diff --git a/runner/src/detect.js b/runner/src/detect.js
index 52952c4..5886985 100644
--- a/runner/src/detect.js
+++ b/runner/src/detect.js
@@ -33,13 +33,14 @@ function installCmd(pm, files) {
       ? ['npm', 'ci', '--include=dev']
       : ['npm', 'install', '--include=dev', '--no-audit', '--no-fund'];
   }
+  if (pm === 'yarn' && files.has('.yarnrc.yml')) return ['yarn', 'install', '--immutable']; // Yarn Berry
   return [pm, 'install', '--frozen-lockfile'];
 }
 
-// npm/pnpm brauchen `--` vor den Skript-Argumenten, yarn/bun reichen sie direkt durch.
+// Nur npm braucht `--` vor den Skript-Argumenten, pnpm/yarn/bun reichen sie direkt durch.
 function devCmdFor(pm, port) {
   const args = ['--host', '0.0.0.0', '--port', String(port)];
-  return pm === 'npm' || pm === 'pnpm' ? [pm, 'run', 'dev', '--', ...args] : [pm, 'run', 'dev', ...args];
+  return pm === 'npm' ? [pm, 'run', 'dev', '--', ...args] : [pm, 'run', 'dev', ...args];
 }
 
 export function detectProject(files, { mode, index, previewJson } = {}) {
diff --git a/runner/test/detect.test.js b/runner/test/detect.test.js
index 01ae6e7..d51b063 100644
--- a/runner/test/detect.test.js
+++ b/runner/test/detect.test.js
@@ -38,7 +38,12 @@ test('pnpm-lock ⇒ pnpm', () => {
   assert.equal(r.pm, 'pnpm');
   assert.deepEqual(r.install, ['pnpm', 'install', '--frozen-lockfile']);
   assert.deepEqual(r.buildCmd, ['pnpm', 'run', 'build']);
-  assert.deepEqual(r.devCmd, ['pnpm', 'run', 'dev', '--', '--host', '0.0.0.0', '--port', '5173']);
+  assert.deepEqual(r.devCmd, ['pnpm', 'run', 'dev', '--host', '0.0.0.0', '--port', '5173']);
+});
+
+test('yarn Berry (.yarnrc.yml) ⇒ --immutable', () => {
+  const r = detectProject(fake({ 'package.json': pkg({ build: 'x' }), 'yarn.lock': '', '.yarnrc.yml': '' }), {});
+  assert.deepEqual(r.install, ['yarn', 'install', '--immutable']);
 });
 
 test('yarn.lock und bun.lockb/bun.lock', () => {