diff --git a/agent/canvas.py b/agent/canvas.py
index 65303ca9e9e..09538faddc3 100644
--- a/agent/canvas.py
+++ b/agent/canvas.py
@@ -119,7 +119,11 @@ def __str__(self):
         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"]:
@@ -128,8 +132,17 @@ def __str__(self):
                 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 9bceb4ce6d9..0f5e84a0f2d 100644
--- a/agent/component/base.py
+++ b/agent/component/base.py
@@ -94,7 +94,12 @@ def _deprecated_params_set(self):
         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/canvas_app.py b/api/apps/canvas_app.py
index 8c896e36add..e3da60b9402 100644
--- a/api/apps/canvas_app.py
+++ b/api/apps/canvas_app.py
@@ -19,6 +19,20 @@
 import logging
 from functools import partial
 from quart import request, Response, make_response
+
+
+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")
 from agent.component import LLM
 from api.db import CanvasCategory
 from api.db.services.canvas_service import CanvasTemplateService, UserCanvasService, API4ConversationService
@@ -235,7 +249,7 @@ async def sse():
         nonlocal canvas, user_id
         try:
             async for ans in canvas.run(query=query, files=files, user_id=user_id, inputs=inputs):
-                yield "data:" + json.dumps(ans, ensure_ascii=False) + "\n\n"
+                yield "data:" + json.dumps(ans, ensure_ascii=False, default=_canvas_json_default) + "\n\n"
 
             commit_ok = CanvasReplicaService.commit_after_run(
                 canvas_id=req["id"],
@@ -293,7 +307,7 @@ async def generate():
                         }
                     )
                     ans.setdefault("data", {})["trace"] = trace_items
-                    answer = "data:" + json.dumps(ans, ensure_ascii=False) + "\n\n"
+                    answer = "data:" + json.dumps(ans, ensure_ascii=False, default=_canvas_json_default) + "\n\n"
                 yield answer
 
             if event not in ["message", "message_end"]: