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
    • §3.1 well-known URL construction, including path insertion for issuers with path components
    • §3.3 issuer validation — reject a cross-origin issuer (AS mix-up guard), warn on a same-origin mismatch (trailing slash / path / case) and 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 an 86-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
• HTTP 429 / 503 handling — honours Retry-After (delta-seconds or HTTP-date) up to a 60-second cap on both 429 (Too Many Requests) and 503 (Service Unavailable) — the two spec-sanctioned Retry-After carriers (RFC 9110 §10.2.3) — then surfaces the status so the client can decide (cf. modelcontextprotocol/typescript-sdk#1892)
• Auto-pagination (Streamable HTTP transport) — transparently follows nextCursor for tools/list / resources/list / resources/templates/list / prompts/list and merges the pages into one response, so clients that drop pages beyond the first still see the full list (cf. anthropics/claude-code#39586)
• Streaming resilience — streams SSE responses in real time; auto-reconnects on mid-stream disconnect
• Line-separator safety — escapes raw U+2028 / U+2029 (legal in JSON, but JavaScript line terminators) in upstream responses so clients that treat them as line breaks cannot mis-frame the output; lossless (cf. modelcontextprotocol/typescript-sdk#2155)
• Argument normalization — rewrites a tools/call request whose arguments is null to {} so strict servers that reject the null form accept the call; on by default, opt out with --no-normalize-arguments (cf. modelcontextprotocol/typescript-sdk#2012)
• Cancellation-aware filtering — tracks request ids cancelled via notifications/cancelled on stdin and drops any late upstream response carrying one of those ids before it reaches the client, per the MCP cancellation spec; on by default (60 s TTL), opt out with --no-cancel-filter (cf. anthropics/claude-code#51073)
• Session recovery — resets MCP session ID on 404 and retries
• Protocol version header — captures the negotiated protocolVersion from the initialize response and injects MCP-Protocol-Version on every subsequent Streamable HTTP request (MCP spec rev 2025-06-18); servers that enforce the header would otherwise reject post-initialize requests with 400 Bad Request
• Token refresh on 401 — automatically refreshes expired OAuth tokens mid-session (OAuth mode only)
• Step-up authorization on 403 — on a Bearer error="insufficient_scope" challenge, re-authorizes for the union of the granted and required scopes (RFC 9470 / MCP step-up; cf. anthropics/claude-code#44652)
• 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

# For an SSE server, pass --transport sse so --check runs the legacy
# GET/endpoint/POST handshake instead of a Streamable HTTP probe:
mcp-stdio --check --transport sse https://your-server.example.com:8080/sse

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)
  --oauth-timeout SECONDS
                         Seconds to wait for the interactive OAuth flow (browser
                         callback / device-code confirmation) before giving up
                         (default: 120; OAuth only)
  --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).
                         Persisted in the token store so proactive refreshes
                         and step-up flows stay consistent
  -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)
  --sse-read-timeout SEC Idle read timeout on the SSE GET stream
                         (default: 300; 0 disables; SSE transport only)
  --no-tcp-keepalive     Disable TCP keepalive on the HTTP socket
  --no-cancel-filter     Disable the cancel-aware response filter (drops late
                         responses for ids cancelled via notifications/cancelled)
  --no-normalize-arguments
                         Disable rewriting a tools/call request's
                         arguments:null to {} before forwarding
  --check                Check connection and exit
  -V, --version          Show version
  -h, --help             Show help

Run mcp-stdio --help for the full per-flag detail (platform notes and issue references are more verbose than this table).

Reverse gateway: serve mode

The default mode bridges stdio → HTTP (client side). The serve subcommand is the mirror image — HTTP → stdio — exposing a local stdio MCP server as a Streamable HTTP MCP endpoint so clients that cannot spawn it locally can reach it over the network:

flowchart BT
    A["MCP client<br>Claude Code / Desktop<br>(or mcp-stdio --oauth)"]
    B("mcp-stdio serve<br><b>HTTP → stdio</b> gateway<br>auth: none / static token /<br>embedded OAuth 2.1 AS")
    C["local stdio<br>MCP server"]
    A <== "Streamable HTTP<br>Bearer / OAuth 2.1 (PKCE)" ==> B
    B <-- "stdio (spawned child)" --> C

This is the mirror of the client-side diagram at the top: there mcp-stdio is stdio → HTTP; here it is HTTP → stdio.

mcp-stdio serve --port 8080 -- python -m my_mcp_server

Then point any MCP client (including mcp-stdio itself) at it:

mcp-stdio --check http://127.0.0.1:8080/mcp

• Stdlib only (http.server) — adds no runtime dependency.
• Implements the Streamable HTTP request/response and notification semantics plus a GET SSE channel for server-initiated messages.
• Authentication is optional and layered:
  • No token — the endpoint is open (run it behind a TLS-terminating proxy).
  • Static token (--auth-token / MCP_STDIO_SERVE_TOKEN) — acts as an OAuth Resource Server: MCP requests require Authorization: Bearer <token>, and a 401 advertises RFC 9728 Protected Resource Metadata at /.well-known/oauth-protected-resource.
  • Embedded OAuth AS (--enable-oauth) — a minimal OAuth 2.1 Authorization Server (PKCE auth-code, RFC 7591 dynamic client registration, refresh, opaque in-memory tokens, stdlib only). The mcp-stdio client's --oauth flow then works against the gateway.
• Single-client assumption for now: JSON-RPC ids pass through verbatim.

Static-token example (token via env so it is not visible in ps):

MCP_STDIO_SERVE_TOKEN=your-secret mcp-stdio serve --port 8080 -- python -m my_mcp_server
mcp-stdio --bearer-token your-secret --check http://127.0.0.1:8080/mcp

Embedded-OAuth example. User authentication is delegated to a fronting reverse proxy that asserts the logged-in user via a header (--trusted-user-header, only trusted behind a proxy that strips client copies). --dev-user is an insecure loopback-only shortcut for local testing:

mcp-stdio serve --enable-oauth --public-url http://127.0.0.1:8080 \
  --dev-user alice --port 8080 -- python -m my_mcp_server
mcp-stdio --oauth http://127.0.0.1:8080/mcp

Options: --host (default 127.0.0.1), --port (default 8080), --path (default /mcp), --auth-token TOKEN (or MCP_STDIO_SERVE_TOKEN, preferred); and for the embedded AS: --enable-oauth, --public-url URL (pins the issuer; recommended behind a proxy), --trusted-user-header HEADER, --dev-user USER (insecure, testing only), --access-token-ttl SECONDS. In-memory tokens mean a restart invalidates issued tokens (the client re-runs --oauth). The backend command follows the options (an optional -- separator is supported).

Workarounds

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

How It Works

1. If --oauth (browser) or --oauth-device (headless, RFC 8628) is set, obtains an access token (cached → refresh → browser/device 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 (OAuth mode only), refreshes the access token and retries; with static --bearer-token / -H auth the 401 is surfaced to the client

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. The negotiated MCP-Protocol-Version header is sent on every post-initialize request (spec rev 2025-06-18).
• 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