feat: parse Google-style docstrings to populate tool parameter descriptions

[Maanik23]

Github-Issue:#226

Summary

FastMCP previously discarded the parameter descriptions in a function's docstring when generating the JSON schema for a tool. Tools showed up to LLMs with bare titles and types but no per-parameter context, which made tool calling less reliable.

This PR adds a small Google-style docstring parser (no new dependencies) that extracts the summary and per-parameter descriptions, then wires them into both the tool description and the generated JSON schema.

Before

def add_numbers(a: float, b: float) -> float:
    """Adds two numbers and returns the result.

    Args:
        a: The first number to add.
        b: The second number to add.
    """
    return a + b

Generated schema:

{
  "properties": {
    "a": {"title": "A", "type": "number"},
    "b": {"title": "B", "type": "number"}
  }
}

The tool description was the entire raw docstring including the Args: block.

After

{
  "properties": {
    "a": {"title": "A", "type": "number", "description": "The first number to add."},
    "b": {"title": "B", "type": "number", "description": "The second number to add."}
  }
}

And the tool description is just the summary: "Adds two numbers and returns the result."

Changes

src/mcp/server/mcpserver/utilities/docstring.py (new file)

• parse_docstring(docstring) -> tuple[str, dict[str, str]] returns a (summary, param_descriptions) tuple
• Hand-written parser, no new dependencies
• Handles multi-line summaries, parameters with or without type annotations, complex annotations like Annotated[list[int], Field(min_length=1)], multi-line continuation of parameter descriptions, all common Args header aliases, and early termination at Returns/Raises/Examples sections

src/mcp/server/mcpserver/utilities/func_metadata.py

• Calls parse_docstring(func.__doc__) once per function
• Passes the matching description (if any) to Field(description=...) when building the dynamic Pydantic model

src/mcp/server/mcpserver/tools/base.py

• Uses the parsed summary as the tool description when no explicit description= is provided
• Falls back to the raw docstring if the parser cannot find a summary
• An explicit description= argument to Tool.from_function() still wins, preserving backward compatibility

Tests

• tests/server/mcpserver/test_docstring.py (new) — 16 unit tests covering: None/empty/whitespace input, summary-only docstrings, multi-line summaries, Google-style with and without type annotations, multi-line parameter descriptions, all Args header aliases (Args, Arguments, Parameters), section termination by Raises/Returns, blank lines inside the Args block, complex Annotated[...] type expressions with nested brackets, and unrecognized continuation lines
• tests/server/mcpserver/test_func_metadata.py — added two integration tests verifying that descriptions land in the JSON schema and that functions without an Args section still work
• tests/server/mcpserver/test_tool_manager.py — added an end-to-end test through ToolManager that confirms both the tool description and parameter descriptions come from the docstring

Compatibility

• Default behaviour is preserved: existing tools without docstrings, with summary-only docstrings, or with non-Google docstring styles continue to work exactly as before
• An explicit description= argument to Tool.from_function() still takes precedence over the parsed summary
• No new runtime dependencies
• Only the Google docstring style is supported. NumPy and Sphinx styles fall back to summary-only behaviour, which is still an improvement over the previous "use the entire raw docstring" behaviour

Why Google style only

The issue suggested supporting google, numpy, and sphinx styles via griffe. Adding griffe is a non-trivial dependency for a feature that, in practice, is overwhelmingly used with Google-style docstrings (the default in FastAPI, Pydantic, LangChain, and most modern Python projects). A self-contained parser keeps the dependency footprint at zero and is easy to extend later if there is real demand for the other two styles.