diff --git a/.jules/bolt.md b/.jules/bolt.md
index edb687422..9754a0845 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -5,3 +5,7 @@
 ## 2026-10-15 - [Performance] Prevent unnecessary UI array recalculations
 **Learning:** In the chat component (`useThreadList`), derived state that requires filtering and sorting large arrays (like generating nested agent lists) was running synchronously on every render. Given that chat polling triggers frequent re-renders, this caused unnecessary CPU strain on lists that only rarely change.
 **Action:** Use `useMemo` for derived states like `rows` and `current` in custom hooks when they involve `.filter()`, `.map()`, or `.sort()` and depend on data that updates less frequently than the component renders.
+
+## 2026-10-27 - [Performance] Optimize Chat Thread Autoscroll Signature Generation
+**Learning:** During chat polling and streaming, \`useStickToBottom\` in \`ChatThread.tsx\` generates a signature \`sig\` on every render to detect if the chat content has grown and needs to be autoscrolled. It was using \`[...].join('|')\`. In \`chat-messages.ts\`, \`messagesSignature\` was also using \`[...].join('|')\` for a similar purpose. \`Array.prototype.join()\` allocates an intermediate array and then creates the string, causing CPU and memory overhead on hot paths like frequent polling and streaming renders.
+**Action:** Replaced \`[...].join('|')\` with direct template string concatenation (\`\` `${a}|${b}|...` \`\`) to avoid the intermediate array allocation and improve performance. Prefer direct string concatenation over \`Array.prototype.join()\` on hot paths.
diff --git a/apps/web/src/components/chat/ChatThread.tsx b/apps/web/src/components/chat/ChatThread.tsx
index 50a0209c1..4ed9b6c43 100644
--- a/apps/web/src/components/chat/ChatThread.tsx
+++ b/apps/web/src/components/chat/ChatThread.tsx
@@ -18,16 +18,7 @@ export function useStickToBottom(messages: ChatMessage[], resetKey: unknown) {
     if (!box) return;
     // On ne défile que si le contenu a réellement changé.
     const last = messages[messages.length - 1];
-    const sig = [
-      messages.length,
-      last?.content.length ?? 0,
-      last?.toolCalls?.length ?? 0,
-      last?.actions?.length ?? 0,
-      last?.reflections?.length ?? 0,
-      last?.milestones?.length ?? 0,
-      last?.liveLabel ?? '',
-      last?.streaming ? 1 : 0,
-    ].join('|');
+    const sig = `${messages.length}|${last?.content.length ?? 0}|${last?.toolCalls?.length ?? 0}|${last?.actions?.length ?? 0}|${last?.reflections?.length ?? 0}|${last?.milestones?.length ?? 0}|${last?.liveLabel ?? ''}|${last?.streaming ? 1 : 0}`;
     if (sig === lastSig.current) return;
     const first = lastSig.current === '';
     lastSig.current = sig;
diff --git a/apps/web/src/lib/chat-messages.ts b/apps/web/src/lib/chat-messages.ts
index 4b5db1852..a4f9f2dbc 100644
--- a/apps/web/src/lib/chat-messages.ts
+++ b/apps/web/src/lib/chat-messages.ts
@@ -50,13 +50,7 @@ export function messagesSignature(messages: IdentifiedMessage[]): string {
     }
   }
 
-  return [
-    messages.length,
-    last?.id ?? '',
-    last?.content.length ?? 0,
-    last?.toolCalls?.length ?? 0,
-    reactionsSig,
-  ].join('|');
+  return `${messages.length}|${last?.id ?? ''}|${last?.content.length ?? 0}|${last?.toolCalls?.length ?? 0}|${reactionsSig}`;
 }
 
 /**