fix: fifth review round (durable mount failures, {#key} hydration slots)

cursor[bot]:
- variant_mount_failed now sets the session's pendingEvent (without
  clobbering a still-pending generate), so a helper restart replays it
  onto /poll and a repair --reply resolves instead of returning
  unknown_poll_reply_id. live-resume's next action names the real event
  id instead of a literal EVENT_ID placeholder.
- Contract v2 text hydration strips {#key} DELIMITERS from the zip
  source (content stays; it always renders), so key blocks can no longer
  shift expression slots against the live DOM.

This work was produced with AI assistance (Claude Code).

Co-Authored-By: Claude Code <noreply@anthropic.com>

skill/scripts/live-browser.js

@@ -5501,9 +5501,13 @@
   }
 
   // Remove balanced {#each}...{/each} and {#if}...{/if} regions (including
-  // the delimiters) from a markup string. Nesting-aware.
+  // the delimiters) from a markup string. Nesting-aware. {#key} blocks keep
+  // their CONTENT (it always renders) but lose their delimiter tokens, which
+  // would otherwise consume live text slots in the zip and shift every
+  // following expression.
   function stripSvelteBlockRegions(markup) {
     let out = String(markup || '');
+    out = stripSvelteKeyDelimiters(out);
     for (const kind of ['each', 'if']) {
       const open = '{#' + kind;
       const close = '{/' + kind + '}';
@@ -5530,6 +5534,30 @@
     return out;
   }
 
+  function stripSvelteKeyDelimiters(markup) {
+    let out = String(markup || '');
+    for (;;) {
+      const start = out.indexOf('{#key');
+      if (start === -1) break;
+      // The opening tag runs to its matching close brace (expressions inside
+      // may nest braces).
+      let depth = 0;
+      let i = start;
+      let openEnd = -1;
+      while (i < out.length) {
+        if (out[i] === '{') depth++;
+        else if (out[i] === '}') {
+          depth--;
+          if (depth === 0) { openEnd = i + 1; break; }
+        }
+        i++;
+      }
+      if (openEnd === -1) break;
+      out = out.slice(0, start) + out.slice(openEnd);
+    }
+    return out.split('{/key}').join('');
+  }
+
   function cloneWithoutElements(rootEl, excludedEls) {
     if (!excludedEls || excludedEls.length === 0) return rootEl;
     const excludedSet = new Set(excludedEls);

skill/scripts/live-resume.mjs

@@ -69,7 +69,7 @@ export function mountFailureAction(snapshot = {}) {
   if (!latest) return null;
   const where = latest.url ? ` from ${latest.url}` : '';
   const why = latest.error ? ` (${latest.error})` : '';
-  return `The browser failed to mount variant ${latest.variant}${where}${why}; nothing is on screen. Fix the variant files, then reply with live-poll.mjs --reply EVENT_ID done --file <manifest or source path> for the queued variant_mount_failed event (or republish) so the browser retries.`;
+  return `The browser failed to mount variant ${latest.variant}${where}${why}; nothing is on screen. Fix the variant files, then reply with live-poll.mjs --reply ${snapshot?.pendingEvent?.id || snapshot?.id || 'SESSION_ID'} done --file <manifest or source path> for the queued variant_mount_failed event (or republish) so the browser retries.`;
 }
 
 function parseArgs(argv) {

skill/scripts/live/session-store.mjs

@@ -410,6 +410,13 @@ function applyEvent(snapshot, entry) {
         },
       ].slice(-MOUNT_FAILURE_HISTORY);
       next.renderState = deriveRenderState(next);
+      // The failure needs an agent reply, so it must survive a helper
+      // restart the same way a generate does. Never clobber a still-pending
+      // generate: a progressive publish can fail an early mount while the
+      // generate event itself is still leased.
+      if (!next.pendingEvent) {
+        next.pendingEvent = toPendingEvent(event);
+      }
       break;
     }
     case 'checkpoint':

tests/live-recovery-commands.test.mjs

@@ -163,7 +163,7 @@ describe('live recovery CLI commands', () => {
       'event=live_resume.mount_failure_action actor=agent operation=recover_session risk=agent_thinks_variants_are_on_screen expected=named failing variant and url actual=' + resume.nextAction,
     );
     assert.match(resume.nextAction, /variant_mount_failed/);
-    assert.match(resume.nextAction, /--reply EVENT_ID done --file/);
+    assert.match(resume.nextAction, /--reply cli-render-2 done --file/);
 
     const status = runJson(STATUS_SCRIPT, [], cwd);
     assert.match(status.recoveryHint, /failed to mount variant 2/);

tests/live-session-store.test.mjs

@@ -730,3 +730,39 @@ describe('live-session-store', () => {
     });
   });
 });
+
+describe('review regressions: durable mount failures', () => {
+  it('variant_mount_failed survives a helper restart as the pending event', () => {
+    const tmp = mkdtempSync(join(tmpdir(), 'impeccable-store-mountfail-'));
+    try {
+      const store = createLiveSessionStore({ cwd: tmp });
+      store.appendEvent({ type: 'generate', id: 'mf123456', count: 3, pageUrl: '/', element: { tagName: 'h1' } });
+      store.appendEvent({ type: 'agent_done', id: 'mf123456' });
+      store.appendEvent({ type: 'variant_mount_failed', id: 'mf123456', variant: 2, url: 'http://x/v2.svelte', error: 'boom' });
+
+      // A second store instance = the restarted helper.
+      const restarted = createLiveSessionStore({ cwd: tmp });
+      const snapshot = restarted.getSnapshot('mf123456');
+      assert.equal(snapshot.pendingEvent?.type, 'variant_mount_failed');
+      assert.equal(snapshot.pendingEvent?.variant, 2);
+
+      // The repair reply retires it.
+      restarted.appendEvent({ type: 'agent_done', id: 'mf123456', sourceEventType: 'variant_mount_failed' });
+      assert.equal(restarted.getSnapshot('mf123456').pendingEvent, null);
+    } finally {
+      rmSync(tmp, { recursive: true, force: true });
+    }
+  });
+
+  it('a mount failure does not clobber a still-pending generate', () => {
+    const tmp = mkdtempSync(join(tmpdir(), 'impeccable-store-mountfail2-'));
+    try {
+      const store = createLiveSessionStore({ cwd: tmp });
+      store.appendEvent({ type: 'generate', id: 'mf223456', count: 3, pageUrl: '/', element: { tagName: 'h1' } });
+      store.appendEvent({ type: 'variant_mount_failed', id: 'mf223456', variant: 1, url: 'http://x/v1.svelte', error: 'early' });
+      assert.equal(store.getSnapshot('mf223456').pendingEvent?.type, 'generate');
+    } finally {
+      rmSync(tmp, { recursive: true, force: true });
+    }
+  });
+});