# 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
```mermaid
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_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:
```bash
cp .env.example .env
# Edit .env: MCP_OPENAPI_SPEC (URL or file path), MCP_API_BASE_URL
```
2. Install, build, and start:
```bash
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](https://modelcontextprotocol.io/docs/tools/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](https://hub.docker.com/r/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):
```bash
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:
```bash
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](https://hub.docker.com/r/evilfreelancer/openapi-to-mcp)).
1. Copy `.env.example` to `.env` and set:
- `MCP_OPENAPI_SPEC (URL like `http://api:3000/openapi.json` or 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:
```bash
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
```bash
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](https://hub.docker.com/r/evilfreelancer/openapi-to-mcp). To build locally:
```bash
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
- **[mcp-openapi-proxy](https://github.com/matthewhand/mcp-openapi-proxy)** (Python) – MCP server that exposes REST APIs from OpenAPI specs as MCP tools. Low-level mode (one tool per endpoint) or FastMCP mode. Auth and endpoint filtering. Install: `uvx mcp-openapi-proxy`.
- **[openapi-mcp-proxy](https://github.com/JacerOmri/openapi-mcp-proxy)** (TypeScript) – CLI that turns an OpenAPI service into an MCP server; middleware between OpenAPI and MCP clients.
- **[openapi-mcp-generator](https://github.com/harsha-iiiv/openapi-mcp-generator)** (TypeScript) – Generates a full MCP server project from OpenAPI 3.0+ (stdio, SSE, Streamable HTTP), with Zod validation and auth. Install: `npm install -g openapi-mcp-generator`.
- **[FastMCP + OpenAPI](https://gofastmcp.com/integrations/openapi)** (Python) – OpenAPI integration for FastMCP: auth, route mapping, parameter handling.
- **[openapi-mcp-codegen](https://github.com/cnoe-io/openapi-mcp-codegen)** – Code generator from OpenAPI to MCP server (Apache 2.0).
- **[Swagger MCP](https://mcpmarket.com/server/swagger-4)** (Vizioz) – AI-driven MCP server generation from Swagger/OpenAPI; stores specs locally.
- **[liblab](https://liblab.com/docs/mcp/howto-generate-mcp-with-liblab)** – Cloud service: generate and deploy MCP server from OpenAPI or Postman collection.
