Unpinned MCP Server Schemas Risk Silent API Breaks on Every Code Commit

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A developer discovered that while pinning dependencies in requirements.txt prevents breaking library updates, it does nothing to guard against unintended changes to the JSON schema that FastMCP auto-generates from Python source code. Every commit that renames a parameter, alters a type hint, or edits a docstring can silently rewrite the contract that AI agents rely on to call tools correctly. Unlike a failed install or runtime crash, these schema shifts produce no errors — agents simply build calls against fields or types that no longer exist. The repository was found to have no snapshot tests, no golden schema fixtures, and no CI checks that would flag such drift. The author highlights this as a blind spot in MCP server development, where the only way to verify what agents actually receive is manual inspection after something has already broken.

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