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