MCP tool definition drift: how approved tools change silently and how to catch it
BlitzBallBruiser · reddit · 2026-11-07
Approving an MCP server only vouches for what it looked like that day. Descriptions, schemas, hints and new tools can all drift silently afterward. The post lays out a practical defense: canonicalize and hash each tool definition on every tools/list, block only mismatched tools, start new tools unapproved, and log approvals. Hashes can't catch behavior drift, though—pin package versions for local stdio servers, validate structuredContent against outputSchema, run known-answer test calls, and watch egress. Remote servers remain an unsolved problem.
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