Add option to skip output validation for mcp client

[germanbecker]

Description

MCP Client session currently validates the output generated by MCP servers agains the reported json schema, and raises ValidationError.
This means that MCP server tools that have a not well maintained output schema, but are otherwise functional, are unusable.
The request is to add an option either at the ClientSession interface and/or the call_tool method so skip validation and return the result even if it does not conform to the output schemma

References

https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/client/session.py#L319-L349

[ernestprovo23]

hey @germanbecker, would the right shape here be a validate_structured_output: bool = True kwarg on ClientSession.__init__? that's a one-flag gate at session.py:319-320 (only validation call site i can find), default true so nothing breaks for existing users.

noticed #1189 went down this road last fall and got closed over the safety-vs-flexibility tradeoff. the difference this time is the framing... real-world driver is servers shipping buggy output schemas that are otherwise functional, not a perf concern. happy to take it on if a maintainer (cc @felixweinberger) thinks the use case clears the bar. if not, totally fine to close.

[germanbecker]

Hey! Thanks for looking into this! yes I think either at the __init__ level or at the call_tool would work. As for the justification, my example is a tool where the schema has a list of literals for an output, but it appears the maintainers forgot one possible literal. Since this will eventually be a text stream sent to an LLM, in some applications (like my case) is better so send the output that doesn't conform to schema than send an empty error

[germanbecker]

For reference the current workaround:

class MyClientSession(ClientSession):
    async def _validate_tool_result(self, name: str, result: CallToolResult) -> None:
        pass