Three AI APIs reject MCP tool lists before the model sees them
A study ran 21,812 MCP server tool lists through the Batch APIs of Claude Haiku 5.5, GPT-5-nano and Gemini 2.5 Flash. Anthropic refused 456 lists, OpenAI 591, and Google 7 plus 135 unexplained. Each API applies its own validation rules to schemas and tool names.
- Anthropic refused 456 lists, OpenAI 591, Google 7 plus 135 unexplained
- Common causes: oneOf/allOf/anyOf at the schema top level and invalid tool names
- OpenAI caps lists at 128 tools, a limit not stated in the Chat Completions docs
- The same list can pass one vendor's API and be refused by another
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