diff --git a/agent/canvas.py b/agent/canvas.py
index 86fd8689ba..15f5cf0449 100644
--- a/agent/canvas.py
+++ b/agent/canvas.py
@@ -121,7 +121,11 @@ class Graph:
         for k in self.dsl.keys():
             if k in ["components"]:
                 continue
-            dsl[k] = deepcopy(self.dsl[k])
+            try:
+                dsl[k] = deepcopy(self.dsl[k])
+            except Exception as e:
+                logging.warning("Graph.__str__: deepcopy failed for dsl key '%s' (type=%s): %s. Using shallow reference.", k, type(self.dsl[k]).__name__, e)
+                dsl[k] = self.dsl[k]
 
         for k, cpn in self.components.items():
             if k not in dsl["components"]:
@@ -130,8 +134,17 @@ class Graph:
                 if c == "obj":
                     dsl["components"][k][c] = json.loads(str(cpn["obj"]))
                     continue
-                dsl["components"][k][c] = deepcopy(cpn[c])
-        return json.dumps(dsl, ensure_ascii=False)
+                try:
+                    dsl["components"][k][c] = deepcopy(cpn[c])
+                except Exception as e:
+                    logging.warning("Graph.__str__: deepcopy failed for component '%s' key '%s' (type=%s): %s. Using shallow reference.", k, c, type(cpn[c]).__name__, e)
+                    dsl["components"][k][c] = cpn[c]
+        def _serialize_default(obj):
+            if callable(obj):
+                return None
+            logging.warning("Graph.__str__: JSON fallback via str() for type=%s", type(obj).__name__)
+            return str(obj)
+        return json.dumps(dsl, ensure_ascii=False, default=_serialize_default)
 
     def reset(self):
         self.path = []
diff --git a/agent/component/base.py b/agent/component/base.py
index c0ddea168a..a91bec70ef 100644
--- a/agent/component/base.py
+++ b/agent/component/base.py
@@ -96,7 +96,12 @@ class ComponentParamBase(ABC):
         return {name: True for name in self.get_feeded_deprecated_params()}
 
     def __str__(self):
-        return json.dumps(self.as_dict(), ensure_ascii=False)
+        def _serialize_default(obj):
+            if callable(obj):
+                return None
+            logging.warning("ComponentParamBase.__str__: JSON fallback via str() for type=%s", type(obj).__name__)
+            return str(obj)
+        return json.dumps(self.as_dict(), ensure_ascii=False, default=_serialize_default)
 
     def as_dict(self):
         def _recursive_convert_obj_to_dict(obj):
diff --git a/api/apps/restful_apis/agent_api.py b/api/apps/restful_apis/agent_api.py
index 859719bba8..8ae6723709 100644
--- a/api/apps/restful_apis/agent_api.py
+++ b/api/apps/restful_apis/agent_api.py
@@ -71,6 +71,20 @@ from peewee import MySQLDatabase, PostgresqlDatabase
 _background_tasks: Set[asyncio.Task] = set()
 
 
+def _canvas_json_default(obj):
+    """Fallback serializer for canvas SSE events.
+
+    Agent components store functools.partial objects as deferred streaming
+    handles (see llm.py, agent_with_tools.py, message.py). These leak into
+    SSE event dicts via component input/output propagation and are not
+    JSON-serializable. This handler converts them to None so that downstream
+    consumers never receive opaque ``str(partial(...))`` representations.
+    """
+    if callable(obj):
+        return None
+    raise TypeError(f"Object of type {type(obj).__name__} is not JSON serializable")
+
+
 def _require_canvas_access_sync(func):
     @wraps(func)
     def wrapper(*args, **kwargs):
@@ -312,7 +326,7 @@ async def _run_workflow_session(
                                 }
                             )
                     final_ans = ans
-                    yield "data:" + json.dumps(ans, ensure_ascii=False) + "\n\n"
+                    yield "data:" + json.dumps(ans, ensure_ascii=False, default=_canvas_json_default) + "\n\n"
 
                 if final_ans:
                     if "data" not in final_ans or not isinstance(final_ans["data"], dict):
@@ -1576,7 +1590,7 @@ async def agent_chat_completion(tenant_id, agent_id=None):
             emitted = False
             async for ans in _iter_session_completion_events(tenant_id, agent_id, req, return_trace):
                 emitted = True
-                yield "data:" + json.dumps(ans, ensure_ascii=False) + "\n\n"
+                yield "data:" + json.dumps(ans, ensure_ascii=False, default=_canvas_json_default) + "\n\n"
             if not emitted:
                 # Parity with the new-session SSE path: if the canvas yields
                 # no events on an existing session (e.g. empty query), still