FirebaseError: Could not detect runtime for functions

Question

I'm trying to deploy a Vite+React+SSR app to Firebase, but Firebase won't recognize the runtime for my server script. I've created a file – server/index.js – running Express.js for SSR rendering of the app, which works fine locally. In the same file I've added onRequest from Firebase to handle SSL.

import express from 'express'
import { onRequest } from 'firebase-functions/v2/https'

const app = express()
app.use(/* Express and Vite settings */)
app.listen(process.env.PORT)

export const ssr = onRequest(app)

Here's my Firebase config:

{
  "hosting": {
    "public": "dist/client",
    "ignore": [
      "**/.*",
      "**/node_modules/**",
      "firebase.json",
      "firebase-debug.log",
      "firebase-debug.*.log"
    ],
    "rewrites": [
      {
        "source": "**",
        "function": "ssr"
      }
    ]
  },
  "functions": [
    {
      "source": "server",
      "runtime": "nodejs22",
      "codebase": "default"
    }
  ]
}

Package.json:

{
  "name": "name",
  "version": "0.1.0",
  "private": true,
  "type": "module",
  "scripts": {
    "build": "rimraf dist && pnpm build:client && pnpm build:server",
    "build:client": "vite build --outDir dist/client --ssrManifest",
    "build:server": "vite build --ssr src/entry-server.tsx --outDir dist/server",
    "deploy": "firebase deploy --only functions",
    "serve": "pnpm build && firebase emulators:start --only functions",
    "shell": "pnpm build && firebase functions:shell",
    "start": "firebase functions:shell"
  },

  // ...Dependencies & devDependencies

  "engines": {
    "node": "22"
  }
}

I've tried with node 20 and node 22, to no avail.