Fix @mcp.tool() Failing to Detect Context Argument When Type Variables Are Used in Context

[jkawamoto]

Motivation and Context

When defining tools to safely access the app context (lifespan context) using Context, it is necessary to include type variables.

However, the current implementation of @mcp.tool() fails to detect the context argument when type variables are specified (as detailed in #357).

This PR addresses and resolves this issue.

How Has This Been Tested?

The following commands were executed locally to verify the fix:

uv sync --frozen --all-extras --dev
uv run --frozen ruff check .
uv run --frozen pytest

Breaking Changes

No.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[Kludex]

I don't think this solves the issue. Add a test, and you'll see.

[jkawamoto]

Actually, it passes the current tests and also works on my local server.

The cause of #357 is that it only checks if param.annotation is Context.
When type variables are specialized, the resulting class is a subclass of the generic class.
Also, a class is a subclass of itself.
So, we just need to use issubclass(param.annotation, Context) instead of param.annotation is Context.

This part is for handling cases where param.annotation is typing.Literal or similar, btw:

if get_origin(param.annotation) is not None:
    continue

[Kludex]

I'm asking to add a test where we use the generic params for Context. The context needs to be passed to the function.

This should work for:

context: Context[SessionParam]  # missing lifespan, since it has a default
context: Context[SessionParam, LifespanParam]  # both
context: Context[SessionParam, LifespanParam] | None = None  # union
context: Annotated[Context[Session, LifespanParam], "metadata"]  # Not necessary for now

[jkawamoto]

I've added a test for the second case.

Regarding the first case:

Context[SessionParam]

This is not only useless but also invalid. It raises the following exception:

Too few parameters for <class 'mcp.server.fastmcp.server.Context'>; actual 1, expected 2

About the third case: I don't believe this situation should occur. If a tool requests a context, it should always be provided.