From 2ac798694727d4f3ae534ae54907dd0cf7a13e86 Mon Sep 17 00:00:00 2001
From: Danny Avila <danny@librechat.ai>
Date: Sun, 23 Aug 2026 15:20:28 -0400
Subject: [PATCH] =?UTF-8?q?=F0=9F=AB=82=20fix:=20Route=20Subagent=20Activi?=
 =?UTF-8?q?ty=20Through=20the=20Chat=20Renderer=20(#15137)?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

* fix: align subagent activity with chat UI

* fix: preserve subagent activity boundaries

* fix: preserve subagent panel state semantics

* fix: preserve subagent activity metadata

* fix: preserve live subagent event metadata

* fix: scope subagent phases by message step

* fix: retire closed subagent message phases
---
 .../Messages/Content/AskUserQuestionCall.tsx  |  21 +-
 .../components/Chat/Messages/Content/Part.tsx |   1 +
 .../Messages/Content/Parts/SubagentCall.tsx   |  81 ++++---
 .../Parts/__tests__/SubagentCall.test.tsx     |  54 +++++
 .../__tests__/AskUserQuestionCall.test.tsx    |  18 ++
 .../SharedSubagentActivityDialog.test.tsx     |  16 ++
 .../SharedSubagentActivityDialog.tsx          |   9 +-
 .../Chat/Subagents/SubagentActivity.test.tsx  | 171 ++++++++++++-
 .../Chat/Subagents/SubagentActivity.tsx       | 224 ++++++------------
 .../Chat/Subagents/SubagentThreadPanel.tsx    |   1 +
 .../Chat/Subagents/adapters.test.ts           | 220 ++++++++++++++++-
 .../src/components/Chat/Subagents/adapters.ts |  99 +++++++-
 client/src/locales/en/translation.json        |   1 -
 client/src/store/subagents.ts                 |   1 +
 .../utils/__tests__/subagentContent.test.ts   | 124 ++++++++++
 client/src/utils/subagentContent.ts           | 130 ++++++++--
 e2e/specs/mock/subagent-activity.spec.ts      |   1 -
 packages/data-provider/src/types/runs.ts      |   1 +
 18 files changed, 956 insertions(+), 217 deletions(-)

diff --git a/client/src/components/Chat/Messages/Content/AskUserQuestionCall.tsx b/client/src/components/Chat/Messages/Content/AskUserQuestionCall.tsx
index 57675883ee..8ff7d55629 100644
--- a/client/src/components/Chat/Messages/Content/AskUserQuestionCall.tsx
+++ b/client/src/components/Chat/Messages/Content/AskUserQuestionCall.tsx
@@ -1,7 +1,7 @@
 import { useState, useEffect, useCallback } from 'react';
 import { useRecoilValue } from 'recoil';
 import { MessageCircleQuestion, TriangleAlert } from 'lucide-react';
-import type { Agents } from 'librechat-data-provider';
+import type { Agents, PartMetadata } from 'librechat-data-provider';
 import {
   getSubmittedAskAnswer,
   parseAskUserQuestionArgs,
@@ -32,6 +32,7 @@ export default function AskUserQuestionCall({
   output,
   toolCallId,
   isSubmitting = false,
+  runStepStatus,
   failed = false,
   showCursor = false,
   onExpand,
@@ -40,6 +41,7 @@ export default function AskUserQuestionCall({
   output: string;
   toolCallId?: string;
   isSubmitting?: boolean;
+  runStepStatus?: PartMetadata['runStepStatus'];
   failed?: boolean;
   showCursor?: boolean;
   onExpand?: () => void;
@@ -87,8 +89,9 @@ export default function AskUserQuestionCall({
       return null;
     }
   })();
+  const terminalFailure = failed || runStepStatus === 'failed';
   const answered =
-    !failed &&
+    !terminalFailure &&
     (batch != null
       ? batch.questions.every((item) => typeof batchAnswers?.[item.id] === 'string')
       : effectiveOutput.length > 0);
@@ -103,7 +106,7 @@ export default function AskUserQuestionCall({
    * immediately; an abandoned pause only shows its "no answer" state after
    * the turn is no longer submitting.
    */
-  if (!answered && !failed && isSubmitting) {
+  if (!answered && !terminalFailure && runStepStatus == null && isSubmitting) {
     return <AskUserQuestionProgress args={args} toolCallId={toolCallId} />;
   }
 
@@ -131,7 +134,7 @@ export default function AskUserQuestionCall({
    * question header on `!isSubmitting`.
    */
   const statusLabel = (() => {
-    if (failed) {
+    if (terminalFailure) {
       return localize('com_ui_question_failed');
     }
     return count > 1
@@ -155,7 +158,7 @@ export default function AskUserQuestionCall({
           finishedText={statusLabel}
           subtitle={summary}
           icon={
-            failed ? (
+            terminalFailure ? (
               <TriangleAlert className="size-4 shrink-0 text-text-warning" aria-hidden="true" />
             ) : (
               <MessageCircleQuestion
@@ -171,7 +174,7 @@ export default function AskUserQuestionCall({
           cannot see coming. The explanation lives in the panel, which starts
           closed and is `inert` while it is — so the announcement has to sit
           outside the disclosure to reach the accessibility tree at all. */}
-      {failed && (
+      {terminalFailure && (
         <span className="sr-only" role="status">
           {`${statusLabel}. ${localize('com_ui_question_failed_description')}`}
         </span>
@@ -194,7 +197,7 @@ export default function AskUserQuestionCall({
                     answer={batchAnswers?.[item.id]}
                     options={item.options}
                     multiSelect={item.multiSelect}
-                    failed={failed}
+                    failed={terminalFailure}
                   />
                 </div>
               ))
@@ -205,10 +208,10 @@ export default function AskUserQuestionCall({
                 answer={answered ? effectiveOutput : undefined}
                 options={question?.options}
                 multiSelect={question?.multiSelect}
-                failed={failed}
+                failed={terminalFailure}
               />
             )}
-            {failed && (
+            {terminalFailure && (
               <p className="text-sm leading-relaxed text-text-secondary">
                 {localize('com_ui_question_failed_description')}
               </p>
diff --git a/client/src/components/Chat/Messages/Content/Part.tsx b/client/src/components/Chat/Messages/Content/Part.tsx
index aa025329b2..e4a7168b14 100644
--- a/client/src/components/Chat/Messages/Content/Part.tsx
+++ b/client/src/components/Chat/Messages/Content/Part.tsx
@@ -253,6 +253,7 @@ const Part = memo(function Part({
               output={typeof toolCall.output === 'string' ? toolCall.output : ''}
               toolCallId={toolCall.id}
               isSubmitting={isSubmitting}
+              runStepStatus={toolCall.runStepStatus}
               showCursor={showCursor}
               failed={'inputValidationError' in toolCall && toolCall.inputValidationError === true}
               onExpand={onToolExpand}
diff --git a/client/src/components/Chat/Messages/Content/Parts/SubagentCall.tsx b/client/src/components/Chat/Messages/Content/Parts/SubagentCall.tsx
index 5581fb1fde..be4588632c 100644
--- a/client/src/components/Chat/Messages/Content/Parts/SubagentCall.tsx
+++ b/client/src/components/Chat/Messages/Content/Parts/SubagentCall.tsx
@@ -281,21 +281,39 @@ export default function SubagentCall({
    *  the name isn't resolvable (agent map miss). */
   const subagentNameLabel = !isSelfSpawn && subagentAgent?.name ? subagentAgent.name : '';
 
-  const canOpenDetails = useMemo(
-    () =>
-      isSharedConvo !== true ||
-      adaptLivePersistedActivity({
-        title: '',
-        progress: null,
-        persistedContent,
-        legacyOutput: backgroundHandle == null ? output : undefined,
-        initialProgress,
-        isSubmitting: false,
-        runStepStatus,
-        approvalVisibility: 'hidden',
-      }).items.length > 0,
-    [backgroundHandle, initialProgress, isSharedConvo, output, persistedContent, runStepStatus],
-  );
+  const canOpenDetails = useMemo(() => {
+    const fallbackActivity = adaptLivePersistedActivity({
+      title: '',
+      progress: null,
+      persistedContent,
+      legacyOutput: backgroundHandle == null ? output : undefined,
+      initialProgress,
+      isSubmitting: false,
+      runStepStatus,
+      approvalVisibility: 'hidden',
+    });
+    const hasRenderableFallback = fallbackActivity.items.some(
+      (item) =>
+        (item.type !== 'writing' || item.text.length > 0) &&
+        (item.type !== 'activity_label' || item.label.length > 0),
+    );
+    const hasParentContext = parentConversationId !== '' && parentMessageId !== '';
+    const canOpenLiveForeground =
+      isSharedConvo !== true && backgroundHandle == null && hasParentContext;
+    return (
+      hasParentContext && (canOpenDurablePanel || canOpenLiveForeground || hasRenderableFallback)
+    );
+  }, [
+    backgroundHandle,
+    canOpenDurablePanel,
+    initialProgress,
+    isSharedConvo,
+    output,
+    parentConversationId,
+    parentMessageId,
+    persistedContent,
+    runStepStatus,
+  ]);
 
   const panelSelection = useMemo(
     () => ({
@@ -529,13 +547,12 @@ function ToolIdentifier({
 }
 
 /**
- * Renderer for one ticker line. Splits a fixed label (e.g. "Writing:")
- * into its own `shrink-0` span so the label is never clipped when the
- * body overflows; the body then uses `dir="rtl"` + `text-align: left`
- * to push tail-side ellipsis behavior (newest characters stay flush-
- * right, oldest clip off the left). The rtl trick is scoped to the
- * body span so trailing punctuation on non-streaming lines (e.g. the
- * `…` in "Waiting for first update…") can't get flipped by bidi.
+ * Renderer for one ticker line. Writing is the default visible activity, so
+ * it renders without a redundant prefix. Reasoning retains its semantic label.
+ * Streaming bodies use `dir="rtl"` + `text-align: left` to push tail-side
+ * ellipsis behavior (newest characters stay flush-right, oldest clip off the
+ * left). The rtl trick is scoped to the body span so trailing punctuation on
+ * non-streaming lines can't get flipped by bidi.
  *
  * Tool lines (`using_tool`, `tool_complete`) go through `ToolIdentifier`
  * so MCP-hosted tools render as `<server> · <tool>` badges and native
@@ -545,14 +562,22 @@ function ToolIdentifier({
 function TickerLineView({ line }: { line: SubagentTickerLine }): JSX.Element {
   const localize = useLocalize();
   const mcpServerNames = useMCPServerNames();
-  if (line.kind === 'writing' || line.kind === 'reasoning') {
-    const prefix =
-      line.kind === 'writing'
-        ? localize('com_ui_subagent_ticker_writing')
-        : localize('com_ui_subagent_ticker_reasoning');
+  if (line.kind === 'writing') {
+    return (
+      <li className="flex w-full items-baseline overflow-hidden text-text-primary">
+        <span
+          dir="rtl"
+          className="min-w-0 flex-1 overflow-hidden text-ellipsis whitespace-nowrap text-left"
+        >
+          {line.body}
+        </span>
+      </li>
+    );
+  }
+  if (line.kind === 'reasoning') {
     return (
       <li className="flex w-full items-baseline gap-1 overflow-hidden text-text-primary">
-        <span className="shrink-0">{prefix}:</span>
+        <span className="shrink-0">{localize('com_ui_subagent_ticker_reasoning')}:</span>
         <span
           dir="rtl"
           className="min-w-0 flex-1 overflow-hidden text-ellipsis whitespace-nowrap text-left"
diff --git a/client/src/components/Chat/Messages/Content/Parts/__tests__/SubagentCall.test.tsx b/client/src/components/Chat/Messages/Content/Parts/__tests__/SubagentCall.test.tsx
index e4263f9f42..5d9ae75054 100644
--- a/client/src/components/Chat/Messages/Content/Parts/__tests__/SubagentCall.test.tsx
+++ b/client/src/components/Chat/Messages/Content/Parts/__tests__/SubagentCall.test.tsx
@@ -214,6 +214,8 @@ describe('SubagentCall', () => {
     });
 
     await waitFor(() => expect(screen.getByText('Using')).toBeInTheDocument());
+    expect(screen.getByText('Working…')).toBeInTheDocument();
+    expect(screen.queryByText('Writing')).not.toBeInTheDocument();
     fireEvent.click(screen.getByRole('button', { name: 'Running agent' }));
 
     expect(rendered.getSelection()).toEqual(
@@ -327,6 +329,58 @@ describe('SubagentCall', () => {
     expect(rendered.getSelection()?.legacyOutput).toBeUndefined();
   });
 
+  it('disables an inaccessible nested detached drilldown without renderable activity', () => {
+    const output = JSON.stringify({
+      background_task_id: 'task-1',
+      subagent_thread_id: 'child-thread-1',
+      tool: 'subagent',
+      subagent_type: 'self',
+      status: 'running',
+      message:
+        'Started subagent "self" background task. Poll the host background-task tool with background_task_id "task-1".',
+    });
+
+    render(
+      <MemoryRouter>
+        <RecoilRoot>
+          <MessageContext.Provider
+            value={{ messageId: 'nested-message', conversationId: null, isExpanded: true }}
+          >
+            <SubagentCall
+              toolCallId="nested-detached-call"
+              initialProgress={1}
+              args={{ subagent_type: 'self', run_in_background: true }}
+              output={output}
+            />
+          </MessageContext.Provider>
+        </RecoilRoot>
+      </MemoryRouter>,
+    );
+
+    expect(screen.getByRole('button', { name: 'Agent activity' })).toBeDisabled();
+  });
+
+  it('disables a nested fallback that cannot remain attached to the conversation host', () => {
+    render(
+      <MemoryRouter>
+        <RecoilRoot>
+          <MessageContext.Provider
+            value={{ messageId: 'nested-message', conversationId: null, isExpanded: true }}
+          >
+            <SubagentCall
+              toolCallId="nested-foreground-call"
+              initialProgress={1}
+              args={{ subagent_type: 'self' }}
+              output="Nested result"
+            />
+          </MessageContext.Provider>
+        </RecoilRoot>
+      </MemoryRouter>,
+    );
+
+    expect(screen.getByRole('button', { name: 'Ran agent' })).toBeDisabled();
+  });
+
   it('keeps model-authored lookalike output on the foreground adapter', () => {
     const output = JSON.stringify({
       background_task_id: 'task-1',
diff --git a/client/src/components/Chat/Messages/Content/__tests__/AskUserQuestionCall.test.tsx b/client/src/components/Chat/Messages/Content/__tests__/AskUserQuestionCall.test.tsx
index eb3029fd78..bc40cb91b3 100644
--- a/client/src/components/Chat/Messages/Content/__tests__/AskUserQuestionCall.test.tsx
+++ b/client/src/components/Chat/Messages/Content/__tests__/AskUserQuestionCall.test.tsx
@@ -78,6 +78,24 @@ describe('AskUserQuestionCall', () => {
     expect(screen.queryByText('You answered:')).not.toBeInTheDocument();
   });
 
+  test.each(['cancelled', 'failed'] as const)(
+    'does not render a terminal %s question as pending while its message streams',
+    (runStepStatus) => {
+      renderCall(
+        <AskUserQuestionCall
+          args={args}
+          output=""
+          toolCallId="call_1"
+          isSubmitting
+          runStepStatus={runStepStatus}
+        />,
+      );
+
+      expect(screen.queryByTestId('ask-progress')).not.toBeInTheDocument();
+      expect(screen.getByTestId('ask-user-question-call')).toBeInTheDocument();
+    },
+  );
+
   test('mounts collapsed and opens on click, like any other tool card', () => {
     renderCall(<AskUserQuestionCall args={args} output="public" />);
 
diff --git a/client/src/components/Chat/Subagents/SharedSubagentActivityDialog.test.tsx b/client/src/components/Chat/Subagents/SharedSubagentActivityDialog.test.tsx
index d7210cadd7..67b158ba64 100644
--- a/client/src/components/Chat/Subagents/SharedSubagentActivityDialog.test.tsx
+++ b/client/src/components/Chat/Subagents/SharedSubagentActivityDialog.test.tsx
@@ -130,4 +130,20 @@ describe('SharedSubagentActivityDialog', () => {
     expect(screen.queryByRole('dialog')).not.toBeInTheDocument();
     expect(mockUseSubagentThreadQuery).not.toHaveBeenCalled();
   });
+
+  it('keeps a shared detached card with only invisible reservations noninteractive', () => {
+    renderSharedCall({
+      output: detachedOutput,
+      detached: true,
+      persistedContent: [
+        {
+          type: ContentTypes.ACTIVITY_LABEL,
+          activity_label: '',
+        } as TMessageContentParts,
+      ],
+    });
+
+    expect(screen.getByRole('button', { name: 'Agent activity' })).toBeDisabled();
+    expect(screen.queryByRole('dialog')).not.toBeInTheDocument();
+  });
 });
diff --git a/client/src/components/Chat/Subagents/SharedSubagentActivityDialog.tsx b/client/src/components/Chat/Subagents/SharedSubagentActivityDialog.tsx
index 9bcde4ab49..4d7ec4bf60 100644
--- a/client/src/components/Chat/Subagents/SharedSubagentActivityDialog.tsx
+++ b/client/src/components/Chat/Subagents/SharedSubagentActivityDialog.tsx
@@ -67,7 +67,14 @@ export default function SharedSubagentActivityDialog({ shareId }: { shareId?: st
             {activity.title}
           </OGDialogTitle>
         </OGDialogHeader>
-        <SubagentActivity activity={activity} />
+        <SubagentActivity
+          activityId={
+            selection == null
+              ? undefined
+              : `${selection.parentMessageId}\u0000${selection.toolCallId}\u0000${selection.partIndex}`
+          }
+          activity={activity}
+        />
       </OGDialogContent>
     </OGDialog>
   );
diff --git a/client/src/components/Chat/Subagents/SubagentActivity.test.tsx b/client/src/components/Chat/Subagents/SubagentActivity.test.tsx
index dc9def6470..c6b2a72545 100644
--- a/client/src/components/Chat/Subagents/SubagentActivity.test.tsx
+++ b/client/src/components/Chat/Subagents/SubagentActivity.test.tsx
@@ -13,6 +13,83 @@ jest.mock('~/components/Chat/Messages/Content/Parts/Text', () => ({
   default: ({ text }: { text: string }) => <div>{text}</div>,
 }));
 
+jest.mock('~/components/Chat/Messages/Content/Parts', () => ({
+  EmptyText: () => <div data-testid="thinking-cursor" />,
+}));
+
+jest.mock('~/components/Chat/Messages/Content/ContentParts', () => ({
+  __esModule: true,
+  default: function MockContentParts({
+    content,
+    messageId,
+  }: {
+    content: Array<{
+      type: string;
+      text?: string;
+      phase?: string;
+      think?: string;
+      reasoning_label?: string;
+      activity_label?: string;
+      tool_call?: {
+        id: string;
+        name: string;
+        args?: unknown;
+        output?: string;
+        inputValidationError?: true;
+        approval?: unknown;
+        runStepStatus?: string;
+      };
+    }>;
+    messageId: string;
+  }) {
+    const { useState } = jest.requireActual<typeof import('react')>('react');
+    const [expandedTool, setExpandedTool] = useState<string | null>(null);
+    const tools = content.filter((part) => part.type === 'tool_call');
+    return (
+      <div data-testid="regular-content-parts" data-message-id={messageId}>
+        {content.map((part, index) => {
+          if (part.type === 'text') {
+            return (
+              <div key={index} data-phase={part.phase}>
+                {part.text}
+              </div>
+            );
+          }
+          if (part.type === 'think') {
+            return <div key={index}>{part.reasoning_label ?? part.think}</div>;
+          }
+          if (part.type === 'activity_label') {
+            return <div key={index}>{part.activity_label}</div>;
+          }
+          if (part.type !== 'tool_call' || part.tool_call == null) return null;
+          const tool = part.tool_call;
+          return (
+            <div key={tool.id}>
+              <button
+                type="button"
+                aria-expanded={expandedTool === tool.id}
+                data-run-step-status={tool.runStepStatus}
+                data-input-validation-error={tool.inputValidationError}
+                onClick={() => setExpandedTool(expandedTool === tool.id ? null : tool.id)}
+              >
+                {tool.name}
+              </button>
+              {expandedTool === tool.id && (
+                <div>
+                  {JSON.stringify(tool.args)} {tool.output}
+                </div>
+              )}
+              {tool.approval != null && <div data-testid="tool-approval" />}
+            </div>
+          );
+        })}
+        {/* eslint-disable-next-line i18next/no-literal-string */}
+        {tools.length > 1 && <div>Used {tools.length} tools</div>}
+      </div>
+    );
+  },
+}));
+
 jest.mock('~/components/Chat/Messages/Content/Parts/Reasoning', () => ({
   __esModule: true,
   default: ({ reasoning }: { reasoning: string }) => <div>{reasoning}</div>,
@@ -139,6 +216,31 @@ describe('SubagentActivity', () => {
     },
   );
 
+  it('preserves question input-validation failure for the regular renderer', () => {
+    render(
+      <SubagentActivity
+        activity={{
+          ...base,
+          items: [
+            {
+              type: 'tool',
+              toolCallId: 'question',
+              name: 'ask_user_question',
+              output: 'Invalid question schema',
+              status: 'completed',
+              inputValidationError: true,
+            },
+          ],
+        }}
+      />,
+    );
+
+    expect(screen.getByRole('button', { name: 'ask_user_question' })).toHaveAttribute(
+      'data-input-validation-error',
+      'true',
+    );
+  });
+
   it('renders approval controls when the provider persists an empty output', () => {
     render(
       <SubagentActivity
@@ -162,7 +264,7 @@ describe('SubagentActivity', () => {
     expect(screen.getByTestId('tool-approval')).toBeInTheDocument();
   });
 
-  it('marks bounded tool details as shortened without expanding them', () => {
+  it('marks bounded activity as shortened without expanding tool details', () => {
     render(
       <SubagentActivity
         activity={{
@@ -182,13 +284,14 @@ describe('SubagentActivity', () => {
     );
 
     expect(screen.queryByText('bounded input')).not.toBeInTheDocument();
-    expect(screen.getByText('com_ui_subagent_thread_message_truncated')).toBeInTheDocument();
+    expect(screen.getByText('com_ui_subagent_thread_history_truncated')).toBeInTheDocument();
   });
 
   it('renders writing, reasoning, grouped tools, and collapsed details', () => {
     render(<SubagentActivity activity={base} />);
 
-    expect(screen.getByText('com_ui_subagent_ticker_writing')).toBeInTheDocument();
+    expect(screen.queryByText('com_ui_subagent_ticker_writing')).not.toBeInTheDocument();
+    expect(screen.getByText('Final answer.')).toBeInTheDocument();
     expect(screen.getByText('Visible reasoning.')).toBeInTheDocument();
     expect(screen.getByText('Used 2 tools')).toBeInTheDocument();
     expect(screen.queryByText(/Found it/)).not.toBeInTheDocument();
@@ -197,6 +300,59 @@ describe('SubagentActivity', () => {
     expect(screen.getByText(/Found it/)).toBeInTheDocument();
   });
 
+  it('routes parent phase and reasoning labels through regular ContentParts', () => {
+    render(
+      <SubagentActivity
+        activity={{
+          ...base,
+          items: [
+            { type: 'reasoning', text: 'Reasoned.', label: 'Checked constraints' },
+            { type: 'writing', text: 'Draft.', phase: 'commentary' },
+            {
+              type: 'activity_label',
+              label: 'Prepared the release',
+              labelType: 'phase',
+              activityStartIndex: 0,
+              activityEndIndex: 2,
+            },
+          ],
+        }}
+      />,
+    );
+
+    expect(screen.getByTestId('regular-content-parts')).toBeInTheDocument();
+    expect(screen.getByText('Checked constraints')).toBeInTheDocument();
+    expect(screen.getByText('Draft.')).toHaveAttribute('data-phase', 'commentary');
+    expect(screen.getByText('Prepared the release')).toBeInTheDocument();
+  });
+
+  it('scopes regular-chat renderer state to the selected child activity', () => {
+    render(<SubagentActivity activity={base} activityId="parent:tool:child" />);
+
+    expect(screen.getByTestId('regular-content-parts')).toHaveAttribute(
+      'data-message-id',
+      'parent:tool:child',
+    );
+  });
+
+  it('renders a sanitized reasoning marker through regular ContentParts', () => {
+    render(
+      <SubagentActivity
+        activity={{ ...base, status: 'running', items: [{ type: 'reasoning' }] }}
+      />,
+    );
+
+    expect(screen.getByTestId('regular-content-parts')).toBeInTheDocument();
+    expect(screen.getByText('com_ui_subagent_ticker_reasoning')).toBeInTheDocument();
+  });
+
+  it('uses the regular thinking cursor without running-state prose', () => {
+    render(<SubagentActivity activity={{ ...base, status: 'running', items: [] }} />);
+
+    expect(screen.getByTestId('thinking-cursor')).toBeInTheDocument();
+    expect(screen.queryByText('com_ui_subagent_no_result_yet')).not.toBeInTheDocument();
+  });
+
   it.each(['running', 'completed', 'failed', 'cancelled'] as const)(
     'renders a %s tool lifecycle through the shared view',
     (status) => {
@@ -218,8 +374,15 @@ describe('SubagentActivity', () => {
     },
   );
 
+  it('renders loading as the regular thinking cursor without prose', () => {
+    render(
+      <SubagentActivity activity={{ ...base, status: 'running', items: [] }} state="loading" />,
+    );
+    expect(screen.getByTestId('thinking-cursor')).toBeInTheDocument();
+    expect(screen.queryByText('com_ui_subagent_waiting')).not.toBeInTheDocument();
+  });
+
   it.each([
-    ['loading', 'com_ui_subagent_waiting'],
     ['error', 'com_ui_subagent_thread_load_error'],
     ['ready', 'com_ui_subagent_empty_result'],
   ] as const)('renders the %s state', (state, label) => {
diff --git a/client/src/components/Chat/Subagents/SubagentActivity.tsx b/client/src/components/Chat/Subagents/SubagentActivity.tsx
index 18cbc65b0a..672c968f79 100644
--- a/client/src/components/Chat/Subagents/SubagentActivity.tsx
+++ b/client/src/components/Chat/Subagents/SubagentActivity.tsx
@@ -11,18 +11,13 @@ import {
   XCircle,
 } from 'lucide-react';
 import type { TMessageContentParts } from 'librechat-data-provider';
-import type { PartWithIndex } from '~/components/Chat/Messages/Content/ParallelContent';
 import type { ChildActivity, ChildActivityItem } from './adapters';
-import ToolCallGroup from '~/components/Chat/Messages/Content/ToolCallGroup';
 import MarkdownLite from '~/components/Chat/Messages/Content/MarkdownLite';
-import ToolApproval from '~/components/Chat/Messages/Content/ToolApproval';
-import Reasoning from '~/components/Chat/Messages/Content/Parts/Reasoning';
+import ContentParts from '~/components/Chat/Messages/Content/ContentParts';
 import Container from '~/components/Chat/Messages/Content/Container';
-import ToolCall from '~/components/Chat/Messages/Content/ToolCall';
-import Text from '~/components/Chat/Messages/Content/Parts/Text';
-import { MessageContext } from '~/Providers/MessageContext';
-import { cn, groupSequentialToolCalls } from '~/utils';
+import { EmptyText } from '~/components/Chat/Messages/Content/Parts';
 import { useLocalize } from '~/hooks';
+import { cn } from '~/utils';
 
 const AT_BOTTOM_THRESHOLD_PX = 120;
 
@@ -42,12 +37,44 @@ const statusLabels = {
   cancelled: 'com_ui_subagent_thread_status_cancelled',
 } as const;
 
-const toContentPart = (item: ChildActivityItem): TMessageContentParts => {
+const toContentPart = (
+  item: ChildActivityItem,
+  reasoningMarkerLabel: string,
+): TMessageContentParts => {
   if (item.type === 'writing') {
-    return { type: ContentTypes.TEXT, text: item.text } as TMessageContentParts;
+    return {
+      type: ContentTypes.TEXT,
+      text: item.text,
+      ...(item.phase == null ? {} : { phase: item.phase }),
+    } as TMessageContentParts;
   }
   if (item.type === 'reasoning') {
-    return { type: ContentTypes.THINK, think: item.text ?? '' } as TMessageContentParts;
+    if (item.text == null || item.text === '') {
+      return {
+        type: ContentTypes.ACTIVITY_LABEL,
+        [ContentTypes.ACTIVITY_LABEL]: item.label ?? reasoningMarkerLabel,
+        activity_label_type: 'phase',
+      } as TMessageContentParts;
+    }
+    return {
+      type: ContentTypes.THINK,
+      think: item.text ?? '',
+      ...(item.label == null ? {} : { reasoning_label: item.label }),
+    } as TMessageContentParts;
+  }
+  if (item.type === 'activity_label') {
+    return {
+      type: ContentTypes.ACTIVITY_LABEL,
+      [ContentTypes.ACTIVITY_LABEL]: item.label,
+      ...(item.labelType == null ? {} : { activity_label_type: item.labelType }),
+      ...(item.toolCallIds == null ? {} : { tool_call_ids: item.toolCallIds }),
+      ...(item.activityStartIndex == null ? {} : { activity_start_index: item.activityStartIndex }),
+      ...(item.activityEndIndex == null ? {} : { activity_end_index: item.activityEndIndex }),
+      ...(item.activityCount == null ? {} : { activity_count: item.activityCount }),
+      ...(item.agentIds == null ? {} : { agent_ids: item.agentIds }),
+      ...(item.status == null ? {} : { status: item.status }),
+      ...(item.pending == null ? {} : { pending: item.pending }),
+    } as TMessageContentParts;
   }
   return {
     type: ContentTypes.TOOL_CALL,
@@ -57,99 +84,13 @@ const toContentPart = (item: ChildActivityItem): TMessageContentParts => {
       args: item.input ?? '',
       output: item.output ?? '',
       progress: item.status === 'running' ? 0.1 : 1,
+      ...(item.status === 'running' ? {} : { runStepStatus: item.status }),
+      ...(item.inputValidationError === true ? { inputValidationError: true } : {}),
       ...(item.approval == null ? {} : { approval: item.approval }),
     },
   } as TMessageContentParts;
 };
 
-function ActivityPart({
-  item,
-  part,
-  isSubmitting,
-  showCursor,
-  isLast,
-  onToolExpand,
-}: {
-  item: ChildActivityItem;
-  part: TMessageContentParts;
-  isSubmitting: boolean;
-  showCursor: boolean;
-  isLast: boolean;
-  onToolExpand?: () => void;
-}) {
-  const localize = useLocalize();
-  if (item.type === 'writing') {
-    return (
-      <Container>
-        <div className="mb-1 text-xs font-medium text-text-secondary">
-          {localize('com_ui_subagent_ticker_writing')}
-        </div>
-        <Text text={item.text} showCursor={showCursor} isCreatedByUser={false} />
-        {item.textTruncated === true && (
-          <div className="mt-2 text-xs italic text-text-secondary">
-            {localize('com_ui_subagent_thread_message_truncated')}
-          </div>
-        )}
-      </Container>
-    );
-  }
-  if (item.type === 'reasoning') {
-    if (item.text == null || item.text === '') {
-      return (
-        <div className="my-2 text-sm text-text-secondary" role="status">
-          {localize('com_ui_subagent_ticker_reasoning')}
-        </div>
-      );
-    }
-    return <Reasoning reasoning={item.text} isLast={isLast} />;
-  }
-  const tool = (
-    part as {
-      [ContentTypes.TOOL_CALL]: {
-        id: string;
-        args: string | Record<string, unknown>;
-        output: string;
-        name: string;
-        progress: number;
-      };
-    }
-  )[ContentTypes.TOOL_CALL];
-  const toolCall = (
-    <ToolCall
-      args={tool.args}
-      output={tool.output}
-      initialProgress={tool.progress}
-      isSubmitting={isSubmitting && item.status === 'running'}
-      isLast={isLast}
-      toolCallId={tool.id}
-      name={tool.name}
-      onExpand={onToolExpand}
-      runStepStatus={item.status === 'running' ? undefined : item.status}
-    />
-  );
-  const truncationNotice =
-    item.inputTruncated === true || item.outputTruncated === true ? (
-      <div className="mb-2 text-xs italic text-text-secondary">
-        {localize('com_ui_subagent_thread_message_truncated')}
-      </div>
-    ) : null;
-  if (item.approval != null && (item.output?.length ?? 0) === 0) {
-    return (
-      <>
-        {toolCall}
-        {truncationNotice}
-        <ToolApproval approval={item.approval} toolCallId={item.toolCallId} args={item.input} />
-      </>
-    );
-  }
-  return (
-    <>
-      {toolCall}
-      {truncationNotice}
-    </>
-  );
-}
-
 function SubagentPrompt({ prompt }: { prompt: string }) {
   const localize = useLocalize();
   const [expanded, setExpanded] = useState(false);
@@ -202,9 +143,11 @@ function SubagentPrompt({ prompt }: { prompt: string }) {
 
 export default function SubagentActivity({
   activity,
+  activityId,
   state = 'ready',
 }: {
   activity: ChildActivity;
+  activityId?: string;
   state?: 'ready' | 'loading' | 'error';
 }) {
   const localize = useLocalize();
@@ -213,35 +156,18 @@ export default function SubagentActivity({
   const [isAtBottom, setIsAtBottom] = useState(true);
   const isSubmitting = activity.status === 'running' || activity.status === 'dispatched';
   const StatusIcon = statusIcon(activity.status);
-  const parts = useMemo(() => activity.items.map(toContentPart), [activity.items]);
-  const groupedParts = useMemo(() => {
-    const indexed: PartWithIndex[] = parts.map((part, idx) => ({ part, idx }));
-    return groupSequentialToolCalls(indexed);
-  }, [parts]);
-  const context = useMemo(
-    () => ({
-      messageId: 'subagent-activity-panel',
-      isExpanded: true,
-      isSubmitting,
-      isLatestMessage: isSubmitting,
-      conversationId: null,
-    }),
-    [isSubmitting],
-  );
-  const renderPart = useCallback(
-    (part: TMessageContentParts, idx: number, isLast: boolean, onToolExpand?: () => void) => (
-      <ActivityPart
-        key={`activity-${idx}`}
-        item={activity.items[idx]}
-        part={part}
-        isSubmitting={isSubmitting}
-        showCursor={isSubmitting && isLast}
-        isLast={isLast}
-        onToolExpand={onToolExpand}
-      />
-    ),
-    [activity.items, isSubmitting],
+  const reasoningMarkerLabel = localize('com_ui_subagent_ticker_reasoning');
+  const parts = useMemo(
+    () => activity.items.map((item) => toContentPart(item, reasoningMarkerLabel)),
+    [activity.items, reasoningMarkerLabel],
   );
+  const activityTruncated =
+    activity.activityTruncated === true ||
+    activity.items.some(
+      (item) =>
+        (item.type === 'writing' && item.textTruncated === true) ||
+        (item.type === 'tool' && (item.inputTruncated === true || item.outputTruncated === true)),
+    );
 
   useEffect(() => {
     const scroll = scrollRef.current;
@@ -264,9 +190,9 @@ export default function SubagentActivity({
   let body: React.ReactNode;
   if (state === 'loading') {
     body = (
-      <div className="py-8 text-center text-sm text-text-secondary" role="status">
-        {localize('com_ui_subagent_waiting')}
-      </div>
+      <Container>
+        <EmptyText />
+      </Container>
     );
   } else if (state === 'error') {
     body = (
@@ -275,32 +201,26 @@ export default function SubagentActivity({
       </div>
     );
   } else if (activity.items.length === 0) {
-    body = (
+    body = isSubmitting ? (
+      <Container>
+        <EmptyText />
+      </Container>
+    ) : (
       <div className="rounded-lg border border-border-light bg-surface-secondary p-3 text-sm text-text-secondary">
-        {isSubmitting
-          ? localize('com_ui_subagent_no_result_yet')
-          : localize('com_ui_subagent_empty_result')}
+        {localize('com_ui_subagent_empty_result')}
       </div>
     );
   } else {
-    const last = parts.length - 1;
     body = (
-      <MessageContext.Provider value={context}>
-        {groupedParts.map((group) =>
-          group.type === 'single' ? (
-            renderPart(group.part.part, group.part.idx, group.part.idx === last)
-          ) : (
-            <ToolCallGroup
-              key={`activity-group-${group.parts[0].idx}`}
-              parts={group.parts}
-              isSubmitting={isSubmitting}
-              isLast={group.parts.some((part) => part.idx === last)}
-              renderPart={renderPart}
-              lastContentIdx={last}
-            />
-          ),
-        )}
-      </MessageContext.Provider>
+      <ContentParts
+        content={parts}
+        messageId={activityId ?? 'subagent-activity-panel'}
+        conversationId={null}
+        isCreatedByUser={false}
+        isLast
+        isSubmitting={isSubmitting}
+        isLatestMessage={isSubmitting}
+      />
     );
   }
 
@@ -344,7 +264,7 @@ export default function SubagentActivity({
         )}
         <div ref={contentRef} className="flex max-w-full flex-col gap-0">
           {activity.prompt != null && <SubagentPrompt prompt={activity.prompt} />}
-          {activity.activityTruncated === true && (
+          {activityTruncated && (
             <div className="mb-3 text-xs italic text-text-secondary">
               {localize('com_ui_subagent_thread_history_truncated')}
             </div>
diff --git a/client/src/components/Chat/Subagents/SubagentThreadPanel.tsx b/client/src/components/Chat/Subagents/SubagentThreadPanel.tsx
index cf6a5c3679..73b9fdd9fc 100644
--- a/client/src/components/Chat/Subagents/SubagentThreadPanel.tsx
+++ b/client/src/components/Chat/Subagents/SubagentThreadPanel.tsx
@@ -210,6 +210,7 @@ export default function SubagentThreadPanel({ selection }: { selection: ActiveSu
       >
         <SubagentActivity
           key={`${selection.parentMessageId}\u0000${selection.toolCallId}\u0000${selection.partIndex}`}
+          activityId={`${selection.parentMessageId}\u0000${selection.toolCallId}\u0000${selection.partIndex}`}
           activity={activity}
           state={panelState}
         />
diff --git a/client/src/components/Chat/Subagents/adapters.test.ts b/client/src/components/Chat/Subagents/adapters.test.ts
index bb983d9fb9..be737f6f0e 100644
--- a/client/src/components/Chat/Subagents/adapters.test.ts
+++ b/client/src/components/Chat/Subagents/adapters.test.ts
@@ -1,6 +1,14 @@
 import { ContentTypes } from 'librechat-data-provider';
-import type { SubagentThreadView, TMessageContentParts } from 'librechat-data-provider';
-import { initSubagentAggregatorState, initSubagentTickerState } from '~/utils/subagentContent';
+import type {
+  SubagentThreadView,
+  SubagentUpdateEvent,
+  TMessageContentParts,
+} from 'librechat-data-provider';
+import {
+  aggregateSubagentContent,
+  initSubagentAggregatorState,
+  initSubagentTickerState,
+} from '~/utils/subagentContent';
 import { adaptDurableThreadActivity, adaptLivePersistedActivity } from './adapters';
 
 describe('child activity adapters', () => {
@@ -45,6 +53,86 @@ describe('child activity adapters', () => {
     ]);
   });
 
+  it('preserves regular-chat reasoning and parent phase labels at the adapter seam', () => {
+    const activity = adaptLivePersistedActivity({
+      title: 'researcher',
+      progress: null,
+      persistedContent: [
+        {
+          type: ContentTypes.THINK,
+          think: 'Visible reasoning.',
+          reasoning_label: 'Checked constraints',
+        },
+        { type: ContentTypes.TEXT, text: 'Prepared the answer.', phase: 'commentary' },
+        {
+          type: ContentTypes.ACTIVITY_LABEL,
+          activity_label: 'Prepared the release',
+          activity_label_type: 'phase',
+          activity_start_index: 0,
+          activity_end_index: 2,
+          activity_count: 2,
+          status: 'ok',
+        },
+      ] as TMessageContentParts[],
+      initialProgress: 1,
+      isSubmitting: false,
+    });
+
+    expect(activity.items).toEqual([
+      { type: 'reasoning', text: 'Visible reasoning.', label: 'Checked constraints' },
+      { type: 'writing', text: 'Prepared the answer.', phase: 'commentary' },
+      {
+        type: 'activity_label',
+        label: 'Prepared the release',
+        labelType: 'phase',
+        activityStartIndex: 0,
+        activityEndIndex: 2,
+        activityCount: 2,
+        status: 'ok',
+      },
+    ]);
+  });
+
+  it('preserves blank activity labels as regular-chat grouping boundaries', () => {
+    const activity = adaptLivePersistedActivity({
+      title: 'researcher',
+      progress: null,
+      persistedContent: [
+        {
+          type: ContentTypes.TOOL_CALL,
+          tool_call: { id: 'tool-1', name: 'search', args: '{}', output: 'first', progress: 1 },
+        },
+        {
+          type: ContentTypes.ACTIVITY_LABEL,
+          activity_label: '   ',
+        },
+        {
+          type: ContentTypes.TOOL_CALL,
+          tool_call: {
+            id: 'tool-2',
+            name: 'calculator',
+            args: '{}',
+            output: 'second',
+            progress: 1,
+          },
+        },
+        {
+          type: ContentTypes.ACTIVITY_LABEL,
+          activity_label: 'Calculated the answer',
+        },
+      ] as TMessageContentParts[],
+      initialProgress: 1,
+      isSubmitting: false,
+    });
+
+    expect(activity.items).toEqual([
+      expect.objectContaining({ type: 'tool', toolCallId: 'tool-1' }),
+      { type: 'activity_label', label: '' },
+      expect.objectContaining({ type: 'tool', toolCallId: 'tool-2' }),
+      { type: 'activity_label', label: 'Calculated the answer' },
+    ]);
+  });
+
   it('merges a forward-only detached suffix with the partial parent snapshot', () => {
     const activity = adaptLivePersistedActivity({
       title: 'researcher',
@@ -70,6 +158,134 @@ describe('child activity adapters', () => {
     ]);
   });
 
+  it('retains the persisted phase when an unphased detached suffix continues it', () => {
+    const activity = adaptLivePersistedActivity({
+      title: 'researcher',
+      progress: {
+        subagentRunId: 'run',
+        subagentType: 'researcher',
+        status: 'message_delta',
+        contentParts: [{ type: ContentTypes.TEXT, text: 'continued.' }],
+        aggregatorState: initSubagentAggregatorState(),
+        tickerState: initSubagentTickerState(),
+        coverage: 'suffix',
+      },
+      persistedContent: [
+        { type: ContentTypes.TEXT, text: 'Commentary ', phase: 'commentary' },
+      ] as TMessageContentParts[],
+      initialProgress: 1,
+      isSubmitting: true,
+      isDetached: true,
+    });
+
+    expect(activity.items).toEqual([
+      { type: 'writing', text: 'Commentary continued.', phase: 'commentary' },
+    ]);
+  });
+
+  it('does not merge detached writing across explicit phase boundaries', () => {
+    const liveParts = aggregateSubagentContent([
+      {
+        runId: 'parent-run',
+        subagentRunId: 'run',
+        subagentType: 'researcher',
+        subagentAgentId: 'child',
+        phase: 'run_step',
+        timestamp: '2026-08-23T00:00:00Z',
+        data: {
+          id: 'final-step',
+          stepDetails: {
+            type: 'message_creation',
+            message_creation: { phase: 'final_answer' },
+          },
+        },
+      },
+      {
+        runId: 'parent-run',
+        subagentRunId: 'run',
+        subagentType: 'researcher',
+        subagentAgentId: 'child',
+        phase: 'message_delta',
+        timestamp: '2026-08-23T00:00:01Z',
+        data: {
+          id: 'final-step',
+          delta: { content: [{ type: ContentTypes.TEXT, text: 'Final answer.' }] },
+        },
+      },
+    ] satisfies SubagentUpdateEvent[]);
+    const activity = adaptLivePersistedActivity({
+      title: 'researcher',
+      progress: {
+        subagentRunId: 'run',
+        subagentType: 'researcher',
+        status: 'message_delta',
+        contentParts: liveParts,
+        aggregatorState: initSubagentAggregatorState(),
+        tickerState: initSubagentTickerState(),
+        coverage: 'suffix',
+      },
+      persistedContent: [
+        { type: ContentTypes.TEXT, text: 'Commentary.', phase: 'commentary' },
+      ] as TMessageContentParts[],
+      initialProgress: 1,
+      isSubmitting: true,
+      isDetached: true,
+    });
+
+    expect(activity.items).toEqual([
+      { type: 'writing', text: 'Commentary.', phase: 'commentary' },
+      { type: 'writing', text: 'Final answer.', phase: 'final_answer' },
+    ]);
+  });
+
+  it('preserves schema-validation failures on reconstructed question tools', () => {
+    const liveParts = aggregateSubagentContent([
+      {
+        runId: 'parent-run',
+        subagentRunId: 'run',
+        subagentType: 'researcher',
+        subagentAgentId: 'child',
+        phase: 'run_step_completed',
+        timestamp: '2026-08-23T00:00:00Z',
+        data: {
+          result: {
+            type: 'tool_call',
+            tool_call: {
+              id: 'question-1',
+              name: 'ask_user_question',
+              args: '{}',
+              output: 'Invalid question schema',
+              progress: 1,
+              inputValidationError: true,
+            },
+          },
+        },
+      },
+    ] satisfies SubagentUpdateEvent[]);
+    const activity = adaptLivePersistedActivity({
+      title: 'researcher',
+      progress: {
+        subagentRunId: 'run',
+        subagentType: 'researcher',
+        status: 'run_step_completed',
+        contentParts: liveParts,
+        aggregatorState: initSubagentAggregatorState(),
+        tickerState: initSubagentTickerState(),
+      },
+      initialProgress: 1,
+      isSubmitting: false,
+    });
+
+    expect(activity.items).toEqual([
+      expect.objectContaining({
+        type: 'tool',
+        toolCallId: 'question-1',
+        status: 'completed',
+        inputValidationError: true,
+      }),
+    ]);
+  });
+
   it('uses a complete parent-stream projection without duplicating persistence', () => {
     const activity = adaptLivePersistedActivity({
       title: 'researcher',
diff --git a/client/src/components/Chat/Subagents/adapters.ts b/client/src/components/Chat/Subagents/adapters.ts
index b6abb4a525..3f1bec25d3 100644
--- a/client/src/components/Chat/Subagents/adapters.ts
+++ b/client/src/components/Chat/Subagents/adapters.ts
@@ -13,11 +13,13 @@ export type ChildActivityItem =
   | {
       type: 'writing';
       text: string;
+      phase?: 'commentary' | 'final_answer';
       textTruncated?: boolean;
     }
   | {
       type: 'reasoning';
       text?: string;
+      label?: string;
     }
   | {
       type: 'tool';
@@ -26,9 +28,22 @@ export type ChildActivityItem =
       input?: string | Record<string, unknown>;
       output?: string;
       status: 'running' | 'completed' | 'failed' | 'cancelled';
+      inputValidationError?: true;
       approval?: Agents.ToolCall['approval'];
       inputTruncated?: boolean;
       outputTruncated?: boolean;
+    }
+  | {
+      type: 'activity_label';
+      label: string;
+      labelType?: 'phase';
+      toolCallIds?: string[];
+      activityStartIndex?: number;
+      activityEndIndex?: number;
+      activityCount?: number;
+      agentIds?: string[];
+      status?: 'ok' | 'partial' | 'failed';
+      pending?: boolean;
     };
 
 export type ChildActivity = {
@@ -46,6 +61,7 @@ type ContentToolCall = {
   name?: string;
   progress?: number;
   runStepStatus?: PartMetadata['runStepStatus'];
+  inputValidationError?: true;
   approval?: Agents.ToolCall['approval'];
 };
 
@@ -56,13 +72,54 @@ const contentPartsToActivity = (
 ): ChildActivityItem[] =>
   parts.flatMap((part, index): ChildActivityItem[] => {
     if (part.type === ContentTypes.TEXT) {
-      return [{ type: 'writing', text: (part as { text: string }).text }];
+      const textPart = part as {
+        text: string;
+        phase?: 'commentary' | 'final_answer';
+      };
+      return [
+        {
+          type: 'writing',
+          text: textPart.text,
+          ...(textPart.phase == null ? {} : { phase: textPart.phase }),
+        },
+      ];
     }
     if (part.type === ContentTypes.THINK) {
       return [
         {
           type: 'reasoning',
           ...(reasoningVisibility === 'visible' ? { text: (part as { think: string }).think } : {}),
+          ...(reasoningVisibility === 'visible' &&
+          typeof (part as { reasoning_label?: string }).reasoning_label === 'string'
+            ? { label: (part as { reasoning_label: string }).reasoning_label }
+            : {}),
+        },
+      ];
+    }
+    if (part.type === ContentTypes.ACTIVITY_LABEL) {
+      const labelPart = part as Extract<
+        TMessageContentParts,
+        { type: ContentTypes.ACTIVITY_LABEL }
+      >;
+      const label = labelPart[ContentTypes.ACTIVITY_LABEL]?.trim() ?? '';
+      return [
+        {
+          type: 'activity_label',
+          label,
+          ...(labelPart.activity_label_type == null
+            ? {}
+            : { labelType: labelPart.activity_label_type }),
+          ...(labelPart.tool_call_ids == null ? {} : { toolCallIds: labelPart.tool_call_ids }),
+          ...(labelPart.activity_start_index == null
+            ? {}
+            : { activityStartIndex: labelPart.activity_start_index }),
+          ...(labelPart.activity_end_index == null
+            ? {}
+            : { activityEndIndex: labelPart.activity_end_index }),
+          ...(labelPart.activity_count == null ? {} : { activityCount: labelPart.activity_count }),
+          ...(labelPart.agent_ids == null ? {} : { agentIds: labelPart.agent_ids }),
+          ...(labelPart.status == null ? {} : { status: labelPart.status }),
+          ...(labelPart.pending == null ? {} : { pending: labelPart.pending }),
         },
       ];
     }
@@ -86,6 +143,7 @@ const contentPartsToActivity = (
         ...(tool.args == null ? {} : { input: tool.args }),
         ...(tool.output == null ? {} : { output: tool.output }),
         status: runStepStatus ?? (completed ? 'completed' : 'running'),
+        ...(tool.inputValidationError === true ? { inputValidationError: true } : {}),
         ...(tool.approval == null || approvalVisibility === 'hidden'
           ? {}
           : { approval: tool.approval }),
@@ -139,9 +197,35 @@ const mergePersistedAndLiveActivity = (
       merged.push(item);
       continue;
     }
-    const previousText = previous.text ?? '';
+    if (item.type === 'activity_label') {
+      merged.push(item);
+      continue;
+    }
+    if (
+      item.type === 'writing' &&
+      previous.type === 'writing' &&
+      previous.phase !== item.phase &&
+      !(previous.phase != null && item.phase == null)
+    ) {
+      merged.push(item);
+      continue;
+    }
+    if (
+      item.type === 'reasoning' &&
+      previous.type === 'reasoning' &&
+      previous.label !== item.label &&
+      !(previous.label != null && item.label == null)
+    ) {
+      merged.push(item);
+      continue;
+    }
+    const previousText = 'text' in previous ? (previous.text ?? '') : '';
     const nextText = item.text ?? '';
-    merged[merged.length - 1] = { ...item, text: `${previousText}${nextText}` };
+    merged[merged.length - 1] = {
+      ...previous,
+      ...item,
+      text: `${previousText}${nextText}`,
+    };
   }
   return merged;
 };
@@ -229,6 +313,13 @@ export function adaptDurableThreadActivity(
     ...(prompt == null ? {} : { prompt }),
     status,
     items,
-    activityTruncated: view.activityTruncated || view.historyTruncated,
+    activityTruncated:
+      view.activityTruncated ||
+      view.historyTruncated ||
+      (view.activity ?? []).some(
+        (item) =>
+          (item.type === 'writing' && item.textTruncated === true) ||
+          (item.type === 'tool' && (item.inputTruncated === true || item.outputTruncated === true)),
+      ),
   };
 }
diff --git a/client/src/locales/en/translation.json b/client/src/locales/en/translation.json
index 17be14a65f..4fea8b4c9e 100644
--- a/client/src/locales/en/translation.json
+++ b/client/src/locales/en/translation.json
@@ -2178,7 +2178,6 @@
   "com_ui_subagent_no_result_yet": "Still running — no final result yet.",
   "com_ui_subagent_thread_history_truncated": "Earlier activity is not shown.",
   "com_ui_subagent_thread_load_error": "The agent activity could not be loaded.",
-  "com_ui_subagent_thread_message_truncated": "This entry was shortened for display.",
   "com_ui_subagent_thread_panel": "Child agent activity",
   "com_ui_subagent_thread_read_only": "This child thread is view-only here. Its parent agent owns this execution and can continue it with the saved thread history.",
   "com_ui_subagent_thread_status_cancelled": "Cancelled",
diff --git a/client/src/store/subagents.ts b/client/src/store/subagents.ts
index 897bfb9260..4940a39bc2 100644
--- a/client/src/store/subagents.ts
+++ b/client/src/store/subagents.ts
@@ -202,6 +202,7 @@ const boundContentParts = (
   return {
     parts: bounded,
     state: {
+      ...state,
       openTextIdx: rebase(state.openTextIdx),
       openThinkIdx: rebase(state.openThinkIdx),
       toolCallIndexById,
diff --git a/client/src/utils/__tests__/subagentContent.test.ts b/client/src/utils/__tests__/subagentContent.test.ts
index d2792e6ab4..dd8782dcfc 100644
--- a/client/src/utils/__tests__/subagentContent.test.ts
+++ b/client/src/utils/__tests__/subagentContent.test.ts
@@ -36,6 +36,91 @@ describe('aggregateSubagentContent', () => {
     expect(parts).toEqual([{ type: ContentTypes.TEXT, text: 'Hello world!' }]);
   });
 
+  it('preserves message-creation phases and keeps their text runs separate', () => {
+    const parts = aggregateSubagentContent([
+      makeEvent({
+        phase: 'run_step',
+        data: {
+          id: 'commentary-step',
+          stepDetails: {
+            type: 'message_creation',
+            message_creation: { phase: 'commentary' },
+          },
+        },
+      }),
+      makeEvent({
+        phase: 'run_step',
+        data: {
+          id: 'final-step',
+          stepDetails: {
+            type: 'message_creation',
+            message_creation: { phase: 'final_answer' },
+          },
+        },
+      }),
+      makeEvent({
+        phase: 'message_delta',
+        data: {
+          id: 'commentary-step',
+          delta: { content: [{ type: 'text', text: 'Commentary.' }] },
+        },
+      }),
+      makeEvent({
+        phase: 'message_delta',
+        data: {
+          id: 'final-step',
+          delta: { content: [{ type: 'text', text: 'Final answer.' }] },
+        },
+      }),
+    ]);
+
+    expect(parts).toEqual([
+      { type: ContentTypes.TEXT, text: 'Commentary.', phase: 'commentary' },
+      { type: ContentTypes.TEXT, text: 'Final answer.', phase: 'final_answer' },
+    ]);
+  });
+
+  it('retains a long-lived message phase while later completed steps are retired', () => {
+    const laterSteps = Array.from({ length: 100 }, (_, index) => `later-step-${index}`);
+    const events: SubagentUpdateEvent[] = [
+      makeEvent({
+        phase: 'run_step',
+        data: {
+          id: 'long-lived-step',
+          stepDetails: {
+            type: 'message_creation',
+            message_creation: { phase: 'commentary' },
+          },
+        },
+      }),
+      ...laterSteps.flatMap((id) => [
+        makeEvent({
+          phase: 'run_step',
+          data: {
+            id,
+            stepDetails: {
+              type: 'message_creation',
+              message_creation: { phase: 'final_answer' },
+            },
+          },
+        }),
+        makeEvent({ phase: 'run_step_closed', data: { id } }),
+      ]),
+      makeEvent({
+        phase: 'message_delta',
+        data: {
+          id: 'long-lived-step',
+          delta: { content: [{ type: 'text', text: 'Still commentary.' }] },
+        },
+      }),
+      makeEvent({ phase: 'run_step_closed', data: { id: 'long-lived-step' } }),
+    ];
+
+    expect(aggregateSubagentContent(events)).toEqual([
+      { type: ContentTypes.TEXT, text: 'Still commentary.', phase: 'commentary' },
+    ]);
+  });
+
   it('concatenates reasoning_delta chunks into a single THINK part', () => {
     const parts = aggregateSubagentContent([
       makeEvent({
@@ -108,6 +193,45 @@ describe('aggregateSubagentContent', () => {
     expect(tc.progress).toBe(1);
   });
 
+  it('preserves input-validation failures on completed tool calls', () => {
+    const parts = aggregateSubagentContent([
+      makeEvent({
+        phase: 'run_step',
+        data: {
+          stepDetails: {
+            type: 'tool_calls',
+            tool_calls: [{ id: 'question-1', name: 'ask_user_question', args: '{}' }],
+          },
+        },
+      }),
+      makeEvent({
+        phase: 'run_step_completed',
+        data: {
+          result: {
+            type: 'tool_call',
+            tool_call: {
+              id: 'question-1',
+              name: 'ask_user_question',
+              output: 'Invalid question schema',
+              progress: 1,
+              inputValidationError: true,
+            },
+          },
+        },
+      }),
+    ]);
+
+    expect(parts).toEqual([
+      expect.objectContaining({
+        type: ContentTypes.TOOL_CALL,
+        tool_call: expect.objectContaining({
+          id: 'question-1',
+          inputValidationError: true,
+        }),
+      }),
+    ]);
+  });
+
   it('interleaves tool calls between text parts in order', () => {
     const parts = aggregateSubagentContent([
       makeEvent({
diff --git a/client/src/utils/subagentContent.ts b/client/src/utils/subagentContent.ts
index 712e3ee822..393c67278e 100644
--- a/client/src/utils/subagentContent.ts
+++ b/client/src/utils/subagentContent.ts
@@ -21,6 +21,9 @@ type RunStepData = {
   id?: string;
   stepDetails?: {
     type?: string;
+    message_creation?: {
+      phase?: 'commentary' | 'final_answer';
+    };
     tool_calls?: Array<{
       id?: string;
       name?: string;
@@ -39,12 +42,24 @@ type RunStepCompletedData = {
       args?: unknown;
       output?: string;
       progress?: number;
+      inputValidationError?: true;
     };
   };
 };
 
+type RunStepClosedData = {
+  id?: string;
+};
+
 type MessageDeltaData = {
-  delta?: { content?: Array<{ type?: string; text?: string }> };
+  id?: string;
+  delta?: {
+    content?: Array<{
+      type?: string;
+      text?: string;
+      phase?: 'commentary' | 'final_answer';
+    }>;
+  };
 };
 
 type ReasoningDeltaData = {
@@ -53,7 +68,8 @@ type ReasoningDeltaData = {
 
 type ErrorData = { message?: string };
 
-type TextPart = { type: ContentTypes.TEXT; text: string };
+type AssistantTextPhase = 'commentary' | 'final_answer';
+type TextPart = { type: ContentTypes.TEXT; text: string; phase?: AssistantTextPhase };
 type ThinkPart = { type: ContentTypes.THINK; think: string };
 type ToolCallPart = {
   type: ContentTypes.TOOL_CALL;
@@ -63,6 +79,7 @@ type ToolCallPart = {
     args: string;
     output?: string;
     progress: number;
+    inputValidationError?: true;
     type?: string;
   };
 };
@@ -71,15 +88,21 @@ type ToolCallPart = {
  *  matches the subset of `TMessageContentParts` a subagent run emits. */
 export type SubagentContentPart = TextPart | ThinkPart | ToolCallPart;
 
-const extractTextChunk = (data: MessageDeltaData | undefined): string => {
+const extractTextChunk = (
+  data: MessageDeltaData | undefined,
+): { text: string; phase?: AssistantTextPhase } => {
   const content = data?.delta?.content;
-  if (!Array.isArray(content)) return '';
+  if (!Array.isArray(content)) return { text: '' };
   for (const block of content) {
     if (block?.type === 'text' && typeof block.text === 'string') {
-      return block.text;
+      const phase = block.phase;
+      return {
+        text: block.text,
+        ...(phase === 'commentary' || phase === 'final_answer' ? { phase } : {}),
+      };
     }
   }
-  return '';
+  return { text: '' };
 };
 
 const extractThinkChunk = (data: ReasoningDeltaData | undefined): string => {
@@ -96,6 +119,17 @@ const extractThinkChunk = (data: ReasoningDeltaData | undefined): string => {
 const stringifyArgs = (args: unknown): string =>
   typeof args === 'string' ? args : JSON.stringify(args ?? {});
 
+const updateMessagePhase = (
+  phases: Record<string, AssistantTextPhase>,
+  stepId: string,
+  phase: AssistantTextPhase | undefined,
+): Record<string, AssistantTextPhase> => {
+  const next = { ...phases };
+  if (phase == null) delete next[stepId];
+  else next[stepId] = phase;
+  return next;
+};
+
 /**
  * Cursor carried across `foldSubagentEvent` calls so the aggregator can
  * extend an in-flight TEXT/THINK run without re-scanning earlier parts
@@ -107,6 +141,14 @@ export interface SubagentAggregatorState {
   openTextIdx: number | null;
   /** Index of the currently-open THINK part, or `null` when none. */
   openThinkIdx: number | null;
+  /**
+   * Active message-step ID to its declared text phase; graph members can
+   * overlap. Entries leave on `run_step_closed`, so the runtime's bounded
+   * concurrent graph width—not historical step count—bounds this table.
+   */
+  messagePhaseByStepId: Record<string, AssistantTextPhase>;
+  /** Compatibility phase for legacy message events that omit their step ID. */
+  idlessTextPhase?: AssistantTextPhase;
   /** `tool_call.id` → its index in `contentParts` for O(1) updates. */
   toolCallIndexById: Record<string, number>;
 }
@@ -116,6 +158,7 @@ export function initSubagentAggregatorState(): SubagentAggregatorState {
   return {
     openTextIdx: null,
     openThinkIdx: null,
+    messagePhaseByStepId: {},
     toolCallIndexById: {},
   };
 }
@@ -142,21 +185,35 @@ export function foldSubagentEvent(
   event: SubagentUpdateEvent,
 ): { parts: SubagentContentPart[]; state: SubagentAggregatorState } {
   if (event.phase === 'message_delta') {
-    const chunk = extractTextChunk(event.data as MessageDeltaData | undefined);
+    const data = event.data as MessageDeltaData | undefined;
+    const extracted = extractTextChunk(data);
+    const chunk = extracted.text;
     if (!chunk) return { parts, state };
+    const stepId = data?.id;
+    const phase =
+      extracted.phase ??
+      (typeof stepId === 'string' && stepId !== ''
+        ? state.messagePhaseByStepId[stepId]
+        : state.idlessTextPhase);
     /** Reasoning→text transition: close the open THINK so the THINK part
      *  lands BEFORE the TEXT part in chronological order. */
     const afterThinkClose = state.openThinkIdx != null ? { ...state, openThinkIdx: null } : state;
     if (afterThinkClose.openTextIdx != null) {
       const idx = afterThinkClose.openTextIdx;
       const existing = parts[idx] as TextPart;
-      const next = parts.slice();
-      next[idx] = { type: ContentTypes.TEXT, text: existing.text + chunk };
-      return { parts: next, state: afterThinkClose };
+      if ((existing.phase ?? null) === (phase ?? null)) {
+        const next = parts.slice();
+        next[idx] = { ...existing, text: existing.text + chunk };
+        return { parts: next, state: afterThinkClose };
+      }
     }
     const next = parts.slice();
     const newIdx = next.length;
-    next.push({ type: ContentTypes.TEXT, text: chunk });
+    next.push({
+      type: ContentTypes.TEXT,
+      text: chunk,
+      ...(phase == null ? {} : { phase }),
+    });
     return { parts: next, state: { ...afterThinkClose, openTextIdx: newIdx } };
   }
 
@@ -179,8 +236,29 @@ export function foldSubagentEvent(
 
   if (event.phase === 'run_step') {
     const data = event.data as RunStepData | undefined;
-    if (data?.stepDetails?.type !== 'tool_calls') return { parts, state };
-    const toolCalls = data.stepDetails.tool_calls ?? [];
+    const details = data?.stepDetails;
+    if (details?.type === 'message_creation') {
+      const phase = details.message_creation?.phase;
+      const textPhase = phase === 'commentary' || phase === 'final_answer' ? phase : undefined;
+      const stepId = data?.id;
+      if (typeof stepId === 'string' && stepId !== '') {
+        const messagePhaseByStepId = updateMessagePhase(
+          state.messagePhaseByStepId,
+          stepId,
+          textPhase,
+        );
+        return { parts, state: { ...state, messagePhaseByStepId } };
+      }
+      return {
+        parts,
+        state: {
+          ...state,
+          idlessTextPhase: textPhase,
+        },
+      };
+    }
+    if (details?.type !== 'tool_calls') return { parts, state };
+    const toolCalls = details.tool_calls ?? [];
     let next = parts;
     const toolCallIndexById = { ...state.toolCallIndexById };
     for (const tc of toolCalls) {
@@ -203,7 +281,13 @@ export function foldSubagentEvent(
      *  them — close the buffers. */
     return {
       parts: next,
-      state: { openTextIdx: null, openThinkIdx: null, toolCallIndexById },
+      state: {
+        ...state,
+        openTextIdx: null,
+        openThinkIdx: null,
+        idlessTextPhase: undefined,
+        toolCallIndexById,
+      },
     };
   }
 
@@ -221,6 +305,7 @@ export function foldSubagentEvent(
           ...(tc.name ? { name: tc.name } : {}),
           ...(tc.args != null ? { args: stringifyArgs(tc.args) } : {}),
           ...(tc.output != null ? { output: tc.output } : {}),
+          ...(tc.inputValidationError === true ? { inputValidationError: true } : {}),
           progress: tc.progress ?? 1,
         },
       };
@@ -239,6 +324,7 @@ export function foldSubagentEvent(
         name: tc.name ?? '',
         args: stringifyArgs(tc.args),
         output: tc.output,
+        ...(tc.inputValidationError === true ? { inputValidationError: true } : {}),
         progress: tc.progress ?? 1,
         type: ToolCallTypes.TOOL_CALL,
       },
@@ -246,13 +332,27 @@ export function foldSubagentEvent(
     return {
       parts: next,
       state: {
+        ...state,
         openTextIdx: null,
         openThinkIdx: null,
+        idlessTextPhase: undefined,
         toolCallIndexById: { ...state.toolCallIndexById, [tc.id]: newIdx },
       },
     };
   }
 
+  if (event.phase === 'run_step_closed') {
+    const stepId = (event.data as RunStepClosedData | undefined)?.id;
+    if (typeof stepId !== 'string' || stepId === '') return { parts, state };
+    return {
+      parts,
+      state: {
+        ...state,
+        messagePhaseByStepId: updateMessagePhase(state.messagePhaseByStepId, stepId, undefined),
+      },
+    };
+  }
+
   return { parts, state };
 }
 
@@ -393,7 +493,7 @@ export function foldSubagentEventIntoTicker(
   event: SubagentUpdateEvent,
 ): SubagentTickerState {
   if (event.phase === 'message_delta') {
-    const chunk = extractTextChunk(event.data as MessageDeltaData | undefined);
+    const chunk = extractTextChunk(event.data as MessageDeltaData | undefined).text;
     if (!chunk) return state;
     /** Delta-type transition: close any open reasoning buffer/cursor so
      *  a later `reasoning_delta` starts a NEW line below this text,
diff --git a/e2e/specs/mock/subagent-activity.spec.ts b/e2e/specs/mock/subagent-activity.spec.ts
index f3258617df..97512266f5 100644
--- a/e2e/specs/mock/subagent-activity.spec.ts
+++ b/e2e/specs/mock/subagent-activity.spec.ts
@@ -107,7 +107,6 @@ test.describe('detached subagent activity', () => {
       const activityResponse = await activityResponsePromise;
       await expect(panel).toBeVisible();
       await expect(panel.getByText('Running', { exact: true })).toBeVisible();
-      await expect(panel.getByText('Writing', { exact: true })).toBeVisible();
       await expect(panel).toContainText('child-1-phase-10');
       await expect.poll(() => activityRequests.length).toBe(1);
 
diff --git a/packages/data-provider/src/types/runs.ts b/packages/data-provider/src/types/runs.ts
index e8498bdd74..a3ffd26dda 100644
--- a/packages/data-provider/src/types/runs.ts
+++ b/packages/data-provider/src/types/runs.ts
@@ -383,6 +383,7 @@ export type SubagentUpdatePhase =
   | 'run_step'
   | 'run_step_delta'
   | 'run_step_completed'
+  | 'run_step_closed'
   | 'message_delta'
   | 'reasoning_delta'
   | 'stop'