Skills

mcp-openapi-proxy

Installation
SKILL.md

mcp-openapi-proxy

Turns any OpenAPI 3.x spec into a lightweight MCP stdio navigator/executor. The server always exposes 3 MCP tools and uses toolName identifiers to refer to individual endpoints.

For exhaustive reference, see the repository README.md. This skill is the short operational guide for installing the binary, wiring it into an MCP client, and using the current tool contract correctly.

Always discover the actual registered tools first; never assume semantic tool names.

Install

go install github.com/rendis/mcp-openapi-proxy/cmd/mcp-openapi-proxy@latest

Quick Start

MCP_SPEC=./openapi.yaml \
MCP_BASE_URL=https://api.example.com \
MCP_AUTH_TOKEN=your-token \
mcp-openapi-proxy

MCP_BASE_URL is optional when the OpenAPI spec resolves to a single absolute server.

Recommended setup flow:

  1. Install mcp-openapi-proxy
  2. Configure your MCP client:
    • Claude Code: create .mcp.json from .mcp.json.example
    • Codex global: run codex mcp add ...
    • Codex project-local: create ./.codex/config.toml manually
  3. Choose auth:
    • Static token: add MCP_AUTH_TOKEN
    • OIDC: add MCP_OIDC_ISSUER and MCP_OIDC_CLIENT_ID, then run one of:
      • mcp-openapi-proxy login
      • mcp-openapi-proxy login <mcp_name>
      • mcp-openapi-proxy login --mcp-config ./path/to/.mcp.json
      • mcp-openapi-proxy login --mcp-config ./path/to/.mcp.json --server <mcp_name>
      • mcp-openapi-proxy login --codex-server <name>
      • mcp-openapi-proxy login --codex-config ~/.codex/config.toml
      • mcp-openapi-proxy login --codex-config ~/.codex/config.toml --server <name>
  4. Start the MCP client
  5. Use list_endpoints, describe_endpoint, and call_endpoint

When login is invoked with .mcp.json or Codex config support, it reads the selected server’s env from that config entry. Shell env still overrides the config file when both are present.

Plain mcp-openapi-proxy login stays env-first. When shell env is not enough, it falls back to:

  1. ./.mcp.json
  2. ~/.codex/config.toml (or $CODEX_HOME/config.toml)

If the server name is omitted, login discovers eligible servers from the selected config file. It only considers direct command values that point to the mcp-openapi-proxy binary (plain name, full path, or .exe path). Wrapper commands such as go, env, docker, or shell scripts are ignored. If more than one eligible server exists, login prints the options and prompts you to choose one.

If the source spec comes from swag init, convert it before using the proxy. swag init emits Swagger 2.0, while mcp-openapi-proxy expects OpenAPI 3.x:

swag init -g cmd/api/main.go
swagger2openapi ./docs/swagger.json -o ./docs/openapi.json
MCP_SPEC=./docs/openapi.json mcp-openapi-proxy

Configuration

Variable Required Default Description
MCP_SPEC Yes — Path or URL to OpenAPI 3.x spec (YAML/JSON)
MCP_BASE_URL No — Explicit API base URL. If omitted, the proxy uses a single absolute OpenAPI server when available
MCP_TOOL_PREFIX No api Prefix for the 3 MCP tools and endpoint toolName identifiers
MCP_AUTH_PROFILE No MCP_TOOL_PREFIX or default Namespace for stored OIDC tokens
MCP_AUTH_TOKEN No — Global bearer token fallback
MCP_OIDC_ISSUER No — OIDC issuer URL
MCP_OIDC_CLIENT_ID No — OIDC client ID
MCP_OIDC_SCOPES No Spec scopes or openid profile email offline_access Override OIDC login scopes
MCP_EXTRA_HEADERS No — Comma-separated key:value pairs for every request
MCP_MAX_BODY_BYTES No 10485760 Maximum response body size to buffer and return
MCP_ALLOW_INSECURE_HTTP No 0 Allow sending credentials over non-loopback http:// URLs
MCP_EXCLUDE_DEPRECATED No 0 Skip deprecated endpoints when generating tools

MCP Client Setup

Claude Code (.mcp.json)

Generic example:

{
  "mcpServers": {
    "my-api": {
      "command": "mcp-openapi-proxy",
      "env": {
        "MCP_SPEC": "./openapi.yaml",
        "MCP_BASE_URL": "https://api.example.com",
        "MCP_TOOL_PREFIX": "myapi",
        "MCP_AUTH_PROFILE": "myapi"
      }
    }
  }
}

Static token variant:

{
  "mcpServers": {
    "my-api": {
      "command": "mcp-openapi-proxy",
      "env": {
        "MCP_SPEC": "./openapi.yaml",
        "MCP_BASE_URL": "https://api.example.com",
        "MCP_TOOL_PREFIX": "myapi",
        "MCP_AUTH_PROFILE": "myapi",
        "MCP_AUTH_TOKEN": "your-token"
      }
    }
  }
}

OIDC variant:

{
  "mcpServers": {
    "my-api": {
      "command": "mcp-openapi-proxy",
      "env": {
        "MCP_SPEC": "./openapi.yaml",
        "MCP_BASE_URL": "https://api.example.com",
        "MCP_TOOL_PREFIX": "myapi",
        "MCP_AUTH_PROFILE": "myapi",
        "MCP_OIDC_ISSUER": "https://auth.example.com/realms/myrealm",
        "MCP_OIDC_CLIENT_ID": "my-client"
      }
    }
  }
}

OIDC login from .mcp.json:

mcp-openapi-proxy login

Or select the server explicitly:

mcp-openapi-proxy login my-api

Or with an explicit config path:

mcp-openapi-proxy login --mcp-config ./path/to/.mcp.json

Or with an explicit config path plus explicit server:

mcp-openapi-proxy login --mcp-config ./path/to/.mcp.json --server my-api

OpenAI Codex (.codex/config.toml)

Global install via Codex CLI:

codex mcp add my-api \
  --env MCP_SPEC=./openapi.yaml \
  --env MCP_BASE_URL=https://api.example.com \
  --env MCP_TOOL_PREFIX=myapi \
  --env MCP_AUTH_TOKEN=your-token \
  -- mcp-openapi-proxy

OIDC variant via Codex CLI:

codex mcp add my-api \
  --env MCP_SPEC=./openapi.yaml \
  --env MCP_BASE_URL=https://api.example.com \
  --env MCP_TOOL_PREFIX=myapi \
  --env MCP_AUTH_PROFILE=myapi \
  --env MCP_OIDC_ISSUER=https://auth.example.com/realms/myrealm \
  --env MCP_OIDC_CLIENT_ID=my-client \
  -- mcp-openapi-proxy

Global Codex installs go into ~/.codex/config.toml.

Project-local Codex config is manual because the visible codex mcp add help does not expose a project-scoped install mode. Use ./.codex/config.toml when you want repo-local configuration:

[mcp_servers.my-api]
command = "mcp-openapi-proxy"

[mcp_servers.my-api.env]
MCP_SPEC = "./openapi.yaml"
MCP_BASE_URL = "https://api.example.com"
MCP_TOOL_PREFIX = "myapi"
MCP_AUTH_TOKEN = "your-token"

For this stdio server, use mcp-openapi-proxy login, not codex mcp login:

mcp-openapi-proxy login --codex-server my-api
mcp-openapi-proxy login --codex-config ~/.codex/config.toml
mcp-openapi-proxy login --codex-config ~/.codex/config.toml --server my-api
mcp-openapi-proxy login --codex-config ./.codex/config.toml --server my-api

codex mcp login is for OAuth that Codex manages itself. Here the proxy owns OIDC discovery, PKCE, and token storage.

If local ./.codex/config.toml is ignored, switch to the global ~/.codex/config.toml path or use a trusted project in Codex.

Gemini CLI (~/.gemini/settings.json)

{
  "mcpServers": {
    "my-api": {
      "command": "mcp-openapi-proxy",
      "env": {
        "MCP_SPEC": "./openapi.yaml",
        "MCP_BASE_URL": "https://api.example.com",
        "MCP_TOOL_PREFIX": "myapi",
        "MCP_AUTH_TOKEN": "your-token"
      }
    }
  }
}

Authentication

Credential resolution order is:

  1. MCP_AUTH_<SCHEME>_*
  2. global MCP_AUTH_TOKEN
  3. OIDC token cache for MCP_AUTH_PROFILE

Static Token (dev/CI)

Set MCP_AUTH_TOKEN to provide a global bearer fallback for bearer-compatible schemes.

For Claude Code, the usual place is .mcp.json -> mcpServers.<name>.env.MCP_AUTH_TOKEN.

Per-Scheme Credentials

Use the OpenAPI security scheme name, uppercased and sanitized by replacing ., -, /, and spaces with underscores.

  • http bearer, oauth2, openIdConnect → MCP_AUTH_<SCHEME>_TOKEN
  • http basic → MCP_AUTH_<SCHEME>_USERNAME and MCP_AUTH_<SCHEME>_PASSWORD
  • apiKey → MCP_AUTH_<SCHEME>_KEY

Example:

MCP_AUTH_PARTNER_AUTH_V2_TOKEN=secret-token
MCP_AUTH_ADMIN_BASIC_USERNAME=alice
MCP_AUTH_ADMIN_BASIC_PASSWORD=s3cr3t
MCP_AUTH_X_API_KEY_KEY=dev-key

OIDC PKCE (production)

Standard OIDC discovery (recommended):

MCP_OIDC_ISSUER=https://auth.example.com/realms/myrealm \
MCP_OIDC_CLIENT_ID=my-client \
mcp-openapi-proxy login

If your MCP client uses .mcp.json or Codex TOML, these forms now read the selected server’s env automatically:

  • mcp-openapi-proxy login
  • mcp-openapi-proxy login my-api
  • mcp-openapi-proxy login --mcp-config ./path/to/.mcp.json
  • mcp-openapi-proxy login --mcp-config ./path/to/.mcp.json --server my-api
  • mcp-openapi-proxy login --codex-server my-api
  • mcp-openapi-proxy login --codex-config ~/.codex/config.toml
  • mcp-openapi-proxy login --codex-config ~/.codex/config.toml --server my-api

Keep the same MCP_AUTH_PROFILE, MCP_OIDC_ISSUER, and MCP_OIDC_CLIENT_ID values there so the running server uses the token cache created by login.

If the server name is omitted, login only considers direct mcp-openapi-proxy commands from the selected config file. Wrapper commands such as go, env, docker, or shell scripts are not eligible for login discovery. When multiple eligible entries exist, login lists them and prompts you to choose one.

If you also set those values in the shell when invoking login, the shell values win.

Application-specific discovery:

MCP_BASE_URL=https://api.example.com mcp-openapi-proxy login

Commands:

  • mcp-openapi-proxy login
  • mcp-openapi-proxy status
  • mcp-openapi-proxy logout

Scopes come from MCP_OIDC_SCOPES when set. Otherwise, if MCP_SPEC is available, the proxy unions scopes from the OpenAPI security requirements. If neither source is available, it falls back to openid profile email offline_access.

Tokens are stored at ~/.mcp-openapi-proxy/<profile>-tokens.json, where <profile> comes from MCP_AUTH_PROFILE or falls back to the tool prefix / default.

Registered Tools and Endpoint IDs

The MCP server always exposes exactly these tools:

  • {prefix}_list_endpoints
  • {prefix}_describe_endpoint
  • {prefix}_call_endpoint

Always inspect the registered tools first. Do not assume the server exposes one MCP tool per endpoint anymore.

Each OpenAPI endpoint still has a stable toolName:

{prefix}_{method}_{sanitized_path}

That toolName is the identifier you pass into describe_endpoint and call_endpoint.

Feature Flags API example with MCP_TOOL_PREFIX=fe:

Operation Endpoint toolName
GET /admin/features fe_get_admin_features
POST /admin/features fe_post_admin_features
GET /admin/features/{key} fe_get_admin_features_key
PATCH /admin/features/{key}/toggle fe_patch_admin_features_key_toggle
GET /admin/workspaces fe_get_admin_workspaces

Names like fe_list_features or fe_toggle_feature are not part of the product contract.

Input and Output Contract

Recommended agent flow:

  1. Call {prefix}_list_endpoints
  2. Pick a toolName
  3. Call {prefix}_describe_endpoint if you need the exact contract
  4. Call {prefix}_call_endpoint

list_endpoints

Input:

{
  "q": "feature",
  "path_prefix": "/admin",
  "method": "PATCH",
  "auth": "bearer",
  "limit": 25
}

Output:

{
  "items": [
    {
      "toolName": "fe_patch_admin_features_key_toggle",
      "method": "PATCH",
      "path": "/admin/features/{key}/toggle",
      "description": "Toggle a feature flag",
      "requiredAuth": "bearer",
      "tags": ["flags", "admin"],
      "deprecated": false
    }
  ],
  "nextCursor": ""
}

describe_endpoint

Input:

{
  "toolName": "fe_patch_admin_features_key_toggle"
}

Output includes:

  • toolName, method, path, summary, description, deprecated
  • requiredAuth and full securityRequirements
  • parameters.path/query/headers/cookies
  • requestBody with media types, schema, examples, and encoding
  • responses with status, description, headers, and body schemas
  • externalDocs, servers, baseURLHint

call_endpoint

Input:

{
  "toolName": "fe_patch_admin_features_key_toggle",
  "path": {
    "key": "checkout"
  },
  "headers": {
    "X-Workspace": "acme"
  },
  "body": {
    "enabled": true
  }
}

X-Workspace is sent in headers per request or configured globally with MCP_EXTRA_HEADERS. Workspace selection is header-based, not a dedicated tool.

call_endpoint returns the HTTP envelope:

{
  "status": 200,
  "content_type": "application/json",
  "headers": {
    "X-Trace": ["abc123"]
  },
  "body": {
    "id": "item-1"
  }
}
  • API 4xx/5xx responses preserve the real HTTP response and set IsError=true
  • Proxy/runtime failures return status: 0 plus proxy_error
  • Binary payloads are represented as base64 wrappers
  • Deprecated endpoints are listed by default and only excluded with MCP_EXCLUDE_DEPRECATED=1

Multiple APIs

Use distinct prefixes to run side-by-side:

{
  "mcpServers": {
    "users-api": { "command": "mcp-openapi-proxy", "env": { "MCP_TOOL_PREFIX": "users", "..." : "..." } },
    "billing-api": { "command": "mcp-openapi-proxy", "env": { "MCP_TOOL_PREFIX": "billing", "..." : "..." } }
  }
}

Common Mistakes

  • Missing MCP_SPEC → startup fails immediately
  • No usable base URL → set MCP_BASE_URL explicitly or declare a single absolute OpenAPI server
  • Spec generated by swag init → detect swagger: "2.0" or a generated swagger.json, convert it with swagger2openapi, then point MCP_SPEC at the resulting OpenAPI 3.x file
  • Missing per-scheme auth → configure MCP_AUTH_<SCHEME>_*, MCP_AUTH_TOKEN, or run mcp-openapi-proxy login
  • Spec URL unreachable → fails at startup; check network/VPN
  • Authenticated calls to non-loopback http:// → blocked unless MCP_ALLOW_INSECURE_HTTP=1
  • Treating endpoints as MCP tools → endpoints are discovered as toolName values; only list_endpoints, describe_endpoint, and call_endpoint are registered
  • Calling call_endpoint without discovery → list first, then describe if needed, then call with toolName

For exhaustive reference, examples, and troubleshooting, see the repository README.md.

Installs
12
Repository
rendis/mcp-openapi-proxy
GitHub Stars
1
First Seen
Mar 20, 2026
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