mcp-stdio

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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[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
    • §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 <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)
• 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 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, session management, 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 with the invalid_redirect_uri error per §3.2.2, refresh, opaque in-memory tokens, stdlib only). An https issuer echoes the RFC 9207 iss parameter on the authorization response (mix-up defence) and advertises it in metadata. The mcp-stdio client's --oauth flow then works against the gateway.
• Embedded-AS token security: the Resource Server validates token audience (RFC 8707 / MCP — a token issued for another resource is rejected); a presented-but-invalid token gets error="invalid_token" (RFC 6750 §3.1); and replaying an authorization code or a rotated refresh token revokes the whole grant family (RFC 6749 §4.1.2 / RFC 9700 §4.14.2), with a brief grace window so a benign client retry is not punished.
• Multi-client isolation by session — each MCP session gets its own spawned backend child, so concurrent clients are isolated by process boundary (a JSON-RPC id collision across clients can never cross responses). Per the MCP Streamable HTTP spec, initialize mints an Mcp-Session-Id, every later request carries it, an unknown/terminated id gets 404 (the client then re-initializes), and a DELETE tears that session's child down. A concurrent-session cap guards an open gateway against unbounded child spawns. When OAuth is enabled each session is bound to the authenticated user — a session id presented with a different user's token is rejected (404), so a leaked id cannot cross tenants.

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); session limits --max-sessions N (default 100; an initialize past the cap gets 503) and --session-idle-ttl SECONDS (evict a session and its child after this much inactivity so a client that disconnects without DELETE does not pin a slot; 0 = disabled, the default); 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, --allow-redirect-uri URL (repeatable; see below), --token-store PATH (see below). Without --token-store, tokens are in-memory only and a restart invalidates them (the client re-runs --oauth). The backend command follows the options (an optional -- separator is supported).

• Non-loopback remote clients — DCR only accepts an RFC 8252 loopback http:// redirect_uri by default, which a browser-based remote MCP client (a web app with a fixed HTTPS OAuth callback, not a locally-run CLI/native app) cannot satisfy. --allow-redirect-uri URL (repeatable) trusts one additional redirect_uri byte-for-byte — no host, prefix, or port matching — so add only a URL you have verified belongs to a client you actually trust; each entry is exactly as trusted as a hardcoded redirect target. It is independent of the loopback path (adding one never widens the other) and requires --enable-oauth.
• Restart-durable tokens — --token-store PATH persists the issued tokens, rotation tombstones, and client registrations to a JSON file (created 0600, written atomically on every state change), so a client that held a valid token before a restart keeps calling tools without a new interactive authorization, and a refresh presented after the restart is honored. This keeps deploys transparent for remote clients that do not re-authorize on 401/invalid_grant (they would otherwise replay the dead token indefinitely and appear connected while their tools silently vanish). Refresh-token reuse detection and grant-family revocation survive the restart too — the consumption ledger is part of the persisted state. The file is credential material: guard it like a private key, and give each serve process its own path — a sidecar .lock file refuses a second process at startup (sharing one store would silently clobber issued tokens), and the path is probe-written at launch so a misconfigured target fails the start instead of silently disabling persistence. Requires --enable-oauth (#277).
• Path-scoped issuer — --public-url retains a path, so several --enable-oauth backends can share one host behind a reverse proxy, each under its own prefix (e.g. --public-url https://gw.example.org/team-a serving https://gw.example.org/team-a/mcp). The issuer becomes https://gw.example.org/team-a, its AS endpoints live under the prefix (/team-a/authorize, /token, /register), and the well-known documents sit at the RFC 8414 §3.1 / RFC 9728 §3.1 root-inserted locations (/.well-known/oauth-authorization-server/team-a, /.well-known/oauth-protected-resource/team-a/mcp) — byte-symmetric with the client's path-aware discovery. A bare-origin --public-url behaves exactly as before (#245).

Multi-user deployment

serve is built for multiple concurrent users. Each MCP session gets its own spawned backend child and — with OAuth enabled — is bound to the authenticated user, so users are isolated by process boundary and a leaked session id cannot cross tenants.

End-user login is delegated to a fronting reverse proxy that performs the real SSO and asserts the user via --trusted-user-header (trusted ONLY because the proxy strips any client-supplied copy). The embedded AS then mints per-user tokens, and the gateway binds each session to that user.

flowchart TD
    UA["User A<br>mcp-stdio --oauth"]
    UB["User B<br>mcp-stdio --oauth"]
    RP["Reverse proxy<br>SSO login, sets X-Forwarded-User<br>strips any client-supplied copy"]
    GW["mcp-stdio serve --enable-oauth<br>--trusted-user-header X-Forwarded-User"]
    CA["stdio child<br>session of A"]
    CB["stdio child<br>session of B"]
    UA == "Streamable HTTP<br>OAuth 2.1 (PKCE)" ==> RP
    UB == "Streamable HTTP<br>OAuth 2.1 (PKCE)" ==> RP
    RP ==> GW
    GW -- "spawn per session" --> CA
    GW -- "spawn per session" --> CB

Gateway (bound to loopback, behind the proxy):

mcp-stdio serve --enable-oauth \
  --public-url https://mcp.example.org \
  --trusted-user-header X-Forwarded-User \
  --max-sessions 200 --session-idle-ttl 900 \
  --host 127.0.0.1 --port 8080 -- python -m my_mcp_server

• --public-url pins the issuer to the external HTTPS URL the proxy serves.
• --trusted-user-header is the header the proxy sets after login; the gateway trusts it only because the proxy strips any client-supplied copy.
• --max-sessions caps concurrent per-user children; --session-idle-ttl reclaims a child after a user disconnects without sending DELETE.

Each user points their client at the gateway, runs the OAuth flow once, and is served by a dedicated child:

mcp-stdio --oauth https://mcp.example.org/mcp

Notes:

• Isolation is by process boundary — user A and user B never share a child, so per-connection backend state (or a JSON-RPC id collision) cannot leak across them.
• The backend command is a template: every session spawns the same command as a fresh child. Identity is enforced at the gateway (session→user binding); it is not injected into the child today, so a backend that needs per-user context should derive it from the request, or run one gateway per backend configuration (optionally multiplexed by path — see Path-scoped issuer).

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
6. In OAuth mode a background timer also refreshes the token shortly before it expires (--oauth-refresh-leeway), independent of request flow — this keeps long sessions alive against gateways that report token expiry as an HTTP 200 tool-error rather than a 401 (opt out with --no-proactive-refresh)

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