diff --git a/ARCHITECTURE.md b/ARCHITECTURE.md
index 7714c37..87538b6 100644
--- a/ARCHITECTURE.md
+++ b/ARCHITECTURE.md
@@ -243,6 +243,17 @@ All admin operations go through PB admin API (Hono proxy or `+page.server.ts`).
 - Audit trail option
 - Consistent error handling
 
+### 5.5 Family Chat
+
+Real-time right-slideout chat panel (TopNav chat icon, slideout on desktop / full-screen on mobile).
+
+- **Writes go through the Hono proxy** (`POST /api/chat/:famId/messages`, `POST /api/chat/:famId/typing`) — the proxy resolves the actor via the `session` cookie / `x-session-*` headers (admin) or `x-device-token` headers (member). `GET /api/chat/me` returns the actor's identity (`{id, type, name, color}`) for both roles.
+- **Reads use the anonymous PB SDK client-side** (`chatStore` in `frontend/src/lib/stores/chat.svelte.ts`), the same pattern as `famStore`. Collections `messages` and `chat_typing` have public `listRule`/`viewRule` (empty string); browser `.subscribe('*')` gives realtime SSE sync for both roles.
+- **Collections:** `messages` (famId, authorType admin/member, authorId, authorName, authorColor, content, `createdAt`), `chat_typing` (transient per-actor presence: famId, actorId, actorType, authorName, authorColor, typing).
+- **History** = last 14 days. Filter uses the custom `createdAt` field — the auto `created`/`updated` fields are NOT filterable in this PocketBase version (400), so a custom date field is set by the proxy at create time.
+- **`createdAt` vs `created`:** all sorting, optimistic-temp messages, and the timestamp label use `createdAt`. The PB auto `created` field is not returned on records, so referencing it throws (`localeCompare` of undefined).
+- **UI** (`Chat.svelte`): @mention tokens, typing indicators, unread badge, optimistic send with reconcile. The panel starts closed (no auto-open on mount) and is toggled via `chatStore.toggle()`.
+
 ---
 
 ## 6. Project Structure