openapi-to-mcp

openapi-to-mcp

Use your OpenAPI specification to expose your API's endpoints as strongly typed tools.

Basic example for https://petstore3.swagger.io/ 🎉

{
  "mcpServers": {
    "petstore": {
      "command": "openapi-to-mcp",
        "args": [
          "https://petstore3.swagger.io/api/v3/openapi.json"
        ]
    }
  }
}

More complex example, using Github's API:

{
    "mcpServers": {
        "github": {
            "command": "openapi-to-mcp",
            "args": [
                "https://raw.githubusercontent.com/github/rest-api-description/refs/heads/main/descriptions/api.github.com/api.github.com.yaml",
                "--bearer-token",
                "github_pat_xxxxxx",
                "--tool-naming-strategy",
                "verbandpath"
            ]
        }
    }
}

This example use the bearer token auth (with a Github Personal Access Token) and force the tool naming strategy to "verb and path", as Github's operation ids are not valid tool names.

Install

As a Nuget tool: openapi-to-mcp

dotnet tool install --global openapi-to-mcp

Or download the executables from the releases

Usage

Usage:
  openapi-to-mcp <open-api> [options]

Arguments:
  <open-api>  You OpenAPI specification (URL or file) [required]

Options:
  -t, --tool-naming-strategy <extension|extension_or_operationid_or_verbandpath|operationid|verbandpath>  How the tool name should be computed [default: extension_or_operationid_or_verbandpath]
  -h, --host-override                                                                                     Host override
  -b, --bearer-token                                                                                      Bearer token
  -o2, --oauth-2-grant-type <client_credentials|password|refresh_token>                                   OAuth2 flow to be used
  -o2_tu, --oauth-2-token-url                                                                             OAuth2 token endpoint URL (override the one defined in your OpenAPI for your chosen OAuth2 flow)
  -o2_ci, --oauth-2-client-id                                                                             OAuth2 client id (for the client_credentials grant_type)
  -o2_cs, --oauth-2-client-secret                                                                         OAuth2 client secret (for the client_credentials grant_type)
  -o2_rt, --oauth-2-refresh-token                                                                         OAuth2 refresh token (for the refresh_token grant_type)
  -o2_un, --oauth-2-username                                                                              OAuth2 username (for the password grant_type)
  -o2_pw, --oauth-2-password                                                                              OAuth2 password (for the password grant_type)
  -i, --instructions                                                                                      MCP instruction to be advertised by the server
  --verbose                                                                                               Log more info (in sdterr) [default: False]
  -?, -h, --help                                                                                          Show help and usage information
  --version                                                                                               Show version information

OpenAPI support

• Currently, OpenAPI 2.0 and 3.0 are supported.
  • 3.1 is not (at least not until microsoft/OpenAPI.NET supports it)
• Specifications can be JSON/YAML and local (file) or remote (URL)
• Only local $refs are supported

OpenAPI custom extensions

A set of custom extensions is available to customize how your API should be exposed:

• info.x-mcp-instructions (string): Textual instructions exposed by the MCP server during the initialize handshake
• operation.x-mcp-tool-name (string): Custom tool name
• operation.x-mcp-tool-description (string): Custom tool description
• operation.x-mcp-tool-enabled (boolean): Enabled/disabled a specific operation (enabled by default)

MCP features

Only STDIO transport is currently supported.

Tools

Operations ("endpoints") from your OpenAPI specification are translated to MCP tools

• All path/query/JSON body parameters are exposed (using their JSON schema)
• Response is returned as-is
• By default, the tool name is computed using first the operation.x-mcp-tool-name extension, then the operation.operationId and then {httpMethod}_{escaped_path}
  • The tool naming strategy can be defined via the --tool-naming-strategy option.
  • ⚠️Tools are discarded if their name don't match ^[a-zA-Z0-9_-]{1,64}$
• Tools description are extracted as follows: operation.x-mcp-tool-description ?? operation.description ?? path.description

Tool call and host

When a tool is called, the MCP server will call the underlying endpoint. To determine which host to call a combination of parameters are used:

• the --host-override option
• your specification first server's URL if it's an absolute URL
• the host of the remote OpenAPI provided
• otherwise, an error is thrown

For example running openapi-to-mcp https://petstore3.swagger.io/api/v3/openapi.json:

• https://petstore3.swagger.io/api/v3/openapi.json defines a server, but its URL is relative (/api/v3)
• so the host of the specification's own URL is used: https://petstore3.swagger.io and the relative path of the server is appended to it

Authorization

Bearer token

A token can be provided as option --bearer-token. It'll be provided to all calls as the Authorization: Bearer {token} header. It'll also be provided when fetching a remote specification.

OAuth2

ClientCredentials, RefreshToken, Password are supported. If your OpenAPI specification declare securitySchemes for those flows, the corresponding tokenUrl will be used.

How to publish

Create a new tag/release 🤷