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AI & agents · A weekendAnywhere· Source · Jun 2026

Python developers building MCP tools lose their argument descriptions, so the model never sees them.

Developers building MCP tool servers in Python

Where it stands

  1. Sourced (done)
  2. Kit (done)
  3. Wanted (next)
  4. Building
  5. Solved

How this problem came to be

  1. Found1 sourcegithub.com/modelcontextprotocol
  2. Quoted3 claimseach checked against its page
  3. BriefWrittenand what solved looks like
  4. Market1 signals16 reactions on the issue
  5. Kit5 steps20 tools · 6 routes
  6. People0 want0 live it · 0 would test

The problem, broken down

Who it hurts
Developers who write tools for AI agents with the Model Context Protocol (MCP) Python SDK's FastMCP.
The moment
When an agent picks the wrong tool or passes the wrong argument because the schema only lists names and types.
What people do today
Developers write argument descriptions in docstrings, but FastMCP builds the schema from type hints only.
Why it's unsolved
Maintainers have not decided whether to require one docstring style or add a parsing library as a dependency.
What solved looks like
A pull request that fills each argument's description in the tool schema from Google, NumPy or Sphinx style docstrings, leaves the schema unchanged when there is no docstring, and has tests for each style.

Evidence

Every claim is tied to the source it comes from
MCP Python SDK issue #226: docstring to tool JSON schema ↗
  1. 1The issue shows FastMCP turning a documented add_numbers(a, b) function into a schema where a and b have only a title and a type; the argument descriptions in the docstring are dropped.
  2. 2The reporter says this affects tool-calling performance and proposes parsing a standard docstring style, possibly with the griffe library.
  3. 3The issue is labelled ‘good first issue’ and ‘needs decision’, and was still open on 2026-10-09.

Written from the cited sources. Anything they don't say is left as a question, not a guess.

Sources

From a public page with a citation.

  1. [1]MCP Python SDK issue #226: docstring to tool JSON schema
What the source says, in short

The issue shows FastMCP turning a documented add_numbers(a, b) function into a schema where a and b have only a title and a type; the argument descriptions in the docstring are dropped. The reporter says this affects tool-calling performance and proposes parsing a standard docstring style, possibly with the griffe library. The issue is labelled ‘good first issue’ and ‘needs decision’, and was still open on 2026-10-09.

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Market read

How big this is, from the evidence

  • GitHub ↗Builders' pull · Medium confidence16 reactions on the issue

How these were measured: Total reactions on the GitHub issue, read from its API.

For whoever takes this up

The builder's path for this problem

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