diff --git a/web/app/components/base/markdown-blocks/think-block.spec.tsx b/web/app/components/base/markdown-blocks/think-block.spec.tsx
new file mode 100644
index 0000000000..a155b240b9
--- /dev/null
+++ b/web/app/components/base/markdown-blocks/think-block.spec.tsx
@@ -0,0 +1,248 @@
+import { act, render, screen } from '@testing-library/react'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { ChatContextProvider } from '@/app/components/base/chat/chat/context'
+import ThinkBlock from './think-block'
+
+// Mock react-i18next
+vi.mock('react-i18next', () => ({
+  useTranslation: () => ({
+    t: (key: string) => {
+      const translations: Record<string, string> = {
+        'chat.thinking': 'Thinking...',
+        'chat.thought': 'Thought',
+      }
+      return translations[key] || key
+    },
+  }),
+}))
+
+// Helper to wrap component with ChatContextProvider
+const renderWithContext = (
+  children: React.ReactNode,
+  isResponding: boolean = true,
+) => {
+  return render(
+    <ChatContextProvider
+      config={undefined}
+      isResponding={isResponding}
+      chatList={[]}
+      showPromptLog={false}
+      questionIcon={undefined}
+      answerIcon={undefined}
+      onSend={undefined}
+      onRegenerate={undefined}
+      onAnnotationEdited={undefined}
+      onAnnotationAdded={undefined}
+      onAnnotationRemoved={undefined}
+      onFeedback={undefined}
+    >
+      {children}
+    </ChatContextProvider>,
+  )
+}
+
+describe('ThinkBlock', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    vi.useFakeTimers()
+  })
+
+  afterEach(() => {
+    vi.useRealTimers()
+  })
+
+  describe('Rendering', () => {
+    it('should render regular details element when data-think is false', () => {
+      render(
+        <ThinkBlock data-think={false}>
+          <p>Regular content</p>
+        </ThinkBlock>,
+      )
+
+      expect(screen.getByText('Regular content')).toBeInTheDocument()
+    })
+
+    it('should render think block with thinking state when data-think is true', () => {
+      renderWithContext(
+        <ThinkBlock data-think={true}>
+          <p>Thinking content</p>
+        </ThinkBlock>,
+        true,
+      )
+
+      expect(screen.getByText(/Thinking\.\.\./)).toBeInTheDocument()
+      expect(screen.getByText('Thinking content')).toBeInTheDocument()
+    })
+
+    it('should render thought state when content has ENDTHINKFLAG', () => {
+      renderWithContext(
+        <ThinkBlock data-think={true}>
+          <p>Completed thinking[ENDTHINKFLAG]</p>
+        </ThinkBlock>,
+        true,
+      )
+
+      expect(screen.getByText(/Thought/)).toBeInTheDocument()
+    })
+  })
+
+  describe('Timer behavior', () => {
+    it('should update elapsed time while thinking', () => {
+      renderWithContext(
+        <ThinkBlock data-think={true}>
+          <p>Thinking...</p>
+        </ThinkBlock>,
+        true,
+      )
+
+      // Initial state should show 0.0s
+      expect(screen.getByText(/\(0\.0s\)/)).toBeInTheDocument()
+
+      // Advance timer by 500ms and run pending timers
+      act(() => {
+        vi.advanceTimersByTime(500)
+      })
+
+      // Should show approximately 0.5s
+      expect(screen.getByText(/\(0\.5s\)/)).toBeInTheDocument()
+    })
+
+    it('should stop timer when isResponding becomes false', () => {
+      const { rerender } = render(
+        <ChatContextProvider
+          config={undefined}
+          isResponding={true}
+          chatList={[]}
+          showPromptLog={false}
+          questionIcon={undefined}
+          answerIcon={undefined}
+          onSend={undefined}
+          onRegenerate={undefined}
+          onAnnotationEdited={undefined}
+          onAnnotationAdded={undefined}
+          onAnnotationRemoved={undefined}
+          onFeedback={undefined}
+        >
+          <ThinkBlock data-think={true}>
+            <p>Thinking content</p>
+          </ThinkBlock>
+        </ChatContextProvider>,
+      )
+
+      // Verify initial thinking state
+      expect(screen.getByText(/Thinking\.\.\./)).toBeInTheDocument()
+
+      // Advance timer
+      act(() => {
+        vi.advanceTimersByTime(1000)
+      })
+
+      // Simulate user clicking stop (isResponding becomes false)
+      rerender(
+        <ChatContextProvider
+          config={undefined}
+          isResponding={false}
+          chatList={[]}
+          showPromptLog={false}
+          questionIcon={undefined}
+          answerIcon={undefined}
+          onSend={undefined}
+          onRegenerate={undefined}
+          onAnnotationEdited={undefined}
+          onAnnotationAdded={undefined}
+          onAnnotationRemoved={undefined}
+          onFeedback={undefined}
+        >
+          <ThinkBlock data-think={true}>
+            <p>Thinking content</p>
+          </ThinkBlock>
+        </ChatContextProvider>,
+      )
+
+      // Should now show "Thought" instead of "Thinking..."
+      expect(screen.getByText(/Thought/)).toBeInTheDocument()
+    })
+
+    it('should NOT stop timer when isResponding is undefined (outside ChatContextProvider)', () => {
+      // Render without ChatContextProvider
+      render(
+        <ThinkBlock data-think={true}>
+          <p>Content without ENDTHINKFLAG</p>
+        </ThinkBlock>,
+      )
+
+      // Initial state should show "Thinking..."
+      expect(screen.getByText(/Thinking\.\.\./)).toBeInTheDocument()
+
+      // Advance timer
+      act(() => {
+        vi.advanceTimersByTime(2000)
+      })
+
+      // Timer should still be running (showing "Thinking..." not "Thought")
+      expect(screen.getByText(/Thinking\.\.\./)).toBeInTheDocument()
+      expect(screen.getByText(/\(2\.0s\)/)).toBeInTheDocument()
+    })
+  })
+
+  describe('ENDTHINKFLAG handling', () => {
+    it('should remove ENDTHINKFLAG from displayed content', () => {
+      renderWithContext(
+        <ThinkBlock data-think={true}>
+          <p>Content[ENDTHINKFLAG]</p>
+        </ThinkBlock>,
+        true,
+      )
+
+      expect(screen.getByText('Content')).toBeInTheDocument()
+      expect(screen.queryByText('[ENDTHINKFLAG]')).not.toBeInTheDocument()
+    })
+
+    it('should detect ENDTHINKFLAG in nested children', () => {
+      renderWithContext(
+        <ThinkBlock data-think={true}>
+          <div>
+            <span>Nested content[ENDTHINKFLAG]</span>
+          </div>
+        </ThinkBlock>,
+        true,
+      )
+
+      // Should show "Thought" since ENDTHINKFLAG is present
+      expect(screen.getByText(/Thought/)).toBeInTheDocument()
+    })
+
+    it('should detect ENDTHINKFLAG in array children', () => {
+      renderWithContext(
+        <ThinkBlock data-think={true}>
+          {['Part 1', 'Part 2[ENDTHINKFLAG]']}
+        </ThinkBlock>,
+        true,
+      )
+
+      expect(screen.getByText(/Thought/)).toBeInTheDocument()
+    })
+  })
+
+  describe('Edge cases', () => {
+    it('should handle empty children', () => {
+      renderWithContext(
+        <ThinkBlock data-think={true}></ThinkBlock>,
+        true,
+      )
+
+      expect(screen.getByText(/Thinking\.\.\./)).toBeInTheDocument()
+    })
+
+    it('should handle null children gracefully', () => {
+      renderWithContext(
+        <ThinkBlock data-think={true}>
+          {null}
+        </ThinkBlock>,
+        true,
+      )
+
+      expect(screen.getByText(/Thinking\.\.\./)).toBeInTheDocument()
+    })
+  })
+})
diff --git a/web/app/components/base/markdown-blocks/think-block.tsx b/web/app/components/base/markdown-blocks/think-block.tsx
index 697fd096cc..f920218152 100644
--- a/web/app/components/base/markdown-blocks/think-block.tsx
+++ b/web/app/components/base/markdown-blocks/think-block.tsx
@@ -59,7 +59,11 @@ const useThinkTimer = (children: any) => {
   }, [startTime, isComplete])
 
   useEffect(() => {
-    if (hasEndThink(children) || !isResponding)
+    // Stop timer when:
+    // 1. Content has [ENDTHINKFLAG] marker (normal completion)
+    // 2. isResponding is explicitly false (user clicked stop button)
+    // Note: Don't stop when isResponding is undefined (component used outside ChatContextProvider)
+    if (hasEndThink(children) || isResponding === false)
       setIsComplete(true)
   }, [children, isResponding])