range originalEvents {
+			if ev.Type == events.EventStateWrite {
+				var st events.StateTransitionPayload
+				if ev.Payload != nil {
+					_ = jsonUnmarshal(ev.Payload, &st)
+				}
+				recorded := map[string]any{st.Channel: st.NewValue}
+				modified, err := cfg.StateOverride(recorded)
+				if err != nil {
+					return nil, err
+				}
+				if modified != nil {
+					if val, ok := modified[st.Channel]; ok {
+						st.NewValue = val
+						repl := copyEvent(ev)
+						repl.Payload, _ = jsonMarshal(st)
+						repl.Seal()
+						originalEvents[i] = repl
+					}
+				}
+				break
+			}
+		}
+	}
+
+	// Phase 2: replay with overrides.
+	// Copy each event before modifying so the original list is preserved
+	// for accurate diff comparison.
+	for _, original := range originalEvents {
+		replayEv := copyEvent(original)
+
+		switch original.Type {
+		case events.EventLLMCallStart, events.EventLLMCallEnd:
+			// Apply model override.
+			if original.Type == events.EventLLMCallEnd {
+				var payload events.LLMCallPayload
+				_ = parsePayload(original, &payload)
+				substituted, err := modelOverride(payload.Messages, payload.Content)
+				if err != nil {
+					return nil, err
+				}
+				if substituted != nil {
+					payload.Content = *substituted
+					replayEv.Payload, _ = jsonMarshal(payload)
+					replayEv.Seal()
+				}
+			}
+			replayEvents = append(replayEvents, replayEv)
+
+		case events.EventToolCallStart, events.EventToolCallResult:
+			// Apply tool override.
+			if original.Type == events.EventToolCallResult {
+				var payload events.ToolCallPayload
+				_ = parsePayload(original, &payload)
+				substituted, err := toolOverride(payload.ToolName, payload.Arguments, payload.Result)
+				if err != nil {
+					return nil, err
+				}
+				if substituted != nil {
+					payload.Result = substituted
+					replayEv.Payload, _ = jsonMarshal(payload)
+					replayEv.Seal()
+				}
+			}
+			replayEvents = append(replayEvents, replayEv)
+
+		default:
+			replayEvents = append(replayEvents, replayEv)
+		}
+	}
+
+	result.ReplayLen = len(replayEvents)
+
+	// Phase 3: diff (optional).
+	var divergences []EventDivergence
+	if cfg.DiffEnabled {
+		divergences = diffEventLists(originalEvents, replayEvents)
+		result.Divergences = divergences
+	}
+
+	// Populate ReplayMetrics.
+	divergenceCount := len(divergences)
+	replayMetrics := ReplayMetrics{
+		TotalEvents:     result.ReplayLen,
+		DivergenceCount: divergenceCount,
+		MatchCount:      result.ReplayLen - divergenceCount,
+	}
+	result.ReplayMetrics = replayMetrics
+
+	// Phase 4: write to output store (optional).
+	if cfg.OutputStore != nil {
+		if err := cfg.OutputStore.Append(ctx, replayEvents...); err != nil {
+			return nil, err
+		}
+		result.Events = replayEvents
+	}
+
+	result.Duration = time.Since(start)
+	return result, nil
+}
+
+// parsePayload unmarshals a typed payload from an event.
+func parsePayload(ev *events.Event, target any) error {
+	if ev.Payload == nil {
+		return nil
+	}
+	return jsonUnmarshal(ev.Payload, target)
+}
+
+// diffEventLists compares original and replayed event lists.
+func diffEventLists(original, replayed []*events.Event) []EventDivergence {
+	var divergences []EventDivergence
+	maxLen := len(original)
+	if len(replayed) > maxLen {
+		maxLen = len(replayed)
+	}
+
+	for i := 0; i < maxLen; i++ {
+		var orig *events.Event
+		var replay *events.Event
+
+		if i < len(original) {
+			orig = original[i]
+		}
+		if i < len(replayed) {
+			replay = replayed[i]
+		}
+
+		if orig == nil && replay != nil {
+			divergences = append(divergences, EventDivergence{
+				Clock:       replay.Clock,
+				ReplayEvent: replay,
+				Type:        DivergenceExtra,
+				Description: "replay produced extra event",
+			})
+			continue
+		}
+		if orig != nil && replay == nil {
+			divergences = append(divergences, EventDivergence{
+				Clock:         orig.Clock,
+				OriginalEvent: orig,
+				Type:          DivergenceMissing,
+				Description:   "original event missing in replay",
+			})
+			continue
+		}
+
+		// Both exist — compare.
+		if orig.Type != replay.Type {
+			divergences = append(divergences, EventDivergence{
+				Clock:         orig.Clock,
+				OriginalEvent: orig,
+				ReplayEvent:   replay,
+				Type:          DivergenceMismatch,
+				Description:   "event type mismatch",
+			})
+		}
+		if orig.Hash != replay.Hash {
+			divergences = append(divergences, EventDivergence{
+				Clock:         orig.Clock,
+				OriginalEvent: orig,
+				ReplayEvent:   replay,
+				Type:          DivergenceMismatch,
+				Description:   "payload mismatch (hash differs)",
+			})
+		}
+	}
+
+	return divergences
+}
diff --git a/internal/harness/replay/replay_test.go b/internal/harness/replay/replay_test.go
new file mode 100644
index 000000000..aea768828
--- /dev/null
+++ b/internal/harness/replay/replay_test.go
@@ -0,0 +1,576 @@
+package replay
+
+import (
+	"context"
+	"fmt"
+	"testing"
+
+	"ragflow/internal/harness/events"
+)
+
+func TestReplayEngine_EmptyTrace(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+
+	engine := NewReplayEngine(store)
+	result, err := engine.Replay(ctx, &ReplayConfig{
+		TraceID: "empty",
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.OriginalLen != 0 {
+		t.Fatalf("expected 0, got %d", result.OriginalLen)
+	}
+	if result.ReplayMetrics.TotalEvents != 0 {
+		t.Fatalf("expected 0, got %d", result.ReplayMetrics.TotalEvents)
+	}
+}
+
+func TestReplayEngine_ExactReplay(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+
+	// Record a simple trace.
+	rec := events.NewEventRecorder(store, events.WithTraceID("exact"))
+	rec.RecordModelCall(ctx, "gpt-4", "openai", []any{"hi"}, "hello", events.TokenUsage{PromptTokens: 5, CompletionTokens: 10, TotalTokens: 15}, 300, 0.001)
+	rec.RecordToolCall(ctx, "search", map[string]any{"q": "test"}, "result1", 100, 0, "")
+
+	engine := NewReplayEngine(store)
+	result, err := engine.Replay(ctx, &ReplayConfig{
+		TraceID:     "exact",
+		DiffEnabled: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.OriginalLen == 0 {
+		t.Fatal("expected non-zero original events")
+	}
+	if result.OriginalLen != result.ReplayLen {
+		t.Fatalf("OriginalLen %d != ReplayLen %d", result.OriginalLen, result.ReplayLen)
+	}
+	if len(result.Divergences) != 0 {
+		t.Fatalf("expected 0 divergences for exact replay, got %d: %+v", len(result.Divergences), result.Divergences)
+	}
+	if result.ReplayMetrics.TotalEvents != result.ReplayLen {
+		t.Fatalf("TotalEvents %d != ReplayLen %d", result.ReplayMetrics.TotalEvents, result.ReplayLen)
+	}
+}
+
+func TestReplayEngine_ModelOverride(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	rec := events.NewEventRecorder(store, events.WithTraceID("mo"))
+	rec.RecordModelCall(ctx, "gpt-4", "openai", []any{"hi"}, "original", events.TokenUsage{}, 100, 0)
+	rec.RecordToolCall(ctx, "search", map[string]any{"q": "test"}, "tool-result", 50, 0, "")
+
+	engine := NewReplayEngine(store)
+	result, err := engine.Replay(ctx, &ReplayConfig{
+		TraceID: "mo",
+		ModelOverride: func(messages []any, recorded string) (*string, error) {
+			sub := "substituted: " + recorded
+			return &sub, nil
+		},
+		DiffEnabled: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Tool events should still match (exact replay for tools).
+	// With model override, LLMEnd has same hash since override doesn't modify the stored event.
+	if result.OriginalLen != result.ReplayLen {
+		t.Fatalf("replay should have same length")
+	}
+}
+
+func TestReplayEngine_ToolOverride(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	rec := events.NewEventRecorder(store, events.WithTraceID("to"))
+	rec.RecordToolCall(ctx, "calc", map[string]any{"expr": "1+1"}, "2", 50, 0, "")
+
+	engine := NewReplayEngine(store)
+	result, err := engine.Replay(ctx, &ReplayConfig{
+		TraceID: "to",
+		ToolOverride: func(name string, args map[string]any, recorded any) (any, error) {
+			return "overridden", nil
+		},
+		DiffEnabled: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.OriginalLen != result.ReplayLen {
+		t.Fatalf("replay should have same length: %d vs %d", result.OriginalLen, result.ReplayLen)
+	}
+}
+
+func TestReplayEngine_StateOverride(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	rec := events.NewEventRecorder(store, events.WithTraceID("so"))
+	rec.RecordStateWrite(ctx, "messages", nil, "initial", "append")
+
+	engine := NewReplayEngine(store)
+	result, err := engine.Replay(ctx, &ReplayConfig{
+		TraceID: "so",
+		StateOverride: func(recorded map[string]any) (map[string]any, error) {
+			recorded["messages"] = "overridden"
+			return recorded, nil
+		},
+		DiffEnabled: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.OriginalLen == 0 {
+		t.Fatal("expected events")
+	}
+	// StateOverride modifies the first EventStateWrite payload.
+	fmt.Printf("Divergences: %+v\n", result.Divergences)
+}
+
+func TestReplayEngine_OutputStore(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	rec := events.NewEventRecorder(store, events.WithTraceID("out"))
+	rec.RecordModelCall(ctx, "gpt-4", "openai", nil, "resp", events.TokenUsage{}, 100, 0)
+
+	output := events.NewMemoryEventStore()
+	engine := NewReplayEngine(store)
+	result, err := engine.Replay(ctx, &ReplayConfig{
+		TraceID:     "out",
+		OutputStore: output,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(result.Events) == 0 {
+		t.Fatal("expected events in result.Events")
+	}
+	outLen, _ := output.Length(ctx)
+	if int(outLen) != result.ReplayLen {
+		t.Fatalf("output store has %d events, expected %d", outLen, result.ReplayLen)
+	}
+}
+
+func TestReplayEngine_MultipleRuns(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	rec := events.NewEventRecorder(store, events.WithTraceID("multi"))
+	for i := 0; i < 5; i++ {
+		rec.RecordToolCall(ctx, fmt.Sprintf("tool-%d", i), nil, fmt.Sprintf("res-%d", i), 10, 0, "")
+	}
+
+	engine := NewReplayEngine(store)
+	for i := 0; i < 3; i++ {
+		result, err := engine.Replay(ctx, &ReplayConfig{TraceID: "multi"})
+		if err != nil {
+			t.Fatal(err)
+		}
+		if result.OriginalLen != 10 { // 5 start + 5 result
+			t.Fatalf("run %d: expected 10 events, got %d", i, result.OriginalLen)
+		}
+	}
+}
+
+func TestFork_FromEvent(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	rec := events.NewEventRecorder(store, events.WithTraceID("fork-test"))
+	rec.RecordModelCall(ctx, "gpt-4", "openai", nil, "resp1", events.TokenUsage{}, 100, 0)
+	rec.RecordModelCall(ctx, "gpt-4", "openai", nil, "resp2", events.TokenUsage{}, 100, 0)
+
+	engine := NewReplayEngine(store)
+
+	// Get the first event's ID.
+	iter := store.Stream(ctx, events.EventFilter{TraceID: "fork-test"})
+	first, _ := iter.Next(ctx)
+	iter.Close()
+
+	result, err := engine.Fork(ctx, &ForkConfig{
+		TraceID: "fork-test",
+		Point:   first.ID,
+		Store:   store,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.ForkTraceID == "" {
+		t.Fatal("expected non-empty fork trace ID")
+	}
+	if result.ParentTraceID != "fork-test" {
+		t.Fatalf("expected parent fork-test, got %s", result.ParentTraceID)
+	}
+	if len(result.ForkEvents) == 0 {
+		t.Fatal("expected fork events")
+	}
+}
+
+func TestDiff_IdenticalTraces(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	rec := events.NewEventRecorder(store, events.WithTraceID("diff-a"))
+	rec.RecordToolCall(ctx, "search", nil, "res", 50, 0, "")
+	rec.RecordModelCall(ctx, "gpt-4", "openai", nil, "final", events.TokenUsage{}, 100, 0)
+
+	// Copy events to a second trace.
+	store2 := events.NewMemoryEventStore()
+	rec2 := events.NewEventRecorder(store2, events.WithTraceID("diff-b"))
+	rec2.RecordToolCall(ctx, "search", nil, "res", 50, 0, "")
+	rec2.RecordModelCall(ctx, "gpt-4", "openai", nil, "final", events.TokenUsage{}, 100, 0)
+
+	result, err := Diff(ctx, store, store2, "diff-a", "diff-b")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(result.MissingInRight) > 0 || len(result.MissingInLeft) > 0 {
+		t.Fatalf("identical traces should have no missing events: left=%d, right=%d",
+			len(result.MissingInLeft), len(result.MissingInRight))
+	}
+}
+
+func TestDiff_DifferentTraces(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	rec := events.NewEventRecorder(store, events.WithTraceID("left"))
+	rec.RecordToolCall(ctx, "search", nil, "res1", 50, 0, "")
+	rec.RecordModelCall(ctx, "gpt-4", "openai", nil, "final1", events.TokenUsage{}, 100, 0)
+
+	store2 := events.NewMemoryEventStore()
+	rec2 := events.NewEventRecorder(store2, events.WithTraceID("right"))
+	rec2.RecordToolCall(ctx, "search", nil, "res2", 50, 0, "")
+	rec2.RecordModelCall(ctx, "gpt-4", "openai", nil, "final2", events.TokenUsage{}, 100, 0)
+
+	result, err := Diff(ctx, store, store2, "left", "right")
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Different payloads → mismatches expected.
+	_ = result
+}
+
+func TestReplayResult_Metrics(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	rec := events.NewEventRecorder(store, events.WithTraceID("metrics"))
+	rec.RecordToolCall(ctx, "t1", nil, "r1", 10, 0, "")
+	rec.RecordToolCall(ctx, "t2", nil, "r2", 20, 0, "err")
+	rec.RecordModelCall(ctx, "gpt-4", "openai", nil, "m1", events.TokenUsage{}, 100, 0)
+
+	engine := NewReplayEngine(store)
+	result, err := engine.Replay(ctx, &ReplayConfig{
+		TraceID:     "metrics",
+		DiffEnabled: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.ReplayMetrics.TotalEvents == 0 {
+		t.Fatal("expected non-zero TotalEvents")
+	}
+	if result.ReplayMetrics.TotalEvents != result.ReplayLen {
+		t.Fatalf("TotalEvents %d != ReplayLen %d", result.ReplayMetrics.TotalEvents, result.ReplayLen)
+	}
+	if result.ReplayMetrics.DivergenceCount+result.ReplayMetrics.MatchCount != result.ReplayMetrics.TotalEvents {
+		t.Fatal("DivergenceCount + MatchCount should equal TotalEvents")
+	}
+}
+
+func TestEventsContains_FindByType(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	rec := events.NewEventRecorder(store, events.WithTraceID("find"))
+	rec.OnNodeStart(ctx, "n1", 0)
+	rec.RecordToolCall(ctx, "search", nil, "res", 10, 0, "")
+
+	iter := store.Stream(ctx, events.EventFilter{TraceID: "find"})
+	var evts []*events.Event
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		evts = append(evts, ev)
+	}
+
+	if !EventsContains(evts, events.EventNodeStart) {
+		t.Fatal("should contain EventNodeStart")
+	}
+	if EventsContains(evts, events.EventGraphStart) {
+		t.Fatal("should not contain EventGraphStart")
+	}
+
+	found := FindEventsOfType(evts, events.EventToolCallStart)
+	if len(found) != 1 {
+		t.Fatalf("expected 1 EventToolCallStart, got %d", len(found))
+	}
+
+	count := EventCount(evts, events.EventNodeStart)
+	if count != 1 {
+		t.Fatalf("expected 1 EventNodeStart, got %d", count)
+	}
+}
+
+func TestReplayLiveTools(t *testing.T) {
+	called := false
+	override := ReplayLiveTools()
+	_, _ = override("search", nil, "recorded")
+	called = true
+	if !called {
+		t.Fatal("override function should return nil")
+	}
+}
+
+func TestReplaySubstituteModel(t *testing.T) {
+	override := ReplaySubstituteModel(func(recorded string) string {
+		return "over:" + recorded
+	})
+	result, err := override(nil, "hello")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if *result != "over:hello" {
+		t.Fatalf("expected 'over:hello', got '%s'", *result)
+	}
+}
+
+func TestReplayExactTools(t *testing.T) {
+	override := ReplayExactTools()
+	result, err := override("test", nil, "exact-value")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result != "exact-value" {
+		t.Fatalf("expected 'exact-value', got '%v'", result)
+	}
+}
+
+func TestIntegration_RecordReplayRoundTrip(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	rec := events.NewEventRecorder(store, events.WithTraceID("rtt"))
+
+	// Record a realistic agent trace.
+	rec.OnRunStart(ctx, "agent", "thread-1")
+	rec.OnStepStart(ctx, 0, 2)
+	rec.OnNodeStart(ctx, "llm_call", 0)
+	rec.RecordModelCall(ctx, "gpt-4", "openai", []any{"user msg"}, "assistant resp", events.TokenUsage{PromptTokens: 50, CompletionTokens: 100, TotalTokens: 150}, 800, 0.003)
+	rec.OnNodeEnd(ctx, "llm_call", 0, nil, nil)
+	rec.OnNodeStart(ctx, "execute_tools", 0)
+	rec.RecordToolCall(ctx, "web_search", map[string]any{"q": "RAG"}, "search results", 1200, 0, "")
+	rec.OnNodeEnd(ctx, "execute_tools", 0, nil, nil)
+	rec.OnStepEnd(ctx, 0, nil)
+	rec.OnRunEnd(ctx, "agent", "thread-1", nil)
+
+	// Replay exactly.
+	engine := NewReplayEngine(store)
+	result, err := engine.Replay(ctx, &ReplayConfig{
+		TraceID:     "rtt",
+		DiffEnabled: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.OriginalLen == 0 {
+		t.Fatal("expected non-zero events")
+	}
+	if result.OriginalLen != result.ReplayLen {
+		t.Fatalf("OriginalLen %d != ReplayLen %d", result.OriginalLen, result.ReplayLen)
+	}
+	if len(result.Divergences) != 0 {
+		t.Fatalf("expected 0 divergences, got %d", len(result.Divergences))
+	}
+	if result.Duration == 0 {
+		t.Fatal("expected non-zero duration")
+	}
+	if result.ReplayMetrics.TotalEvents != result.ReplayLen {
+		t.Fatal("ReplayMetrics.TotalEvents mismatch")
+	}
+}
+
+func TestIntegration_ForkThenReplay(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	rec := events.NewEventRecorder(store, events.WithTraceID("fork-rtt"))
+
+	rec.RecordModelCall(ctx, "gpt-4", "openai", nil, "resp1", events.TokenUsage{}, 100, 0)
+	rec.RecordToolCall(ctx, "search", nil, "res1", 50, 0, "")
+	rec.RecordModelCall(ctx, "gpt-4", "openai", nil, "resp2", events.TokenUsage{}, 100, 0)
+
+	// Fork from the last model call.
+	iter := store.Stream(ctx, events.EventFilter{TraceID: "fork-rtt"})
+	var allEvents []*events.Event
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		allEvents = append(allEvents, ev)
+	}
+	iter.Close()
+
+	// Find the last LLMCallStart event.
+	var forkPoint events.EventID
+	for _, ev := range allEvents {
+		if ev.Type == events.EventLLMCallStart {
+			forkPoint = ev.ID
+		}
+	}
+
+	engine := NewReplayEngine(store)
+	forkResult, err := engine.Fork(ctx, &ForkConfig{
+		TraceID: "fork-rtt",
+		Point:   forkPoint,
+		Store:   store,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if forkResult.ForkTraceID == "" {
+		t.Fatal("expected non-empty fork ID")
+	}
+
+	// Fork events should include the original events up to fork point + fork marker.
+	if len(forkResult.ForkEvents) == 0 {
+		t.Fatal("expected fork events")
+	}
+}
+
+// ---- True replay: BuildCheckpoint ----
+
+func TestBuildCheckpoint_StateWrite(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	rec := events.NewEventRecorder(store, events.WithTraceID("cp-test"))
+
+	rec.RecordStateWrite(ctx, "messages", nil, []any{"hello"}, "append")
+	rec.RecordStateWrite(ctx, "counter", nil, 42, "add")
+	rec.OnNodeEnd(ctx, "agent", 0, nil, nil)
+	rec.OnStepEnd(ctx, 0, nil)
+
+	iter := store.Stream(ctx, events.EventFilter{TraceID: "cp-test"})
+	var evts []*events.Event
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		evts = append(evts, ev)
+	}
+
+	cp, cpID := BuildCheckpoint(evts, "test-thread")
+	if cpID == "" {
+		t.Fatal("expected non-empty checkpoint ID")
+	}
+	if cp["messages"] == nil {
+		t.Fatal("expected messages in checkpoint")
+	}
+	if val, ok := cp["counter"].(int); !ok || val != 42 {
+		if val, ok := cp["counter"].(float64); !ok || val != 42 {
+			t.Fatalf("expected counter=42, got %v (type %T)", cp["counter"], cp["counter"])
+		}
+	}
+	if val, ok := cp["__step__"].(float64); !ok || val != 0 {
+		t.Fatalf("expected step=0, got %v (type %T)", cp["__step__"], cp["__step__"])
+	}
+	if val, ok := cp["__last_completed_node__"].(string); !ok || val != "agent" {
+		t.Fatalf("expected last_completed_node='agent', got %v (type %T)", cp["__last_completed_node__"], cp["__last_completed_node__"])
+	}
+}
+
+func TestBuildCheckpoint_EmptyEvents(t *testing.T) {
+	cp, cpID := BuildCheckpoint(nil, "empty")
+	if cpID == "" {
+		t.Fatal("expected checkpoint ID even with no events")
+	}
+	// last_state is always set for non-nil input
+	_ = cp
+}
+
+// ---- True replay: Fork with Engine ----
+
+func TestFork_WithEngine(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	rec := events.NewEventRecorder(store, events.WithTraceID("fork-eng"))
+
+	// Record two state writes, then a tool call.
+	rec.RecordStateWrite(ctx, "results", nil, []any{"first"}, "append")
+	rec.RecordStateWrite(ctx, "count", nil, 1, "add")
+	rec.RecordToolCall(ctx, "search", map[string]any{"q": "test"}, "result", 100, 0, "")
+
+	// Find the fork point (second event, a state write).
+	iter := store.Stream(ctx, events.EventFilter{TraceID: "fork-eng"})
+	var allEvents []*events.Event
+	for {
+		ev, ok := iter.Next(ctx)
+		if !ok {
+			break
+		}
+		allEvents = append(allEvents, ev)
+	}
+	if len(allEvents) < 2 {
+		t.Fatal("need at least 2 events")
+	}
+
+	// Fork from the second state write (after count=1 is persisted).
+	forkPoint := allEvents[1].ID // EventStateWrite for "count"
+
+	engine := NewReplayEngine(store)
+	result, err := engine.Fork(ctx, &ForkConfig{
+		TraceID: "fork-eng",
+		Point:   forkPoint,
+		Store:   store,
+		// No ForkEngine — should use deterministic replay path.
+		OutputStore: events.NewMemoryEventStore(),
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.FinalState != nil {
+		t.Fatal("expected nil FinalState without ForkEngine")
+	}
+	if result.Duration == 0 {
+		t.Fatal("expected non-zero duration")
+	}
+}
+
+func TestFork_EventNotFound(t *testing.T) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	engine := NewReplayEngine(store)
+
+	_, err := engine.Fork(ctx, &ForkConfig{
+		TraceID: "nonexistent",
+		Point:   "no-such-event",
+		Store:   store,
+	})
+	if err == nil {
+		t.Fatal("expected error for nonexistent event")
+	}
+}
+
+// ---- bench ----
+
+func BenchmarkReplay(b *testing.B) {
+	ctx := context.Background()
+	store := events.NewMemoryEventStore()
+	rec := events.NewEventRecorder(store, events.WithTraceID("bench-replay"))
+
+	// Record 100 events.
+	for i := 0; i < 50; i++ {
+		rec.RecordModelCall(ctx, "gpt-4", "openai", nil, "resp", events.TokenUsage{}, 100, 0)
+		rec.RecordToolCall(ctx, "search", nil, "res", 50, 0, "")
+	}
+
+	engine := NewReplayEngine(store)
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, err := engine.Replay(ctx, &ReplayConfig{TraceID: "bench-replay"})
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}