mcp-stdio 0.13.0

Stdio-to-HTTP gateway — connects MCP clients to remote HTTP MCP servers

mcp-stdio

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Stdio-to-HTTP gateway — connects MCP clients to remote HTTP MCP servers.

Overview

MCP clients like Claude Desktop and Claude Code see mcp-stdio as a locally running self-hosted MCP server, while it relays all requests to a remote MCP server with support for various authentication methods:

flowchart BT
    A[Claude<br>Desktop/Code] <-- stdio --> B(mcp-stdio)
    B <== "<b>HTTPS</b><br>Streamable HTTP / SSE<br>Bearer Token<br>Header<br>OAuth" ==> C[Remote<br>MCP Server]
    B -. "OAuth 2.1<br>(PKCE)" .-> D[Authorization<br>Server]
    D -. callback .-> B
    style B fill:#4a5,stroke:#333,color:#fff

Bearer tokens, custom headers, and OAuth 2.1 credentials are forwarded to the remote server.

Features

• Both MCP transports supported — Streamable HTTP (current spec, default) and SSE (MCP 2024-11-05 legacy), selectable with --transport. SSE parser follows the WHATWG Server-Sent Events spec.
• OAuth 2.1 client — built-in authorization code flow with PKCE, dynamic client registration, token refresh, and secure token persistence. Implements the full MCP authorization spec at the section level:
  • RFC 9728 Protected Resource Metadata
    • §3 discovery of authorization servers via /.well-known/oauth-protected-resource
    • §3.1 path-aware well-known URL construction for path-based reverse-proxy deployments, with host-root fallback; preserves the resource URL's query component on the constructed metadata URL
    • §3.3 resource field validation — warn on mismatch, continue
    • §5.1 WWW-Authenticate: Bearer resource_metadata= hint — probes the server before discovery so servers that publish PRM at a non-standard URL are found without well-known path guessing
  • RFC 8414 Authorization Server Metadata
    • §3 well-known URL construction, including path insertion for issuers with path components
    • §3 issuer validation — warn on mismatch, continue
  • RFC 8707 Resource Indicators
    • §2 resource parameter in authorization, token exchange, and refresh requests
  • RFC 7636 PKCE
    • §4.1–4.2 S256 code_challenge_method with a 96-char code_verifier
  • RFC 8628 Device Authorization Grant
    • §3.1 device authorization request with resource indicator (RFC 8707)
    • §3.4–3.5 token polling with authorization_pending / slow_down (interval +=5 s) / expired_token / access_denied handling
    • DCR registers urn:ietf:params:oauth:grant-type:device_code in grant_types (RFC 7591 §2)
  • RFC 7591 Dynamic Client Registration
    • §3 client registration request; token_endpoint_auth_method chosen from token_endpoint_auth_methods_supported in AS metadata (prefers none → client_secret_post → client_secret_basic)
    • §3.2.1 client_secret_expires_at handling — auto re-register on expiry
  • RFC 6749 OAuth 2.0
    • §2.3.1 client_secret_basic: Authorization: Basic header with percent-encoded credentials (applied to code exchange, token refresh, and Device Authorization Grant polling)
  • RFC 6750 Bearer Token usage
    • §2.1 Authorization: Bearer <token> request header
• Retry with backoff — retries up to 3 times on connection errors
• Streaming resilience — streams SSE responses in real time; auto-reconnects on mid-stream disconnect
• Session recovery — resets MCP session ID on 404 and retries
• Token refresh on 401 — automatically refreshes expired OAuth tokens mid-session
• Bearer token auth — via --bearer-token flag or MCP_BEARER_TOKEN env var
• Custom headers — pass any header with -H / --header
• Graceful shutdown — handles SIGTERM/SIGINT
• Proxy support — respects HTTP_PROXY, HTTPS_PROXY, NO_PROXY env vars via httpx
• Minimal dependencies — only httpx; OAuth uses stdlib only

Install

pip install mcp-stdio

Or with uv:

uv tool install mcp-stdio

Or run directly without installing:

uvx mcp-stdio https://your-server.example.com:8080/mcp

Or with Homebrew:

brew install shigechika/tap/mcp-stdio

Quick Start

mcp-stdio https://your-server.example.com:8080/mcp

With Bearer token authentication:

# Recommended: use env var (token is hidden from `ps`)
MCP_BEARER_TOKEN=YOUR_TOKEN mcp-stdio https://your-server.example.com:8080/mcp

# Or pass directly (token is visible in `ps` output)
mcp-stdio https://your-server.example.com:8080/mcp --bearer-token YOUR_TOKEN

With custom headers:

mcp-stdio https://your-server.example.com:8080/mcp --header "X-API-Key: YOUR_KEY"

With OAuth 2.1 authentication (for servers that require it):

mcp-stdio --oauth https://your-server.example.com:8080/mcp

# With a pre-registered client ID (skips dynamic registration)
mcp-stdio --oauth --client-id YOUR_CLIENT_ID https://your-server.example.com:8080/mcp

With OAuth 2.1 Device Authorization Grant (RFC 8628, for headless/SSH environments):

mcp-stdio --oauth-device https://your-server.example.com:8080/mcp

For legacy MCP servers using the 2024-11-05 SSE transport:

mcp-stdio --transport sse https://your-server.example.com:8080/sse

Check connectivity before use:

mcp-stdio --check https://your-server.example.com:8080/mcp

Claude Desktop Configuration

Add to claude_desktop_config.json:

{
  "mcpServers": {
    "my-remote-server": {
      "command": "mcp-stdio",
      "args": ["https://your-server.example.com:8080/mcp"],
      "env": {
        "MCP_BEARER_TOKEN": "YOUR_TOKEN"
      }
    }
  }
}

Config file locations:

• macOS: ~/Library/Application Support/Claude/claude_desktop_config.json
• Windows: %APPDATA%\Claude\claude_desktop_config.json
• Linux: ~/.config/Claude/claude_desktop_config.json

Claude Code Configuration

claude mcp add my-remote-server \
  -e MCP_BEARER_TOKEN=YOUR_TOKEN \
  -- mcp-stdio https://your-server.example.com:8080/mcp

Usage

mcp-stdio [OPTIONS] URL

Arguments:
  URL                    Remote MCP server URL

Options:
  --bearer-token TOKEN   Bearer token (or set MCP_BEARER_TOKEN env var)
  --oauth                Enable OAuth 2.1 authentication (browser flow)
  --oauth-device         Enable OAuth 2.1 Device Authorization Grant (RFC 8628, headless)
  --client-id ID         Pre-registered OAuth client ID (or set MCP_OAUTH_CLIENT_ID)
  --oauth-scope SCOPE    OAuth scope to request
  --oauth-refresh-leeway SECONDS
                         Proactively refresh tokens this many seconds before
                         expiry (default: 60, or MCP_OAUTH_REFRESH_LEEWAY)
  --no-resource-indicator
                         Omit the RFC 8707 resource parameter from all OAuth
                         requests. Required for AS that reject it, such as
                         Microsoft Entra ID v2 with api:// scopes (AADSTS9010010)
  -H, --header 'Key: Value'  Custom header (can be repeated)
  --transport {streamable-http,sse}
                         Transport type (default: streamable-http)
  --timeout-connect SEC  Connection timeout (default: 10)
  --timeout-read SEC     Read timeout (default: 120)
  --check                Check connection and exit
  -V, --version          Show version
  -h, --help             Show help

Workarounds

See WORKAROUNDS.md for known issues in Claude Code, mcp-remote, and Windows that mcp-stdio addresses.

How It Works

1. If --oauth is set, obtains an access token (cached → refresh → browser flow)
2. Reads JSON-RPC messages from stdin (sent by Claude Desktop/Code)
3. Relays them over HTTPS to the remote MCP server
4. Parses responses and writes them to stdout
5. On 401, refreshes the OAuth token and retries

Transport details:

• Streamable HTTP (default) — each stdin message is a single POST; session state is tracked via the Mcp-Session-Id header and re-initialized automatically on 404.
• SSE (MCP 2024-11-05 legacy) — a persistent GET stream delivers responses and the initial endpoint event containing the POST URL; the stream auto-reconnects on disconnect.

OAuth tokens are stored in ~/.config/mcp-stdio/tokens.json (permissions 0600).

License

MIT