Metadata-Version: 2.4
Name: mcp-stdio
Version: 0.42.1
Summary: Stdio-to-HTTP gateway — connects MCP clients to remote HTTP MCP servers
Project-URL: Homepage, https://github.com/shigechika/mcp-stdio
Project-URL: Issues, https://github.com/shigechika/mcp-stdio/issues
Author: Shigechika AIKAWA
License-Expression: MIT
License-File: LICENSE
Keywords: bridge,claude,gateway,mcp,oauth,proxy,relay,sse,stdio,streamable-http
Classifier: Development Status :: 4 - Beta
Classifier: Environment :: Console
Classifier: Intended Audience :: Developers
Classifier: License :: OSI Approved :: MIT License
Classifier: Programming Language :: Python :: 3
Classifier: Programming Language :: Python :: 3.10
Classifier: Programming Language :: Python :: 3.11
Classifier: Programming Language :: Python :: 3.12
Classifier: Programming Language :: Python :: 3.13
Classifier: Programming Language :: Python :: 3.14
Classifier: Topic :: Software Development :: Libraries
Requires-Python: >=3.10
Requires-Dist: httpx>=0.25.0
Provides-Extra: dev
Requires-Dist: pytest-httpx>=0.35.0; extra == 'dev'
Requires-Dist: pytest-timeout>=2.3; extra == 'dev'
Requires-Dist: pytest>=8.0; extra == 'dev'
Provides-Extra: firestore
Requires-Dist: google-cloud-firestore>=2.0.0; extra == 'firestore'
Provides-Extra: integration
Requires-Dist: mcp<3,>=2.0; extra == 'integration'
Requires-Dist: uvicorn>=0.30; extra == 'integration'
Description-Content-Type: text/markdown

<!-- mcp-name: io.github.shigechika/mcp-stdio -->

# mcp-stdio

English | [日本語](README.ja.md)

A stdio↔HTTP MCP gateway that works in **both directions** — the same
binary is both an MCP-over-HTTP **client** gateway and, via `serve` mode, a
full MCP-over-HTTP **server** (with an optional embedded OAuth 2.1
Authorization Server), so it can sit on either end of an MCP connection.

📖 **New here? Start with the [user guide](https://shigechika.github.io/mcp-stdio/)** — task-oriented docs for connecting a client or publishing a server. This README is the full reference.

## Overview

**Client gateway (default mode) — stdio → HTTP.** [MCP](https://modelcontextprotocol.io/) 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:

```mermaid
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.

**Reverse gateway (`mcp-stdio serve`) — HTTP → stdio.** The mirror image: takes a
local stdio MCP server (any language, any framework) and exposes it as a
Streamable HTTP endpoint, with optional bearer-token or embedded-OAuth-2.1
authentication, per-user backend isolation when OAuth is enabled, and a
restart-durable token store —
see [Reverse gateway: `serve` mode](#reverse-gateway-serve-mode) below. This
is how mcp-stdio can stand in for a framework's own HTTP/OAuth hosting layer
(e.g. instead of depending on a Python web framework's built-in server for
just that part) when the tool definitions themselves already run fine over
stdio.

## 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](https://html.spec.whatwg.org/multipage/server-sent-events.html).
- **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](https://www.rfc-editor.org/rfc/rfc9728) 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](https://www.rfc-editor.org/rfc/rfc8414) 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](https://www.rfc-editor.org/rfc/rfc8707) Resource Indicators
    - §2 `resource` parameter in authorization, token exchange, **and refresh** requests
  - [RFC 7636](https://www.rfc-editor.org/rfc/rfc7636) PKCE
    - §4.1–4.2 S256 `code_challenge_method` with an 86-char `code_verifier`
  - [RFC 8628](https://www.rfc-editor.org/rfc/rfc8628) 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](https://www.rfc-editor.org/rfc/rfc7591) 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](https://www.rfc-editor.org/rfc/rfc8252) §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](https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization#client-id-metadata-documents) (MCP 2025-11-25 / [draft-ietf-oauth-client-id-metadata-document-00](https://datatracker.ietf.org/doc/html/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](https://www.rfc-editor.org/rfc/rfc8252) §7.3 / §8.4)
  - [RFC 6749](https://www.rfc-editor.org/rfc/rfc6749) 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](https://www.rfc-editor.org/rfc/rfc6750) Bearer Token usage
    - §2.1 `Authorization: Bearer <token>` request header
- **Works with MCP 2026-07-28 servers** — add `--protocol-era auto` and mcp-stdio asks the server which protocol it speaks and adapts; your MCP client needs no changes. Without the flag nothing changes, so upgrading is safe. `mcp-stdio serve` answers both old and new clients on the same address automatically — or only the newer ones with `--modern-only`, and `--modern-idle-ttl` reclaims idle backends. Newer clients can also hold one connection open and hear when your server's tool, prompt or resource lists change, or when a specific resource they named is updated (mcp-stdio subscribes on their behalf when your server declares `resources.subscribe`). Verified end to end against python-sdk v2.0.0 in both directions. → [Working with MCP 2026-07-28 servers](https://shigechika.github.io/mcp-stdio/modes/#protocol-eras)
- **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](https://www.rfc-editor.org/rfc/rfc9470) / 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](https://www.python-httpx.org/)
- **Minimal dependencies** — only [httpx](https://www.python-httpx.org/); OAuth uses stdlib only

## Install

```bash
pip install mcp-stdio
```

Or with [uv](https://docs.astral.sh/uv/):

```bash
uv tool install mcp-stdio
```

Or run directly without installing:

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

Or with [Homebrew](https://brew.sh/):

```bash
brew install shigechika/tap/mcp-stdio
```

## Quick Start

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

With Bearer token authentication:

```bash
# 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:

```bash
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):

```bash
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):

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

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

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

Check connectivity before use:

```bash
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`:

```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

```bash
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
  --oauth-resource URI   Send this exact RFC 8707 resource value on every OAuth
                         request instead of the server-URL-derived one. Required
                         for AS that demand a specific resource identifier, e.g.
                         Microsoft Entra ID's App ID URI api://<app-id>. Persisted
                         in the token store. Mutually exclusive with
                         --no-resource-indicator
  -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:

```mermaid
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**.

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

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

```bash
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](https://www.rfc-editor.org/rfc/rfc9728) 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](https://www.rfc-editor.org/rfc/rfc7591)
    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](https://www.rfc-editor.org/rfc/rfc9207) `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](https://www.rfc-editor.org/rfc/rfc8707) / MCP — a token issued for
  another resource is rejected); a presented-but-invalid token gets
  `error="invalid_token"` ([RFC 6750](https://www.rfc-editor.org/rfc/rfc6750)
  §3.1); and replaying an authorization code or a rotated refresh token revokes
  the whole grant family ([RFC 6749](https://www.rfc-editor.org/rfc/rfc6749)
  §4.1.2 / [RFC 9700](https://www.rfc-editor.org/rfc/rfc9700) §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`):

```bash
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:

```bash
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`), `--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 `--max-sessions-per-owner N`
(on a new `initialize`, LRU-evict that OAuth user's older sessions down to `N`,
reclaiming ghosts left by a client that reconnects without `DELETE`; `0` =
disabled, the default; static-token and open-gateway sessions are exempt);
`--user-env VAR` (inject the authenticated principal into each spawned
child's environment under this name, so a multi-user-aware backend can read
its caller's identity without its own OAuth stack — requires
`--enable-oauth`; the open-gateway and shared static-token principals are
exempt, same as `--max-sessions-per-owner`; refuses a search-path /
dynamic-linker variable name such as `PATH`, `LD_PRELOAD`, or `PYTHONPATH`
that the child's own runtime needs to start); 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` or `--token-store-firestore COLLECTION/DOCUMENT`
(mutually exclusive, see below). Without either, 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).
  - *Restart-durable tokens, no local disk* — `--token-store-firestore
    COLLECTION/DOCUMENT` persists the same state as `--token-store` (same
    JSON-shaped snapshot, same probe-write-at-launch fail-fast behavior) to
    one [Firestore](https://cloud.google.com/firestore) document instead of
    a local file, for a deployment with no durable local disk (Cloud Run and
    similar). The GCP project is resolved the standard google-cloud way
    (`GOOGLE_CLOUD_PROJECT` env var, or ADC on Cloud Run) — there is no
    separate `--project` flag. Requires the `google-cloud-firestore` package
    (`pip install mcp-stdio[firestore]`, an optional extra — plain
    `pip install mcp-stdio` never needs it) and `--enable-oauth`. **Unlike
    `--token-store` there is no lock against two processes sharing one
    document** — safe ONLY when exactly one `serve` process ever writes to a
    given document at a time (e.g. Cloud Run with `min-instances=1` and
    `max-instances=1`); two concurrent writers silently last-writer-wins
    clobber each other's issued tokens. Mutually exclusive with
    `--token-store`.
  - *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](https://www.rfc-editor.org/rfc/rfc8414) §3.1 /
    [RFC 9728](https://www.rfc-editor.org/rfc/rfc9728) §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.

```mermaid
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):

```bash
mcp-stdio serve --enable-oauth \
  --public-url https://mcp.example.org \
  --trusted-user-header X-Forwarded-User \
  --max-sessions 200 --session-idle-ttl 900 --max-sessions-per-owner 1 \
  --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`.
- `--max-sessions-per-owner` reclaims a user's *previous* sessions the moment
  they re-initialize, so a client that reconnects without `DELETE` (common for
  hosted connectors) no longer leaves ghosts. Because ghosts are reclaimed at
  reconnect rather than only by the idle reaper, a longer `--session-idle-ttl`
  no longer risks ghosts filling `--max-sessions` — which lets an interactive
  client with pauses avoid frequent re-initialization.

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

```bash
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](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
