diff --git a/integrations/build.js b/integrations/build.js
index d07ba5194..20aef339e 100644
--- a/integrations/build.js
+++ b/integrations/build.js
@@ -40,6 +40,8 @@ function build({
   entry = 'index.ts',
 }) {
   const buildOptions = {
+    entryPoints: bundle ? [`./src/${entry}`] : ['./src/**/*.ts'],
+    outdir: `./dist/${targetDir}`,
     target: platformBuildTargets[platform],
     platform,
     format,
@@ -55,14 +57,6 @@ function build({
     buildOptions.banner = {js: licenseHeaderText};
   }
 
-  if (bundle) {
-    buildOptions.entryPoints = [`./src/${entry}`];
-    buildOptions.outfile = `./dist/${targetDir}/index.js`;
-  } else {
-    buildOptions.entryPoints = ['./src/**/*.ts'];
-    buildOptions.outdir = `./dist/${targetDir}`;
-  }
-
   if (format === 'esm') {
     buildOptions.banner = {
       js:
diff --git a/tests/integration/build_setup/integrations_build_test.ts b/tests/integration/build_setup/integrations_build_test.ts
new file mode 100644
index 000000000..a81bcc8bd
--- /dev/null
+++ b/tests/integration/build_setup/integrations_build_test.ts
@@ -0,0 +1,93 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+import {exec} from 'node:child_process';
+import * as fs from 'node:fs/promises';
+import * as path from 'node:path';
+import {promisify} from 'node:util';
+import {describe, expect, it} from 'vitest';
+
+const execAsync = promisify(exec);
+
+const repoRoot = process.cwd();
+const packageDir = path.join(repoRoot, 'integrations');
+const distDir = path.join(packageDir, 'dist');
+
+/**
+ * Budget (ms) for one `rm -rf dist` plus one full workspace build. A build is
+ * `tsc --emitDeclarationOnly` followed by three esbuild passes and takes ~2s on
+ * a warm checkout; 60s absorbs a cold, loaded CI runner.
+ */
+const BUILD_TIMEOUT_MS = 60000;
+
+interface PackageManifest {
+  browser: string;
+  main: string;
+  module: string;
+}
+
+async function readManifest(): Promise<PackageManifest> {
+  const raw = await fs.readFile(path.join(packageDir, 'package.json'), 'utf8');
+  return JSON.parse(raw) as PackageManifest;
+}
+
+/**
+ * Removes `integrations/dist` and reruns `script`, so that what the assertions
+ * see was emitted by this build and not left behind by an earlier one.
+ */
+async function buildFromClean(script: string): Promise<void> {
+  await fs.rm(distDir, {recursive: true, force: true});
+  await execAsync(`npm run ${script} --workspace integrations`, {
+    cwd: repoRoot,
+  });
+}
+
+async function expectEmitted(entryPointField: string, relativePath: string) {
+  const absolute = path.join(packageDir, relativePath);
+  await expect(
+    fs.access(absolute),
+    `"${entryPointField}": "${relativePath}" was not emitted`,
+  ).resolves.toBeUndefined();
+  const {size} = await fs.stat(absolute);
+  expect(size, `${relativePath} is empty`).toBeGreaterThan(0);
+}
+
+// Both cases rebuild the real `integrations/dist`. The bundled build runs
+// first so the tree is left holding the plain build's output, which is what
+// the rest of the repo expects. Nothing else reads `integrations/dist`.
+describe('integrations build output', () => {
+  it(
+    'emits every entry point the manifest names, bundled',
+    async () => {
+      await buildFromClean('build:bundle');
+
+      const manifest = await readManifest();
+      await expectEmitted('browser', manifest.browser);
+      await expectEmitted('main', manifest.main);
+      await expectEmitted('module', manifest.module);
+
+      const webDir = path.join(packageDir, path.dirname(manifest.browser));
+      const emitted = await fs.readdir(webDir);
+      expect(
+        emitted.filter((name) => name.endsWith('.js')),
+        'the bundled build must emit one bundle, not one file per source',
+      ).toEqual([path.basename(manifest.browser)]);
+    },
+    BUILD_TIMEOUT_MS,
+  );
+
+  it(
+    'emits every entry point the manifest names, unbundled',
+    async () => {
+      await buildFromClean('build');
+
+      const manifest = await readManifest();
+      await expectEmitted('browser', manifest.browser);
+      await expectEmitted('main', manifest.main);
+      await expectEmitted('module', manifest.module);
+    },
+    BUILD_TIMEOUT_MS,
+  );
+});