Vercel Edge Runtime で Node.js API（fs / child_process 等）が使えない

$ npx next --version
Next.js v16.3.5

$ cat app/api/x/route.js   # runtime = "edge" のまま fs を使う
import fs from "fs";
export const runtime = "edge";
export async function GET() {
  const s = fs.readFileSync("/etc/hostname", "utf8");
  return new Response(s);
}

$ npx next build
▲ Next.js 16.3.5 (Turbopack)
✓ Running next.config took 6ms
 
  Creating an optimized production build ...
✓ Compiled successfully in 1769ms
  Running TypeScript ...
  Finished TypeScript in 3ms ...
  Collecting page data using 5 workers ...
⚠ The Edge Runtime is deprecated. You can use the "nodejs" runtime instead. Learn more: https://nextjs.org/docs/messages/edge-runtime-deprecated
⚠ Using edge runtime on a page currently disables static generation for that page
  Generating static pages using 5 workers (0/3) ...
✓ Generating static pages using 5 workers (3/3) in 262ms
Turbopack build encountered 1 warning:
./app/api/x/route.js:1:1
Warning: A Node.js module is loaded ('fs' at line 1) which is not supported in the Edge Runtime.
    Learn More: https://nextjs.org/docs/messages/node-module-in-edge-runtime
> 1 | import fs from "fs";
    | ^^^^^^^^^^^^^^^^^^^^
  2 | export const runtime = "edge";
  3 | export async function GET() {
  4 |   const s = fs.readFileSync("/etc/hostname", "utf8");
 
Ecmascript file had an error
 
  Finalizing page optimization ...
 
Route (app)
┌ ○ /
├ ○ /_not-found
└ ƒ /api/x
 
○  (Static)   prerendered as static content
ƒ  (Dynamic)  server-rendered on demand
 
終了コード: 0

$ npx next start -p 3000 > start.log 2>&1 &

$ curl -s -o /dev/null -w 'HTTP %{http_code}\n' http://localhost:3000/api/x
HTTP 500

$ grep -m1 -A1 "Error:" start.log
⨯ Error: The edge runtime does not support Node.js 'fs' module.
Learn More: https://nextjs.org/docs/messages/node-module-in-edge-runtime

$ sed -i 's/runtime = "edge"/runtime = "nodejs"/' app/api/x/route.js

$ npx next build
▲ Next.js 16.3.5 (Turbopack)
✓ Running next.config took 5ms
 
  Creating an optimized production build ...
✓ Compiled successfully in 375ms
  Running TypeScript ...
  Finished TypeScript in 1ms ...
  Collecting page data using 5 workers ...
  Generating static pages using 5 workers (0/4) ...
  Generating static pages using 5 workers (1/4) 
  Generating static pages using 5 workers (2/4) 
  Generating static pages using 5 workers (3/4) 
✓ Generating static pages using 5 workers (4/4) in 271ms
  Finalizing page optimization ...
 
Route (app)
┌ ○ /
├ ○ /_not-found
└ ƒ /api/x
 
○  (Static)   prerendered as static content
ƒ  (Dynamic)  server-rendered on demand
 
終了コード: 0

$ npx next start -p 3000 > start.log 2>&1 &

$ curl -s -o /dev/null -w 'HTTP %{http_code}\n' http://localhost:3000/api/x
HTTP 200

// app/api/foo/route.ts
export const runtime = "nodejs";  // 'edge' を外す