diff --git a/__tests__/unit/services/cat-post-mentions.test.ts b/__tests__/unit/services/cat-post-mentions.test.ts
new file mode 100644
index 000000000..29a619a97
--- /dev/null
+++ b/__tests__/unit/services/cat-post-mentions.test.ts
@@ -0,0 +1,104 @@
+/**
+ * Wall posts reach the queue through a database trigger, because posts are
+ * written straight from the browser to a Postgres function and there is no
+ * server seam to hook. That trigger is a PREFILTER: it queues anything
+ * containing the substring "@cat", so `@catalogue` arrives here too.
+ *
+ * The point of these tests is that the prefilter is not the rule. Detection has
+ * ONE implementation — the resolver — and the worker is where its verdict is
+ * applied. A second copy of "what counts as a mention", written in SQL, is
+ * exactly the kind of duplication that drifts silently.
+ */
+
+const replyToPostMention = jest.fn().mockResolvedValue(true);
+const resolveMentions = jest.fn();
+const claimCatMentions = jest.fn();
+const completeCatMention = jest.fn();
+const failCatMention = jest.fn();
+
+jest.mock('@/services/mentions/cat-account', () => ({
+  ensureCatAccount: jest.fn().mockResolvedValue({ id: 'cat-1', username: 'cat' }),
+}));
+jest.mock('@/services/mentions/cat-post-reply', () => ({
+  replyToPostMention: (...a: unknown[]) => replyToPostMention(...a),
+}));
+jest.mock('@/services/mentions/cat-reply', () => ({
+  replyToConversationMention: jest.fn().mockResolvedValue(true),
+}));
+jest.mock('@/services/mentions/resolve', () => ({
+  resolveMentions: (...a: unknown[]) => resolveMentions(...a),
+}));
+jest.mock('@/services/mentions/queue', () => ({
+  claimCatMentions: (...a: unknown[]) => claimCatMentions(...a),
+  completeCatMention: (...a: unknown[]) => completeCatMention(...a),
+  failCatMention: (...a: unknown[]) => failCatMention(...a),
+  MAX_ATTEMPTS: 3,
+}));
+
+import { runCatMentions } from '@/services/mentions/worker';
+
+const postMention = {
+  id: 'q1',
+  source_type: 'timeline_event',
+  source_id: 'e1',
+  requester_id: 'u1',
+  conversation_id: null,
+  parent_event_id: 'e1',
+  attempts: 1,
+};
+
+/** Admin stub returning one post's text. */
+const admin = (description: string) =>
+  ({
+    from: () => ({
+      select: () => ({ eq: () => ({ maybeSingle: () => Promise.resolve({ data: { title: null, description }, error: null }) }) }),
+    }),
+  }) as never;
+
+beforeEach(() => {
+  replyToPostMention.mockClear().mockResolvedValue(true);
+  resolveMentions.mockReset();
+  claimCatMentions.mockReset().mockResolvedValue([postMention]);
+  completeCatMention.mockReset();
+  failCatMention.mockReset();
+});
+
+describe('wall-post mentions', () => {
+  it('answers a post that really tags the Cat', async () => {
+    resolveMentions.mockResolvedValue({ mentions: [], mentionsCat: true });
+    const result = await runCatMentions(admin('@cat is this goal realistic?'));
+    expect(replyToPostMention).toHaveBeenCalledWith(expect.anything(), {
+      eventId: 'e1',
+      catId: 'cat-1',
+    });
+    expect(result.answered).toBe(1);
+  });
+
+  it('discards the prefilter’s over-selection without replying', async () => {
+    // The trigger queued this because it contains "@cat" as a substring. The
+    // resolver says otherwise, and the resolver is the authority.
+    resolveMentions.mockResolvedValue({ mentions: [], mentionsCat: false });
+    const result = await runCatMentions(admin('browsing the @catalogue today'));
+    expect(replyToPostMention).not.toHaveBeenCalled();
+    expect(result.answered).toBe(1);
+    expect(result.failed).toBe(0);
+  });
+
+  it('treats a discarded mention as resolved, not failed', async () => {
+    resolveMentions.mockResolvedValue({ mentions: [], mentionsCat: false });
+    await runCatMentions(admin('the @catalogue'));
+    // Marking it failed would retry it three times and then log an error about
+    // a post that never asked the Cat anything.
+    expect(completeCatMention).toHaveBeenCalled();
+    expect(failCatMention).not.toHaveBeenCalled();
+  });
+
+  it('does not consult the resolver for a private-message mention', async () => {
+    claimCatMentions.mockResolvedValue([
+      { ...postMention, conversation_id: 'c1', parent_event_id: null },
+    ]);
+    await runCatMentions(admin('irrelevant'));
+    // Those arrive through an API route that already resolved them.
+    expect(resolveMentions).not.toHaveBeenCalled();
+  });
+});
diff --git a/src/services/mentions/cat-post-reply.ts b/src/services/mentions/cat-post-reply.ts
new file mode 100644
index 000000000..3dfa76314
--- /dev/null
+++ b/src/services/mentions/cat-post-reply.ts
@@ -0,0 +1,129 @@
+/**
+ * Answering a mention on the wall.
+ *
+ * Same promise as in a private message — the Cat answers where it was asked —
+ * but the context is different in a way that matters. On X, tagging Grok under
+ * a reply gets you an answer about that reply. Here the Cat reads the THREAD:
+ * the post that started it and the replies leading to the one that tagged it.
+ * "@cat is this realistic?" three replies deep is a question about the
+ * conversation, and answering it from the last sentence alone is the difference
+ * between an assistant and an autocomplete.
+ */
+
+import { CAT_CONTEXT_MESSAGE_WINDOW, CAT_DISPLAY_NAME, CAT_MENTION } from '@/config/cat-identity';
+import { DATABASE_TABLES } from '@/config/database-tables';
+import { callPlatformJson, parseJsonLoose } from '@/services/cat/platform-llm';
+import { writeTimelineReply } from '@/services/timeline/write-timeline-reply';
+import { logger } from '@/utils/logger';
+import type { SupabaseClient } from '@supabase/supabase-js';
+
+export const CAT_POST_FALLBACK =
+  "I couldn't work that one out just now — tag me again and I'll try afresh.";
+
+const SYSTEM_PROMPT = [
+  `You are ${CAT_DISPLAY_NAME}, the OrangeCat agent, replying publicly under someone's post.`,
+  `You were tagged with ${CAT_MENTION}. Answer the question using the thread for context.`,
+  'This is a public reply: be brief, specific and useful to anyone reading, not just the asker.',
+  'Two or three sentences. No preamble, no restating the question, no sign-off.',
+  'If you do not know something, say so in one clause and say what would settle it.',
+  'Respond as JSON: {"reply": "<your message>"}',
+].join('\n');
+
+interface ThreadEvent {
+  id: string;
+  actor_id: string;
+  title: string | null;
+  description: string | null;
+  parent_event_id: string | null;
+  thread_id: string | null;
+  created_at: string;
+}
+
+/**
+ * The post that was tagged, plus the thread it belongs to, oldest first.
+ *
+ * Falls back to the tagged post alone when it starts a thread of its own — a
+ * top-level post has no ancestors, and that is not a failure.
+ */
+export async function loadThreadContext(
+  admin: SupabaseClient,
+  eventId: string
+): Promise<ThreadEvent[]> {
+  const { data: tagged } = await admin
+    .from(DATABASE_TABLES.TIMELINE_EVENTS)
+    .select('id, actor_id, title, description, parent_event_id, thread_id, created_at')
+    .eq('id', eventId)
+    .maybeSingle();
+
+  if (!tagged) {
+    return [];
+  }
+  const event = tagged as ThreadEvent;
+  const threadId = event.thread_id ?? event.parent_event_id ?? event.id;
+
+  const { data: thread } = await admin
+    .from(DATABASE_TABLES.TIMELINE_EVENTS)
+    .select('id, actor_id, title, description, parent_event_id, thread_id, created_at')
+    .or(`id.eq.${threadId},thread_id.eq.${threadId}`)
+    .eq('is_deleted', false)
+    .order('created_at', { ascending: true })
+    .limit(CAT_CONTEXT_MESSAGE_WINDOW);
+
+  const rows = (thread ?? []) as ThreadEvent[];
+  // The tagged post itself must be present even if the thread query missed it
+  // (a brand-new row, or one whose thread_id was never set).
+  return rows.some(r => r.id === event.id) ? rows : [...rows, event];
+}
+
+export function buildThreadPrompt(events: ThreadEvent[], taggedId: string): string {
+  const lines = events.map(e => {
+    const body = (e.description ?? e.title ?? '').trim();
+    const marker = e.id === taggedId ? ' <- tagged you here' : '';
+    return `- ${body}${marker}`;
+  });
+  return ['Thread (oldest first):', lines.join('\n'), '', 'Reply to the post that tagged you.'].join(
+    '\n'
+  );
+}
+
+/**
+ * @returns true when a reply was posted. As in a private message, a failure to
+ *   think still produces an answer rather than silence — under a public post
+ *   that matters more, not less, because everyone can see nothing happened.
+ */
+export async function replyToPostMention(
+  admin: SupabaseClient,
+  params: { eventId: string; catId: string }
+): Promise<boolean> {
+  const { eventId, catId } = params;
+
+  const thread = await loadThreadContext(admin, eventId);
+  if (thread.length === 0) {
+    logger.warn('Cat tagged on a post it cannot read', { eventId }, 'CatPostReply');
+    return false;
+  }
+
+  let reply = '';
+  try {
+    const raw = await callPlatformJson(SYSTEM_PROMPT, buildThreadPrompt(thread, eventId), {
+      timeoutMs: 30_000,
+    });
+    reply = (parseJsonLoose<{ reply?: string }>(raw)?.reply ?? '').trim();
+  } catch (error) {
+    logger.error(
+      'Cat post reply generation failed',
+      { eventId, error: error instanceof Error ? error.message : String(error) },
+      'CatPostReply'
+    );
+  }
+
+  await writeTimelineReply(admin, {
+    parentEventId: eventId,
+    actorId: catId,
+    description: reply || CAT_POST_FALLBACK,
+    // Marked so the UI can render a Cat reply distinctly rather than leaving a
+    // reader to work out from the avatar that this one was written by an agent.
+    metadata: { is_cat_reply: true, answered_event_id: eventId },
+  });
+  return true;
+}
diff --git a/src/services/mentions/worker.ts b/src/services/mentions/worker.ts
index 1c6e02b86..ffaf74aee 100644
--- a/src/services/mentions/worker.ts
+++ b/src/services/mentions/worker.ts
@@ -9,6 +9,9 @@
 
 import { ensureCatAccount } from '@/services/mentions/cat-account';
 import { replyToConversationMention } from '@/services/mentions/cat-reply';
+import { replyToPostMention } from '@/services/mentions/cat-post-reply';
+import { resolveMentions } from '@/services/mentions/resolve';
+import { DATABASE_TABLES } from '@/config/database-tables';
 import {
   claimCatMentions,
   completeCatMention,
@@ -94,7 +97,37 @@ async function answer(
       catId,
     });
   }
-  // Wall posts are queued by the same table but answered by a later change;
-  // until then such a row would be retried forever, so it fails fast instead.
+
+  if (mention.parent_event_id) {
+    // The database trigger is a PREFILTER: it queues anything containing the
+    // substring "@cat", so `@catalogue` and `bob@catering.com` arrive here too.
+    // The resolver is the authority, and this is where its verdict is applied —
+    // detection has one implementation, not one per surface.
+    if (!(await postActuallyTagsTheCat(admin, mention.parent_event_id))) {
+      return true; // Nothing owed. Resolved, not failed.
+    }
+    return replyToPostMention(admin, { eventId: mention.parent_event_id, catId });
+  }
+
   return false;
 }
+
+/** Ask the real resolver whether the post's text mentions the Cat. */
+async function postActuallyTagsTheCat(
+  admin: SupabaseClient,
+  eventId: string
+): Promise<boolean> {
+  const { data } = await admin
+    .from(DATABASE_TABLES.TIMELINE_EVENTS)
+    .select('title, description')
+    .eq('id', eventId)
+    .maybeSingle();
+
+  if (!data) {
+    return false;
+  }
+  const row = data as { title: string | null; description: string | null };
+  const text = `${row.description ?? ''}\n${row.title ?? ''}`;
+  const { mentionsCat } = await resolveMentions(admin, text);
+  return mentionsCat;
+}
diff --git a/src/services/timeline/write-timeline-reply.ts b/src/services/timeline/write-timeline-reply.ts
new file mode 100644
index 000000000..1ac9a492b
--- /dev/null
+++ b/src/services/timeline/write-timeline-reply.ts
@@ -0,0 +1,97 @@
+/**
+ * Writing a timeline reply as a trusted server caller.
+ *
+ * The same split as features/messaging/server/write-message.ts, for the same
+ * reason. `create_timeline_event` re-imposes the RLS actor check by hand —
+ * `v_actor_id IS DISTINCT FROM auth.uid()` raises "Actor mismatch" — which is
+ * exactly right when a browser calls it, and impossible for the Cat: a worker
+ * has no `auth.uid()` at all, so every Cat reply would raise.
+ *
+ * Relaxing that check is not an option; it is the only thing standing between
+ * one authenticated user and posting as another. So authorization stays there
+ * for callers that have a session, and this exists for callers that have
+ * already established authority some other way — which is why it takes an admin
+ * client explicitly instead of creating one.
+ */
+
+import { DATABASE_TABLES } from '@/config/database-tables';
+import { logger } from '@/utils/logger';
+import type { SupabaseClient } from '@supabase/supabase-js';
+
+export interface WriteTimelineReplyInput {
+  parentEventId: string;
+  actorId: string;
+  description: string;
+  metadata?: Record<string, unknown>;
+}
+
+/** @returns the new event id, or null if it could not be written. */
+export async function writeTimelineReply(
+  admin: SupabaseClient,
+  input: WriteTimelineReplyInput
+): Promise<string | null> {
+  const { data: parent } = await admin
+    .from(DATABASE_TABLES.TIMELINE_EVENTS)
+    .select('id, thread_id, thread_depth, visibility')
+    .eq('id', input.parentEventId)
+    .maybeSingle();
+
+  if (!parent) {
+    logger.warn('Cannot reply to a post that is not there', { id: input.parentEventId }, 'TimelineWrite');
+    return null;
+  }
+
+  const parentRow = parent as {
+    id: string;
+    thread_id: string | null;
+    thread_depth: number | null;
+    visibility: string | null;
+  };
+
+  const title = input.description.slice(0, 140) || 'Update';
+  const now = new Date().toISOString();
+
+  const { data, error } = await admin
+    .from(DATABASE_TABLES.TIMELINE_EVENTS)
+    .insert({
+      event_type: 'status_update',
+      actor_id: input.actorId,
+      actor_type: 'user',
+      subject_type: 'profile',
+      subject_id: input.actorId,
+      title,
+      description: input.description,
+      content: { text: input.description },
+      // A reply inherits its parent's audience. Answering a followers-only post
+      // in public would republish the question to people who could not see it.
+      visibility: parentRow.visibility ?? 'public',
+      metadata: input.metadata ?? {},
+      parent_event_id: parentRow.id,
+      thread_id: parentRow.thread_id ?? parentRow.id,
+      thread_depth: (parentRow.thread_depth ?? 0) + 1,
+      event_timestamp: now,
+      created_at: now,
+      updated_at: now,
+    })
+    .select('id')
+    .single();
+
+  if (error || !data) {
+    logger.error('Failed to write a timeline reply', { error: error?.message }, 'TimelineWrite');
+    return null;
+  }
+
+  const eventId = (data as { id: string }).id;
+
+  // Without a visibility row the reply exists and appears on nobody's timeline —
+  // the same trap create_post_with_visibility exists to avoid.
+  const { error: visError } = await admin
+    .from('timeline_event_visibility')
+    .insert({ event_id: eventId, timeline_type: 'profile', timeline_owner_id: input.actorId });
+
+  if (visError) {
+    logger.warn('Cat reply written but not routed to a timeline', { eventId, error: visError.message }, 'TimelineWrite');
+  }
+
+  return eventId;
+}
diff --git a/supabase/migrations/20260826170000_cat_mention_trigger_timeline.sql b/supabase/migrations/20260826170000_cat_mention_trigger_timeline.sql
new file mode 100644
index 000000000..8e1ff5bd1
--- /dev/null
+++ b/supabase/migrations/20260826170000_cat_mention_trigger_timeline.sql
@@ -0,0 +1,73 @@
+-- Notice when a post or reply tags the Cat.
+--
+-- Private messages are written through an API route, so there is a server seam
+-- to notice a mention in. Wall posts are not: post-composer.ts calls a Postgres
+-- function straight from the BROWSER. There is nowhere in the app to hook, so
+-- the notice has to happen in the database.
+--
+-- A trigger is also the stronger choice on its own merits: it cannot be skipped
+-- by a client that forgets to call an endpoint, or by a second client written
+-- later, and it costs one small insert on a table that is already being written.
+--
+-- THIS TRIGGER IS A PREFILTER, NOT THE RULE.
+-- Mention detection has ONE implementation — domain/mentions/parse.ts plus
+-- services/mentions/resolve.ts — and it stays there, because getting `@cat.` and
+-- `@catalog` and `bob@example.com` right is not something to write twice in two
+-- languages and hope they agree. So this asks only the cheap, permissive
+-- question "does this text contain @cat at all?" and the worker decides for
+-- real. It may over-select (a post about `@catalogue` is queued and then
+-- discarded); it must never under-select, which is why the pattern is a plain
+-- case-insensitive substring rather than a clever one.
+
+CREATE OR REPLACE FUNCTION public.note_cat_mention_on_timeline_event()
+RETURNS trigger
+    LANGUAGE plpgsql SECURITY DEFINER
+    SET search_path TO 'public'
+    AS $$
+DECLARE
+  v_cat_id uuid;
+BEGIN
+  -- Nothing to do for the overwhelming majority of posts.
+  IF COALESCE(NEW.description, '') !~* '@cat'
+     AND COALESCE(NEW.title, '') !~* '@cat' THEN
+    RETURN NEW;
+  END IF;
+
+  SELECT id INTO v_cat_id FROM profiles WHERE username = 'cat';
+
+  -- No Cat account yet: nothing can be owed to a sender that does not exist.
+  IF v_cat_id IS NULL THEN
+    RETURN NEW;
+  END IF;
+
+  -- The Cat never answers itself. Without this, a reply of its own that quoted
+  -- the handle would queue another reply, forever.
+  IF NEW.actor_id = v_cat_id THEN
+    RETURN NEW;
+  END IF;
+
+  -- ON CONFLICT DO NOTHING is what makes the trigger safe to fire more than
+  -- once: the unique key on (source_type, source_id) means one post owes one
+  -- reply however many times this runs.
+  INSERT INTO cat_mention_queue (source_type, source_id, requester_id, parent_event_id)
+  VALUES ('timeline_event', NEW.id, NEW.actor_id, NEW.id)
+  ON CONFLICT (source_type, source_id) DO NOTHING;
+
+  RETURN NEW;
+END;
+$$;
+
+COMMENT ON FUNCTION public.note_cat_mention_on_timeline_event IS
+  'Prefilter only: queues a post that MIGHT tag the Cat. services/mentions/resolve.ts is the authority on whether it actually does.';
+
+DROP TRIGGER IF EXISTS trg_note_cat_mention ON public.timeline_events;
+
+-- AFTER INSERT: the post is already stored, so a fault here can never cost
+-- somebody their words. The trigger is not deferrable for the same reason —
+-- queueing late is fine, losing the post is not.
+CREATE TRIGGER trg_note_cat_mention
+  AFTER INSERT ON public.timeline_events
+  FOR EACH ROW
+  EXECUTE FUNCTION public.note_cat_mention_on_timeline_event();
+
+NOTIFY pgrst, 'reload schema';