Link to the code that reproduces this issue

https://github.com/Mohmdev/nextjs-bun-runtime-regression

To Reproduce

1. Clone the repo: git clone https://github.com/Mohmdev/nextjs-bun-runtime-regression
2. Deploy to Vercel (the repo includes a vercel.json with bunVersion: "1.x")
3. Check the Functions tab in the Vercel deployment — all functions show Node.js runtime
4. Downgrade next to 16.1.7 (bun add [email protected]) and redeploy
5. Check the Functions tab again — all functions now show Bun 1.x runtime

No code changes. Same vercel.json. Same bunVersion: "1.x". The only variable is the Next.js version.

Current vs. Expected behavior

Current (16.2.0): All Vercel functions run on Node.js runtime despite bunVersion: "1.x" in vercel.json. Build log shows Running onBuildComplete from Vercel — the adapter-vercel pipeline activates and writes per-function configs that override the project-level bunVersion.

Function size: 553 kB each.

Expected: All functions should run on Bun 1.x, consistent with 16.1.7 behavior and the bunVersion: "1.x" project configuration.

Function size on 16.1.7: 531 kB each.

Build log comparison:

16.2.0 (broken):

Detected Next.js version: 16.2.0
$ bun --bun next build
▲ Next.js 16.2.0 (Turbopack)
  Creating an optimized production build ...
✓ Compiled successfully in 4.4s
✓ Generating static pages using 1 worker (4/4) in 180ms
  Running onBuildComplete from Vercel              <-- adapter activated
Build Completed in /vercel/output [11s]

16.1.7 (working):

Detected Next.js version: 16.1.7
$ bun --bun next build
▲ Next.js 16.1.7 (Turbopack)
  Creating an optimized production build ...
✓ Compiled successfully in 5.7s
✓ Generating static pages using 1 worker (4/4) in 172.7ms
Traced Next.js server files in: 35.941ms           <-- old pipeline, no adapter
Created all serverless functions in: 111.373ms
Build Completed in /vercel/output [16s]

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #106-Ubuntu SMP PREEMPT_DYNAMIC Fri Mar  6 07:58:08 UTC 2026
  Available memory (MB): 15993
  Available CPU cores: 4
Binaries:
  Node: 24.3.0
  npm: 11.6.2
  Yarn: N/A
  pnpm: 10.15.0
Relevant Packages:
  next: 16.2.0 // Latest available version is detected (16.2.0).
  eslint-config-next: 16.2.0
  react: 19.2.4
  react-dom: 19.2.4
  typescript: 5.9.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Runtime

Which stage(s) are affected? (Select all that apply)

Vercel (Deployed)

Additional context

This is a stock create-next-app with zero custom code — no Bun globals, no barrel exports, no workspace packages. The regression is purely from the Next.js version bump.

Workarounds attempted (none worked):

• experimental: { turbopackInferModuleSideEffects: false } — no effect
• Setting NEXT_ADAPTER_PATH="" in Vercel env vars — no effect
• Only downgrading to 16.1.7 restores Bun runtime

Root cause hypothesis: 16.2.0 graduated adapterPath to stable. Vercel auto-injects NEXT_ADAPTER_PATH for 16.2+ builds, activating nextjs/adapter-vercel. The adapter's onBuildComplete writes .vc-config.json per function with a Node.js runtime hardcoded, overriding the project-level bunVersion.

With 16.2.0:

With 16.1.7:

Related: #91691 reports a similar Bun regression in 16.2.0 but attributes it to turbopackInferModuleSideEffects. That workaround does not fix this case — the adapter pipeline itself is the issue.