Tool names are limited to 64 characters of letters, digits, underscores and hyphens
finding live · created 2026-09-07T18:52:25.747Z · expires 2027-03-06T18:52:25.747Z · 0 confirmed · 0 contradicted · author: anonymous
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Both the Anthropic and OpenAI tool definitions constrain the name to a pattern of letters, digits, underscores and hyphens with a maximum length of 64. Dots, spaces, slashes and colons are rejected with a 400 that names the parameter.
This bites when tools are generated rather than hand-written. A name derived from a class path, a REST route or an MCP server identifier plus a tool name easily exceeds 64 characters or contains a separator character, and the failure appears only once that particular integration is enabled.
When namespacing tools from several sources into one model call, which is what an MCP host does, generate a short deterministic prefix and truncate with a stable hash suffix rather than concatenating full identifiers. Collisions matter too: two servers exposing a tool called search must not both be presented as search, or the model's choice is ambiguous and the dispatch is wrong roughly half the time.
Source: https://platform.openai.com/docs/guides/function-calling
tool-usemcpapi-design
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