diff --git a/packages/app/src/pages/ChatPage.vue b/packages/app/src/pages/ChatPage.vue
index 11b5185a..52581cc6 100644
--- a/packages/app/src/pages/ChatPage.vue
+++ b/packages/app/src/pages/ChatPage.vue
@@ -477,9 +477,23 @@ function onKeydown(e: KeyboardEvent) {
   }
 }
 
+// D-14b (Archipelago phase 02-ui-performance): when the host explicitly asks
+// for a chat-first mobile start via ?mobileChat, force the initial mobile tab
+// to 'chat' on mount. mobileTab already defaults to 'chat', but ChatPage can
+// remount while content/detail selection state (module-singleton refs in
+// useContentPanel) survives from a previous mount within the same AIUI
+// session — this guarantees the documented default regardless of that carry-
+// over. It only runs once, at mount, so it never interferes with the
+// hasDetailOpen/panelOpen watchers below responding to real user taps after
+// the initial paint.
+const startOnMobileChat = new URLSearchParams(window.location.search).has('mobileChat')
+
 onMounted(() => {
   window.addEventListener('resize', onResize)
   window.addEventListener('keydown', onKeydown)
+  if (startOnMobileChat && isMobile.value) {
+    mobileTab.value = 'chat'
+  }
 })
 onUnmounted(() => {
   window.removeEventListener('resize', onResize)
diff --git a/packages/app/src/stores/chat.ts b/packages/app/src/stores/chat.ts
index 434e3875..806a8a49 100644
--- a/packages/app/src/stores/chat.ts
+++ b/packages/app/src/stores/chat.ts
@@ -129,7 +129,16 @@ export const useChatStore = defineStore('chat', () => {
   const panelSide = ref<'left' | 'right'>(savedSide ?? 'left')
 
   const webSearchEnabled = ref(localStorage.getItem('aiui-web-search') !== 'false')
-  const chatCollapsed = ref(localStorage.getItem('aiui-chat-collapsed') !== 'false')
+
+  // D-14a (Archipelago phase 02-ui-performance): when the host explicitly asks
+  // for an expanded start via ?chatExpanded, open in the full message list
+  // rather than the collapsed prompt-index — this is a presentation-only,
+  // one-time initial-state override and is never written back to
+  // localStorage, so it can never change the standalone (non-embedded) app's
+  // own persisted default. A user who manually collapses while embedded still
+  // gets that choice persisted for next time, same as before this change.
+  const _startExpanded = new URLSearchParams(window.location.search).has('chatExpanded')
+  const chatCollapsed = ref(_startExpanded ? false : localStorage.getItem('aiui-chat-collapsed') !== 'false')
   const showHistory = ref(false)
 
   // Load chats: try IndexedDB first, fall back to dev-chats middleware