openapi-contract-mcp

An MCP server that lets a coding agent check requests against a live OpenAPI contract instead of guessing.

Five read-only tools: find an endpoint by docs anchor or path, read what it accepts and returns, and diff a real request body against the schema. Tested with unit tests, protocol-level tests, and agent-level evals that measure whether a model actually picks the right tool.

Why

On a multi-tenant CRM I work on, a large share of "frontend bugs" turned out to be contract bugs, and the arguments about them went in circles because both sides reasoned from memory. Two things kept wasting time:

• The docs anchor is not the path. A colleague links #/accounts/change-email, you search the code for change-email and find nothing, because the operationId is change-email and the path is /auth/set_email/.

• Copying fields from a GET response into a PUT body. id and created_at come back from the server but must not be sent; firstName vs first_name gets silently dropped by the backend.

The workflow started as a Claude Code skill I use on that project, with a small Python script. This server is the same idea as a proper tool any MCP client can use, with the answers computed rather than eyeballed.

Related MCP server: OpenAPI MCP Server

Tools

Example, against the public Petstore schema:

// diff_payload { method: "POST", path: "/pet", payload: { "name": "Rex", "photoUrl": "x", "status": "sleeping" } }
{
  "operation": "POST /pet",
  "matchesContract": false,
  "issues": [
    { "path": "$.photoUrls", "kind": "missing", "message": "is required but missing" },
    { "path": "$.photoUrl",  "kind": "unknown", "message": "is not in the schema, did you mean \"photoUrls\"?" },
    { "path": "$.status",    "kind": "enum",    "message": "is \"sleeping\", allowed: \"available\", \"pending\", \"sold\"" }
  ]
}

A wrong path returns an error the model can recover from, not a dead end:

No operation DELETE /pets in the schema. Did you mean:
- DELETE /pet/{petId}
- ...

Setup

Requires Node 20+. The schema must be OpenAPI 3.x as JSON, from a URL or a local file.

git clone https://github.com/Arag0rn/openapi-contract-mcp && cd openapi-contract-mcp
npm ci && npm run build

Claude Code

claude mcp add openapi-contract -- node /absolute/path/to/openapi-contract-mcp/dist/index.js https://petstore3.swagger.io/api/v3/openapi.json

Cursor / any client with a JSON config

{
  "mcpServers": {
    "openapi-contract": {
      "command": "node",
      "args": ["/absolute/path/to/openapi-contract-mcp/dist/index.js", "https://your-api/schema/?format=json"]
    }
  }
}

Design decisions

• Read-only by construction. The only network request the server makes is a GET for the schema itself, with a 15 s timeout. No tool calls the API the schema describes. Every tool carries readOnlyHint: true, destructiveHint: false, and the server sends instructions on connect that say so.

• Short cache, explicit refresh. A stale schema looks authoritative, which makes it worse than none. Concurrent tool calls share one download.

• Errors the model can act on. Misses return isError: true with suggestions: same path with other methods, then operations with the requested method, singular and plural path forms tried.

• Context budget. Output is capped at 20 000 characters with a hint on how to narrow the request. diff_payload returns only the problems, not the schema.

• Logic separate from protocol. spec.ts, operations.ts and diff.ts know nothing about MCP and are tested directly; server.ts only wires them to tools.

• Not a full JSON Schema validator. diff_payload covers what breaks requests in practice. allOf is merged, oneOf/anyOf pass if any variant matches (otherwise the closest variant is reported). Formats, min/max, patterns and remote $refs are not checked. Swagger 2.0 and YAML are not supported.

Tests

npm test          # 36 unit and protocol tests (vitest)
npm run smoke     # starts dist/index.js over stdio and calls every tool against a real schema

test/server.test.ts talks to the server through the MCP protocol in memory, so it checks what an agent actually sees: tool list, annotations, argument validation, recoverable errors.

Evals

Unit tests show the code is right. They do not show that a model picks the right tool, because that depends on tool names, descriptions and server instructions. evals/ measures exactly that.

• Nine scenarios in evals/cases.ts, each a question a developer would ask: resolving a docs anchor, debugging a 400, a oneOf payload, a wrong path, a required header, refreshing after a deploy, and a request to delete a user (guardrail).

• Each run starts Claude Code headless with only this server: built-in tools disabled, user settings, CLAUDE.md and other MCP servers ignored, no hints in the prompt.

• Each scenario is graded on the trajectory (the expected tools called in order, with the expected arguments) and the answer (required facts present, forbidden claims absent).

npm run eval -- --model haiku --repeat 3      # uses the logged-in Claude Code account, no API key

Results with Claude Haiku 4.5:

What the evals changed:

1. Asked to debug a 400 with a concrete payload, the agent read the schema and compared by hand instead of calling diff_payload. The answers were right on small bodies, but that does not scale. Fix: routing sentences in both descriptions ("if you already have a concrete payload, call diff_payload").

2. Asked to delete a user, the agent offered to "construct and send the DELETE", although the server cannot reach the API and the schema has no such operation. Fix: server instructions stating the tools are read-only and telling the agent to check whether an operation is documented at all.

3. The agent quoted /users/{id} for /users/{id}/. For Django-style backends that is a redirect or a 404. Fix: get_request_schema adds a note when a path ends with a slash. The eval stays strict on this.

The remaining failure is real: in one of three runs the agent refused the delete correctly but did not check the schema and mentioned a DELETE /users/{id} that does not exist.

Two grader bugs were fixed along the way, both where the agent had behaved correctly and a regex did not recognise the wording. Regex graders are brittle for meaning; an LLM judge would be the next step. The rule I followed: a grader may only be loosened when the transcript shows the agent was right.

Raw transcripts of every run are in evals/results/.

Project layout

src/
  spec.ts         loading, caching, $ref resolution
  operations.ts   endpoint search, request and response contracts, suggestions
  diff.ts         payload vs schema
  server.ts       MCP tools and server instructions
  index.ts        stdio entry point
test/             vitest, fixture schema with the tricky cases
evals/            agent-level scenarios and runner
scripts/smoke.ts  stdio smoke test

License

MIT