diff --git a/apps/web/src/components/chat/MessagesTimeline.tsx b/apps/web/src/components/chat/MessagesTimeline.tsx
index 483859ba03005fb62c090ff93b2ac307ed1fa5be..03775a22f1090754f506dff04839d1132327525e 100644
--- a/apps/web/src/components/chat/MessagesTimeline.tsx
+++ b/apps/web/src/components/chat/MessagesTimeline.tsx
@@ -2641,6 +2641,7 @@ function ActivityGroupTimelineRow({
         details.push(
           <ReasoningTraceBlock
             key={entry.id}
+            anchorKey={row.id}
             messages={messages}
             live={row.active && index === row.entries.length - 1}
             showHeader={work.length > 0}
@@ -2667,7 +2668,7 @@ function ActivityGroupTimelineRow({
           shimmer={thinking}
         />
       </button>
-      {row.expanded ? <div className="mt-2 space-y-2">{details}</div> : null}
+      {row.expanded ? <div className="mt-2">{details}</div> : null}
     </div>
   );
 }
@@ -2685,15 +2686,16 @@ function ThinkingTimelineRow() {
 }
 
 /**
- * Thinking inside an expanded activity group: the trace is already one click
- * deep, so the text renders under its "Thought" header without another toggle.
+ * Thinking inside a tool group has its own disclosure, preserved across recycling.
  * A group whose row already reads "Thought" (no visible tool) skips the header.
  */
 function ReasoningTraceBlock({
+  anchorKey,
   messages,
   live,
   showHeader,
 }: {
+  anchorKey: string;
   messages: ReadonlyArray<ChatMessage>;
   live: boolean;
   showHeader: boolean;
@@ -2701,6 +2703,7 @@ function ReasoningTraceBlock({
   const ctx = use(TimelineRowCtx);
   const { isWorking, unsettledTurnId } = use(TimelineRowActivityCtx);
   const first = messages[0]!;
+  const expanded = !showHeader || ctx.expandedReasoningMessageIds.has(first.id);
   const streaming =
     live &&
     messages.some((reasoningMessage) => reasoningMessage.streaming) &&
@@ -2714,10 +2717,17 @@ function ReasoningTraceBlock({
     return null;
   }
   const label = streaming ? "Thinking" : "Thought";
+  const collapsedPreview = messages.find((message) => message.text.trim().length > 0)?.text.trim();
+  const headerText = expanded ? label : (collapsedPreview ?? label);
   return (
     <div className="flex flex-col">
       {showHeader ? (
-        <div className="flex min-h-6 select-none items-center gap-1.5 px-0.5 py-0.5 text-sm leading-relaxed">
+        <button
+          type="button"
+          aria-expanded={expanded}
+          onClick={() => ctx.onToggleReasoning(first.id, !expanded, anchorKey)}
+          className="flex min-h-6 cursor-pointer select-none items-center gap-1.5 rounded-md px-0.5 text-start text-sm leading-relaxed transition-colors hover:bg-accent/20 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-inset focus-visible:ring-ring/70"
+        >
           <span className="flex size-6 shrink-0 items-center justify-center text-icon-muted">
             <BrainIcon aria-hidden className="block size-4 shrink-0 stroke-[1.8] opacity-70" />
           </span>
@@ -2725,28 +2735,38 @@ function ReasoningTraceBlock({
             ref={streaming ? observeVisibleAnimation : undefined}
             className="relative min-w-0 flex-1 truncate text-secondary-label"
           >
-            {label}
-            {streaming ? <ActivityShimmerOverlay>{label}</ActivityShimmerOverlay> : null}
+            {headerText}
+            {streaming ? <ActivityShimmerOverlay>{headerText}</ActivityShimmerOverlay> : null}
           </span>
+          <span className="flex size-4 shrink-0 items-center justify-center" aria-hidden>
+            <ChevronRightIcon
+              className={cn(
+                "size-3 shrink-0 text-icon-muted opacity-70 transition-transform duration-200",
+                expanded && "rotate-90",
+              )}
+            />
+          </span>
+        </button>
+      ) : null}
+      {expanded ? (
+        <div className="ms-7 flex max-h-96 flex-col gap-3 overflow-auto px-0.5 py-1 select-text">
+          {messages.map((reasoningMessage) => (
+            <ChatMarkdown
+              key={reasoningMessage.id}
+              className="text-foreground"
+              text={reasoningMessage.text}
+              cwd={ctx.markdownCwd}
+              threadRef={ctx.threadRef ?? undefined}
+              isStreaming={streaming && reasoningMessage.streaming}
+              lineBreaks
+              skills={ctx.skills}
+              headingLevelOffset={MESSAGE_HEADING_LEVEL}
+              onUseArtifactTemplate={ctx.onUseArtifactTemplate}
+              onImageExpand={ctx.onImageExpand}
+            />
+          ))}
         </div>
       ) : null}
-      <div className="ms-7 flex max-h-96 flex-col gap-3 overflow-auto px-0.5 py-1 select-text">
-        {messages.map((reasoningMessage) => (
-          <ChatMarkdown
-            key={reasoningMessage.id}
-            className="text-foreground"
-            text={reasoningMessage.text}
-            cwd={ctx.markdownCwd}
-            threadRef={ctx.threadRef ?? undefined}
-            isStreaming={streaming && reasoningMessage.streaming}
-            lineBreaks
-            skills={ctx.skills}
-            headingLevelOffset={MESSAGE_HEADING_LEVEL}
-            onUseArtifactTemplate={ctx.onUseArtifactTemplate}
-            onImageExpand={ctx.onImageExpand}
-          />
-        ))}
-      </div>
     </div>
   );
 }