diff --git a/packages/ui/src/components/message-part.css b/packages/ui/src/components/message-part.css
index 78d574d8a135..ae3c6bd88bdd 100644
--- a/packages/ui/src/components/message-part.css
+++ b/packages/ui/src/components/message-part.css
@@ -277,13 +277,35 @@
   }
 }
 
+[data-component="reasoning-part-trigger"] {
+  width: 100%;
+  min-height: 24px;
+  display: flex;
+  align-items: center;
+  justify-content: flex-start;
+  gap: 0px;
+  cursor: pointer;
+
+  [data-slot="reasoning-part-label"] {
+    font-family: var(--font-family-sans);
+    font-size: 14px;
+    font-weight: var(--font-weight-medium);
+    line-height: var(--line-height-large);
+    color: var(--text-strong);
+  }
+
+  [data-slot="collapsible-arrow"] {
+    color: var(--icon-weaker);
+  }
+}
+
 [data-component="reasoning-part"] {
   width: 100%;
   color: var(--text-base);
   line-height: var(--line-height-normal);
 
   [data-component="markdown"] {
-    margin-top: 16px;
+    margin-top: 4px;
     font-style: normal;
     font-size: 13px;
     color: var(--text-weak);
diff --git a/packages/ui/src/components/message-part.tsx b/packages/ui/src/components/message-part.tsx
index 35912e119d40..859c0300e7ad 100644
--- a/packages/ui/src/components/message-part.tsx
+++ b/packages/ui/src/components/message-part.tsx
@@ -181,6 +181,7 @@ export interface MessagePartProps {
   virtualizeDiff?: boolean
   showAssistantCopyPartID?: string | null
   turnDurationMs?: number
+  working?: boolean
 }
 
 export type PartComponent = Component<MessagePartProps>
@@ -705,6 +706,8 @@ export function AssistantParts(props: {
                   return part().get(entry.ref.messageID)?.get(entry.ref.partID)
                 })
 
+                const tail = createMemo(() => entryAccessor().key === last())
+
                 return (
                   <Show when={message()}>
                     <Show when={item()}>
@@ -714,6 +717,7 @@ export function AssistantParts(props: {
                         showAssistantCopyPartID={props.showAssistantCopyPartID}
                         turnDurationMs={props.turnDurationMs}
                         defaultOpen={partDefaultOpen(item()!, props.shellToolDefaultOpen, props.editToolDefaultOpen)}
+                        working={props.working && tail()}
                       />
                     </Show>
                   </Show>
@@ -1274,6 +1278,7 @@ export function Part(props: MessagePartProps) {
         virtualizeDiff={props.virtualizeDiff}
         showAssistantCopyPartID={props.showAssistantCopyPartID}
         turnDurationMs={props.turnDurationMs}
+        working={props.working}
       />
     </Show>
   )
@@ -1580,19 +1585,35 @@ PART_MAPPING["text"] = function TextPartDisplay(props) {
 
 PART_MAPPING["reasoning"] = function ReasoningPartDisplay(props) {
   const data = useData()
+  const i18n = useI18n()
   const part = () => props.part as ReasoningPart
   const streaming = createMemo(
     () => props.message.role === "assistant" && typeof (props.message as AssistantMessage).time.completed !== "number",
   )
   const text = () => readPartText(data.store.part_text_accum_delta, part())
+  const [open, setOpen] = createSignal(true)
 
   return (
     <Show when={text()}>
-      <div data-component="reasoning-part" data-timeline-part-id={part().id}>
-        <Show when={streaming()} fallback={<Markdown text={text()} cacheKey={part().id} streaming={false} />}>
-          <PacedMarkdown text={text()} cacheKey={part().id} streaming={streaming()} />
-        </Show>
-      </div>
+      <Collapsible open={open()} onOpenChange={setOpen} variant="ghost">
+        <Collapsible.Trigger>
+          <div data-component="reasoning-part-trigger">
+            <span data-slot="reasoning-part-label">
+              <Show when={props.working && streaming()} fallback={i18n.t("ui.sessionTurn.status.reasoningSummary")}>
+                <TextShimmer text={i18n.t("ui.sessionTurn.status.reasoning")} />
+              </Show>
+            </span>
+            <Collapsible.Arrow />
+          </div>
+        </Collapsible.Trigger>
+        <Collapsible.Content>
+          <div data-component="reasoning-part" data-timeline-part-id={part().id}>
+            <Show when={streaming()} fallback={<Markdown text={text()} cacheKey={part().id} streaming={false} />}>
+              <PacedMarkdown text={text()} cacheKey={part().id} streaming={streaming()} />
+            </Show>
+          </div>
+        </Collapsible.Content>
+      </Collapsible>
     </Show>
   )
 }
diff --git a/packages/ui/src/i18n/ar.ts b/packages/ui/src/i18n/ar.ts
index dd8cfed6f98f..c1824e9c0d8d 100644
--- a/packages/ui/src/i18n/ar.ts
+++ b/packages/ui/src/i18n/ar.ts
@@ -67,6 +67,8 @@ export const dict = {
   "ui.sessionTurn.status.thinkingWithTopic": "تفكير - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "جمع الأفكار",
   "ui.sessionTurn.status.consideringNextSteps": "النظر في الخطوات التالية",
+  "ui.sessionTurn.status.reasoning": "التفكير",
+  "ui.sessionTurn.status.reasoningSummary": "ملخّص التفكير",
 
   "ui.messagePart.questions.dismissed": "تم رفض الأسئلة",
   "ui.messagePart.compaction": "تم ضغط الجلسة",
diff --git a/packages/ui/src/i18n/br.ts b/packages/ui/src/i18n/br.ts
index 4b2c418ca501..1652653188d4 100644
--- a/packages/ui/src/i18n/br.ts
+++ b/packages/ui/src/i18n/br.ts
@@ -67,6 +67,8 @@ export const dict = {
   "ui.sessionTurn.status.thinkingWithTopic": "Pensando - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "Organizando pensamentos",
   "ui.sessionTurn.status.consideringNextSteps": "Considerando próximos passos",
+  "ui.sessionTurn.status.reasoning": "Raciocínio",
+  "ui.sessionTurn.status.reasoningSummary": "Resumo do Raciocínio",
 
   "ui.messagePart.questions.dismissed": "Perguntas descartadas",
   "ui.messagePart.compaction": "Sessão compactada",
diff --git a/packages/ui/src/i18n/bs.ts b/packages/ui/src/i18n/bs.ts
index be041f147133..c187dff561f8 100644
--- a/packages/ui/src/i18n/bs.ts
+++ b/packages/ui/src/i18n/bs.ts
@@ -71,6 +71,8 @@ export const dict = {
   "ui.sessionTurn.status.thinkingWithTopic": "Razmišljanje - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "Sređivanje misli",
   "ui.sessionTurn.status.consideringNextSteps": "Razmatranje sljedećih koraka",
+  "ui.sessionTurn.status.reasoning": "Razmišljanje",
+  "ui.sessionTurn.status.reasoningSummary": "Sažetak Razmišljanja",
 
   "ui.messagePart.questions.dismissed": "Pitanja odbačena",
   "ui.messagePart.compaction": "Sesija sažeta",
diff --git a/packages/ui/src/i18n/da.ts b/packages/ui/src/i18n/da.ts
index 91adc52e9d45..e69a9bff305d 100644
--- a/packages/ui/src/i18n/da.ts
+++ b/packages/ui/src/i18n/da.ts
@@ -66,6 +66,8 @@ export const dict = {
   "ui.sessionTurn.status.thinkingWithTopic": "Tænker - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "Samler tanker",
   "ui.sessionTurn.status.consideringNextSteps": "Overvejer næste skridt",
+  "ui.sessionTurn.status.reasoning": "Ræsonnering",
+  "ui.sessionTurn.status.reasoningSummary": "Ræsonnerings Oversigt",
 
   "ui.messagePart.questions.dismissed": "Spørgsmål afvist",
   "ui.messagePart.compaction": "Session komprimeret",
diff --git a/packages/ui/src/i18n/de.ts b/packages/ui/src/i18n/de.ts
index 7b36859bf6de..bfcb83feeb7c 100644
--- a/packages/ui/src/i18n/de.ts
+++ b/packages/ui/src/i18n/de.ts
@@ -72,6 +72,8 @@ export const dict = {
   "ui.sessionTurn.status.thinkingWithTopic": "Denken - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "Gedanken sammeln",
   "ui.sessionTurn.status.consideringNextSteps": "Nächste Schritte erwägen",
+  "ui.sessionTurn.status.reasoning": "Überlegung",
+  "ui.sessionTurn.status.reasoningSummary": "Überlegungs Zusammenfassung",
 
   "ui.messagePart.questions.dismissed": "Fragen verworfen",
   "ui.messagePart.compaction": "Sitzung komprimiert",
diff --git a/packages/ui/src/i18n/en.ts b/packages/ui/src/i18n/en.ts
index 0f691a214cac..4a6a7e4fe5df 100644
--- a/packages/ui/src/i18n/en.ts
+++ b/packages/ui/src/i18n/en.ts
@@ -74,6 +74,8 @@ export const dict: Record<string, string> = {
   "ui.sessionTurn.status.thinkingWithTopic": "Thinking - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "Gathering thoughts",
   "ui.sessionTurn.status.consideringNextSteps": "Considering next steps",
+  "ui.sessionTurn.status.reasoning": "Reasoning",
+  "ui.sessionTurn.status.reasoningSummary": "Reasoning Summary",
 
   "ui.messagePart.diagnostic.error": "Error",
   "ui.messagePart.title.edit": "Edit",
diff --git a/packages/ui/src/i18n/es.ts b/packages/ui/src/i18n/es.ts
index f883531031b0..d4a229b3c1db 100644
--- a/packages/ui/src/i18n/es.ts
+++ b/packages/ui/src/i18n/es.ts
@@ -67,6 +67,8 @@ export const dict = {
   "ui.sessionTurn.status.thinkingWithTopic": "Pensando - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "Recopilando pensamientos",
   "ui.sessionTurn.status.consideringNextSteps": "Considerando siguientes pasos",
+  "ui.sessionTurn.status.reasoning": "Razonamiento",
+  "ui.sessionTurn.status.reasoningSummary": "Resumen del Razonamiento",
 
   "ui.messagePart.questions.dismissed": "Preguntas descartadas",
   "ui.messagePart.compaction": "Sesión compactada",
diff --git a/packages/ui/src/i18n/fr.ts b/packages/ui/src/i18n/fr.ts
index 3ae122d35eeb..f3cf08aee35e 100644
--- a/packages/ui/src/i18n/fr.ts
+++ b/packages/ui/src/i18n/fr.ts
@@ -67,6 +67,8 @@ export const dict = {
   "ui.sessionTurn.status.thinkingWithTopic": "Réflexion - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "Rassemblement des idées",
   "ui.sessionTurn.status.consideringNextSteps": "Examen des prochaines étapes",
+  "ui.sessionTurn.status.reasoning": "Raisonnement",
+  "ui.sessionTurn.status.reasoningSummary": "Résumé du Raisonnement",
 
   "ui.messagePart.questions.dismissed": "Questions ignorées",
   "ui.messagePart.compaction": "Session compactée",
diff --git a/packages/ui/src/i18n/ja.ts b/packages/ui/src/i18n/ja.ts
index 1b50cb5ff158..1110f4301a32 100644
--- a/packages/ui/src/i18n/ja.ts
+++ b/packages/ui/src/i18n/ja.ts
@@ -66,6 +66,8 @@ export const dict = {
   "ui.sessionTurn.status.thinkingWithTopic": "思考中 - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "考えをまとめています",
   "ui.sessionTurn.status.consideringNextSteps": "次のステップを検討中",
+  "ui.sessionTurn.status.reasoning": "推論",
+  "ui.sessionTurn.status.reasoningSummary": "推論サマリー",
 
   "ui.messagePart.questions.dismissed": "質問をスキップしました",
   "ui.messagePart.compaction": "セッションを圧縮しました",
diff --git a/packages/ui/src/i18n/ko.ts b/packages/ui/src/i18n/ko.ts
index d6c480364d3f..8dae09a3436f 100644
--- a/packages/ui/src/i18n/ko.ts
+++ b/packages/ui/src/i18n/ko.ts
@@ -67,6 +67,8 @@ export const dict = {
   "ui.sessionTurn.status.thinkingWithTopic": "생각 중 - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "생각 정리 중",
   "ui.sessionTurn.status.consideringNextSteps": "다음 단계 고려 중",
+  "ui.sessionTurn.status.reasoning": "추론",
+  "ui.sessionTurn.status.reasoningSummary": "추론 요약",
 
   "ui.messagePart.questions.dismissed": "질문 무시됨",
   "ui.messagePart.compaction": "세션 압축됨",
diff --git a/packages/ui/src/i18n/no.ts b/packages/ui/src/i18n/no.ts
index ecf67219d52f..53a826e2f288 100644
--- a/packages/ui/src/i18n/no.ts
+++ b/packages/ui/src/i18n/no.ts
@@ -70,6 +70,8 @@ export const dict: Record<Keys, string> = {
   "ui.sessionTurn.status.thinkingWithTopic": "Tenker - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "Samler tanker",
   "ui.sessionTurn.status.consideringNextSteps": "Vurderer neste trinn",
+  "ui.sessionTurn.status.reasoning": "Resonnering",
+  "ui.sessionTurn.status.reasoningSummary": "Resonnerings Sammendrag",
 
   "ui.messagePart.questions.dismissed": "Spørsmål avvist",
   "ui.messagePart.compaction": "Økt komprimert",
diff --git a/packages/ui/src/i18n/pl.ts b/packages/ui/src/i18n/pl.ts
index 6ef40d2dafdc..ee509ba50941 100644
--- a/packages/ui/src/i18n/pl.ts
+++ b/packages/ui/src/i18n/pl.ts
@@ -66,6 +66,8 @@ export const dict = {
   "ui.sessionTurn.status.thinkingWithTopic": "Myślenie - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "Zbieranie myśli",
   "ui.sessionTurn.status.consideringNextSteps": "Rozważanie kolejnych kroków",
+  "ui.sessionTurn.status.reasoning": "Rozumowanie",
+  "ui.sessionTurn.status.reasoningSummary": "Podsumowanie Rozumowania",
 
   "ui.messagePart.questions.dismissed": "Pytania odrzucone",
   "ui.messagePart.compaction": "Sesja skompaktowana",
diff --git a/packages/ui/src/i18n/ru.ts b/packages/ui/src/i18n/ru.ts
index ed5c4e8f4f7c..1f583831055e 100644
--- a/packages/ui/src/i18n/ru.ts
+++ b/packages/ui/src/i18n/ru.ts
@@ -66,6 +66,8 @@ export const dict = {
   "ui.sessionTurn.status.thinkingWithTopic": "Размышление - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "Сбор мыслей",
   "ui.sessionTurn.status.consideringNextSteps": "Рассмотрение следующих шагов",
+  "ui.sessionTurn.status.reasoning": "Рассуждение",
+  "ui.sessionTurn.status.reasoningSummary": "Сводка Рассуждений",
 
   "ui.messagePart.questions.dismissed": "Вопросы отклонены",
   "ui.messagePart.compaction": "Сессия сжата",
diff --git a/packages/ui/src/i18n/th.ts b/packages/ui/src/i18n/th.ts
index e2534041ab20..1e5d014e6c3d 100644
--- a/packages/ui/src/i18n/th.ts
+++ b/packages/ui/src/i18n/th.ts
@@ -68,6 +68,8 @@ export const dict = {
   "ui.sessionTurn.status.thinkingWithTopic": "กำลังคิด - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "รวบรวมความคิด",
   "ui.sessionTurn.status.consideringNextSteps": "พิจารณาขั้นตอนถัดไป",
+  "ui.sessionTurn.status.reasoning": "การใช้เหตุผล",
+  "ui.sessionTurn.status.reasoningSummary": "สรุปการใช้เหตุผล",
 
   "ui.messagePart.questions.dismissed": "ละทิ้งคำถามแล้ว",
   "ui.messagePart.compaction": "บีบอัดเซสชันแล้ว",
diff --git a/packages/ui/src/i18n/tr.ts b/packages/ui/src/i18n/tr.ts
index 0ce4a73b6f52..356766592feb 100644
--- a/packages/ui/src/i18n/tr.ts
+++ b/packages/ui/src/i18n/tr.ts
@@ -73,6 +73,8 @@ export const dict = {
   "ui.sessionTurn.status.thinkingWithTopic": "Düşünüyor - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "Düşünceler toplanıyor",
   "ui.sessionTurn.status.consideringNextSteps": "Sonraki adımlar değerlendiriliyor",
+  "ui.sessionTurn.status.reasoning": "Akıl Yürütme",
+  "ui.sessionTurn.status.reasoningSummary": "Akıl Yürütme Özeti",
 
   "ui.messagePart.questions.dismissed": "Sorular reddedildi",
   "ui.messagePart.compaction": "Oturum sıkıştırıldı",
diff --git a/packages/ui/src/i18n/uk.ts b/packages/ui/src/i18n/uk.ts
index 8bfb3e4404dc..e8d04fc01889 100644
--- a/packages/ui/src/i18n/uk.ts
+++ b/packages/ui/src/i18n/uk.ts
@@ -65,6 +65,8 @@ export const dict: Record<string, string> = {
   "ui.sessionTurn.status.thinkingWithTopic": "Міркування — {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "Збирання думок",
   "ui.sessionTurn.status.consideringNextSteps": "Розгляд наступних кроків",
+  "ui.sessionTurn.status.reasoning": "Міркування",
+  "ui.sessionTurn.status.reasoningSummary": "Підсумок міркувань",
 
   "ui.messagePart.diagnostic.error": "Помилка",
   "ui.messagePart.title.edit": "Редагувати",
diff --git a/packages/ui/src/i18n/zh.ts b/packages/ui/src/i18n/zh.ts
index 1a385620f191..8e4ebab5bad0 100644
--- a/packages/ui/src/i18n/zh.ts
+++ b/packages/ui/src/i18n/zh.ts
@@ -70,6 +70,8 @@ export const dict = {
   "ui.sessionTurn.status.thinkingWithTopic": "思考：{{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "正在整理思路",
   "ui.sessionTurn.status.consideringNextSteps": "正在考虑下一步",
+  "ui.sessionTurn.status.reasoning": "推理",
+  "ui.sessionTurn.status.reasoningSummary": "推理摘要",
 
   "ui.messagePart.questions.dismissed": "问题已忽略",
   "ui.messagePart.compaction": "会话已压缩",
diff --git a/packages/ui/src/i18n/zht.ts b/packages/ui/src/i18n/zht.ts
index 941e1df636b7..8e969a2f5bb2 100644
--- a/packages/ui/src/i18n/zht.ts
+++ b/packages/ui/src/i18n/zht.ts
@@ -70,6 +70,8 @@ export const dict = {
   "ui.sessionTurn.status.thinkingWithTopic": "思考 - {{topic}}",
   "ui.sessionTurn.status.gatheringThoughts": "正在整理思緒",
   "ui.sessionTurn.status.consideringNextSteps": "正在考慮下一步",
+  "ui.sessionTurn.status.reasoning": "推理",
+  "ui.sessionTurn.status.reasoningSummary": "推理摘要",
 
   "ui.messagePart.questions.dismissed": "問題已略過",
   "ui.messagePart.compaction": "工作階段已壓縮",