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fix(sessions): prevent PydanticSerializationError when session state contains non-serializable objects #4748

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fix(sessions): prevent PydanticSerializationError when session state contains non-serializable objects #4748

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32 changes: 32 additions & 0 deletions src/google/adk/events/event_actions.py
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,7 @@

from __future__ import annotations

import json
from typing import Any
from typing import Optional

Expand All @@ -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__}>'
Comment on lines +39 to +47
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@gemini-code-assist gemini-code-assist bot Mar 7, 2026

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medium

The current implementation of _make_json_serializable doesn't handle Pydantic BaseModel instances, which are not directly serializable by the standard json library. If a Pydantic model is present in the state, it will be incorrectly marked as non-serializable (<not serializable: ...>), leading to data that could have been persisted being lost.

To make this more robust, you should add a specific check for BaseModel instances and serialize them using obj.model_dump() before proceeding with the recursive serialization. This will ensure that Pydantic models within the state are correctly persisted.

Suggested change
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__}>'
if isinstance(obj, BaseModel):
return _make_json_serializable(obj.model_dump())
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."""

Expand Down Expand Up @@ -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."""
Expand Down Expand Up @@ -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."""