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OpenAPI to MCP CLI

The cargo-rustapi CLI includes rustapi mcp generate. It takes any OpenAPI 3.x spec (from FastAPI, Express, Go, Spring, etc.) and instantly spins up an MCP server. Tool calls are proxied to the real backend.

This is a major growth feature: non-Rust teams can get first-class agent tools with zero Rust code.

Installation

cargo install cargo-rustapi
# or from source
cargo install --path crates/cargo-rustapi

Basic Usage

# From a local spec file
rustapi mcp generate \
  --spec ./openapi.json \
  --target http://localhost:8000 \
  --port 9090 \
  --tags public,agent

# From a URL
rustapi mcp generate \
  --url https://api.example.com/openapi.json \
  --target https://api.example.com \
  --port 9090

# Point at a running service (auto-fetches /openapi.json)
rustapi mcp generate \
  --api http://localhost:8080 \
  --port 9090

# Best for RustAPI projects: no flags needed!
# Auto-generates the OpenAPI spec from your code (no running server or openapi.json required)
rustapi mcp generate --stdio

Flags

FlagDescription
--spec <FILE>Local JSON or YAML OpenAPI file
--url <URL>Fetch spec from HTTP URL
--api <BASE>Use <BASE>/openapi.json as spec source + default target
--target <URL>Backend that tool calls proxy to (required unless --api)
--port <N>MCP listen port (default 9090)
--name <NAME>Server name advertised to clients
--tags <TAGS>Comma-separated tags to expose
--allow-path-prefixOnly include paths starting with prefix
--stdioUse stdio transport instead of HTTP

Example Walkthrough with a Python FastAPI

  1. Start your FastAPI app on port 8000 (it serves OpenAPI at /openapi.json).

  2. In another terminal:

rustapi mcp generate --api http://localhost:8000 --port 9090 --tags public

  1. Test with curl or connect Claude Desktop / Cursor to http://localhost:9090.

  2. Agents see your FastAPI endpoints as tools. Calls go through the real backend (auth, validation, DB, etc.).

How It Works

  • Uses McpServer::from_spec(config, &parsed_openapi).
  • Reuses the same discovery and proxy logic as native RustAPI MCP.
  • No RustAPI server required on the target side.

Security Note

The generated MCP server is only as secure as the filters you apply (--tags, --allow-path-prefix). Never expose internal/admin routes to agents unless you intend to.