OpenAPI to MCP

Standalone proxy that turns any OpenAPI/Swagger-described HTTP API into an MCP (Model Context Protocol) server. It loads the spec at startup, filters operations by include/exclude, and registers one MCP tool per API operation. Tool calls are executed as HTTP requests to the backend API.

Useful when you already have (or want) a REST API with an OpenAPI/Swagger spec: the same spec drives both human-readable API docs and MCP tools for AI clients.

How it works

flowchart LR
  subgraph startup["Startup"]
    A[OpenAPI spec<br/>URL or file] --> B[Load and filter<br/>include or exclude]
    B --> C[N MCP tools<br/>one per operation]
  end

  subgraph runtime["Runtime"]
    D[MCP client] <-->|Streamable HTTP<br/>POST/GET /mcp| E[openapi-to-mcp]
    E <-->|HTTP| F[Backend API]
  end

  C -.->|registered in| E

Logging and Correlation IDs

The server includes comprehensive logging with correlation ID support for request tracking:

• Correlation ID: Extracted from X-Correlation-ID header (case-insensitive) or auto-generated for each request
• Log levels: DEBUG, INFO, WARN, ERROR (configurable via MCP_LOG_LEVEL env var, default: INFO)
• Log format: [correlation_id] LEVEL message with optional context data
• Request tracking: All logs include correlation ID for tracing requests through the system

For E2E testing, pass X-Correlation-ID header with your request to track it across all logs.

1. Load OpenAPI spec from MCP_OPENAPI_SPEC (URL starting with http:// or https://, or file path).
2. Collect operations (method + path). Filter: if MCP_INCLUDE_ENDPOINTS is set, keep only those; otherwise drop any in MCP_EXCLUDE_ENDPOINTS. Include has priority over exclude.
3. For each operation create an MCP tool: name = MCP_TOOL_PREFIX + path segment (e.g. api_ + messages = api_messages). Path parameters are included in the tool name (e.g. /channels/{username} becomes channels_username). If the same path segment is used by more than one method (e.g. GET and PUT on /pet/{id}), the tool name is made unique by appending the method (e.g. pet_id_get, pet_id_put). Input schema from parameters and requestBody (Zod), handler = HTTP call to MCP_API_BASE_URL.
4. Load MCP server instructions: by default uses info.description from OpenAPI spec. Optionally, load custom instructions from MCP_INSTRUCTIONS_FILE and combine with OpenAPI description according to MCP_INSTRUCTIONS_MODE (default/replace/append/prepend). If file loading fails, server logs a warning and continues with OpenAPI instructions only.

Transport: Streamable HTTP. Endpoint: POST /mcp and GET /mcp.

Environment variables (MCP_ prefix)

Environment variables are loaded from .env file in the project root (using dotenv). You can also set them directly in your shell environment. See .env.example for a template.

| Variable | Description | Default | |----------|-------------|---------| | MCP_API_BASE_URL | Base URL for API requests | http://127.0.0.1:3000 | | MCP_API_BASIC_AUTH | Basic auth for API requests: username:password. Use when the remote API is protected by HTTP Basic Auth. If both this and MCP_API_BEARER_TOKEN are set, Bearer is used. | - | | MCP_API_BEARER_TOKEN | Bearer token for API requests. Use when the remote API expects Authorization: Bearer <token>. Takes precedence over MCP_API_BASIC_AUTH when both set. | - | | MCP_OPENAPI_SPEC | OpenAPI spec source: URL (starts with http:// or https://) or file path (e.g. http://api:3000/openapi.json or ./openapi.json). Automatically detects URL vs file. | - | | MCP_INCLUDE_ENDPOINTS | Comma-separated method:path (e.g. get:/messages,get:/channels). If set, only these become tools. | - | | MCP_EXCLUDE_ENDPOINTS | Comma-separated method:path to exclude. Ignored for endpoints in include. | - | | MCP_TOOL_PREFIX | Prefix for tool names (e.g. api_ -> api_messages, api_channels) | (empty) | | MCP_SERVER_NAME | Server name reported to MCP clients | openapi-to-mcp | | MCP_PORT | Port for Streamable HTTP server | 3100 | | MCP_HOST | Bind host | 0.0.0.0 | | MCP_LOG_LEVEL | Log level: DEBUG, INFO, WARN, ERROR (case-insensitive) | INFO | | MCP_INSTRUCTIONS_FILE | Path to custom instructions file (text file with MCP server instructions) | - | | MCP_INSTRUCTIONS_MODE | How to combine custom instructions with OpenAPI spec description: default (use only OpenAPI description, ignore custom file), replace (use only custom file, ignore OpenAPI), append (OpenAPI + custom file), prepend (custom file + OpenAPI). Case-insensitive. | default | | MCP_CONVERT_HTML_TO_MARKDOWN | Convert HTML tags in operation descriptions to Markdown format. Set to false to disable. | true |

MCP_OPENAPI_SPEC must be set. If it starts with http:// or https://, it's treated as a URL; otherwise, it's treated as a file path.

Backward compatibility: MCP_OPENAPI_SPEC_URL and MCP_OPENAPI_SPEC_FILE are still supported but deprecated. MCP_OPENAPI_SPEC takes precedence if set.

Run with npm (local)

1. Copy .env.example to .env and set at least the OpenAPI spec source and API base URL:

   cp .env.example .env
   # Edit .env: MCP_OPENAPI_SPEC (URL or file path), MCP_API_BASE_URL

2. Install, build, and start:

   npm ci
   npm run build
   npm run start

3. The server listens on http://<MCP_HOST>:<MCP_PORT> (default http://0.0.0.0:3100). Connect MCP clients to POST/GET http://localhost:3100/mcp (Streamable HTTP).

Ensure the backend API is reachable at MCP_API_BASE_URL and that the OpenAPI spec URL (or file) returns a valid OpenAPI 3.x JSON.

Using MCP Inspector

To test the server with MCP Inspector:

1. Start the MCP server (see above).
2. Run MCP Inspector: npx @modelcontextprotocol/inspector
3. In the Inspector UI, select "streamable-http" transport type (not STDIO).
4. Enter the server URL: http://localhost:3100/mcp
5. Click "Connect".

The server includes CORS support for browser-based MCP clients and maintains sessions for Streamable HTTP transport.

Run with Docker

Image on Docker Hub: evilfreelancer/openapi-to-mcp. Use tag latest or a version tag (e.g. v1.0.0).

1. Pull and run with env vars (example: spec from URL, API at host):

   docker run --rm -p 3100:3100 \
     -e MCP_OPENAPI_SPEC=http://host.docker.internal:3000/openapi.json \
     -e MCP_API_BASE_URL=http://host.docker.internal:3000 \
     evilfreelancer/openapi-to-mcp:latest

   On Linux you may need --add-host=host.docker.internal:host-gateway or use the host network. Alternatively pass a file path and mount the spec:

   docker run --rm -p 3100:3100 \
     -v $(pwd)/openapi.json:/app/openapi.json:ro \
     -e MCP_OPENAPI_SPEC=/app/openapi.json \
     -e MCP_API_BASE_URL=http://host.docker.internal:3000 \
     evilfreelancer/openapi-to-mcp:latest

   To build the image locally instead: docker build -t openapi-to-mcp . and use openapi-to-mcp as the image name in the commands above.

Run with Docker Compose

A minimal docker-compose.yaml is included so you can run the MCP server and optionally point it at an existing API. It uses the image from Docker Hub (evilfreelancer/openapi-to-mcp).

1. Copy .env.example to .env and set:

   • MCP_OPENAPI_SPEC (URL like http://api:3000/openapi.jsonor file path like./openapi.json`)
   • MCP_API_BASE_URL (e.g. http://api:3000 if the API runs in another container)

2. From the project root:

   docker compose up -d

3. The MCP server will be available at http://localhost:3100/mcp (Streamable HTTP).

To use a local OpenAPI file instead of a URL, set MCP_OPENAPI_SPEC to the file path and mount the file into the container (see docker-compose.yaml comments if present).

Tests

npm test

Tests cover: config (env vars, include/exclude, defaults), OpenAPI loader (URL and file detection, error when unset), instructions loader (file loading and combination modes), and openapi-to-tools (filtering, prefix, handler calling API with success and error). HTTP is mocked (axios-mock-adapter).

Dockerfile

The project includes a Dockerfile (Node 20 Alpine): install deps, build TypeScript, production prune, run node dist/index.js. No dev dependencies or tests in the image. Pre-built images are published to Docker Hub. To build locally:

docker build -t openapi-to-mcp .

CI - Docker image on Docker Hub

A GitHub Actions workflow (.github/workflows/docker-publish.yml) runs tests, then builds the image and pushes it to Docker Hub.

• Triggers: manually (Actions → "Docker build and push" → Run workflow) or on push of any git tag.
• Version: on tag push the image tag equals the git tag (e.g. v1.0.0); on manual run you can set a version (default latest).
• Main only: when triggered by a tag, the workflow checks that the tag points to a commit on main; otherwise the run fails.

Required repository secrets (Settings → Secrets and variables → Actions):

| Secret | Description | |--------|-------------| | DOCKERHUB_USERNAME | Docker Hub username (image will be DOCKERHUB_USERNAME/openapi-to-mcp) | | DOCKERHUB_TOKEN | Docker Hub access token (recommended) or password |

Similar projects

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