MCP SSE demo

What is MCP SSE demo?

A reproducible demo that shows an MCP SSE server fails when run with the bun runtime but succeeds when run with Node.js.

How to use MCP SSE demo?

Install dependencies with bun install (or npm/yarn/pnpm). Run the MCP inspector with npm run inspect and open it at http://localhost:5173. Select SSE as the transport and enter http://localhost:3500/sse as the URL. In a separate terminal, start the server with npm run start:bun to observe the failure, then kill it and run npm run start:node to see the success.

Key features

• Compares MCP SSE server behavior on bun vs Node.js
• Uses the MCP inspector for testing connections
• Focuses on SSE transport connectivity issues
• Provides clear steps to reproduce the difference

Use cases of MCP SSE demo

• Debugging SSE transport compatibility in MCP implementations
• Testing runtime-specific behavior of MCP servers
• Validating that an MCP SSE server works with Node.js

FAQ from MCP SSE demo

Which runtime causes the MCP SSE server to fail?

The bun runtime causes the SSE connection to remain pending and fail.

Which runtime works correctly with the MCP SSE server?

Node.js works correctly, showing a successful SSE connection and message posting.

What transport does the demo use?

The demo uses the SSE (Server-Sent Events) transport.

What tool is used to test the connection?

The MCP inspector, opened at http://localhost:5173, is used to test the connection.

How can I inspect network traffic during the test?

Open the browser’s developer tools network tab while using the MCP inspector.