diff --git a/web/src/pages/ChatPage.tsx b/web/src/pages/ChatPage.tsx
index 6bca795bde..a152d9c331 100644
--- a/web/src/pages/ChatPage.tsx
+++ b/web/src/pages/ChatPage.tsx
@@ -205,6 +205,16 @@ export default function ChatPage({ isActive = true }: { isActive?: boolean }) {
   useEffect(() => {
     setHasActivated((prev) => latchChatActivation(prev, isActive));
   }, [isActive]);
+
+  // Clear mobile-input tracking refs when the tab is hidden so stale state
+  // from a previous /chat visit doesn't cause the mobile-replacement logic
+  // to misfire on the next activation (#106403: repeated last character).
+  useEffect(() => {
+    if (!isActive) {
+      ptyInputLineRef.current = "";
+      mobileReplacementInputUntilRef.current = 0;
+    }
+  }, [isActive]);
   const [searchParams, setSearchParams] = useSearchParams();
   // Lazy-init: the missing-token check happens at construction so the effect
   // body doesn't have to setState (React 19's set-state-in-effect rule).