"Your MCP server's tool list can be refused before the model sees it: the rules three AI APIs applied to 21,812 lists"
We read the tool lists of MCP servers in the official MCP registry. Then we sent each list, once, to three small AI models through their vendors' Batch APIs: Claude Haiku 5.5, GPT-5-nano and Gemini 2.5 Flash. Each model was set to its lowest reasoning level and got one fixed request. That was 21,81
Posted on Oct 28 • Originally published at protocolprobe.com • AI-assisted
We read the tool lists of MCP servers in the official MCP registry. Then we sent each list, once, to three small AI models through their vendors' Batch APIs: Claude Haiku 5.5, GPT-5-nano and Gemini 2.5 Flash. Each model was set to its lowest reasoning level and got one fixed request. That was 21,812 tool lists, one per server, with answers dated 2026-10-25 to 2026-10-26 (UTC).
No MCP server was contacted and no tool was run. We sent only the tool names, descriptions and input schemas, exactly as each server publishes them.
The finding most useful to server authors: each API checks the tool list before its model sees it, and each API has its own rules. The same list can be accepted by one API and refused by another. A refusal is the API's answer about the list as written. It is not a fault in the server's code, but it does mean that client never offers your tools to the model.
Counts are tool lists, one per server, from our run. An API can have other rules, and can change them.
The 128-tool limit is not stated on the chat completions API reference page we read on 2026-10-25. The refused lists ranged from 129 to 1,711 tools.
For 4 of those 7 lists, the names started with a digit. That meets the docs text we read, which gives no rule for the first character, but the API still refused them. Google's API also answered Request contains an invalid argument. for 135 more lists without naming a rule. Of those 135, 34 had tuple-style items (an array of schemas) and 30 had two tools with the same name. No list that Google's API answered had either. That is an association we measured, not a reason Google gave.
From the rules above, a tool list is accepted by all three APIs in our run more often when:
They describe three small models on the dates shown, each at its lowest reasoning setting. They do not describe the vendors' other models or a real client with its own system prompt. They are not a judgment of any server or publisher. We do not rank the models. Each model's own figures are on the full page, and each server's own API message is on its server page.