diff --git a/tests/uat/extension-page-harness.html b/tests/uat/extension-page-harness.html
index 84a5134..94888be 100644
--- a/tests/uat/extension-page-harness.html
+++ b/tests/uat/extension-page-harness.html
@@ -19,6 +19,42 @@
     <p>This page lives at <code>chrome-extension://&lt;id&gt;/tests/uat/extension-page-harness.html</code>.</p>
     <p>Puppeteer navigates a tab here and drives assertions via <code>window.__mokoshHarness.*</code>.</p>
     <pre id="status">Ready.</pre>
+    <!--
+      Plan 03-01 Wave 1 — probe HTML for A29 rrweb DOM verification
+      (SPEC §10 #4). Provides form + table + modal-trigger so rrweb's
+      record() emits Meta + FullSnapshot + IncrementalSnapshot events
+      against this page. Per RESEARCH Pitfall 4: this fixture omits
+      the rrweb-alpha.4-leaky multi-line input element (rrweb-io/rrweb
+      issue #1596 leaks its value even with maskInputOptions set);
+      only single-line inputs are used below. Per UI-SPEC Section
+      "Test Fixture Conventions": probe HTML uses plain data-test-*
+      attributes; no data-mokosh-* (production-welcome-page reserved);
+      no tokens.css import (the harness head already imports the
+      canonical tokens for A18/A21 — probe sub-tree should appear
+      unstyled to keep rrweb snapshots focused on structural DOM).
+    -->
+    <form id="probe-form" data-test="probe-form">
+      <input type="text" id="probe-text" data-test="probe-text" />
+      <input type="email" id="probe-email" data-test="probe-email" />
+      <input type="password" id="probe-password" data-test="probe-password" />
+      <button type="submit" id="probe-submit" data-test="probe-submit">Submit</button>
+    </form>
+    <table id="probe-table" data-test="probe-table">
+      <thead>
+        <tr><th>col-a</th><th>col-b</th></tr>
+      </thead>
+      <tbody>
+        <tr><td>row-1-a</td><td>row-1-b</td></tr>
+        <tr><td>row-2-a</td><td>row-2-b</td></tr>
+      </tbody>
+    </table>
+    <button id="probe-modal-trigger" data-test="probe-modal-trigger"
+      onclick="(function(){var m=document.getElementById('probe-modal');if(m){m.style.display=(m.style.display==='block'?'none':'block');}})()">
+      Toggle modal
+    </button>
+    <div id="probe-modal" data-test="probe-modal" style="display:none;">
+      <p>Probe modal content (Plan 03-01 A29 IncrementalSnapshot trigger).</p>
+    </div>
     <script type="module" src="./extension-page-harness.ts"></script>
   </body>
 </html>