diff --git a/surface/server/src/event-types.ts b/surface/server/src/event-types.ts
index 79b6e6e1..21e3fe1e 100644
--- a/surface/server/src/event-types.ts
+++ b/surface/server/src/event-types.ts
@@ -1,3 +1,5 @@
+import { CHANNEL_UNREAD_EVENT_TYPES } from '@atrium/surface-client/timeline';
+
 export const MODIFIER_EVENT_TYPES = [
   'message.edited',
   'message.deleted',
@@ -100,14 +102,6 @@ export const SYNC_CATCHUP_RAW_EVENT_TYPES = [
   ...SYNC_EVENT_TYPES.filter((type) => !TIMELINE_EVENT_TYPE_SET.has(type)),
 ];
 
-// === the unread rule ===
-
-// Events that make a channel unread. A thread reply counts only when it is
-// broadcast: the agent's answer is an ordinary channel message and marks the
-// channel unread like one, or the very thing you asked for lands below the fold
-// with nothing to say it arrived.
-const UNREAD_EVENT_TYPES = ['message.posted', 'session.spawned', 'session.replied'] as const;
-
 /**
  * Per-channel `latest_event_id` — the newest event that should draw attention.
  * Correlates against `c.id`, so it belongs inside a LATERAL over `channels c`.
@@ -127,6 +121,6 @@ export const CHANNEL_LATEST_EVENT_ID_SQL = `
        FROM events e
        LEFT JOIN message_state ms ON ms.event_id = e.id
        WHERE e.channel_id = c.id
-         AND e.type IN ${sqlTypeList(UNREAD_EVENT_TYPES)}
+         AND e.type IN ${sqlTypeList(CHANNEL_UNREAD_EVENT_TYPES)}
          AND (e.thread_root_event_id IS NULL OR (e.payload->>'broadcast')::boolean IS TRUE)
          AND NOT COALESCE(ms.is_deleted, false)`;
diff --git a/surface/shared/package.json b/surface/shared/package.json
index 40989a7a..395be87c 100644
--- a/surface/shared/package.json
+++ b/surface/shared/package.json
@@ -8,6 +8,7 @@
   "exports": {
     ".": "./src/index.ts",
     "./handle": "./src/handle.ts",
+    "./timeline": "./src/timeline.ts",
     "./mentions": "./src/mentions.ts",
     "./reactions": "./src/reactions.ts",
     "./prefs": "./src/prefs.ts",
diff --git a/surface/shared/src/appState.ts b/surface/shared/src/appState.ts
index 7413a450..7ef39c10 100644
--- a/surface/shared/src/appState.ts
+++ b/surface/shared/src/appState.ts
@@ -7,6 +7,7 @@ import {
   applyLocalEditOverlay,
   applyLocalReactionOverlay,
   applyEvent,
+  CHANNEL_UNREAD_EVENT_TYPES,
   confirmLocalOverlay,
   emptyTimeline,
   markFailed,
@@ -340,6 +341,8 @@ function isMainTimelineVisibleEvent(ev: WireEvent): boolean {
   return ev.threadRootEventId == null || ev.broadcast === true || ev.payload?.broadcast === true;
 }
 
+const CHANNEL_UNREAD_EVENT_TYPE_SET = new Set<string>(CHANNEL_UNREAD_EVENT_TYPES);
+
 export function appReducer(state: AppState, action: AppAction): AppState {
   switch (action.type) {
     case 'init-me':
@@ -517,7 +520,7 @@ export function appReducer(state: AppState, action: AppAction): AppState {
       for (const ev of action.events) {
         const alreadySeen = seenIds.has(ev.id);
         seenIds.add(ev.id);
-        const isNewMessage = (ev.type === 'message.posted' || ev.type === 'session.spawned') && !alreadySeen;
+        const isNewMessage = CHANNEL_UNREAD_EVENT_TYPE_SET.has(ev.type) && !alreadySeen;
         if (!isNewMessage || !ev.channelId) continue;
         if (isMainTimelineVisibleEvent(ev)) {
           next = {
@@ -644,7 +647,7 @@ export function appReducer(state: AppState, action: AppAction): AppState {
           applyEvent(t, ev, action.catchupCursor !== undefined ? { catchupCursor: action.catchupCursor } : {}),
         );
       }
-      const isNewMessage = (ev.type === 'message.posted' || ev.type === 'session.spawned') && !alreadySeen;
+      const isNewMessage = CHANNEL_UNREAD_EVENT_TYPE_SET.has(ev.type) && !alreadySeen;
       if (isNewMessage && isMainTimelineVisibleEvent(ev)) {
         // Live events must advance the cold counter — the unread divider and
         // unmute re-derivation compare latestEventId against lastReadEventId.
diff --git a/surface/shared/src/read-cursor.test.ts b/surface/shared/src/read-cursor.test.ts
index 86469f24..180f7ade 100644
--- a/surface/shared/src/read-cursor.test.ts
+++ b/surface/shared/src/read-cursor.test.ts
@@ -207,6 +207,29 @@ describe('monotonic channel snapshots', () => {
   });
 });
 
+describe('live unread counters', () => {
+  it('advances latestEventId and unread for a broadcast session reply', () => {
+    const state = appReducer(seed(10, 10), { type: 'select-channel', channelId: null });
+    const reply: WireEvent = {
+      id: 21,
+      workspaceId: 'w1',
+      channelId: 'c1',
+      threadRootEventId: 7,
+      type: 'session.replied',
+      actorId: null,
+      payload: { session_id: 's1', text: 'Done', broadcast: true },
+      createdAt: '2026-07-16T12:00:00.000Z',
+      author: { id: 'agent:s1', handle: 'agent', displayName: 'Agent' },
+      broadcast: true,
+    };
+
+    const next = appReducer(state, { type: 'server-event', event: reply });
+
+    expect(next.channels[0]?.latestEventId).toBe(21);
+    expect(next.unread.c1).toBe(true);
+  });
+});
+
 describe('newestConfirmedMainEventId', () => {
   it('ignores a trailing invisible modifier and unconfirmed rows', () => {
     let state = appReducer(seed(), {
diff --git a/surface/shared/src/timeline.ts b/surface/shared/src/timeline.ts
index b563e23e..00a9f9f3 100644
--- a/surface/shared/src/timeline.ts
+++ b/surface/shared/src/timeline.ts
@@ -295,6 +295,19 @@ export type TimelineOverlay =
       previousHad: boolean;
     };
 
+/**
+ * Events that make a channel unread. The one list: the server's `latest_event_id`
+ * SQL and the live client reducer both read it, so the cold counter and the live
+ * badge cannot disagree about what counts.
+ *
+ * `session.replied` is here because an agent's answer is an ordinary channel
+ * message and marks the channel unread like one — leave it out and the very thing
+ * you asked for lands below the fold with nothing to say it arrived. A thread-rooted
+ * event still only counts when it is broadcast; that clause is applied separately by
+ * each caller.
+ */
+export const CHANNEL_UNREAD_EVENT_TYPES = ['message.posted', 'session.spawned', 'session.replied'] as const;
+
 /** Event types that produce a timeline row. */
 function isRowEvent(type: string): boolean {
   return (