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TutorialSeries: OpenAPI to MCP

Build a Custom MCP Server from Any REST API (No Coding)

June 13, 2026 · 5 min read

The fastest way to get an MCP server running is to convert an existing OpenAPI spec. With MCP-Bridge, you can turn any REST API into a fully functional MCP server in under a minute — no code, no CLI, no setup.

What You'll Build

By the end of this tutorial, you'll have a working MCP server for any REST API of your choice, configured in Claude Desktop and ready to use through natural language.

Step 1: Find an OpenAPI Spec

Most popular APIs publish their OpenAPI specification publicly. Here are three we'll use in this tutorial:

  • Petstore — https://petstore3.swagger.io/api/v3/openapi.json
  • JSONPlaceholder — https://jsonplaceholder.typicode.com/openapi.json
  • GitHub API — already in our directory

If your target API doesn't publish an OpenAPI spec, check our directory of 1,000+ APIs — chances are it's already converted.

Step 2: Convert with MCP-Bridge

Visit the converter tool and paste your OpenAPI URL:

  1. Open mcpbridge.org/convert
  2. Enter the OpenAPI JSON URL in the input field
  3. Click "Convert"
  4. Copy the generated MCP config

The converter extracts every endpoint, parameter, and auth requirement from the spec and generates a ready-to-use MCP server configuration. It handles:

  • All HTTP methods (GET, POST, PUT, DELETE, PATCH)
  • Path and query parameters
  • Request bodies and content types
  • API key and bearer token authentication

Pro tip: The converter runs entirely in your browser. Your API spec never hits a server — great for internal or private APIs.

Step 3: Add to Claude Desktop

Open your claude_desktop_config.json and paste the generated config:

{
  "mcpServers": {
    "my-custom-api": {
      "command": "npx",
      "args": ["-y", "@mcp/my-custom-api"],
      "env": {
        "API_KEY": "your-key-here"
      }
    }
  }
}

Save the file and restart Claude Desktop. Your AI assistant now has access to the entire API.

Step 4: Test It

Try asking Claude something like:

  • "List all pets from the Petstore API"
  • "Find users with the name 'Leanne Graham' from JSONPlaceholder"
  • "Create a new repository on GitHub"

Troubleshooting

Config not showing up?

Make sure your claude_desktop_config.json is valid JSON. Use a JSON validator if you're unsure.

API key errors?

Some APIs require authentication. Check the API's documentation for how to get an API key and add it to the env section.

Spec not parsing?

Make sure the OpenAPI spec URL returns raw JSON, not an HTML page. Some APIs require authentication to access their spec.

Next Steps

Once you've mastered the basics, check out the rest of the OpenAPI to MCP series: