Fastify Websocket with @fastify/vite

[ShaharIlany]

What version of Bun is running?

1.2.14

What platform is your computer?

Darwin 24.5.0 arm64 arm

What steps can reproduce the bug?

Following the fix in #18685, a new warning appears when using Fastify with both @fastify/websocket and @fastify/vite.

Reproduction Steps:

1. Run the server using bun dev
2. Open web browser and go to localhost:3000
3. Trigger a reload by:
   • Saving server.ts (with --watch)
   • Or stopping and restarting the server
4. Observe the console output:

[21:28:26 UTC] INFO: Server listening at http://127.0.0.1:3000
[21:28:26 UTC] INFO: Server listening at http://192.168.1.221:3000
[21:28:27 UTC] WARN: Reply was already sent, did you forget to "return reply" in "/" (GET)?
    reqId: "req-1"
    err: {
      "type": "FastifyError",
      "message": "Reply was already sent, did you forget to \"return reply\" in \"/\" (GET)?",
      "stack":
          Error
              at <anonymous> (...../test/node_modules/fastify/lib/reply.js:134:30)
              at preHandlerCallbackInner (...../test/node_modules/fastify/lib/handleRequest.js:169:13)
              at preHandlerCallback (...../test/node_modules/fastify/lib/handleRequest.js:129:5)
              at validationCompleted (...../test/node_modules/fastify/lib/handleRequest.js:119:5)
              at preValidationCallback (...../test/node_modules/fastify/lib/handleRequest.js:96:5)
              at handler (...../test/node_modules/fastify/lib/handleRequest.js:73:7)
              at handleRequest (...../test/node_modules/fastify/lib/handleRequest.js:29:5)
              at runPreParsing (...../test/node_modules/fastify/lib/route.js:571:19)
              at next (...../test/node_modules/fastify/lib/hooks.js:236:9)
              at handleResolve (...../test/node_modules/fastify/lib/hooks.js:253:7)
      "code": "FST_ERR_REP_ALREADY_SENT",
      "name": "FastifyError",
      "statusCode": 500
    }

Notes:

• Does not occur on Bun v1.2.5
• Reproduced consistently on Bun v1.2.9 to v1.2.14
• The issue started after the websocket bug fix introduced post v1.2.6

Suspected Cause
This appears to be a side effect of recent changes to Bun’s HTTP upgrade handling or module caching related to WebSocket hot reload behavior. Possibly, @fastify/vite is trying to serve a response to an already handled WebSocket upgrade route.

What is the expected behavior?

No response

What do you see instead?

No response

Additional information

Source code for reproduction:
https://github.com/ShaharIlany/bun-fastify-bug