diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react/src/routes/__root.tsx b/dev-packages/e2e-tests/test-applications/tanstackstart-react/src/routes/__root.tsx
index 0a268a350e34..204276e1bdf2 100644
--- a/dev-packages/e2e-tests/test-applications/tanstackstart-react/src/routes/__root.tsx
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react/src/routes/__root.tsx
@@ -1,4 +1,4 @@
-import type { ReactNode } from 'react';
+import { useEffect, type ReactNode } from 'react';
 import { Outlet, createRootRoute, HeadContent, Scripts } from '@tanstack/react-router';
 
 export const Route = createRootRoute({
@@ -20,6 +20,14 @@ export const Route = createRootRoute({
 });
 
 function RootComponent() {
+  // Mark the document as hydrated so tests can wait for React to attach event
+  // handlers before clicking. `toBeVisible()` only confirms the SSR HTML is
+  // rendered; on slow CI runs Playwright can click before hydration completes,
+  // the onClick handler isn't yet attached, and tests asserting on the
+  // resulting client-side error time out (see #20641, #20685, #20867).
+  useEffect(() => {
+    document.documentElement.setAttribute('data-hydrated', 'true');
+  }, []);
   return (
     <RootDocument>
       <Outlet />
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react/tests/errors.test.ts b/dev-packages/e2e-tests/test-applications/tanstackstart-react/tests/errors.test.ts
index a49b77a293b1..235ae6af5dca 100644
--- a/dev-packages/e2e-tests/test-applications/tanstackstart-react/tests/errors.test.ts
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react/tests/errors.test.ts
@@ -13,6 +13,11 @@ test('Sends client-side error to Sentry with auto-instrumentation', async ({ pag
 
   await page.goto(`/`);
 
+  // Wait for React to hydrate (see __root.tsx) before clicking — the SSR HTML
+  // renders the button before the onClick handler is attached, and clicking
+  // pre-hydration would fire no handler and produce no error.
+  await page.locator('html[data-hydrated="true"]').waitFor();
+
   await expect(page.locator('button').filter({ hasText: 'Break the client' })).toBeVisible();
 
   await page.locator('button').filter({ hasText: 'Break the client' }).click();
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react/tests/tunnel.test.ts b/dev-packages/e2e-tests/test-applications/tanstackstart-react/tests/tunnel.test.ts
index 27f200b8ef62..4024f668cde1 100644
--- a/dev-packages/e2e-tests/test-applications/tanstackstart-react/tests/tunnel.test.ts
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react/tests/tunnel.test.ts
@@ -22,6 +22,11 @@ test('Sends client-side errors through the configured tunnel route', async ({ pa
   await page.goto('/');
   const pageOrigin = new URL(page.url()).origin;
 
+  // Wait for React to hydrate (see __root.tsx) before clicking — the SSR HTML
+  // renders the button before the onClick handler is attached, and clicking
+  // pre-hydration would fire no handler and produce no error.
+  await page.locator('html[data-hydrated="true"]').waitFor();
+
   await expect(page.locator('button').filter({ hasText: 'Break the client' })).toBeVisible();
 
   const managedTunnelResponsePromise = page.waitForResponse(response => {