copyResources skips merging public/ directory in monorepo standalone builds

[bill-lunigy]

Bug Description

In monorepo (Turborepo) setups with Next.js, the copyResources function in packages/@apphosting/adapter-nextjs/src/utils.ts skips copying the full public/ directory from the app source into the standalone output because a partial public/ directory already exists from Next.js file tracing.

Steps to Reproduce

1. Create a Turborepo monorepo with a Next.js app in packages/web
2. Place static assets in packages/web/public/ (e.g., logo.svg, favicons)
3. Reference these assets via string paths (src="/logo.svg") rather than static imports
4. Have at least one subdirectory in public/ that IS traced by webpack (e.g., public/legal/ read via fs in server components)
5. Deploy to Firebase App Hosting with rootDirectory: packages/web

Expected Behavior

All files from packages/web/public/ should be present in the deployed container and served correctly.

Actual Behavior

Only files traced by Next.js standalone (e.g., public/legal/) are present. All other public assets (logo.svg, favicons, other SVGs) return 404.

Root Cause

In utils.ts, the copyResources function uses a top-level existence check:

const existsInOutputBundle = await exists(join(outputBundleAppDir, path));
if (!isbundleYamlDir && !existsInOutputBundle && !isApphostingYaml) {
    await copy(join(appDir, path), join(outputBundleAppDir, path));
}

When Next.js standalone tracing creates a partial public/ directory (containing only traced files like legal/), exists() returns true for the public entry, causing the entire directory copy to be skipped. The function should merge directories rather than skip them.

Suggested Fix

Use fs-extra's copy with { overwrite: false } to merge directories without overwriting existing files:

// Instead of skipping if exists, merge directories
await copy(join(appDir, path), join(outputBundleAppDir, path), { overwrite: false });

Or check at the file level rather than the directory level.

Environment

• @apphosting/adapter-nextjs: v14.0.21
• Next.js: 15.x
• Monorepo: Turborepo
• Root directory: packages/web
• Node.js: 22.x

[vitoras]

For anyone having the same issue, the workaround is to create a postbuild script (e.g., scripts/copy-public-to-standalone.mjs) that recursively copies all files from public/ into .next/standalone/public/ after next build:

// Firebase App Hosting workaround: App Hosting runs the app from
// `.next/standalone/` (via `node .next/standalone/server.js`), so static
// public assets must exist under `.next/standalone/public/` to be served.
// Next.js does not automatically copy `public/` into the standalone output,
// so we do it here after every build.

import fs from "fs";
import path from "path";
import { fileURLToPath } from "url";

const __dirname = path.dirname(fileURLToPath(import.meta.url));
const root = path.resolve(__dirname, "..");

const srcDir = path.join(root, "public");
const destDir = path.join(root, ".next", "standalone", "public");

if (!fs.existsSync(srcDir)) {
  console.log("[copy-public-to-standalone] public/ does not exist — skipping");
  process.exit(0);
}

fs.mkdirSync(destDir, { recursive: true });

let copiedCount = 0;

function copyRecursive(src, dest) {
  const entries = fs.readdirSync(src, { withFileTypes: true });
  for (const entry of entries) {
    if (entry.name === ".DS_Store") continue;
    const srcPath = path.join(src, entry.name);
    const destPath = path.join(dest, entry.name);
    if (entry.isDirectory()) {
      fs.mkdirSync(destPath, { recursive: true });
      copyRecursive(srcPath, destPath);
    } else {
      fs.copyFileSync(srcPath, destPath);
      copiedCount++;
    }
  }
}

copyRecursive(srcDir, destDir);
console.log(`[copy-public-to-standalone] Copied ${copiedCount} files from public/ to .next/standalone/public/`);

On package.json:

"scripts": {
  "dev": "next dev",
  "build": "next build",
  "postbuild": "node scripts/copy-public-to-standalone.mjs",
  "start": "next start",
  "lint": "eslint"
},