Mcp Stdio

mcp-stdio

English | [日本語](README.ja.md)

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

📖 New here? Start with the user guide — task-oriented docs for connecting a client or publishing a server. This README is the full reference.

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[ClaudeDesktop/Code]  B(mcp-stdio)
    B HTTPSStreamable HTTP / SSEBearer TokenHeaderOAuth" ==> C[RemoteMCP Server]
    B -. "OAuth 2.1(PKCE)" .-> D[AuthorizationServer]
    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
• §3 OpenID Connect Discovery 1.0 fallback — when the OAuth well-known 404s, probe /.well-known/openid-configuration (path-append and path-insertion) for ASes that expose only the OIDC form (Auth0, Okta, Azure AD, Google)
• 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
• application_type: "native" in DCR (RFC 8252 §8.4 / MCP SEP-837): the loopback auth-code and headless device flows are native clients, so the loopback redirect is not rejected as the RFC 7591 default "web"
• Client ID Metadata Documents (MCP 2025-11-25 / draft-ietf-oauth-client-id-metadata-document-00)
• --client-metadata-url presents an operator-hosted HTTPS document URL as client_id, skipping Dynamic Client Registration; honoured when set even if the AS metadata does not (yet) advertise client_id_metadata_document_supported (warns instead of silently falling back), and outranked by a pre-registered client_id (--client-id or MCP_OAUTH_CLIENT_ID) (#60)
• the hosted document's redirect_uris must include mcp-stdio's loopback callback without a port (http://127.0.0.1/callback) — the actual callback binds a fresh ephemeral port every run, and the AS must accept any port for a loopback redirect URI (RFC 8252 §7.3 / §8.4)
• 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 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)
• SSE in-flight error synthesis — on the legacy SSE transport, replies arrive only on the long-lived GET stream; when that stream drops, requests already POSTed would otherwise hang forever. mcp-stdio tracks the ids in flight on the current stream and synthesizes a JSON-RPC -32000 error for each on a drop — so the client can retry instead of hanging — while auto-reconnecting; cancelled ids are skipped (cf. anthropics/claude-code#60061)
• 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)
• Proactive token refresh — a background timer refreshes the OAuth token shortly before it expires (lead time: --oauth-refresh-leeway), so a long-lived session survives gateways that signal token expiry as an HTTP 200 tool-error instead of a transport 401 (e.g. Atlassian's MCP gateway); on by default in OAuth mode, opt out with --no-proactive-refresh (#242)
• 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)
• Cold-start (--oauth-eager) — answers initialize locally and runs the interactive OAuth flow on a background thread, so a 30–180 s browser/SSO/MFA login does not exceed the client's ~60 s initialize timeout. Gated methods return -32002 until login completes, then notifications/*/list_changed tells the client to fetch the now-available lists. Streamable HTTP only; a warm (valid/refreshable) cache is unaffected (#296)
• 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)
  --client-metadata-url URL
                         HTTPS URL of a Client ID Metadata Document you host
                         (draft-ietf-oauth-client-id-metadata-document-00), used
                         as client_id instead of Dynamic Client Registration.
                         Ignored if --client-id is also given (#60)
  --oauth-scope SCOPE    OAuth scope to request
  --oauth-use-id-token   Present the OIDC id_token as the Bearer credential
                         instead of the access_token (AWS Bedrock AgentCore /
                         Cognito); falls back to access_token if none is returned (#59)
  --oauth-eager          Cold-start: answer initialize locally and run the
                         interactive OAuth flow in the background, so a long
                         browser/SSO/MFA login does not blow the client's ~60 s
                         initialize timeout. Streamable HTTP only; ignored on
                         --transport sse. Warm cache unaffected (#296)
  --oauth-refresh-leeway SECONDS
                         Proactively refresh tokens this many seconds before
                         expiry (default: 60, or MCP_OAUTH_REFRESH_LEEWAY)
  --no-proactive-refresh
                         Disable the background timer that refreshes the OAuth
                         token before it expires. On by default in OAuth mode;
                         keeps long sessions alive against gateways that signal
                         expiry as an HTTP 200 tool-error rather than a 401 (#242)
  --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 clientClaude Code / Desktop(or mcp-stdio --oauth)"]
    B("mcp-stdio serveHTTP → stdio gatewayauth: none / static token /embedded OAuth 2.1 AS")
    C["local stdioMCP server"]
    A Bearer / OAuth 2.1 (PKCE)" ==> B
    B  C

This is the mirror of the client-side

…

Source & license

This open-source MCP server is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: shigechika
• Source: shigechika/mcp-stdio
• License: MIT

Install and usage instructions live in the source repository linked above.