pydantic.errors.PydanticSchemaGenerationError: Unable to generate pydantic-core schema for <class 'mcp.client.session.ClientSession'>.

[kpacabes12]

pydantic.errors.PydanticSchemaGenerationError: Unable to generate pydantic-core schema for <class 'mcp.client.session.ClientSession'>. Set arbitrary_types_allowed=True in the model_config to ignore this error or implement __get_pydantic_core_schema__ on your type to fully support it.

If you got this error by calling handler() within __get_pydantic_core_schema__ then you likely need to call handler.generate_schema(<some type>) since we do not call __get_pydantic_core_schema__ on <some type> otherwise to avoid infinite recursion.

For further information visit https://errors.pydantic.dev/2.12/u/schema-for-unknown-type

i am facing this issue when i am loading the swagger url

url : http://localhost:8000/docs

lib:
google-adk == 1.21.0
fastapi ==0.124.0
pydantic==2.12.5

pythonversion:
python 3.13.2

Can anyone help me to resolve this issue ?

[kpacabes12]

[jimpeebles]

Possibly (but maybe not) related:
#3082 (comment)

There was an issue, a patch was released, but that patch was unintentionally reverted in the 1.21 release. You can try rolling back to 1.20 and see if it helps.

[kpacabes12]

i tried it 1.20 version it was also thrown error , i think 1.15 version its worked in my case this version not supporting the agent as tool feature
File "C:\Users\kpaca\Documents\POC\Acabes_RND\Agent runtime\ACABES-RnD.venv\Lib\site-packages\pydantic\json_schema.py", line 511, in handler_func
json_schema = generate_for_schema_type(schema_or_field)
File "C:\Users\kpaca\Documents\POC\Acabes_RND\Agent runtime\ACABES-RnD.venv\Lib\site-packages\pydantic\json_schema.py", line 921, in is_instance_schema
return self.handle_invalid_for_json_schema(schema, f'core_schema.IsInstanceSchema ({schema["cls"]})')
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\kpaca\Documents\POC\Acabes_RND\Agent runtime\ACABES-RnD.venv\Lib\site-packages\pydantic\json_schema.py", line 2442, in handle_invalid_for_json_schema
raise PydanticInvalidForJsonSchema(f'Cannot generate a JsonSchema for {error_info}')
pydantic.errors.PydanticInvalidForJsonSchema: Cannot generate a JsonSchema for core_schema.IsInstanceSchema (<class 'httpx.Client'>)

For further information visit https://errors.pydantic.dev/2.12/u/invalid-for-json-schema

[alaliaa]

I am experiencing the same issue.

[surajksharma07]

@kpacabes12 Thanks for reporting this!

The issue occurs because McpToolset's MCPSessionManager contains ClientSession objects that Pydantic can't serialize for OpenAPI schema generation.

The fix requires modifying mcp_toolset.py line 135 to mark _mcp_session_manager as private (rename to __mcp_session_manager), or add model_config = ConfigDict(arbitrary_types_allowed=True) to McpToolsetConfig at line 247.

As a workaround, you can monkey-patch ClientSession with __get_pydantic_core_schema__ to return core_schema.any_schema() before creating your FastAPI app.

Team is looking more into it.

[surajksharma07]

@DeanChensj Please have a look into it.