fix(sessions): prevent PydanticSerializationError when state contains non-serializable objects

When Python callables or other non-JSON-serializable values are stored in
session state (e.g. via MCP tool callbacks), EventActions.state_delta and
agent_state would cause DatabaseSessionService.append_event to crash with
PydanticSerializationError during event.model_dump(mode="json").

Add field serializers for state_delta and agent_state that recursively
convert non-serializable leaf values to a descriptive string, allowing
the event to be persisted without data loss for serializable fields.

Fixes #4724

Author: Raman369AI

src/google/adk/events/event_actions.py

@@ -14,6 +14,7 @@
 
 from __future__ import annotations
 
+import json
 from typing import Any
 from typing import Optional
 
@@ -22,11 +23,30 @@
 from pydantic import BaseModel
 from pydantic import ConfigDict
 from pydantic import Field
+from pydantic import field_serializer
 
 from ..auth.auth_tool import AuthConfig
 from ..tools.tool_confirmation import ToolConfirmation
 
 
+def _make_json_serializable(obj: Any) -> Any:
+  """Recursively converts an object to a JSON-serializable form.
+
+  Non-serializable leaf values (e.g. Python callables stored in session state)
+  are replaced with a descriptive string so the overall structure can still be
+  persisted without crashing.
+  """
+  if isinstance(obj, dict):
+    return {k: _make_json_serializable(v) for k, v in obj.items()}
+  if isinstance(obj, (list, tuple)):
+    return [_make_json_serializable(v) for v in obj]
+  try:
+    json.dumps(obj)
+    return obj
+  except (TypeError, ValueError):
+    return f'<not serializable: {type(obj).__name__}>'
+
+
 class EventCompaction(BaseModel):
   """The compaction of the events."""
 
@@ -66,6 +86,10 @@ class EventActions(BaseModel):
   state_delta: dict[str, object] = Field(default_factory=dict)
   """Indicates that the event is updating the state with the given delta."""
 
+  @field_serializer('state_delta', mode='plain')
+  def _serialize_state_delta(self, value: dict[str, object]) -> dict[str, Any]:
+    return _make_json_serializable(value)
+
   artifact_delta: dict[str, int] = Field(default_factory=dict)
   """Indicates that the event is updating an artifact. key is the filename,
   value is the version."""
@@ -106,5 +130,13 @@ class EventActions(BaseModel):
   """The agent state at the current event, used for checkpoint and resume. This
   should only be set by ADK workflow."""
 
+  @field_serializer('agent_state', mode='plain')
+  def _serialize_agent_state(
+      self, value: Optional[dict[str, Any]]
+  ) -> Optional[dict[str, Any]]:
+    if value is None:
+      return None
+    return _make_json_serializable(value)
+
   rewind_before_invocation_id: Optional[str] = None
   """The invocation id to rewind to. This is only set for rewind event."""