ERROR: Cannot set headers after they are sent to the client : Cannot set headers after they are sent to the client

Hello I made nuxt-nitro demo clone without nitro. URL is here. https://nuxt-without-nitro-demo.vercel.app/

I try with /api/hello made with h3, It failed after deploy. When I test on local, It make error.

 ERROR  Cannot set headers after they are sent to the client                                                                                                              20:39:12

  at ServerResponse.setHeader (_http_outgoing.js:518:11)
  at sendError (node_modules/h3/dist/index.js:502:7)
  at node_modules/h3/dist/index.js:514:14

My server middleware is https://github.com/ChangJoo-Park/nuxt-without-nitro-demo/blob/main/server/hello.js#L4-L8. I tested with remove res.setHeader('Access-Control-Allow-Origin', '*') . but it fails too.

[ChangJoo-Park]

Thank you for reply, @danielroe ! I’ll make repo without Nuxt.

Here you are. https://github.com/ChangJoo-Park/h3-issue-21/

It is not happen. It seems to have occurred because of h3 with Nuxt.

I met this error https://github.com/ChangJoo-Park/nuxt-without-nitro-demo/blob/65786ca495f8a24939451923ddaa0f7ebf54f125/server/hello.js#L1-L8 . Using h3 with Nuxt

[lewebsimple]

Also getting this error systematically when trying to implement server-sent events in a fresh Nuxt3 project.