fix(nextjs): copy public/ into the standalone output

packages/@apphosting/adapter-nextjs/e2e/app.spec.ts

@@ -112,6 +112,14 @@ describe("app", () => {
     assert.notEqual(initialUUID, newUUID, "UUID should change after revalidation");
   });
 
+  it("serves public assets", async () => {
+    for (const asset of ["between-links.svg", "between-cards.svg"]) {
+      const response = await fetch(posix.join(host, asset));
+      assert.ok(response.ok, `${asset} should be served from public/`);
+      assert.ok((await response.text()).includes("<svg"), `${asset} should be an SVG`);
+    }
+  });
+
   it(`404`, async () => {
     const response = await fetch(posix.join(host, Math.random().toString()));
     assert.equal(response.status, 404);

packages/@apphosting/adapter-nextjs/src/bin/build.spec.ts

@@ -295,6 +295,37 @@ outputFiles:
     };
     validateTestFiles(tmpDir, expectedFiles);
   });
+
+  it("merges the public directory when the standalone output is partially populated", async () => {
+    const { generateBuildOutput, validateOutputDirectory } = await importUtils;
+    const files = {
+      ".next/standalone/.next/package.json": "",
+      ".next/standalone/server.js": "",
+      ".next/static/staticfile": "",
+      "public/a.svg": "a",
+      "public/b.svg": "b",
+      // Next already copied a.svg here while tracing a server component's file reads,
+      // so public/ exists in the output but is incomplete.
+      ".next/standalone/public/a.svg": "a",
+    };
+    generateTestFiles(tmpDir, files);
+    await generateBuildOutput(
+      tmpDir,
+      tmpDir,
+      outputBundleOptions,
+      path.join(tmpDir, ".next"),
+      defaultNextVersion,
+      adapterMetadata,
+    );
+    await validateOutputDirectory(outputBundleOptions, path.join(tmpDir, ".next"));
+
+    const expectedFiles = {
+      ".next/standalone/public/a.svg": "a",
+      ".next/standalone/public/b.svg": "b",
+    };
+    validateTestFiles(tmpDir, expectedFiles);
+  });
+
   it("test populate output bundle options", async () => {
     const { populateOutputBundleOptions } = await importUtils;
     const expectedOutputBundleOptions = {

packages/@apphosting/adapter-nextjs/src/utils.ts

@@ -156,6 +156,7 @@ export async function generateBuildOutput(
   const staticDirectory = join(nextBuildDirectory, "static");
   await Promise.all([
     copy(staticDirectory, opts.outputStaticDirectoryPath, { overwrite: true }),
+    copyPublicDirectory(appDir, opts.outputPublicDirectoryPath),
     copyResources(appDir, opts.outputDirectoryAppPath, opts.bundleYamlPath),
     generateBundleYaml(opts, rootDir, nextVersion, adapterMetadata),
   ]);
@@ -166,6 +167,16 @@ export async function generateBuildOutput(
   return;
 }
 
+// Copy the app's public directory into the standalone output. Next.js' standalone output does not
+// reliably include it: the webpack builder omits it entirely, and file tracing can pre-create it
+// holding only the assets a server component reads. Merging per file completes either case.
+// https://nextjs.org/docs/app/api-reference/config/next-config-js/output#automatically-copying-traced-files
+async function copyPublicDirectory(appDir: string, outputPublicDir: string): Promise<void> {
+  const publicDirectory = join(appDir, "public");
+  if (!(await exists(publicDirectory))) return;
+  await copy(publicDirectory, outputPublicDir, { overwrite: true });
+}
+
 // Copy all files and directories to apphosting output directory.
 // Files are skipped if there is already a file with the same name in the output directory
 async function copyResources(
@@ -181,7 +192,9 @@ async function copyResources(
     const existsInOutputBundle = await exists(join(outputBundleAppDir, path));
     // Keep apphosting.yaml files in the root directory still, as later steps expect them to be there
     const isApphostingYaml = path === "apphosting_preprocessed" || path === "apphosting.yaml";
-    if (!isbundleYamlDir && !existsInOutputBundle && !isApphostingYaml) {
+    // The public directory is handled by copyPublicDirectory.
+    const isPublicDir = path === "public";
+    if (!isbundleYamlDir && !existsInOutputBundle && !isApphostingYaml && !isPublicDir) {
       await copy(join(appDir, path), join(outputBundleAppDir, path));
     }
   }