I generated 207 MCP tools from an OpenAPI spec. Generating them was the easy part.

Every MCP server I've read that wraps a third-party REST API has the same shape: someone picked fifteen or twenty endpoints that seemed useful, hand-wrote a Zod schema for each, and shipped it. That works for about three months. Then the API adds a field, deprecates an enum value, renames a query parameter. The wrapper doesn't notice, because nothing in it is connected to the API's own description of itself. The schemas drift. The model starts getting rejected by the upstream API for reasons it can't see, and you get to debug an LLM guessing at a shape that stopped being true in February. The other failure is quieter. You need endpoint twenty-one — the one about persistent disks, or scaling, or environment groups — and the author skipped it. Now you're back to curl , except the agent is holding half the context and you're holding the other half. I wanted neither, so I built render-useful-mcp : an MCP server for Render where every API tool is generated from Render's own OpenAPI document. All 207 endpoints, no curation. The generating part took a weekend. Everything after that was the actual work. Make the generator refuse to guess The tempting way to write a spec-to-tools generator is to make it forgiving. Skip what you can't parse, fall back to { type: "object" } when a $ref gets hairy, log a warning and move on. You get 207 tools on the first run and feel great. You also get tools that lie. A parameter typed as a free-form object when the API actually wants one of four enum values is worse than no tool at all — the model will confidently produce garbage, and the failure surfaces three layers away from the cause. So the generator is fail-closed. It aborts the build, loudly, on: An operation whose tag doesn't map to a known toolset. Render added a resource category and I haven't classified it yet. That's my problem to solve, not something to paper over. A tool name collision. Two operations deriving the same name means my naming scheme is wrong. A cyclic $ref it can'