fix(ui): smooth reasoning collapse animation

packages/ui/src/components/chat/message/parts/ReasoningPart.tsx

@@ -209,7 +209,7 @@ export const ReasoningTimelineBlock: React.FC<ReasoningTimelineBlockProps> = ({
 
                     {isStreaming ? (
                         <span className="flex items-center gap-1 typography-meta font-medium" style={{ color: 'var(--tools-title)' }}>
-                            <span>{t('chat.reasoningTrace.reasoning')}</span>
+                            <span>{t(variant === 'justification' ? 'chat.reasoningTrace.justification' : 'chat.reasoningTrace.thinking')}</span>
                             <BusyDots />
                         </span>
                     ) : isExpanded ? (
@@ -244,12 +244,17 @@ export const ReasoningTimelineBlock: React.FC<ReasoningTimelineBlockProps> = ({
                 </div>
             </div>
 
-            {/* Expanded content — left border matching ToolPart */}
+            {/* Expanded content — keep mounted so auto-collapse can animate smoothly. */}
-            {isExpanded && (
             <div
                 id={contentId}
-                    className="relative ml-2 pl-3 pb-1 pt-0.5"
+                aria-hidden={!isExpanded}
+                className={cn(
+                    'grid transition-[grid-template-rows,opacity] duration-[350ms] ease-out motion-reduce:transition-none',
+                    isExpanded ? 'grid-rows-[1fr] opacity-100' : 'grid-rows-[0fr] opacity-0',
+                )}
             >
+                <div className="min-h-0 overflow-hidden">
+                    <div className="relative ml-2 pl-3 pb-1 pt-0.5">
                         <span
                             aria-hidden="true"
                             className="pointer-events-none absolute left-0 top-0 bottom-0 w-px"
@@ -282,7 +287,8 @@ export const ReasoningTimelineBlock: React.FC<ReasoningTimelineBlockProps> = ({
                             ) : null}
                         </ScrollableOverlay>
                     </div>
-            )}
+                </div>
+            </div>
         </div>
     );
 };

packages/ui/src/lib/i18n/messages/en.ts

@@ -1687,7 +1687,6 @@ export const dict = {
   'chat.modelControls.modeValue.all': 'All',
   'chat.modelControls.modeValue.none': '—',
   'chat.reasoningTrace.thinking': 'Thinking',
-  'chat.reasoningTrace.reasoning': 'Reasoning',
   'chat.reasoningTrace.justification': 'Justification',
   'chat.reasoningTrace.expandAria': 'Expand reasoning trace',
   'chat.reasoningTrace.collapseAria': 'Collapse reasoning trace',

packages/ui/src/lib/i18n/messages/es.ts

@@ -1653,7 +1653,6 @@ export const dict: Record<I18nKey, string> = {
   "chat.modelControls.modeValue.all": "Todos",
   "chat.modelControls.modeValue.none": "—",
   "chat.reasoningTrace.thinking": "Pensando",
-  "chat.reasoningTrace.reasoning": "Razonando",
   "chat.reasoningTrace.justification": "Justificación",
   "chat.reasoningTrace.expandAria": "Expandir traza de razonamiento",
   "chat.reasoningTrace.collapseAria": "Contraer traza de razonamiento",

packages/ui/src/lib/i18n/messages/ko.ts

@@ -1687,7 +1687,6 @@ export const dict: Record<I18nKey, string> = {
   'chat.modelControls.modeValue.all': '전체',
   'chat.modelControls.modeValue.none': '—',
   'chat.reasoningTrace.thinking': '추론',
-  'chat.reasoningTrace.reasoning': '추론 중',
   'chat.reasoningTrace.justification': '근거',
   'chat.reasoningTrace.expandAria': '추론 트레이스 펼치기',
   'chat.reasoningTrace.collapseAria': '추론 트레이스 접기',

packages/ui/src/lib/i18n/messages/pl.ts

@@ -890,7 +890,6 @@ export const dict: Record<I18nKey, string> = {
   'chat.messageBody.subtask.showPrompt': 'Pokaż prompt',
   'chat.messageBody.subtask.title': 'Zadanie delegowane',
   'chat.reasoningTrace.thinking': 'Myślenie',
-  'chat.reasoningTrace.reasoning': 'Rozumowanie',
   'chat.reasoningTrace.justification': 'Uzasadnienie',
   'chat.reasoningTrace.expandAria': 'Rozwiń ślad rozumowania',
   'chat.reasoningTrace.collapseAria': 'Zwiń ślad rozumowania',

packages/ui/src/lib/i18n/messages/pt-BR.ts

@@ -1653,7 +1653,6 @@ export const dict: Record<I18nKey, string> = {
   "chat.modelControls.modeValue.all": "Todos",
   "chat.modelControls.modeValue.none": "—",
   "chat.reasoningTrace.thinking": "Pensando",
-  "chat.reasoningTrace.reasoning": "Raciocinando",
   "chat.reasoningTrace.justification": "Justificativa",
   "chat.reasoningTrace.expandAria": "Expandir rastro de raciocínio",
   "chat.reasoningTrace.collapseAria": "Recolher rastro de raciocínio",

packages/ui/src/lib/i18n/messages/uk.ts

@@ -1653,7 +1653,6 @@ export const dict: Record<I18nKey, string> = {
   "chat.modelControls.modeValue.all": "всі",
   "chat.modelControls.modeValue.none": "—",
   "chat.reasoningTrace.thinking": "Міркування",
-  "chat.reasoningTrace.reasoning": "Міркую",
   "chat.reasoningTrace.justification": "Обґрунтування",
   "chat.reasoningTrace.expandAria": "Розгорнути слід міркування",
   "chat.reasoningTrace.collapseAria": "Згорнути слід міркування",

packages/ui/src/lib/i18n/messages/zh-CN.ts

@@ -1653,7 +1653,6 @@ export const dict: Record<I18nKey, string> = {
   'chat.modelControls.modeValue.all': '全部',
   'chat.modelControls.modeValue.none': '—',
   'chat.reasoningTrace.thinking': '思考',
-  'chat.reasoningTrace.reasoning': '推理',
   'chat.reasoningTrace.justification': '理由',
   'chat.reasoningTrace.expandAria': '展开推理轨迹',
   'chat.reasoningTrace.collapseAria': '折叠推理轨迹',