google__adk-docs/docs/workflows/patterns.md

Multi-agent workflow patterns

Supported in ADKPython v0.1.0TypeScript v0.2.0Go v0.1.0Java v0.1.0Kotlin v0.1.0

This guide provides a number of agent patterns which you can implement with Agent Development Kit (ADK), including code examples. These patterns are useful across a broad set of applications and you should evaluate and test them against your project requirements before committing to a full implementation.

Coordinator and dispatcher

• Structure: A central LlmAgent (Coordinator) manages several specialized sub_agents.
• Goal: Route incoming requests to the appropriate specialist agent.
• ADK Primitives Used:
  • Hierarchy: Coordinator has specialists listed in sub_agents.
  • Interaction: Primarily uses LLM-Driven Delegation (requires clear descriptions on sub-agents and appropriate instruction on Coordinator) or Explicit Invocation (AgentTool) (Coordinator includes AgentTool-wrapped specialists in its tools).

=== "Python"

```python
# Conceptual Code: Coordinator using LLM Transfer
from google.adk.agents import LlmAgent


billing_agent = LlmAgent(name="Billing", description="Handles billing inquiries.")
support_agent = LlmAgent(name="Support", description="Handles technical support requests.")


coordinator = LlmAgent(
    name="HelpDeskCoordinator",
    model="gemini-flash-latest",
    instruction="Route user requests: Use Billing agent for payment issues, Support agent for technical problems.",
    description="Main help desk router.",
    # allow_transfer=True is often implicit with sub_agents in AutoFlow
    sub_agents=[billing_agent, support_agent]
)
# User asks "My payment failed" -> Coordinator's LLM should call transfer_to_agent(agent_name='Billing')
# User asks "I can't log in" -> Coordinator's LLM should call transfer_to_agent(agent_name='Support')
```

=== "TypeScript"

```typescript
// Conceptual Code: Coordinator using LLM Transfer
import { LlmAgent } from '@google/adk';

const billingAgent = new LlmAgent({name: 'Billing', description: 'Handles billing inquiries.'});
const supportAgent = new LlmAgent({name: 'Support', description: 'Handles technical support requests.'});

const coordinator = new LlmAgent({
    name: 'HelpDeskCoordinator',
    model: 'gemini-flash-latest',
    instruction: 'Route user requests: Use Billing agent for payment issues, Support agent for technical problems.',
    description: 'Main help desk router.',
    // allowTransfer=true is often implicit with subAgents in AutoFlow
    subAgents: [billingAgent, supportAgent]
});
// User asks "My payment failed" -> Coordinator's LLM should call {functionCall: {name: 'transfer_to_agent', args: {agent_name: 'Billing'}}}
// User asks "I can't log in" -> Coordinator's LLM should call {functionCall: {name: 'transfer_to_agent', args: {agent_name: 'Support'}}}
```

=== "Go"

```go
import (
    "google.golang.org/adk/v2/agent"
    "google.golang.org/adk/v2/agent/llmagent"
)

--8<-- "examples/go/snippets/agents/multi-agent/main.go:coordinator-pattern"
```

=== "Java"

```java
// Conceptual Code: Coordinator using LLM Transfer
import com.google.adk.agents.LlmAgent;

LlmAgent billingAgent = LlmAgent.builder()
    .name("Billing")
    .description("Handles billing inquiries and payment issues.")
    .build();

LlmAgent supportAgent = LlmAgent.builder()
    .name("Support")
    .description("Handles technical support requests and login problems.")
    .build();

LlmAgent coordinator = LlmAgent.builder()
    .name("HelpDeskCoordinator")
    .model("gemini-flash-latest")
    .instruction("Route user requests: Use Billing agent for payment issues, Support agent for technical problems.")
    .description("Main help desk router.")
    .subAgents(billingAgent, supportAgent)
    // Agent transfer is implicit with sub agents in the Autoflow, unless specified
    // using .disallowTransferToParent or disallowTransferToPeers
    .build();

// User asks "My payment failed" -> Coordinator's LLM should call
// transferToAgent(agentName='Billing')
// User asks "I can't log in" -> Coordinator's LLM should call
// transferToAgent(agentName='Support')
```

=== "Kotlin"

```kotlin
--8<-- "examples/kotlin/snippets/agents/multi-agent/MultiAgentExample.kt:coordinator_pattern"
```

Sequential pipeline

• Structure: A SequentialAgent contains sub_agents executed in a fixed order.
• Goal: Implement a multistep process where the output of one-step feeds into the next.
• ADK Primitives Used:
  • Workflow: SequentialAgent defines the order.
  • Communication: Primarily uses Shared Session State. Earlier agents write results (often via output_key), later agents read those results from context.state.

=== "Python"

```python
# Conceptual Code: Sequential Data Pipeline
from google.adk.agents import SequentialAgent, LlmAgent


validator = LlmAgent(name="ValidateInput", instruction="Validate the input.", output_key="validation_status")
processor = LlmAgent(name="ProcessData", instruction="Process data if {validation_status} is 'valid'.", output_key="result")
reporter = LlmAgent(name="ReportResult", instruction="Report the result from {result}.")


data_pipeline = SequentialAgent(
    name="DataPipeline",
    sub_agents=[validator, processor, reporter]
)
# validator runs -> saves to state['validation_status']
# processor runs -> reads state['validation_status'], saves to state['result']
# reporter runs -> reads state['result']
```

=== "TypeScript"

```typescript
// Conceptual Code: Sequential Data Pipeline
import { SequentialAgent, LlmAgent } from '@google/adk';

const validator = new LlmAgent({name: 'ValidateInput', instruction: 'Validate the input.', outputKey: 'validation_status'});
const processor = new LlmAgent({name: 'ProcessData', instruction: 'Process data if {validation_status} is "valid".', outputKey: 'result'});
const reporter = new LlmAgent({name: 'ReportResult', instruction: 'Report the result from {result}.'});

const dataPipeline = new SequentialAgent({
    name: 'DataPipeline',
    subAgents: [validator, processor, reporter]
});
// validator runs -> saves to state['validation_status']
// processor runs -> reads state['validation_status'], saves to state['result']
// reporter runs -> reads state['result']
```

=== "Go"

```go
import (
    "google.golang.org/adk/v2/agent"
    "google.golang.org/adk/v2/agent/llmagent"
    "google.golang.org/adk/v2/agent/workflowagents/sequentialagent"
)

--8<-- "examples/go/snippets/agents/multi-agent/main.go:sequential-pipeline-pattern"
```

=== "Java"

```java
// Conceptual Code: Sequential Data Pipeline
import com.google.adk.agents.SequentialAgent;


LlmAgent validator = LlmAgent.builder()
    .name("ValidateInput")
    .instruction("Validate the input")
    .outputKey("validation_status") // Saves its main text output to session.state["validation_status"]
    .build();


LlmAgent processor = LlmAgent.builder()
    .name("ProcessData")
    .instruction("Process data if {validation_status} is 'valid'")
    .outputKey("result") // Saves its main text output to session.state["result"]
    .build();


LlmAgent reporter = LlmAgent.builder()
    .name("ReportResult")
    .instruction("Report the result from {result}")
    .build();


SequentialAgent dataPipeline = SequentialAgent.builder()
    .name("DataPipeline")
    .subAgents(validator, processor, reporter)
    .build();


// validator runs -> saves to state['validation_status']
// processor runs -> reads state['validation_status'], saves to state['result']
// reporter runs -> reads state['result']
```

=== "Kotlin"

```kotlin
--8<-- "examples/kotlin/snippets/agents/multi-agent/MultiAgentExample.kt:sequential_pipeline_pattern"
```

Parallel fan-out and gather

• Structure: A ParallelAgent runs multiple sub_agents concurrently, often followed by a later agent (in a SequentialAgent) that aggregates results.
• Goal: Execute independent tasks simultaneously to reduce latency, then combine their outputs.
• ADK Primitives Used:
  • Workflow: ParallelAgent for concurrent execution (Fan-Out). Often nested within a SequentialAgent to handle the subsequent aggregation step (Gather).
  • Communication: Sub-agents write results to distinct keys in Shared Session State. The subsequent "Gather" agent reads multiple state keys.

=== "Python"

```python
# Conceptual Code: Parallel Information Gathering
from google.adk.agents import SequentialAgent, ParallelAgent, LlmAgent


fetch_api1 = LlmAgent(name="API1Fetcher", instruction="Fetch data from API 1.", output_key="api1_data")
fetch_api2 = LlmAgent(name="API2Fetcher", instruction="Fetch data from API 2.", output_key="api2_data")


gather_concurrently = ParallelAgent(
    name="ConcurrentFetch",
    sub_agents=[fetch_api1, fetch_api2]
)


synthesizer = LlmAgent(
    name="Synthesizer",
    instruction="Combine results from {api1_data} and {api2_data}."
)


overall_workflow = SequentialAgent(
    name="FetchAndSynthesize",
    sub_agents=[gather_concurrently, synthesizer] # Run parallel fetch, then synthesize
)
# fetch_api1 and fetch_api2 run concurrently, saving to state.
# synthesizer runs afterwards, reading state['api1_data'] and state['api2_data'].
```

=== "TypeScript"

```typescript
// Conceptual Code: Parallel Information Gathering
import { SequentialAgent, ParallelAgent, LlmAgent } from '@google/adk';

const fetchApi1 = new LlmAgent({name: 'API1Fetcher', instruction: 'Fetch data from API 1.', outputKey: 'api1_data'});
const fetchApi2 = new LlmAgent({name: 'API2Fetcher', instruction: 'Fetch data from API 2.', outputKey: 'api2_data'});

const gatherConcurrently = new ParallelAgent({
    name: 'ConcurrentFetch',
    subAgents: [fetchApi1, fetchApi2]
});

const synthesizer = new LlmAgent({
    name: 'Synthesizer',
    instruction: 'Combine results from {api1_data} and {api2_data}.'
});

const overallWorkflow = new SequentialAgent({
    name: 'FetchAndSynthesize',
    subAgents: [gatherConcurrently, synthesizer] // Run parallel fetch, then synthesize
});
// fetchApi1 and fetchApi2 run concurrently, saving to state.
// synthesizer runs afterwards, reading state['api1_data'] and state['api2_data'].
```

=== "Go"

```go
import (
    "google.golang.org/adk/v2/agent"
    "google.golang.org/adk/v2/agent/llmagent"
    "google.golang.org/adk/v2/agent/workflowagents/parallelagent"
    "google.golang.org/adk/v2/agent/workflowagents/sequentialagent"
)

--8<-- "examples/go/snippets/agents/multi-agent/main.go:parallel-gather-pattern"
```

=== "Java"

```java
// Conceptual Code: Parallel Information Gathering
import com.google.adk.agents.LlmAgent;
import com.google.adk.agents.ParallelAgent;
import com.google.adk.agents.SequentialAgent;

LlmAgent fetchApi1 = LlmAgent.builder()
    .name("API1Fetcher")
    .instruction("Fetch data from API 1.")
    .outputKey("api1_data")
    .build();

LlmAgent fetchApi2 = LlmAgent.builder()
    .name("API2Fetcher")
    .instruction("Fetch data from API 2.")
    .outputKey("api2_data")
    .build();

ParallelAgent gatherConcurrently = ParallelAgent.builder()
    .name("ConcurrentFetcher")
    .subAgents(fetchApi2, fetchApi1)
    .build();

LlmAgent synthesizer = LlmAgent.builder()
    .name("Synthesizer")
    .instruction("Combine results from {api1_data} and {api2_data}.")
    .build();

SequentialAgent overallWorfklow = SequentialAgent.builder()
    .name("FetchAndSynthesize") // Run parallel fetch, then synthesize
    .subAgents(gatherConcurrently, synthesizer)
    .build();

// fetch_api1 and fetch_api2 run concurrently, saving to state.
// synthesizer runs afterwards, reading state['api1_data'] and state['api2_data'].
```

=== "Kotlin"

```kotlin
--8<-- "examples/kotlin/snippets/agents/multi-agent/MultiAgentExample.kt:parallel_gather_pattern"
```

Hierarchical task decomposition

• Structure: A multi-level tree of agents where higher-level agents break down complex goals and delegate sub-tasks to lower-level agents.
• Goal: Solve complex problems by recursively breaking them down into simpler, executable steps.
• ADK Primitives Used:
  • Hierarchy: Multi-level parent_agent/sub_agents structure.
  • Interaction: Primarily LLM-Driven Delegation or Explicit Invocation (AgentTool) used by parent agents to assign tasks to subagents. Results are returned up the hierarchy (via tool responses or state).

=== "Python"

```python
# Conceptual Code: Hierarchical Research Task
from google.adk.agents import LlmAgent
from google.adk.tools import agent_tool


# Low-level tool-like agents
web_searcher = LlmAgent(name="WebSearch", description="Performs web searches for facts.")
summarizer = LlmAgent(name="Summarizer", description="Summarizes text.")


# Mid-level agent combining tools
research_assistant = LlmAgent(
    name="ResearchAssistant",
    model="gemini-flash-latest",
    description="Finds and summarizes information on a topic.",
    tools=[agent_tool.AgentTool(agent=web_searcher), agent_tool.AgentTool(agent=summarizer)]
)


# High-level agent delegating research
report_writer = LlmAgent(
    name="ReportWriter",
    model="gemini-flash-latest",
    instruction="Write a report on topic X. Use the ResearchAssistant to gather information.",
    tools=[agent_tool.AgentTool(agent=research_assistant)]
    # Alternatively, could use LLM Transfer if research_assistant is a sub_agent
)
# User interacts with ReportWriter.
# ReportWriter calls ResearchAssistant tool.
# ResearchAssistant calls WebSearch and Summarizer tools.
# Results flow back up.
```

=== "TypeScript"

```typescript
// Conceptual Code: Hierarchical Research Task
import { LlmAgent, AgentTool } from '@google/adk';

// Low-level tool-like agents
const webSearcher = new LlmAgent({name: 'WebSearch', description: 'Performs web searches for facts.'});
const summarizer = new LlmAgent({name: 'Summarizer', description: 'Summarizes text.'});

// Mid-level agent combining tools
const researchAssistant = new LlmAgent({
    name: 'ResearchAssistant',
    model: 'gemini-flash-latest',
    description: 'Finds and summarizes information on a topic.',
    tools: [new AgentTool({agent: webSearcher}), new AgentTool({agent: summarizer})]
});

// High-level agent delegating research
const reportWriter = new LlmAgent({
    name: 'ReportWriter',
    model: 'gemini-flash-latest',
    instruction: 'Write a report on topic X. Use the ResearchAssistant to gather information.',
    tools: [new AgentTool({agent: researchAssistant})]
    // Alternatively, could use LLM Transfer if researchAssistant is a subAgent
});
// User interacts with ReportWriter.
// ReportWriter calls ResearchAssistant tool.
// ResearchAssistant calls WebSearch and Summarizer tools.
// Results flow back up.
```

=== "Go"

```go
import (
    "google.golang.org/adk/v2/agent/llmagent"
    "google.golang.org/adk/v2/tool"
    "google.golang.org/adk/v2/tool/agenttool"
)

--8<-- "examples/go/snippets/agents/multi-agent/main.go:hierarchical-pattern"
```

=== "Java"

```java
// Conceptual Code: Hierarchical Research Task
import com.google.adk.agents.LlmAgent;
import com.google.adk.tools.AgentTool;


// Low-level tool-like agents
LlmAgent webSearcher = LlmAgent.builder()
    .name("WebSearch")
    .description("Performs web searches for facts.")
    .build();


LlmAgent summarizer = LlmAgent.builder()
    .name("Summarizer")
    .description("Summarizes text.")
    .build();


// Mid-level agent combining tools
LlmAgent researchAssistant = LlmAgent.builder()
    .name("ResearchAssistant")
    .model("gemini-flash-latest")
    .description("Finds and summarizes information on a topic.")
    .tools(AgentTool.create(webSearcher), AgentTool.create(summarizer))
    .build();


// High-level agent delegating research
LlmAgent reportWriter = LlmAgent.builder()
    .name("ReportWriter")
    .model("gemini-flash-latest")
    .instruction("Write a report on topic X. Use the ResearchAssistant to gather information.")
    .tools(AgentTool.create(researchAssistant))
    // Alternatively, could use LLM Transfer if research_assistant is a subAgent
    .build();


// User interacts with ReportWriter.
// ReportWriter calls ResearchAssistant tool.
// ResearchAssistant calls WebSearch and Summarizer tools.
// Results flow back up.
```

=== "Kotlin"

```kotlin
--8<-- "examples/kotlin/snippets/agents/multi-agent/MultiAgentExample.kt:hierarchical_pattern"
```

Generate and review pattern

• Structure: Typically involves two agents within a SequentialAgent: a generator agent and a critic reviewer agent.
• Goal: Improve the quality or validity of generated output by having a dedicated agent review it.
• ADK Primitives Used:
  • Workflow: SequentialAgent ensures generation happens before review.
  • Communication: Shared Session State (Generator uses output_key to save output; Reviewer reads that state key). The Reviewer might save its feedback to another state key for subsequent steps.

=== "Python"

```python
# Conceptual Code: Generator-Critic
from google.adk.agents import SequentialAgent, LlmAgent


generator = LlmAgent(
    name="DraftWriter",
    instruction="Write a short paragraph about subject X.",
    output_key="draft_text"
)


reviewer = LlmAgent(
    name="FactChecker",
    instruction="Review the text in {draft_text} for factual accuracy. Output 'valid' or 'invalid' with reasons.",
    output_key="review_status"
)


# Optional: Further steps based on review_status


review_pipeline = SequentialAgent(
    name="WriteAndReview",
    sub_agents=[generator, reviewer]
)
# generator runs -> saves draft to state['draft_text']
# reviewer runs -> reads state['draft_text'], saves status to state['review_status']
```

=== "TypeScript"

```typescript
// Conceptual Code: Generator-Critic
import { SequentialAgent, LlmAgent } from '@google/adk';

const generator = new LlmAgent({
    name: 'DraftWriter',
    instruction: 'Write a short paragraph about subject X.',
    outputKey: 'draft_text'
});

const reviewer = new LlmAgent({
    name: 'FactChecker',
    instruction: 'Review the text in {draft_text} for factual accuracy. Output "valid" or "invalid" with reasons.',
    outputKey: 'review_status'
});

// Optional: Further steps based on review_status

const reviewPipeline = new SequentialAgent({
    name: 'WriteAndReview',
    subAgents: [generator, reviewer]
});
// generator runs -> saves draft to state['draft_text']
// reviewer runs -> reads state['draft_text'], saves status to state['review_status']
```

=== "Go"

```go
import (
    "google.golang.org/adk/v2/agent"
    "google.golang.org/adk/v2/agent/llmagent"
    "google.golang.org/adk/v2/agent/workflowagents/sequentialagent"
)

--8<-- "examples/go/snippets/agents/multi-agent/main.go:generator-critic-pattern"
```

=== "Java"

```java
// Conceptual Code: Generator-Critic
import com.google.adk.agents.LlmAgent;
import com.google.adk.agents.SequentialAgent;


LlmAgent generator = LlmAgent.builder()
    .name("DraftWriter")
    .instruction("Write a short paragraph about subject X.")
    .outputKey("draft_text")
    .build();


LlmAgent reviewer = LlmAgent.builder()
    .name("FactChecker")
    .instruction("Review the text in {draft_text} for factual accuracy. Output 'valid' or 'invalid' with reasons.")
    .outputKey("review_status")
    .build();


// Optional: Further steps based on review_status


SequentialAgent reviewPipeline = SequentialAgent.builder()
    .name("WriteAndReview")
    .subAgents(generator, reviewer)
    .build();


// generator runs -> saves draft to state['draft_text']
// reviewer runs -> reads state['draft_text'], saves status to state['review_status']
```

=== "Kotlin"

```kotlin
--8<-- "examples/kotlin/snippets/agents/multi-agent/MultiAgentExample.kt:generator_critic_pattern"
```

Iterative refinement

• Structure: Uses a LoopAgent containing one or more agents that work on a task over multiple iterations.
• Goal: Progressively improve a result (e.g., code, text, plan) stored in the session state until a quality threshold is met or a maximum number of iterations is reached.
• ADK Primitives Used:
  • Workflow: LoopAgent manages the repetition.
  • Communication: Shared Session State is essential for agents to read the previous iteration's output and save the refined version.
  • Termination: The loop typically ends based on max_iterations or a dedicated checking agent setting escalate=True in the Event Actions when the result is satisfactory.

=== "Python"

```python
# Conceptual Code: Iterative Code Refinement
from google.adk.agents import LoopAgent, LlmAgent, BaseAgent
from google.adk.events import Event, EventActions
from google.adk.agents.invocation_context import InvocationContext
from typing import AsyncGenerator


# Agent to generate/refine code based on state['current_code'] and state['requirements']
code_refiner = LlmAgent(
    name="CodeRefiner",
    instruction="Read state['current_code'] (if exists) and state['requirements']. Generate/refine Python code to meet requirements. Save to state['current_code'].",
    output_key="current_code" # Overwrites previous code in state
)


# Agent to check if the code meets quality standards
quality_checker = LlmAgent(
    name="QualityChecker",
    instruction="Evaluate the code in state['current_code'] against state['requirements']. Output 'pass' or 'fail'.",
    output_key="quality_status"
)


# Custom agent to check the status and escalate if 'pass'
class CheckStatusAndEscalate(BaseAgent):
    async def _run_async_impl(self, ctx: InvocationContext) -> AsyncGenerator[Event, None]:
        status = ctx.session.state.get("quality_status", "fail")
        should_stop = (status == "pass")
        yield Event(author=self.name, actions=EventActions(escalate=should_stop))


refinement_loop = LoopAgent(
    name="CodeRefinementLoop",
    max_iterations=5,
    sub_agents=[code_refiner, quality_checker, CheckStatusAndEscalate(name="StopChecker")]
)
# Loop runs: Refiner -> Checker -> StopChecker
# State['current_code'] is updated each iteration.
# Loop stops if QualityChecker outputs 'pass' (leading to StopChecker escalating) or after 5 iterations.
```

=== "TypeScript"

```typescript
// Conceptual Code: Iterative Code Refinement
import { LoopAgent, LlmAgent, BaseAgent, InvocationContext } from '@google/adk';
import type { Event, createEvent, createEventActions } from '@google/genai';

// Agent to generate/refine code based on state['current_code'] and state['requirements']
const codeRefiner = new LlmAgent({
    name: 'CodeRefiner',
    instruction: 'Read state["current_code"] (if exists) and state["requirements"]. Generate/refine TypeScript code to meet requirements. Save to state["current_code"].',
    outputKey: 'current_code' // Overwrites previous code in state
});

// Agent to check if the code meets quality standards
const qualityChecker = new LlmAgent({
    name: 'QualityChecker',
    instruction: 'Evaluate the code in state["current_code"] against state["requirements"]. Output "pass" or "fail".',
    outputKey: 'quality_status'
});

// Custom agent to check the status and escalate if 'pass'
class CheckStatusAndEscalate extends BaseAgent {
    async *runAsyncImpl(ctx: InvocationContext): AsyncGenerator<Event> {
        const status = ctx.session.state.quality_status;
        const shouldStop = status === 'pass';
        if (shouldStop) {
            yield createEvent({
                author: 'StopChecker',
                actions: createEventActions(),
            });
        }
    }

    async *runLiveImpl(ctx: InvocationContext): AsyncGenerator<Event> {
        // This agent doesn't have a live implementation
        yield createEvent({ author: 'StopChecker' });
    }
}

// Loop runs: Refiner -> Checker -> StopChecker
// State['current_code'] is updated each iteration.
// Loop stops if QualityChecker outputs 'pass' (leading to StopChecker escalating) or after 5 iterations.
const refinementLoop = new LoopAgent({
    name: 'CodeRefinementLoop',
    maxIterations: 5,
    subAgents: [codeRefiner, qualityChecker, new CheckStatusAndEscalate({name: 'StopChecker'})]
});
```

=== "Go"

```go
import (
    "iter"
    "google.golang.org/adk/v2/agent"
    "google.golang.org/adk/v2/agent/llmagent"
    "google.golang.org/adk/v2/agent/workflowagents/loopagent"
    "google.golang.org/adk/v2/session"
)

--8<-- "examples/go/snippets/agents/multi-agent/main.go:iterative-refinement-pattern"
```

=== "Java"

```java
// Conceptual Code: Iterative Code Refinement
import com.google.adk.agents.BaseAgent;
import com.google.adk.agents.LlmAgent;
import com.google.adk.agents.LoopAgent;
import com.google.adk.events.Event;
import com.google.adk.events.EventActions;
import com.google.adk.agents.InvocationContext;
import io.reactivex.rxjava3.core.Flowable;
import java.util.List;


// Agent to generate/refine code based on state['current_code'] and state['requirements']
LlmAgent codeRefiner = LlmAgent.builder()
    .name("CodeRefiner")
    .instruction("Read state['current_code'] (if exists) and state['requirements']. Generate/refine Java code to meet requirements. Save to state['current_code'].")
    .outputKey("current_code") // Overwrites previous code in state
    .build();


// Agent to check if the code meets quality standards
LlmAgent qualityChecker = LlmAgent.builder()
    .name("QualityChecker")
    .instruction("Evaluate the code in state['current_code'] against state['requirements']. Output 'pass' or 'fail'.")
    .outputKey("quality_status")
    .build();


BaseAgent checkStatusAndEscalate = new BaseAgent(
    "StopChecker","Checks quality_status and escalates if 'pass'.", List.of(), null, null) {


  @Override
  protected Flowable<Event> runAsyncImpl(InvocationContext invocationContext) {
    String status = (String) invocationContext.session().state().getOrDefault("quality_status", "fail");
    boolean shouldStop = "pass".equals(status);


    EventActions actions = EventActions.builder().escalate(shouldStop).build();
    Event event = Event.builder()
        .author(this.name())
        .actions(actions)
        .build();
    return Flowable.just(event);
  }
};


LoopAgent refinementLoop = LoopAgent.builder()
    .name("CodeRefinementLoop")
    .maxIterations(5)
    .subAgents(codeRefiner, qualityChecker, checkStatusAndEscalate)
    .build();


// Loop runs: Refiner -> Checker -> StopChecker
// State['current_code'] is updated each iteration.
// Loop stops if QualityChecker outputs 'pass' (leading to StopChecker escalating) or after 5
// iterations.
```

=== "Kotlin"

```kotlin
--8<-- "examples/kotlin/snippets/agents/multi-agent/MultiAgentExample.kt:iterative_refinement_pattern"
```

Human-in-the-loop

• Structure: Integrates human intervention points within an agent workflow.
• Goal: Allow for human oversight, approval, correction, or tasks that AI cannot perform.
• ADK Primitives Used (Conceptual):
  • Interaction: Can be implemented using a custom Tool that pauses execution and sends a request to an external system (e.g., a UI, ticketing system) waiting for human input. The tool then returns the human's response to the agent.
  • Workflow: Could use LLM-Driven Delegation (transfer_to_agent) targeting a conceptual "Human Agent" that triggers the external workflow, or use the custom tool within an LlmAgent.
  • State/Callbacks: State can hold task details for the human; callbacks can manage the interaction flow.
  • Note: ADK doesn't have a built-in "Human Agent" type, so this requires custom integration.

=== "Python"

```python
# Conceptual Code: Using a Tool for Human Approval
from google.adk.agents import LlmAgent, SequentialAgent
from google.adk.tools import FunctionTool


# --- Assume external_approval_tool exists ---
# This tool would:
# 1. Take details (e.g., request_id, amount, reason).
# 2. Send these details to a human review system (e.g., via API).
# 3. Poll or wait for the human response (approved/rejected).
# 4. Return the human's decision.
# async def external_approval_tool(amount: float, reason: str) -> str: ...
approval_tool = FunctionTool(func=external_approval_tool)


# Agent that prepares the request
prepare_request = LlmAgent(
    name="PrepareApproval",
    instruction="Prepare the approval request details based on user input. Store amount and reason in state.",
    # ... likely sets state['approval_amount'] and state['approval_reason'] ...
)


# Agent that calls the human approval tool
request_approval = LlmAgent(
    name="RequestHumanApproval",
    instruction="Use the external_approval_tool with amount from state['approval_amount'] and reason from state['approval_reason'].",
    tools=[approval_tool],
    output_key="human_decision"
)


# Agent that proceeds based on human decision
process_decision = LlmAgent(
    name="ProcessDecision",
    instruction="Check {human_decision}. If 'approved', proceed. If 'rejected', inform user."
)


approval_workflow = SequentialAgent(
    name="HumanApprovalWorkflow",
    sub_agents=[prepare_request, request_approval, process_decision]
)
```

=== "TypeScript"

```typescript
// Conceptual Code: Using a Tool for Human Approval
import { LlmAgent, SequentialAgent, FunctionTool } from '@google/adk';
import { z } from 'zod';

// --- Assume externalApprovalTool exists ---
// This tool would:
// 1. Take details (e.g., request_id, amount, reason).
// 2. Send these details to a human review system (e.g., via API).
// 3. Poll or wait for the human response (approved/rejected).
// 4. Return the human's decision.
async function externalApprovalTool(params: {amount: number, reason: string}): Promise<{decision: string}> {
  // ... implementation to call external system
  return {decision: 'approved'}; // or 'rejected'
}

const approvalTool = new FunctionTool({
  name: 'external_approval_tool',
  description: 'Sends a request for human approval.',
  parameters: z.object({
    amount: z.number(),
    reason: z.string(),
  }),
  execute: externalApprovalTool,
});


// Agent that prepares the request
const prepareRequest = new LlmAgent({
    name: 'PrepareApproval',
    instruction: 'Prepare the approval request details based on user input. Store amount and reason in state.',
    // ... likely sets state['approval_amount'] and state['approval_reason'] ...
});

// Agent that calls the human approval tool
const requestApproval = new LlmAgent({
    name: 'RequestHumanApproval',
    instruction: 'Use the external_approval_tool with amount from state["approval_amount"] and reason from state["approval_reason"].',
    tools: [approvalTool],
    outputKey: 'human_decision'
});

// Agent that proceeds based on human decision
const processDecision = new LlmAgent({
    name: 'ProcessDecision',
    instruction: 'Check {human_decision}. If "approved", proceed. If "rejected", inform user.'
});

const approvalWorkflow = new SequentialAgent({
    name: 'HumanApprovalWorkflow',
    subAgents: [prepareRequest, requestApproval, processDecision]
});
```

=== "Go"

```go
import (
    "google.golang.org/adk/v2/agent"
    "google.golang.org/adk/v2/agent/llmagent"
    "google.golang.org/adk/v2/agent/workflowagents/sequentialagent"
    "google.golang.org/adk/v2/tool"
)

--8<-- "examples/go/snippets/agents/multi-agent/main.go:human-in-loop-pattern"
```

=== "Java"

```java
// Conceptual Code: Using a Tool for Human Approval
import com.google.adk.agents.LlmAgent;
import com.google.adk.agents.SequentialAgent;
import com.google.adk.tools.FunctionTool;


// --- Assume external_approval_tool exists ---
// This tool would:
// 1. Take details (e.g., request_id, amount, reason).
// 2. Send these details to a human review system (e.g., via API).
// 3. Poll or wait for the human response (approved/rejected).
// 4. Return the human's decision.
// public boolean externalApprovalTool(float amount, String reason) { ... }
FunctionTool approvalTool = FunctionTool.create(externalApprovalTool);


// Agent that prepares the request
LlmAgent prepareRequest = LlmAgent.builder()
    .name("PrepareApproval")
    .instruction("Prepare the approval request details based on user input. Store amount and reason in state.")
    // ... likely sets state['approval_amount'] and state['approval_reason'] ...
    .build();


// Agent that calls the human approval tool
LlmAgent requestApproval = LlmAgent.builder()
    .name("RequestHumanApproval")
    .instruction("Use the external_approval_tool with amount from state['approval_amount'] and reason from state['approval_reason'].")
    .tools(approvalTool)
    .outputKey("human_decision")
    .build();


// Agent that proceeds based on human decision
LlmAgent processDecision = LlmAgent.builder()
    .name("ProcessDecision")
    .instruction("Check {human_decision}. If 'approved', proceed. If 'rejected', inform user.")
    .build();


SequentialAgent approvalWorkflow = SequentialAgent.builder()
    .name("HumanApprovalWorkflow")
    .subAgents(prepareRequest, requestApproval, processDecision)
    .build();
```

=== "Kotlin"

```kotlin
--8<-- "examples/kotlin/snippets/agents/multi-agent/MultiAgentExample.kt:human_in_loop_pattern"
```

Human in the loop with Policy

A more advanced and structured way to implement Human-in-the-Loop is by using a PolicyEngine. This approach allows you to define policies that can trigger a confirmation step from a user before a tool is executed. The SecurityPlugin intercepts a tool call, consults the PolicyEngine, and if the policy dictates, it will automatically request user confirmation. This pattern is more robust for enforcing governance and security rules.

Here's how it works:

1. SecurityPlugin: You add this plugin to your Runner. It acts as an interceptor for all tool calls.
2. BasePolicyEngine: You create a custom class that implements this interface. Its evaluate() method contains your logic to decide if a tool call needs confirmation.
3. PolicyOutcome.CONFIRM: When your evaluate() method returns this outcome, the SecurityPlugin pauses the tool execution and generates a special FunctionCall using getAskUserConfirmationFunctionCalls.
4. Application Handling: Your application code receives this special function call and presents the confirmation request to the user.
5. User Confirmation: Once the user confirms, your application sends a FunctionResponse back to the agent, which allows the SecurityPlugin to proceed with the original tool execution.

!!! Note "TypeScript Recommended Pattern" The Policy-based pattern is the recommended approach for implementing Human-in-the-Loop workflows in TypeScript. Support in other ADK languages is planned for future releases.

A conceptual example of using a CustomPolicyEngine to require user confirmation before executing any tool is shown below.

=== "TypeScript"

```typescript
const rootAgent = new LlmAgent({
  name: 'weather_time_agent',
  model: 'gemini-flash-latest',
  description:
      'Agent to answer questions about the time and weather in a city.',
  instruction:
      'You are a helpful agent who can answer user questions about the time and weather in a city.',
  tools: [getWeatherTool],
});

class CustomPolicyEngine implements BasePolicyEngine {
  async evaluate(_context: ToolCallPolicyContext): Promise<PolicyCheckResult> {
    // Default permissive implementation
    return Promise.resolve({
      outcome: PolicyOutcome.CONFIRM,
      reason: 'Needs confirmation for tool call',
    });
  }
}

const runner = new InMemoryRunner({
    agent: rootAgent,
    appName,
    plugins: [new SecurityPlugin({policyEngine: new CustomPolicyEngine()})]
});
```

You can find the full code sample [here](https://github.com/google/adk-docs/blob/main/examples/typescript/snippets/agents/workflow-agents/hitl_confirmation_agent.ts).