diff --git a/core/build.js b/core/build.js
index 99d01f9d7..c015ac3f6 100644
--- a/core/build.js
+++ b/core/build.js
@@ -69,7 +69,10 @@ function build({
     buildOptions.outdir = `./dist/${targetDir}`;
   }
 
-  if (format === 'esm') {
+  // The createRequire preamble lets Node ESM output reach CommonJS-only
+  // dependencies. It is meaningless in a browser, where 'module' is an
+  // unresolvable bare specifier, so restrict it to the Node targets.
+  if (platform === 'node' && format === 'esm') {
     buildOptions.banner = {
       js:
         (buildOptions.banner?.js || '') +
diff --git a/integrations/build.js b/integrations/build.js
index d07ba5194..cf307e1bc 100644
--- a/integrations/build.js
+++ b/integrations/build.js
@@ -63,7 +63,10 @@ function build({
     buildOptions.outdir = `./dist/${targetDir}`;
   }
 
-  if (format === 'esm') {
+  // The createRequire preamble lets Node ESM output reach CommonJS-only
+  // dependencies. It is meaningless in a browser, where 'module' is an
+  // unresolvable bare specifier, so restrict it to the Node targets.
+  if (platform === 'node' && format === 'esm') {
     buildOptions.banner = {
       js:
         (buildOptions.banner?.js || '') +
diff --git a/tests/integration/build_output/web_output_test.ts b/tests/integration/build_output/web_output_test.ts
new file mode 100644
index 000000000..24dd03eec
--- /dev/null
+++ b/tests/integration/build_output/web_output_test.ts
@@ -0,0 +1,66 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+import * as fs from 'node:fs/promises';
+import * as path from 'node:path';
+import {beforeAll, describe, expect, it} from 'vitest';
+
+const repoRoot = process.cwd();
+
+/**
+ * Matches an import of the Node `module` builtin, with or without the `node:`
+ * protocol, so the check holds whichever specifier the build banner uses.
+ */
+const MODULE_BUILTIN_IMPORT = /from\s*['"](?:node:)?module['"]/;
+
+/** Lists the emitted `.js` files under `dir`, relative to `dir`. */
+async function listEmittedJs(dir: string): Promise<string[]> {
+  const entries = await fs.readdir(dir, {recursive: true});
+  return entries.filter((entry) => entry.endsWith('.js'));
+}
+
+describe.each(['core', 'integrations'])('%s browser build', (pkg: string) => {
+  const webDir = path.join(repoRoot, pkg, 'dist', 'web');
+  let emittedJs: string[];
+
+  beforeAll(async () => {
+    const stats = await fs.stat(webDir).catch(() => undefined);
+    if (!stats?.isDirectory()) {
+      expect.fail(`${webDir} is missing. Run \`npm run build\` first.`);
+    }
+
+    emittedJs = await listEmittedJs(webDir);
+    if (emittedJs.length === 0) {
+      expect.fail(`${webDir} holds no .js files. Run \`npm run build\` first.`);
+    }
+  });
+
+  it('emits the entry point the package browser field resolves to', () => {
+    expect(emittedJs).toContain('index_web.js');
+  });
+
+  it('never imports the Node module builtin', async () => {
+    const offenders: string[] = [];
+    for (const file of emittedJs) {
+      const contents = await fs.readFile(path.join(webDir, file), 'utf8');
+      if (MODULE_BUILTIN_IMPORT.test(contents)) {
+        offenders.push(file);
+      }
+    }
+
+    expect(offenders).toEqual([]);
+  });
+});
+
+describe('core Node ESM build', () => {
+  it('keeps the createRequire preamble', async () => {
+    const contents = await fs.readFile(
+      path.join(repoRoot, 'core', 'dist', 'esm', 'index.js'),
+      'utf8',
+    );
+
+    expect(contents).toMatch(MODULE_BUILTIN_IMPORT);
+  });
+});