fix(build): include assets in standalone output

With output: "standalone", next build writes server.js under
.next/standalone but leaves generated static/public assets in the
project root, so starting the standalone server directly (e.g. via PM2)
404s on JS/CSS/font/favicon requests and /login stays stuck loading.
Add a postbuild step that copies .next/static and public into the
standalone directory, skipping the workspace-traced CLI build which
already copies its own assets.

package.json

@@ -7,6 +7,8 @@
     "dev": "next dev --port 20127",
     "dev:webpack": "next dev --webpack --port 20127",
     "build": "next build --webpack",
+    "postbuild": "node scripts/copy-standalone-assets.mjs",
+    "postbuild:bun": "node scripts/copy-standalone-assets.mjs",
     "start": "next start --port 20127",
     "dev:bun": "bun --bun next dev --webpack --port 20127",
     "build:bun": "bun --bun next build --webpack",

scripts/copy-standalone-assets.mjs

@@ -0,0 +1,36 @@
+import { cpSync, existsSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { dirname, resolve } from "node:path";
+
+export function copyStandaloneAssets({ projectRoot = process.cwd(), distDir = process.env.NEXT_DIST_DIR || ".next" } = {}) {
+  if (process.env.NEXT_TRACING_ROOT_MODE === "workspace") {
+    console.log("[standalone-assets] Skipping workspace-traced CLI build; CLI packaging handles assets");
+    return;
+  }
+
+  const buildDir = resolve(projectRoot, distDir);
+  const standaloneDir = resolve(buildDir, "standalone");
+
+  if (!existsSync(standaloneDir)) {
+    console.log(`[standalone-assets] No standalone build found at ${standaloneDir}`);
+    return;
+  }
+
+  const staticSource = resolve(buildDir, "static");
+  const staticDestination = resolve(standaloneDir, distDir, "static");
+  if (existsSync(staticSource)) {
+    cpSync(staticSource, staticDestination, { recursive: true, force: true });
+    console.log(`[standalone-assets] Copied static assets to ${staticDestination}`);
+  }
+
+  const publicSource = resolve(projectRoot, "public");
+  const publicDestination = resolve(standaloneDir, "public");
+  if (existsSync(publicSource)) {
+    cpSync(publicSource, publicDestination, { recursive: true, force: true });
+    console.log(`[standalone-assets] Copied public assets to ${publicDestination}`);
+  }
+}
+
+if (process.argv[1] && resolve(process.argv[1]) === resolve(dirname(fileURLToPath(import.meta.url)), "copy-standalone-assets.mjs")) {
+  copyStandaloneAssets();
+}

tests/unit/standalone-assets.test.js

@@ -0,0 +1,55 @@
+import { mkdirSync, readFileSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { mkdtempSync } from "node:fs";
+import { describe, expect, it } from "vitest";
+import { copyStandaloneAssets } from "../../scripts/copy-standalone-assets.mjs";
+
+function createBuildFixture(distDir) {
+  const projectRoot = mkdtempSync(join(tmpdir(), "9router-standalone-assets-"));
+  const buildRoot = join(projectRoot, distDir);
+  mkdirSync(join(buildRoot, "standalone"), { recursive: true });
+  mkdirSync(join(buildRoot, "static", "chunks"), { recursive: true });
+  mkdirSync(join(projectRoot, "public"), { recursive: true });
+  writeFileSync(join(buildRoot, "static", "chunks", "app.js"), "static asset");
+  writeFileSync(join(projectRoot, "public", "favicon.svg"), "public asset");
+  return projectRoot;
+}
+
+describe("standalone build assets", () => {
+  it("copies static and public assets into the default standalone layout", () => {
+    const projectRoot = createBuildFixture(".next");
+
+    copyStandaloneAssets({ projectRoot, distDir: ".next" });
+
+    expect(readFileSync(join(projectRoot, ".next", "standalone", ".next", "static", "chunks", "app.js"), "utf8"))
+      .toBe("static asset");
+    expect(readFileSync(join(projectRoot, ".next", "standalone", "public", "favicon.svg"), "utf8"))
+      .toBe("public asset");
+  });
+
+  it("uses a custom Next dist directory", () => {
+    const projectRoot = createBuildFixture(".next-cli-build");
+
+    copyStandaloneAssets({ projectRoot, distDir: ".next-cli-build" });
+
+    expect(readFileSync(join(projectRoot, ".next-cli-build", "standalone", ".next-cli-build", "static", "chunks", "app.js"), "utf8"))
+      .toBe("static asset");
+  });
+
+  it("does not modify workspace-traced CLI builds", () => {
+    const projectRoot = createBuildFixture(".next-cli-build");
+    const previousMode = process.env.NEXT_TRACING_ROOT_MODE;
+    process.env.NEXT_TRACING_ROOT_MODE = "workspace";
+
+    try {
+      copyStandaloneAssets({ projectRoot, distDir: ".next-cli-build" });
+    } finally {
+      if (previousMode === undefined) delete process.env.NEXT_TRACING_ROOT_MODE;
+      else process.env.NEXT_TRACING_ROOT_MODE = previousMode;
+    }
+
+    expect(() => readFileSync(join(projectRoot, ".next-cli-build", "standalone", ".next-cli-build", "static", "chunks", "app.js")))
+      .toThrow();
+  });
+});