Gcloud Adc Proxy

日本語

An auth proxy for accessing remote MCP servers using Google Cloud Application Default Credentials (ADC)

Overview

This tool runs as a stdio MCP server and forwards all requests to a remote MCP server, automatically attaching an Authorization header with a Google Cloud Application Default Credentials (ADC) token.

It allows you to connect to remote MCP servers hosted on IAM-protected services such as Cloud Run.

Usage

Prerequisites

You need to configure Google Cloud authentication. Choose one of the following methods:

# Method 1: User authentication using gcloud CLI
gcloud auth application-default login

# Method 2: Using service account key
export GOOGLE_APPLICATION_CREDENTIALS="path/to/service-account.json"

See the Google Cloud documentation for more details.

Basic Usage

# Start MCP proxy
npx mcp-gcloud-adc-proxy --url https://your-cloud-run-service.run.app

# With service account impersonation
npx mcp-gcloud-adc-proxy --url https://your-cloud-run-service.run.app --impersonate-service-account [email protected]

# With custom audience
npx mcp-gcloud-adc-proxy --url https://your-cloud-run-service.run.app --audiences https://example.com

Service Account Impersonation

You can use service account impersonation to generate ID tokens for a specific service account instead of using the default ADC credentials:

npx mcp-gcloud-adc-proxy \
  --url https://your-cloud-run-service.run.app \
  --impersonate-service-account [email protected]

Requirements:

• The ADC principal must have the roles/iam.serviceAccountTokenCreator role on the target service account
• The target service account must have the necessary permissions to access the remote MCP server

Forwarding the original user's identity

When impersonation is enabled and --forward-impersonator-token is passed, the proxy also attaches an X-Impersonator-Id-Token header containing an ID token of the original ADC user (the human who ran the proxy), in addition to the impersonated service account token in Authorization.

npx mcp-gcloud-adc-proxy \
  --url https://your-cloud-run-service.run.app \
  --impersonate-service-account [email protected] \
  --forward-impersonator-token

This lets a remote MCP server that authenticates via the service account still learn who the real caller is (e.g. to scope per-user permissions). It is off by default; without the flag, only the service account token is sent. The header is attached only when the ADC is a user credential (gcloud auth application-default login); it is omitted for service-account keys and other non-user credentials. If the original user's token cannot be obtained, the request still proceeds without the header.

Custom Audience

By default, the target URL is used as the audience for the ID token. You can override this with the --audiences option:

npx mcp-gcloud-adc-proxy \
  --url https://your-cloud-run-service.run.app \
  --audiences https://custom-audience.example.com

Setup to Claude Code

# Add to user scope (available across all projects)
claude mcp add foobar -s user -- npx -y mcp-gcloud-adc-proxy -u https://foobar.run.app

# Or add to project scope to share with your team
claude mcp add foobar -s project -- npx -y mcp-gcloud-adc-proxy -u https://foobar.run.app

# With service account impersonation
claude mcp add foobar -s user -- npx -y mcp-gcloud-adc-proxy -u https://foobar.run.app --impersonate-service-account [email protected]

License

Apache 2.0 License