Nitro Bun preset fails on Vercel (dist missing) while plain Nitro crashes at runtime

[vdmkotai]

Environment

Environment

• Bun 1.3.2
• Vercel project using Bun runtime
• Nitro ^3.0.1-alpha.1, TanStack React Start ^1.132.31, Vite ^7.0.2
• Deployment URL example: <your-deployment-url>
• Logs show internal error right after [env] VITE_SERVER_URL resolved … (no additional stack trace).

Reproduction

1. Repo configured with:

   • Scripts

     {
     "dev": "bun --bun vite dev",
     "build": "bun --bun vite build",
     "serve": "bun --bun vite preview"
     }

     • Vite config

     export default defineConfig({
     plugins: [
     tsconfigPaths(),
     tailwindcss(),
     tanstackStart(),
     nitro({ preset: "bun" }),
     viteReact(),
     ],
     });
     2. Push to Vercel (Bun runtime).

2. Build fails with “dist directory not found”. If I remove { preset: "bun" }, build succeeds but runtime 500s when Vercel starts the server.

Describe the bug

Summary
I’m running a TanStack React Start app with Vite and Nitro.

This is an app I'm trying to deploy (with my neon db an redis configured):
https://github.com/vdmkotai/tanstack-faster

When I run the Vite scripts via Bun (bun --bun vite dev|build|preview) using plain nitro() in vite config - everything builds but the serverless runtime on Vercel crashes immediately.

If I switch Nitro to nitro({ preset: "bun" }) the Vercel build itself crashes saying it can’t find dist folder

Meanwhile, everything works perfectly on Railway.

Additional context

Additional Context

• Manually switching Vercel’s output folder to .output didn’t help.
• Railway deployments (Node runtime) work fine with the same code.

Logs

[pi0]

Thanks for detailed issue.

For deploying to vercel, you should not have preset: bun config so that it can generate proper build output.

Can you check your functions tab when function crashes what it says?

[vdmkotai]

UPD:
After I posted this on X, Bun updated their docs

The new recommended way is to use

export default defineConfig({
  plugins: [
    tanstackStart(),
    nitro({
      vercel: { 
        functions: { 
          runtime: "bun1.x", 
        }, 
      }, 
    }),
  ],
});

I tried, but unfortunately it also didn't help

@pi0 as I said, I tried all three optons:

• plain nitro()
• nitro({ preset: "bun" })
• and now the one I mentioned above (from updated bun docs)

None is working

As for functions tab - not sure what tab do you mean

This one?

[vdmkotai]

I guess the problem is how Vercel or Nitro bundles environment variables.

I need dynamic runtime variables (like VITE_SERVER_URL) for server-side image prefetching, where the URL changes depending on the environment (preview vs production).

Again - on Railway it was totally fine

I was able to fix it by using https://github.com/t3-oss/t3-env

Here's the commit that finally made it work.
vdmkotai/tanstack-faster@2afe445#diff-4ebb1483bb81cb9efb8a047783b4bae57c9fa999ff30d63399a82577714d901f

[pi0]

So this is nothing bun specific i guess?