diff --git a/package.json b/package.json
index 2ef035d..046fe4f 100644
--- a/package.json
+++ b/package.json
@@ -89,6 +89,7 @@
     "@babel/register": "^7.29.7",
     "@eslint/js": "^10.0.1",
     "@rolldown/plugin-babel": "0.2.3",
+    "acorn": "^8.18.0",
     "ava": "^3.0.0",
     "babel-plugin-add-module-exports": "^1.0.4",
     "babel-plugin-istanbul": "^8.0.0",
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index c6dc8d1..a3f2c14 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -42,6 +42,9 @@ importers:
       '@rolldown/plugin-babel':
         specifier: 0.2.3
         version: 0.2.3(@babel/core@7.29.7(supports-color@7.2.0))(rolldown@1.2.0)
+      acorn:
+        specifier: ^8.18.0
+        version: 8.18.0
       ava:
         specifier: ^3.0.0
         version: 3.15.0(supports-color@7.2.0)
@@ -1330,8 +1333,8 @@ packages:
     resolution: {integrity: sha512-HEHNfbars9v4pgpW6SO1KSPkfoS0xVOM/9UzkJltjlsHZmJasxg8aXkuZa7SMf8vKGIBhpUsPluQSqhJFCqebw==}
     engines: {node: '>=0.4.0'}
 
-  acorn@8.17.0:
-    resolution: {integrity: sha512-xRQbDb9BnwDafYNn6Vwl839DYVjqXYb1XVGtWAZ1kcDc6iwAL4hg3B1dZlRiuENFeO2H53gFG3in621AdERVAg==}
+  acorn@8.18.0:
+    resolution: {integrity: sha512-lGq+9yr1/GuAWaVYIHRjvvySG5/4VfKIvC8EWxStPdcDh/Ka7FG3twP6v4d5BkravUilhIAsG4Qj83t02LWUPQ==}
     engines: {node: '>=0.4.0'}
     hasBin: true
 
@@ -5853,15 +5856,15 @@ snapshots:
       mime-types: 3.0.2
       negotiator: 1.0.0
 
-  acorn-jsx@5.3.2(acorn@8.17.0):
+  acorn-jsx@5.3.2(acorn@8.18.0):
     dependencies:
-      acorn: 8.17.0
+      acorn: 8.18.0
 
   acorn-walk@8.3.5:
     dependencies:
-      acorn: 8.17.0
+      acorn: 8.18.0
 
-  acorn@8.17.0: {}
+  acorn@8.18.0: {}
 
   aggregate-error@3.1.0:
     dependencies:
@@ -6006,7 +6009,7 @@ snapshots:
   ava@3.15.0(supports-color@7.2.0):
     dependencies:
       '@concordance/react': 2.0.0
-      acorn: 8.17.0
+      acorn: 8.18.0
       acorn-walk: 8.3.5
       ansi-styles: 5.2.0
       arrgv: 1.0.2
@@ -6993,14 +6996,14 @@ snapshots:
 
   espree@10.4.0:
     dependencies:
-      acorn: 8.17.0
-      acorn-jsx: 5.3.2(acorn@8.17.0)
+      acorn: 8.18.0
+      acorn-jsx: 5.3.2(acorn@8.18.0)
       eslint-visitor-keys: 4.2.1
 
   espree@11.2.0:
     dependencies:
-      acorn: 8.17.0
-      acorn-jsx: 5.3.2(acorn@8.17.0)
+      acorn: 8.18.0
+      acorn-jsx: 5.3.2(acorn@8.18.0)
       eslint-visitor-keys: 5.0.1
 
   esprima@4.0.1: {}
@@ -8915,7 +8918,7 @@ snapshots:
   terser@5.49.0:
     dependencies:
       '@jridgewell/source-map': 0.3.11
-      acorn: 8.17.0
+      acorn: 8.18.0
       commander: 2.20.3
       source-map-support: 0.5.21
 
diff --git a/test/acceptance/test-es5-conformance.js b/test/acceptance/test-es5-conformance.js
new file mode 100644
index 0000000..a5266c3
--- /dev/null
+++ b/test/acceptance/test-es5-conformance.js
@@ -0,0 +1,50 @@
+import test from 'ava';
+import fs from 'fs';
+import path from 'path';
+import * as acorn from 'acorn';
+
+/**
+ * `es5.js` and `bundled.js` exist to serve browsers that predate ES2015. If a
+ * single arrow function or template literal reaches either file, the whole
+ * script is a SyntaxError there and bowser is not merely degraded, it is dead.
+ *
+ * A grep for backticks is not enough. When the webpack build was replaced by
+ * tsdown, rolldown's `__commonJS` interop helper — appended *after* babel runs,
+ * and left alone by terser, which avoids introducing new syntax but does not
+ * transpile — shipped arrow functions into `bundled.js`:
+ *
+ *     var t=(t,e)=>()=>(e||(t((e={exports:{}}).exports,e),t=null),e.exports)
+ *
+ * Parsing the emitted files at `ecmaVersion: 5` is the only check that covers
+ * the whole file, including helpers no source-level transform ever sees.
+ *
+ * These are build outputs — run `pnpm build` before `pnpm test`.
+ */
+const root = path.join(__dirname, '..', '..');
+
+const legacyBundles = ['es5.js', 'bundled.js'];
+
+legacyBundles.forEach((file) => {
+  test(`${file} parses as ES5`, (t) => {
+    const source = fs.readFileSync(path.join(root, file), 'utf8');
+    t.notThrows(
+      () => acorn.parse(source, { ecmaVersion: 5 }),
+      `${file} contains syntax newer than ES5 — it will throw on load in the `
+      + 'browsers this bundle exists to support',
+    );
+  });
+
+  test(`${file} contains no template literals`, (t) => {
+    // Backticks inside string literals are fine (core-js has a few). Only a
+    // real template-literal token is a problem, so tokenise rather than grep.
+    const source = fs.readFileSync(path.join(root, file), 'utf8');
+    const templates = [...acorn.tokenizer(source, { ecmaVersion: 2020 })]
+      .filter((token) => token.type.label === '`' || token.type.label === 'template');
+    t.is(templates.length, 0, `${file} contains a template literal`);
+  });
+});
+
+test('bowser.mjs is a valid ES module', (t) => {
+  const source = fs.readFileSync(path.join(root, 'bowser.mjs'), 'utf8');
+  t.notThrows(() => acorn.parse(source, { ecmaVersion: 'latest', sourceType: 'module' }));
+});
diff --git a/tsdown.config.ts b/tsdown.config.ts
index 60f85d3..f634ba2 100644
--- a/tsdown.config.ts
+++ b/tsdown.config.ts
@@ -1,6 +1,7 @@
 import { defineConfig } from 'tsdown';
 import babel from '@rolldown/plugin-babel';
 import { minify } from 'terser';
+import { transformAsync } from '@babel/core';
 
 const banner = `/*!
  * Bowser - a browser detector
@@ -34,6 +35,51 @@ const legacyBabel = (useBuiltIns: false | 'entry') => babel({
   }]],
 });
 
+/**
+ * Lowers the *emitted chunk* to ES5, after bundling and before terser.
+ *
+ * `legacyBabel()` above only transforms input modules. Rolldown appends its own
+ * runtime helpers afterwards — notably the `__commonJS` wrapper it injects for
+ * CommonJS dependencies — and emits them in modern syntax:
+ *
+ *     var t=(t,e)=>()=>(e||(t((e={exports:{}}).exports,e),t=null),e.exports)
+ *
+ * terser's `ecma: 5` does not transpile; it only avoids *introducing* newer
+ * syntax. So those arrow functions survived into the published `bundled.js`,
+ * making the whole file a SyntaxError in the ES5 engines it exists to serve.
+ * `es5.js` has no CommonJS dependencies, so it never got a helper — which is
+ * why only `bundled.js` was affected, and why this has to run on the output
+ * rather than being folded into `legacyBabel()`.
+ *
+ * `useBuiltIns: false` here on purpose: `bundled.js` already has its polyfills
+ * inlined by the input pass, and re-expanding them would recurse.
+ */
+const lowerChunkToEs5 = () => ({
+  name: 'bowser:babel-output',
+  async renderChunk(code: string, chunk: { fileName: string }) {
+    const result = await transformAsync(code, {
+      babelrc: false,
+      configFile: false,
+      // The emitted chunk is a UMD IIFE, i.e. a script, not a module.
+      sourceType: 'script',
+      // core-js is large and already ES5; skipping its size guard keeps babel
+      // from silently bailing out of compiling `bundled.js`.
+      compact: false,
+      generatorOpts: { comments: true },
+      presets: [['@babel/preset-env', {
+        modules: false,
+        loose: true,
+        useBuiltIns: false,
+        targets: legacyTargets,
+      }]],
+    });
+    if (typeof result?.code !== 'string') {
+      throw new Error(`babel produced no output for ${chunk.fileName}`);
+    }
+    return { code: result.code };
+  },
+});
+
 /**
  * Minifies the UMD chunks with terser instead of rolldown's built-in (oxc)
  * minifier.
@@ -78,7 +124,7 @@ const umd = (name: string, entry: string, useBuiltIns: false | 'entry') => ({
   },
   outDir: '.',
   platform: 'browser' as const,
-  plugins: [legacyBabel(useBuiltIns), terser()],
+  plugins: [legacyBabel(useBuiltIns), lowerChunkToEs5(), terser()],
   // webpack ran in `mode: 'production'`; minification happens in `terser()`
   // above, so rolldown's own minifier stays off. See its comment for why.
   minify: false,