diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c0e1d36760..046ad335f3 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -43,6 +43,23 @@ jobs:
       - run: pnpm test
         env:
           CI: true
+  TestNoAsync:
+    permissions: {}
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@v4
+      - uses: pnpm/action-setup@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: pnpm
+      - run: pnpm install --frozen-lockfile
+      - run: pnpm playwright install chromium
+      - run: pnpm test runtime-runes
+        env:
+          CI: true
+          SVELTE_NO_ASYNC: true
   Lint:
     permissions: {}
     runs-on: ubuntu-latest
diff --git a/packages/svelte/tests/helpers.js b/packages/svelte/tests/helpers.js
index b17bc6502a..410838829e 100644
--- a/packages/svelte/tests/helpers.js
+++ b/packages/svelte/tests/helpers.js
@@ -194,7 +194,7 @@ if (typeof window !== 'undefined') {
 
 export const fragments = /** @type {'html' | 'tree'} */ (process.env.FRAGMENTS) ?? 'html';
 
-export const async_mode = false; // process.env.SVELTE_NO_ASYNC !== 'true';
+export const async_mode = process.env.SVELTE_NO_ASYNC !== 'true';
 
 /**
  * @param {any[]} logs