5 ways MCP tools silently vanish from Claude, Cursor and ChatGPT
Future_AGI · reddit · 2026-10-13
Drawing on real incidents, the author catalogs five ways MCP tools silently disappear from clients: a legal oneOf at the inputSchema root (Claude/Cursor drop it), invalid x-mcp-header annotation types in the Python SDK (fixed in v2.3.0), tool names like ids[] that Claude Code rejects, tool-count/size budgets (ChatGPT showed 43 of 109 tools, cut off at letter K, and caches tools/list at install), and client updates like Codex silently changing tool-calling. Dropped tools never produce failed calls, so the fix is re-listing tools per client and running real tasks after every change — the author's open-source eval kit adds an offline toolselectionaccuracy check.
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