MCP Auth Playground – Local Bridge

About

The MCP Auth Playground – Local Bridge is a companion extension for the MCP Auth Playground (https://mcp-auth.playground.gravitee.io), an interactive workshop that teaches the OAuth 2.1 authorization flow used by Model Context Protocol (MCP) servers.

WHY IS THIS NEEDED?

When using the playground from the hosted URL, your browser enforces the same-origin policy, preventing direct requests to localhost, 127.0.0.1, or Docker-internal hostnames. This extension acts as a lightweight in-browser proxy to bypass that restriction.

HOW IT WORKS

1. The playground web app sends a request via window.postMessage
2. The extension's content script forwards it to the background service worker
3. The service worker performs the actual HTTP fetch (which can reach any URL)
4. The response is sent back to the playground

All communication stays entirely within your browser. No data is sent to external servers.

FEATURES

• Zero configuration – auto-detected by the playground
• Reach localhost, 127.0.0.1, and Docker-exposed ports
• Request counter in the toolbar popup
• Fully open source

PERMISSIONS

This extension requests host access to all URLs because users may connect to any MCP server. The extension ONLY makes requests when explicitly triggered through the playground UI — it never fetches anything on its own.

SOURCE CODE

https://github.com/gravitee-io/mcp-oauth-flow-workshop/tree/main/auth-playground-app/chrome-extension