vercel workflows cannot run on `bun` runtime

Help
1

Tried to setup vercel workflow inside my existing hono app with bun + nitro runtime

the bun + nitro is running ok with bun runtime but the workflows executed is in node making bun module not found

workflow log

# Vercel Runtime Log

## Request
ID: gzqmx-1779349375497-544675829693
Time: 2026-05-21T07:42:55.497Z
POST /.well-known/workflow/v1/step → 500
Host: myhost.vercel.app
Error: FUNCTION_INVOCATION_FAILED
Duration: 2808ms
Cache: MISS
Region: iad1
User Agent: vercel-queue/v2beta

## Lifecycle

### Function
Status: 500
Duration: 1375ms
Runtime: nodejs22.x
Memory: -1MB / 2048MB
Region: iad1

### Function
Status: 500
Duration: 1272ms
Runtime: nodejs22.x
Memory: -1MB / 2048MB
Region: iad1

### Cache
Status: 500
Cache: MISS
Region: iad1

Workflow Run: wrun_01KS4QSH6Q60S49HKCRNEBTC6P
Workflow Step: step_01KS4QSJ1VY94E3CPG1XZ779PH

## Deployment
ID: dpl_JDF5E93SXQQskWzk2CJxo7qZvQPx
Environment: production
Branch: split-pr/workflow-typescript

other log with bun runtime in the same application

# Vercel Runtime Log

## Request
ID: gqxvs-1779349237645-0834d3ada9fd
Time: 2026-05-21T07:40:37.645Z
GET /health → 200
Host: my host
Duration: 499ms
Cache: MISS
Region: sin1
User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/148.0.0.0 Safari/537.36

## Lifecycle

### Function
Status: 200
Route: /__server
Duration: 5ms
Runtime: bun1.x
Memory: 316MB / 2048MB
Region: iad1

## Deployment
ID: dpl_JDF5E93SXQQskWzk2CJxo7qZvQPx
Environment: production
Branch: split-pr/workflow-typescript

my config

// vercel.json
{
  "$schema": "https://openapi.vercel.sh/vercel.json",
  "framework": "nitro",
  "bunVersion": "1.x",
  "buildCommand": "cd ../.. && bunx turbo run build --filter=backend"
}


// nitro.config.ts
import { defineConfig } from "nitro";

export default defineConfig({
  vercel: {
    functions: {
      runtime: "bun1.x",
    },
  },
  modules: ["workflow/nitro"],
  preset: "vercel",
  routes: {
    "/**": "./src/index.ts",
  },
});

kinda confused why it can have seperate runtime in the same application/project. Help is appreciated. thanks!

2

Apparently based on docs you need to add in nitro config but my typescript keep getting error and you also need to import "workflow/nitro"; to remove the type eror

workflow: {
    runtime: "bun1.x",
  },

the final config is like this

import { defineConfig } from "nitro";

import "workflow/nitro";

export default defineConfig({
  vercel: {
    functions: {
      runtime: "bun1.x",
    },
  },
  modules: ["workflow/nitro"],
  preset: "vercel",
  workflow: {
    runtime: "bun1.x",
  },
  routes: {
    "/**": "./src/index.ts",
  },
});