As an additional test, I also removed Bun as the runtime for the build while keeping Bun enabled for the Serverless Functions.

The build completed successfully, but every Server Function immediately returned an Internal Server Error at runtime.

The only configuration that currently works reliably for me is to use Bun only as the package manager, while letting Next.js run with the default runtime for both the build and the Serverless Functions.

Hopefully this additional information helps narrow down where the issue might be.

Pretty much the same problem here.

Since updating to 16.3, `bun run build` (1.3.14) fails on my Railway deployment, although it works locally. Reverting back to 16.2.12 fixed the issue

Hi CoffeePuma55644,

Since you reproduced it with a fresh Next.js 16.3 app, I’d treat this as a compatibility issue between Next.js 16.3 and the Bun runtime path, not just your application code.

There are two separate things to separate here:

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# Bun as package manager
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bun install
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# Bun as the runtime used to execute the Next build
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bun run --bun next build
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For the Function runtime, Vercel’s Bun config currently uses:

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{
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  "bunVersion": "1.x"
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}
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and Vercel manages the minor/patch version, so I wouldn’t assume you can force Functions specifically onto 1.3.14 from vercel.json.

For the build/install side, Vercel has a KB article for pinning Bun in the Install Command, so you could try this as a narrow test:

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bunx bun@1.3.14 install
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https://vercel.com/kb/guide/how-to-pin-a-specific-bun-version-for-vercel-builds

vercel.com

How to pin a specific Bun version for Vercel builds? | Vercel Knowledge Base

Learn how to use a specific Bun version for Vercel builds.

If that still fails during “Collecting page data,” then the issue probably is not just the install-time Bun version. In that case, your current workaround is the one I’d use for production: keep Bun only as the package manager if you want, remove the Bun runtime config, use the normal next build path, or temporarily stay on Next.js 16.2.12.

If you can share the minimal repro’s package.json scripts and vercel.json, that should make it much easier for Vercel/Bun maintainers to confirm whether the failure is in the build runtime, Functions runtime, or the Next.js 16.3 output.