MCP SSE demo

by the-vampiire

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demo of MCP SSE server limitations using the bun runtime

Details

Author
the-vampiire
Downloads
317
Categories
Other

- 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

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.

MCP SSE demo

reproducible demo that shows an MCP SSE server fails using the bun runtime but succeeds using nodejs.

steps

1. install dependencies

# or npm | yarn | pnpm install, same effect
bun install

2. run the MCP inspector

npm run inspect

3. open the inspector http://localhost:5173

4. select SSE as the transport and enter http://localhost:3500/sse as the url

5. open dev tools network tab to inspect traffic

6. in another terminal tab / session run the server using bun

npm run start:bun

7. try to connect to the server using the inspector, observe the network (pending SSE connection), server and inspector logs

8. kill the bun server then run the server using nodejs

npm run start:node

9. try to connect to the server using the inspector, observe the network (successful SSE connection and message posting), server and inspector logs

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