iap-mcp-proxy

A client-side bridge that lets generic MCP clients (Claude Desktop, Claude Code, Cursor, ...) connect to remote MCP servers protected by Google Cloud Identity-Aware Proxy (IAP).

This is a Python port of knwoop/iap-mcp-proxy.

┌──────────────┐        stdio          ┌───────────────┐   HTTPS + ID token   ┌─────┐      ┌────────────┐
│  MCP client  │ ────────────────────► │ iap-mcp-proxy │ ───────────────────► │ IAP │ ───► │ MCP server │
│ (Claude etc.)│                       │  (this tool)  │  Proxy-Authorization │     │      │ (Cloud Run)│
└──────────────┘                       └───────────────┘                      └─────┘      └────────────┘

IAP expects a Google-issued OIDC ID token (or, for managed Cloud Run IAP, a self-signed service-account JWT). The MCP spec's OAuth 2.1 flow cannot produce these tokens, so generic clients get a 401/redirect and stop. This proxy runs locally, obtains and refreshes Google credentials, attaches them as Proxy-Authorization (IAP consumes and strips this header), and forwards MCP traffic (Streamable HTTP) upstream.

Install

From PyPI:

pip install iap-mcp-proxy
# or
uv pip install iap-mcp-proxy

ExtensibilityAI also publishes the same versions to the private Artifact Registry PyPI index for internal consumers.

Or install from source with uv:

git clone https://github.com/ExtensibilityAI/iap-mcp-proxy.git
cd iap-mcp-proxy
uv sync
uv run iap-mcp-proxy --version

Quick start

1. Make sure you have credentials IAP will accept (see Credentials):

   gcloud auth application-default login
   

2. Add the proxy to your MCP client config. Cursor / Claude Desktop:

   {
     "mcpServers": {
       "internal-tools": {
         "command": "iap-mcp-proxy",
         "args": [
           "--audience", "1234567890-abc.apps.googleusercontent.com",
           "https://mcp.internal.example.com/mcp"
         ]
       }
     }
   }
   

   Claude Code:

   claude mcp add internal-tools -- iap-mcp-proxy \
     --audience 1234567890-abc.apps.googleusercontent.com \
     https://mcp.internal.example.com/mcp
   

Usage

iap-mcp-proxy [flags] <UPSTREAM_URL>

Credentials

With --credentials=auto (the default), sources are tried in this order:

1. Impersonation — if --impersonate-service-account is set, mint ID tokens via the IAM Credentials API (generateIdToken) using your ADC as the base identity. Requires roles/iam.serviceAccountTokenCreator on the target SA. Best for CI and shared team setups.
2. ADC — if Application Default Credentials are a service account key or workload credential, mint an ID token directly.
3. Desktop OAuth — gcloud user credentials cannot mint arbitrary-audience ID tokens, so the proxy falls back to an installed-app OAuth flow: first run opens a browser for Google sign-in; the refresh token is stored in your OS keychain (fallback: 0600 file under your user config dir). Requires a desktop OAuth client in the same project as the IAP resource, supplied via IAP_MCP_OAUTH_CLIENT_ID / IAP_MCP_OAUTH_CLIENT_SECRET — see Google's docs on programmatic IAP authentication.

These OIDC modes need IAP to use a custom OAuth client (LB-backed IAP, or a managed-client service with a separate allow-listed OAuth client). For modern managed-client direct Cloud Run IAP, --credentials=signjwt is the simplest route — it needs no OAuth client (see Supported IAP configurations).

The principal must hold roles/iap.httpsResourceAccessor on the IAP resource.

Supported IAP configurations

• Since Google shut down the IAP OAuth Admin API (March 2026), newly-secured apps default to a Google-managed OAuth client, which rejects Google-issued OIDC ID tokens. Use --credentials=signjwt --impersonate-service-account=<SA>.
• Managed-client IAP can also accept OIDC ID tokens via a separate allow-listed OAuth client; then --credentials=impersonate/adc work with that client ID as --audience.

Notes

• The IAP token travels in Proxy-Authorization, which IAP consumes and strips — your app never sees it. If your app has its own auth, pass it with --downstream-auth and it is forwarded verbatim as Authorization.
• On a 401 (or a 302 into Google sign-in) the proxy refreshes the token and retries once; a second failure is surfaced to the MCP client as a JSON-RPC error with an actionable message on stderr.
• If the upstream reports the session expired (HTTP 404 — e.g. after a Cloud Run redeploy), the proxy transparently replays the cached initialize handshake to obtain a fresh session and retries the request; the stdio client never notices.
• If a streaming (SSE) response drops mid-tool-call and the server tags events with IDs, the proxy resumes it with Last-Event-ID instead of losing the response.
• After initialize, the proxy opens the standalone GET SSE stream so server-initiated messages reach the client, reconnecting with Last-Event-ID if the stream drops.
• Exit codes: 0 clean shutdown, 1 fatal error, 2 auth bootstrap failure.

See docs/setup-gcp.md for setting up IAP in both deployment modes.

Development

uv sync --extra dev
uv run pytest
uv run ruff check
uv build

Releasing

Public PyPI uploads run only when a GitHub Release is published (publish-pypi.yml + Trusted Publishing).

1. Bump version in pyproject.toml and __version__ in src/iap_mcp_proxy/__init__.py on main.
2. Merge to main (Deploy may publish that version to Artifact Registry as usual).
3. Create tag vX.Y.Z and Publish a GitHub Release for that tag.
4. Approve the pypi GitHub Environment job if required reviewers are set.
5. Verify: pip install iap-mcp-proxy==X.Y.Z from a clean environment (no Artifact Registry auth).

Routine merges to main do not publish to public PyPI.

License

Apache-2.0