diff --git a/frontend-modern/src/api/__tests__/aiChat.test.ts b/frontend-modern/src/api/__tests__/aiChat.test.ts
index 01381f393..f9d98ed4e 100644
--- a/frontend-modern/src/api/__tests__/aiChat.test.ts
+++ b/frontend-modern/src/api/__tests__/aiChat.test.ts
@@ -247,6 +247,55 @@ describe('AIChatAPI', () => {
     });
   });
 
+  it('runs the reasoning-leak dev stream fixture without opening a provider request', async () => {
+    const onEvent = vi.fn();
+
+    await AIChatAPI.chat(
+      '/fixture reasoning-leak',
+      undefined,
+      'openrouter:deepseek/deepseek-chat',
+      onEvent,
+    );
+
+    expect(apiFetchMock).not.toHaveBeenCalled();
+    expect(onEvent.mock.calls.map(([event]) => event.type)).toEqual([
+      'session',
+      'workflow_state',
+      'content',
+      'content',
+      'tool_start',
+      'tool_end',
+      'content',
+      'done',
+    ]);
+    expect(onEvent.mock.calls[2][0]).toMatchObject({
+      type: 'content',
+      data: {
+        text: expect.stringContaining('Thinking'),
+      },
+    });
+    expect(onEvent.mock.calls[3][0]).toMatchObject({
+      type: 'content',
+      data: {
+        text: expect.stringContaining('pulse_read'),
+      },
+    });
+    expect(onEvent.mock.calls[4][0]).toMatchObject({
+      type: 'tool_start',
+      data: {
+        id: 'fixture-tool-reasoning-leak',
+        name: 'pulse_read',
+        input: expect.stringContaining('ls /dev | wc -l'),
+      },
+    });
+    expect(onEvent.mock.calls[6][0]).toMatchObject({
+      type: 'content',
+      data: {
+        text: 'There are 4,358 entries under `/dev`.',
+      },
+    });
+  });
+
   it('runs the workflow-burst dev stream fixture without opening a provider request', async () => {
     const onEvent = vi.fn();
 
@@ -557,6 +606,7 @@ describe('AIChatAPI', () => {
     expect(AI_CHAT_DEV_STREAM_FIXTURE_NAMES).toContain('provider-retry');
     expect(AI_CHAT_DEV_STREAM_FIXTURE_NAMES).toContain('send-hold');
     expect(AI_CHAT_DEV_STREAM_FIXTURE_NAMES).toContain('command-tool');
+    expect(AI_CHAT_DEV_STREAM_FIXTURE_NAMES).toContain('reasoning-leak');
     expect(AI_CHAT_DEV_STREAM_FIXTURE_NAMES).not.toContain('/fixture provider-retry');
     expect(AI_CHAT_DEV_STREAM_FIXTURE_ALIAS_NAMES).toEqual(['burst-tool', 'queued-follow-up']);
   });
diff --git a/frontend-modern/src/api/aiChatDevStreamFixture.ts b/frontend-modern/src/api/aiChatDevStreamFixture.ts
index 18acebd4e..188088776 100644
--- a/frontend-modern/src/api/aiChatDevStreamFixture.ts
+++ b/frontend-modern/src/api/aiChatDevStreamFixture.ts
@@ -5,6 +5,7 @@ export const AI_CHAT_DEV_STREAM_FIXTURE_PROMPTS = [
   '/fixture assistant-stream',
   '/fixture send-hold',
   '/fixture tool-burst',
+  '/fixture reasoning-leak',
   '/fixture workflow-burst',
   '/fixture context-group',
   '/fixture status-boundary',
@@ -372,6 +373,67 @@ const buildToolBurstFixtureEvents = (model?: string): AIChatStreamEvent[] => [
   },
 ];
 
+const buildReasoningLeakFixtureEvents = (model?: string): AIChatStreamEvent[] => [
+  {
+    type: 'session',
+    data: { id: 'dev-fixture-reasoning-leak' },
+  },
+  {
+    type: 'workflow_state',
+    data: {
+      phase: 'request_start',
+      message: 'Preparing Pulse context.',
+    },
+  },
+  {
+    type: 'content',
+    data: {
+      text: 'Thinking\nWe need to inspect the prompt and count device nodes before answering.',
+    },
+  },
+  {
+    type: 'content',
+    data: {
+      text: '\npulse_read(target_host="current_resource", command="ls /dev | wc -l")',
+    },
+  },
+  {
+    type: 'tool_start',
+    data: {
+      id: 'fixture-tool-reasoning-leak',
+      name: 'pulse_read',
+      input: '{"action":"exec","target_host":"current_resource","command":"ls /dev | wc -l"}',
+      raw_input: 'pulse_read(target_host="current_resource", command="ls /dev | wc -l")',
+    },
+  },
+  {
+    type: 'tool_end',
+    data: {
+      id: 'fixture-tool-reasoning-leak',
+      name: 'pulse_read',
+      input: '{"action":"exec","target_host":"current_resource","command":"ls /dev | wc -l"}',
+      raw_input: 'pulse_read(target_host="current_resource", command="ls /dev | wc -l")',
+      output: '4358',
+      success: true,
+    },
+  },
+  {
+    type: 'content',
+    data: {
+      text: 'There are 4,358 entries under `/dev`.',
+    },
+  },
+  {
+    type: 'done',
+    data: {
+      session_id: 'dev-fixture-reasoning-leak',
+      model: assistantFixtureModel(model),
+      input_tokens: 88,
+      output_tokens: 24,
+    },
+  },
+];
+
 const buildWorkflowBurstFixtureEvents = (model?: string): AIChatStreamEvent[] => [
   {
     type: 'session',
@@ -1009,6 +1071,9 @@ const buildFixtureEvents = (prompt: string, model?: string): AIChatStreamEvent[]
   if (normalized === '/fixture tool-burst') {
     return buildToolBurstFixtureEvents(model);
   }
+  if (normalized === '/fixture reasoning-leak') {
+    return buildReasoningLeakFixtureEvents(model);
+  }
   if (normalized === '/fixture workflow-burst') {
     return buildWorkflowBurstFixtureEvents(model);
   }
diff --git a/frontend-modern/src/components/AI/Chat/__tests__/assistantOutputHygiene.test.ts b/frontend-modern/src/components/AI/Chat/__tests__/assistantOutputHygiene.test.ts
index 2690b149d..135fc6d39 100644
--- a/frontend-modern/src/components/AI/Chat/__tests__/assistantOutputHygiene.test.ts
+++ b/frontend-modern/src/components/AI/Chat/__tests__/assistantOutputHygiene.test.ts
@@ -45,6 +45,17 @@ describe('stripAssistantOutputArtifacts', () => {
     expect(result.stripped).toBe(true);
   });
 
+  it('suppresses content-channel reasoning preludes before raw function-call leaks', () => {
+    const result = stripAssistantOutputArtifacts(
+      'Thinking\nWe need to interpret the user question and count the device nodes.\npulse_read(target_host="current_resource", command="ls /dev | wc -l")',
+    );
+
+    expect(result).toEqual({
+      text: '',
+      stripped: true,
+    });
+  });
+
   it('leaves ordinary prose and unrelated function calls alone', () => {
     expect(stripAssistantOutputArtifacts('Call helper(target="x") in the example.')).toEqual({
       text: 'Call helper(target="x") in the example.',
@@ -115,6 +126,59 @@ describe('stripAssistantOutputArtifacts', () => {
     expect(flushPendingAssistantOutputText(state)).toBe('');
   });
 
+  it('holds content-channel reasoning preludes so they never flash before tool leaks', () => {
+    const state = createAssistantOutputArtifactStreamState();
+
+    expect(
+      appendVisibleTextBeforeAssistantOutputArtifacts(
+        state,
+        'Thinking\nWe need to count device nodes before answering.',
+      ),
+    ).toEqual({
+      text: '',
+      stripped: false,
+    });
+    expect(
+      appendVisibleTextBeforeAssistantOutputArtifacts(
+        state,
+        '\npulse_read(target_host="current_resource", command="ls /dev | wc -l")',
+      ),
+    ).toEqual({
+      text: '',
+      stripped: true,
+    });
+    expect(flushPendingAssistantOutputText(state)).toBe('');
+  });
+
+  it('drops held content-channel reasoning if no answer text follows before stream end', () => {
+    const state = createAssistantOutputArtifactStreamState();
+
+    expect(
+      appendVisibleTextBeforeAssistantOutputArtifacts(
+        state,
+        'Thinking\nWe need to inspect the prompt before answering.',
+      ),
+    ).toEqual({
+      text: '',
+      stripped: false,
+    });
+    expect(flushPendingAssistantOutputText(state)).toBe('');
+  });
+
+  it('releases normal prose that only starts with a similar word', () => {
+    const state = createAssistantOutputArtifactStreamState();
+
+    expect(appendVisibleTextBeforeAssistantOutputArtifacts(state, 'Think')).toEqual({
+      text: '',
+      stripped: false,
+    });
+    expect(appendVisibleTextBeforeAssistantOutputArtifacts(state, ' about the result this way.')).toEqual({
+      text: 'Think about the result this way.',
+      stripped: false,
+    });
+    expect(flushPendingAssistantOutputText(state)).toBe('');
+  });
+
   it('releases a held prefix when the next delta proves it is normal prose', () => {
     const state = createAssistantOutputArtifactStreamState();
 
diff --git a/frontend-modern/src/components/AI/Chat/__tests__/useChat.test.ts b/frontend-modern/src/components/AI/Chat/__tests__/useChat.test.ts
index a3c2a7bdf..700c86337 100644
--- a/frontend-modern/src/components/AI/Chat/__tests__/useChat.test.ts
+++ b/frontend-modern/src/components/AI/Chat/__tests__/useChat.test.ts
@@ -1109,6 +1109,30 @@ describe('useChat', () => {
       dispose();
     });
 
+    it('keeps content-channel reasoning and raw tool-call leaks out of assistant content', async () => {
+      const { getFireEvent } = setupWithEventCapture();
+      const { value: chat, dispose } = withRoot(() => useChat({ sessionId: 's' }));
+
+      await chat.sendMessage('how many devices?');
+      const fire = getFireEvent();
+
+      fire({
+        type: 'content',
+        data: 'Thinking\nWe need to inspect the prompt and count device nodes first.',
+      });
+      fire({
+        type: 'content',
+        data: '\npulse_read(target_host="current_resource", command="ls /dev | wc -l")',
+      });
+
+      const assistant = chat.messages().find((m) => m.role === 'assistant')!;
+      expect(assistant.content).toBe('');
+      expect(assistant.streamEvents?.filter((event) => event.type === 'content') ?? []).toEqual(
+        [],
+      );
+      dispose();
+    });
+
     it('processes thinking events — merges consecutive thinking', async () => {
       const { getFireEvent } = setupWithEventCapture();
       const { value: chat, dispose } = withRoot(() => useChat({ sessionId: 's' }));
diff --git a/frontend-modern/src/components/AI/Chat/assistantOutputHygiene.ts b/frontend-modern/src/components/AI/Chat/assistantOutputHygiene.ts
index 4c9bc7ad2..bf042c6df 100644
--- a/frontend-modern/src/components/AI/Chat/assistantOutputHygiene.ts
+++ b/frontend-modern/src/components/AI/Chat/assistantOutputHygiene.ts
@@ -30,6 +30,10 @@ const functionToolCallLeakRe = /(?:^|[^a-zA-Z0-9_])([a-zA-Z_][a-zA-Z0-9_]*)[ \t\
 const minimaxToolCallLeakRe = /^minimax:tool_call\b/m;
 const visibleInternalToolIdentifierRe =
   /`?\b((?:pulse|patrol)_[a-zA-Z0-9_]+|run_command)\b`?(?:[ \t]+(tool|command|query|call))?/g;
+const internalReasoningHeadingRe =
+  /^\s*(?:#{1,6}[ \t]*)?(thinking|thoughts?|reasoning|analysis)[ \t]*:?[ \t]*(?:\r?\n|$)/i;
+const internalReasoningCueRe =
+  /\b(?:we need to|i need to|let me|let's|need to|the user (?:asks|asked|is asking)|user question|before answering|before responding|tool call|call the|use the|inspect the prompt)\b/i;
 
 const VISIBLE_INTERNAL_TOOL_IDENTIFIER_LABELS: Record<string, string> = {
   patrol_collect: 'collection',
@@ -65,7 +69,7 @@ export function stripAssistantOutputArtifacts(content: string): {
   }
   const visiblePrefix = content.slice(0, idx).trim();
   return {
-    text: isCompactedToolPrelude(visiblePrefix)
+    text: shouldSuppressBeforeAssistantOutputArtifact(visiblePrefix)
       ? ''
       : normalizeAssistantVisibleInternalIdentifiers(visiblePrefix),
     stripped: true,
@@ -93,7 +97,7 @@ export function appendVisibleTextBeforeAssistantOutputArtifacts(
   const candidate = existingRawText + text;
   const idx = assistantOutputArtifactIndex(candidate);
   if (idx < 0) {
-    if (!existingRawText && isCompactedToolPrelude(text)) {
+    if (!existingRawText && shouldHoldPotentialAssistantOutputPrelude(text)) {
       state.pendingText = text;
       return { text: '', stripped: false };
     }
@@ -107,7 +111,7 @@ export function appendVisibleTextBeforeAssistantOutputArtifacts(
   }
 
   const safeText = candidate.slice(0, idx).trimEnd();
-  if (isCompactedToolPrelude(safeText)) {
+  if (shouldSuppressBeforeAssistantOutputArtifact(safeText)) {
     const previousVisibleText = state.visibleText;
     state.visibleText = '';
     state.rawVisibleText = '';
@@ -142,7 +146,7 @@ export function flushPendingAssistantOutputText(state: AssistantOutputArtifactSt
   }
   const text = state.pendingText;
   state.pendingText = '';
-  if (isCompactedToolPrelude(text)) {
+  if (shouldSuppressBeforeAssistantOutputArtifact(text)) {
     return '';
   }
   const normalizedText = normalizeAssistantVisibleInternalIdentifiers(text);
@@ -273,3 +277,36 @@ function isCompactedToolPrelude(content: string): boolean {
   if (whitespace === 0) return true;
   return letters >= 48 && whitespace <= 1;
 }
+
+function shouldSuppressBeforeAssistantOutputArtifact(content: string): boolean {
+  return isCompactedToolPrelude(content) || isContentChannelReasoningPrelude(content);
+}
+
+function shouldHoldPotentialAssistantOutputPrelude(content: string): boolean {
+  return isCompactedToolPrelude(content) || isPotentialContentChannelReasoningPrelude(content);
+}
+
+function isContentChannelReasoningPrelude(content: string): boolean {
+  const trimmed = content.trim();
+  if (!trimmed) return false;
+
+  const match = trimmed.match(internalReasoningHeadingRe);
+  if (!match) return false;
+
+  const body = trimmed.slice(match[0].length).trim();
+  if (!body) return true;
+  if (internalReasoningCueRe.test(body)) return true;
+
+  const words = body.split(/\s+/).filter(Boolean);
+  return words.length >= 8;
+}
+
+function isPotentialContentChannelReasoningPrelude(content: string): boolean {
+  const trimmed = content.trimStart();
+  if (!trimmed) return false;
+
+  const lower = trimmed.toLowerCase();
+  if (lower.length < 'thinking'.length && 'thinking'.startsWith(lower)) return true;
+
+  return internalReasoningHeadingRe.test(trimmed);
+}