diff --git a/docs/adr/0001-piper-es-module-worker.md b/docs/adr/0001-piper-es-module-worker.md
new file mode 100644
index 0000000..c7d79a2
--- /dev/null
+++ b/docs/adr/0001-piper-es-module-worker.md
@@ -0,0 +1,13 @@
+# ADR 0001: Run Piper in an ES module worker
+
+## Decision
+
+Bundle the Piper worker as an ES module and start it with `new Worker(url, { type: 'module' })`.
+
+## Context
+
+Piper loads ONNX Runtime Web and its WebAssembly loader dynamically. The current IIFE/classic worker bundle reports `TypeError: "undefined" cannot be parsed as a URL` during WASM backend initialization, even though the pinned same-origin ONNX loader and binary are available. This is consistent with module-relative URL metadata being lost in the classic bundle.
+
+## Consequences
+
+The browser must support module workers. Current supported browser targets already require modern WebAssembly and worker support. CI asserts that the worker is built and started in module mode; production acceptance remains the real deployed browser flow.
diff --git a/piper-runtime.js b/piper-runtime.js
index 82e606d..dfd9cee 100644
--- a/piper-runtime.js
+++ b/piper-runtime.js
@@ -6,7 +6,9 @@ const pending = new Map();
 
 function getWorker() {
   if (worker) return worker;
-  worker = new Worker(workerUrl);
+  // ONNX Runtime resolves its WASM loader relative to the worker module URL.
+  // Keep the worker as ESM so import.meta.url remains available at runtime.
+  worker = new Worker(workerUrl, {type: 'module'});
   worker.onmessage = event => {
     const message = event.data || {};
     const request = pending.get(message.id);
diff --git a/scripts/build-piper.mjs b/scripts/build-piper.mjs
index 5b00a01..9357367 100644
--- a/scripts/build-piper.mjs
+++ b/scripts/build-piper.mjs
@@ -25,7 +25,7 @@ await build({
   entryPoints: [resolve(root, 'piper-worker.js')],
   outfile: join(workerRoot, 'piper-worker.js'),
   bundle: true,
-  format: 'iife',
+  format: 'esm',
   platform: 'browser',
   target: ['es2022'],
   external: ['fs', 'path'],
diff --git a/scripts/check-frontend.sh b/scripts/check-frontend.sh
index 6df01a2..5ddce64 100755
--- a/scripts/check-frontend.sh
+++ b/scripts/check-frontend.sh
@@ -45,7 +45,7 @@ grep -q 'COPY public/tts/piper-wasm-v1/' Dockerfile
 grep -q 'build:piper' .gitea/workflows/publish.yaml
 grep -q 'Piper' piper-runtime.js
 grep -q 'sv_SE-nst-medium' piper-worker.js
-grep -q 'new Worker(workerUrl)' piper-runtime.js
+grep -q "new Worker(workerUrl, {type: 'module'})" piper-runtime.js
 grep -q 'worker.js?v=' piper-runtime.js
 grep -q 'worker.onmessageerror' piper-runtime.js
 grep -q 'localPiperModelUrl' piper-worker.js
diff --git a/tests/piper-worker-format.test.cjs b/tests/piper-worker-format.test.cjs
new file mode 100644
index 0000000..0768694
--- /dev/null
+++ b/tests/piper-worker-format.test.cjs
@@ -0,0 +1,13 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const {readFile} = require('node:fs/promises');
+const {join} = require('node:path');
+
+test('Piper worker is built and started as an ES module', async () => {
+  const [buildScript, runtime] = await Promise.all([
+    readFile(join(__dirname, '..', 'scripts', 'build-piper.mjs'), 'utf8'),
+    readFile(join(__dirname, '..', 'piper-runtime.js'), 'utf8'),
+  ]);
+  assert.match(buildScript, /outfile: join\(workerRoot, 'piper-worker\.js'\),\s*bundle: true,\s*format: 'esm'/);
+  assert.match(runtime, /new Worker\(workerUrl, \{type: 'module'\}\)/);
+});