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If an API publishes an OpenAPI 3 document, the gateway can expose its operations as tools. Agents keep connecting to the gateway; when one calls a tool, the gateway makes the matching HTTP request and returns the response. There is no MCP server to write or host. An OpenAPI backend provides tools only — no prompts, no resources. Other servers on the same application still contribute theirs.

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What you need

A URL to the raw document — .json or .yaml, version 3 — not the Swagger UI page you read in a browser. And whatever the API expects from a service account: nothing, a fixed header, or OAuth client credentials. The data plane has to reach both the document and the API; if they live on a private network, that means Hybrid.

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Validation, and how to read it

Nothing saves until Validate OpenAPI passes. It fetches the document, parses it, compiles the tools, and fails at a named stage: On success it shows the tool count, the API title, the resolved base address, and one line per tool. Warnings do not block:
  • Operations without operationId — names are derived from method and path, so GET /v1/models-catalog becomes get_v1_models_catalog. Tool names are the first thing a model reads when deciding what to call; if you own the API, give every operation an operationId.
  • Operations skipped — file uploads, form posts, XML. The rest compiles.
  • More than 80 tools — long tool lists crowd out the model’s context. Restrict the set on the application.

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Authenticating to the API

This is how the gateway signs in to the API; how agents sign in to the gateway is set on the application and is unrelated. One service credential for every call. The API sees the gateway, not the person. If you need each user to authorise individually, that needs a real MCP server with forwarded OAuth — this integration cannot do it.

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What it does not do

One call is one request. If an operation paginates, page or cursor become tool arguments and the agent asks for the next page itself. The gateway never walks a collection. An agent that needs everything in one call needs an endpoint that returns everything. JSON only. Uploads, forms and XML are skipped. So are references to other files: publish the document as one file. Limits. 5 MB and 500 operations per document; 10 MB per tool response.