fix: filter non-serializable objects from stream events in prepare()

src/strands/types/_events.py

@@ -5,6 +5,8 @@
 agent lifecycle.
 """
 
+import json
+import uuid
 from collections.abc import Sequence
 from typing import TYPE_CHECKING, Any, cast
 
@@ -24,6 +26,44 @@
     from ..multiagent.base import MultiAgentResult, NodeResult
 
 
+def _is_json_serializable(value: Any) -> bool:
+    """Check if a value is JSON-serializable.
+
+    Args:
+        value: The value to check.
+
+    Returns:
+        True if the value can be serialized to JSON, False otherwise.
+    """
+    try:
+        json.dumps(value)
+        return True
+    except (TypeError, ValueError, OverflowError):
+        return False
+
+
+def _sanitize_invocation_state(invocation_state: dict) -> dict:
+    """Filter invocation_state to only include JSON-serializable values.
+
+    Non-serializable objects (Agent instances, OpenTelemetry Spans, etc.) are
+    silently dropped so that events yielded from stream_async() remain
+    JSON-serializable. UUID values are converted to their string representation.
+
+    Args:
+        invocation_state: The raw invocation state dict.
+
+    Returns:
+        A new dict containing only JSON-serializable key-value pairs.
+    """
+    result = {}
+    for k, v in invocation_state.items():
+        if isinstance(v, uuid.UUID):
+            result[k] = str(v)
+        elif _is_json_serializable(v):
+            result[k] = v
+    return result
+
+
 class TypedEvent(dict):
     """Base class for all typed events in the agent system."""
 
@@ -69,7 +109,7 @@ def __init__(self) -> None:
 
     @override
     def prepare(self, invocation_state: dict) -> None:
-        self.update(invocation_state)
+        self.update(_sanitize_invocation_state(invocation_state))
 
 
 class StartEvent(TypedEvent):
@@ -138,7 +178,7 @@ def is_callback_event(self) -> bool:
     @override
     def prepare(self, invocation_state: dict) -> None:
         if "delta" in self:
-            self.update(invocation_state)
+            self.update(_sanitize_invocation_state(invocation_state))
 
 
 class ToolUseStreamEvent(ModelStreamEvent):
@@ -270,7 +310,7 @@ def __init__(self, delay: int) -> None:
 
     @override
     def prepare(self, invocation_state: dict) -> None:
-        self.update(invocation_state)
+        self.update(_sanitize_invocation_state(invocation_state))
 
 
 class ToolResultEvent(TypedEvent):

tests/strands/agent/hooks/test_agent_events.py

@@ -39,10 +39,7 @@ def mock_sleep():
 
 
 any_props = {
-    "agent": ANY,
     "event_loop_cycle_id": ANY,
-    "event_loop_cycle_span": ANY,
-    "event_loop_cycle_trace": ANY,
     "request_state": {},
 }
 
@@ -177,7 +174,6 @@ async def test_stream_e2e_success(alist):
             "delta": {"text": "Invoking async tool"},
             "event_loop_parent_cycle_id": ANY,
             "messages": ANY,
-            "model": ANY,
             "system_prompt": None,
             "tool_config": tool_config,
         },
@@ -191,7 +187,6 @@ async def test_stream_e2e_success(alist):
             "delta": {"toolUse": {"input": "{}"}},
             "event_loop_parent_cycle_id": ANY,
             "messages": ANY,
-            "model": ANY,
             "system_prompt": None,
             "tool_config": tool_config,
             "type": "tool_use_stream",
@@ -235,7 +230,6 @@ async def test_stream_e2e_success(alist):
             "delta": {"text": "Invoking streaming tool"},
             "event_loop_parent_cycle_id": ANY,
             "messages": ANY,
-            "model": ANY,
             "system_prompt": None,
             "tool_config": tool_config,
         },
@@ -249,7 +243,6 @@ async def test_stream_e2e_success(alist):
             "delta": {"toolUse": {"input": "{}"}},
             "event_loop_parent_cycle_id": ANY,
             "messages": ANY,
-            "model": ANY,
             "system_prompt": None,
             "tool_config": tool_config,
             "type": "tool_use_stream",
@@ -301,7 +294,6 @@ async def test_stream_e2e_success(alist):
             "delta": {"text": "I invoked the tools!"},
             "event_loop_parent_cycle_id": ANY,
             "messages": ANY,
-            "model": ANY,
             "system_prompt": None,
             "tool_config": tool_config,
         },

tests/strands/agent/test_agent.py

@@ -722,34 +722,25 @@ def test_agent__call__callback(mock_model, agent, callback_handler, agenerator):
         unittest.mock.call(event={"contentBlockDelta": {"delta": {"toolUse": {"input": '{"value"}'}}}}),
         unittest.mock.call(
             type="tool_use_stream",
-            agent=agent,
             current_tool_use={"toolUseId": "123", "name": "test", "input": {}},
             delta={"toolUse": {"input": '{"value"}'}},
             event_loop_cycle_id=unittest.mock.ANY,
-            event_loop_cycle_span=unittest.mock.ANY,
-            event_loop_cycle_trace=unittest.mock.ANY,
             request_state={},
         ),
         unittest.mock.call(event={"contentBlockStop": {}}),
         unittest.mock.call(event={"contentBlockStart": {"start": {}}}),
         unittest.mock.call(event={"contentBlockDelta": {"delta": {"reasoningContent": {"text": "value"}}}}),
         unittest.mock.call(
-            agent=agent,
             delta={"reasoningContent": {"text": "value"}},
             event_loop_cycle_id=unittest.mock.ANY,
-            event_loop_cycle_span=unittest.mock.ANY,
-            event_loop_cycle_trace=unittest.mock.ANY,
             reasoning=True,
             reasoningText="value",
             request_state={},
         ),
         unittest.mock.call(event={"contentBlockDelta": {"delta": {"reasoningContent": {"signature": "value"}}}}),
         unittest.mock.call(
-            agent=agent,
             delta={"reasoningContent": {"signature": "value"}},
             event_loop_cycle_id=unittest.mock.ANY,
-            event_loop_cycle_span=unittest.mock.ANY,
-            event_loop_cycle_trace=unittest.mock.ANY,
             reasoning=True,
             reasoning_signature="value",
             request_state={},
@@ -758,12 +749,9 @@ def test_agent__call__callback(mock_model, agent, callback_handler, agenerator):
         unittest.mock.call(event={"contentBlockStart": {"start": {}}}),
         unittest.mock.call(event={"contentBlockDelta": {"delta": {"text": "value"}}}),
         unittest.mock.call(
-            agent=agent,
             data="value",
             delta={"text": "value"},
             event_loop_cycle_id=unittest.mock.ANY,
-            event_loop_cycle_span=unittest.mock.ANY,
-            event_loop_cycle_trace=unittest.mock.ANY,
             request_state={},
         ),
         unittest.mock.call(event={"contentBlockStop": {}}),
@@ -1075,7 +1063,7 @@ async def test_event_loop(*args, **kwargs):
 
     tru_events = await alist(stream)
     exp_events = [
-        {"init_event_loop": True, "callback_handler": mock_callback},
+        {"init_event_loop": True},
         {"data": "First chunk"},
         {"data": "Second chunk"},
         {"complete": True, "data": "Final chunk"},

tests/strands/types/test__events.py

@@ -162,6 +162,30 @@ def test_prepare_without_delta(self):
         event.prepare(invocation_state)
         assert "request_id" not in event
 
+    def test_prepare_filters_non_serializable(self):
+        """Test prepare method filters out non-JSON-serializable values."""
+        import json
+
+        class NonSerializable:
+            pass
+
+        event = ModelStreamEvent({"delta": "content"})
+        invocation_state = {
+            "request_id": "456",
+            "agent": NonSerializable(),
+            "span": NonSerializable(),
+            "cycle_id": 123,
+        }
+        event.prepare(invocation_state)
+        # Serializable values should be present
+        assert event["request_id"] == "456"
+        assert event["cycle_id"] == 123
+        # Non-serializable values should be filtered out
+        assert "agent" not in event
+        assert "span" not in event
+        # The resulting event dict must be JSON-serializable
+        json.dumps(dict(event))  # should not raise
+
 
 class TestToolUseStreamEvent:
     """Tests for ToolUseStreamEvent."""