[Bug]: TypeError: Object of type IntentMandate is not JSON serializable - sqlite_session_service fails to serialize Pydantic models in session state

[diptendulkar]

What happened?

**Environment** ADK Version: 1.13.0 (via google-adk) Python Version: 3.13.5 OS: macOS (darwin 24.5.0, arm64) Installation: pip/uv

Description
When storing a Pydantic BaseModel subclass in the ADK session state, sqlite_session_service.py raises a TypeError because json.dumps() is called directly on the Pydantic model object instead of its serialized representation.

Steps to Reproduce

1. Create a Pydantic model and store it in the session state:

class IntentMandate(BaseModel):    
  natural_language_description: str    i
  ntent_expiry: str    # ... other fields

# In an agent tool or executor:
ctx.state["mandate"] = IntentMandate(    natural_language_description="Buy red shoes",    
intent_expiry="2025-12-30T00:00:00Z"
)

2. The ADK attempts to persist the session state to SQLite
3. Error is raised

Expected Behavior
The session service should handle Pydantic models gracefully, either by:
Calling .model_dump() on Pydantic BaseModel instances before JSON serialization
Using a custom JSON encoder that handles Pydantic models
Documenting that session state values must be JSON-serializable primitives

Actual Behavior
TypeError: Object of type IntentMandate is not JSON serializable (ref screenshot)

Relevant log output

2025-12-29 20:31:45,807 - ERROR - adk_web_server.py:1509 - Error in event_generator: Object of type IntentMandate is not JSON serializable
Traceback (most recent call last):
  File ".venv/lib/python3.13/site-packages/google/adk/cli/adk_web_server.py", line 1499, in event_generator
    async for event in agen:
    ...
      yield f"data: {sse_event}\n\n"
  File ".venv/lib/python3.13/site-packages/google/adk/runners.py", line 505, in run_async
    async for event in agen:
      yield event
  File ".venv/lib/python3.13/site-packages/google/adk/runners.py", line 493, in _run_with_trace
    async for event in agen:
      yield event
  File ".venv/lib/python3.13/site-packages/google/adk/runners.py", line 769, in _exec_with_plugin
    await self.session_service.append_event(
        session=session, event=event
    )
  File ".venv/lib/python3.13/site-packages/google/adk/cli/utils/local_storage.py", line 200, in append_event
    return await service.append_event(session, event)
  File ".venv/lib/python3.13/site-packages/google/adk/sessions/sqlite_session_service.py", line 369, in append_event
    await self._update_session_state_in_db(...)
  File ".venv/lib/python3.13/site-packages/google/adk/sessions/sqlite_session_service.py", line 504, in _update_session_state_in_db
    json.dumps(delta),
  File "/opt/homebrew/Cellar/python@3.13/3.13.5/.../json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
  ...
TypeError: Object of type IntentMandate is not JSON serializable

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