diff --git a/src/renderer/src/hooks/useAgentStatus.ts b/src/renderer/src/hooks/useAgentStatus.ts
index e86aec562..256a64894 100644
--- a/src/renderer/src/hooks/useAgentStatus.ts
+++ b/src/renderer/src/hooks/useAgentStatus.ts
@@ -106,7 +106,8 @@ export function useAgentStatus(): AgentStatusState & AgentStatusActions {
             sessionId: currentSession.agentSessionId,
             update: {
               sessionUpdate: 'user_message',
-              content: content
+              content: content,
+              _internal: false
             }
           }
         } as unknown as StoredSessionUpdate
diff --git a/tests/unit/renderer/utils/sessionUpdates.test.ts b/tests/unit/renderer/utils/sessionUpdates.test.ts
new file mode 100644
index 000000000..58f39245b
--- /dev/null
+++ b/tests/unit/renderer/utils/sessionUpdates.test.ts
@@ -0,0 +1,186 @@
+import { describe, it, expect } from 'vitest'
+import { mergeSessionUpdates } from '../../../../src/renderer/src/utils/sessionUpdates'
+import type { StoredSessionUpdate } from '../../../../src/shared/types'
+
+function makeUpdate(
+  overrides: Partial<StoredSessionUpdate> & { update: StoredSessionUpdate['update'] }
+): StoredSessionUpdate {
+  return {
+    timestamp: new Date().toISOString(),
+    ...overrides
+  }
+}
+
+describe('mergeSessionUpdates', () => {
+  it('should return existing when incoming is empty', () => {
+    const existing = [
+      makeUpdate({ update: { sessionId: 's1', update: { sessionUpdate: 'text_delta' } } })
+    ]
+    const result = mergeSessionUpdates(existing, [])
+    expect(result).toBe(existing)
+  })
+
+  it('should combine non-overlapping updates', () => {
+    const existing = [
+      makeUpdate({ update: { sessionId: 's1', update: { sessionUpdate: 'text_delta' } } })
+    ]
+    const incoming = [
+      makeUpdate({ update: { sessionId: 's1', update: { sessionUpdate: 'end_turn' } } })
+    ]
+    const result = mergeSessionUpdates(existing, incoming)
+    expect(result).toHaveLength(2)
+  })
+
+  it('should deduplicate by sequence number', () => {
+    const update = {
+      sessionId: 's1',
+      update: { sessionUpdate: 'text_delta' }
+    } as StoredSessionUpdate['update']
+
+    const existing = [makeUpdate({ sequenceNumber: 1, update })]
+    const incoming = [makeUpdate({ sequenceNumber: 1, update })]
+    const result = mergeSessionUpdates(existing, incoming)
+    expect(result).toHaveLength(1)
+  })
+
+  it('should deduplicate by payload when no sequence number', () => {
+    const update = {
+      sessionId: 's1',
+      update: { sessionUpdate: 'text_delta' }
+    } as StoredSessionUpdate['update']
+
+    const existing = [makeUpdate({ update })]
+    const incoming = [makeUpdate({ update })]
+    const result = mergeSessionUpdates(existing, incoming)
+    expect(result).toHaveLength(1)
+  })
+
+  it('should replace payload-keyed item with sequence-numbered version', () => {
+    const update = {
+      sessionId: 's1',
+      update: {
+        sessionUpdate: 'user_message',
+        content: [{ type: 'text', text: 'hello' }],
+        _internal: false
+      }
+    } as StoredSessionUpdate['update']
+
+    const optimistic = makeUpdate({ update })
+    const fromDb = makeUpdate({ sequenceNumber: 5, update })
+
+    const result = mergeSessionUpdates([optimistic], [fromDb])
+    expect(result).toHaveLength(1)
+    expect(result[0].sequenceNumber).toBe(5)
+  })
+
+  it('should deduplicate optimistic user_message with DB version when _internal matches', () => {
+    const content = [{ type: 'text', text: 'hello world' }]
+    const agentSessionId = 'agent-session-1'
+
+    // Optimistic update (frontend) - with _internal: false to match DB
+    const optimistic = makeUpdate({
+      update: {
+        sessionId: agentSessionId,
+        update: {
+          sessionUpdate: 'user_message',
+          content,
+          _internal: false
+        }
+      } as StoredSessionUpdate['update']
+    })
+
+    // DB-stored version (returned by focus sync)
+    const fromDb = makeUpdate({
+      sequenceNumber: 10,
+      update: {
+        sessionId: agentSessionId,
+        update: {
+          sessionUpdate: 'user_message',
+          content,
+          _internal: false
+        }
+      } as StoredSessionUpdate['update']
+    })
+
+    const result = mergeSessionUpdates([optimistic], [fromDb])
+    expect(result).toHaveLength(1)
+    expect(result[0].sequenceNumber).toBe(10)
+  })
+
+  it('should NOT deduplicate when _internal field is missing (the bug scenario)', () => {
+    const content = [{ type: 'text', text: 'hello world' }]
+    const agentSessionId = 'agent-session-1'
+
+    // Optimistic update WITHOUT _internal (the old buggy format)
+    const optimistic = makeUpdate({
+      update: {
+        sessionId: agentSessionId,
+        update: {
+          sessionUpdate: 'user_message',
+          content
+        }
+      } as StoredSessionUpdate['update']
+    })
+
+    // DB-stored version WITH _internal
+    const fromDb = makeUpdate({
+      sequenceNumber: 10,
+      update: {
+        sessionId: agentSessionId,
+        update: {
+          sessionUpdate: 'user_message',
+          content,
+          _internal: false
+        }
+      } as StoredSessionUpdate['update']
+    })
+
+    // This demonstrates the bug: mismatched payloads cause duplication
+    const result = mergeSessionUpdates([optimistic], [fromDb])
+    expect(result).toHaveLength(2)
+  })
+
+  it('should skip payload-only item when seq-keyed version already exists', () => {
+    const update = {
+      sessionId: 's1',
+      update: { sessionUpdate: 'text_delta' }
+    } as StoredSessionUpdate['update']
+
+    const fromDb = makeUpdate({ sequenceNumber: 1, update })
+    const duplicate = makeUpdate({ update })
+
+    const result = mergeSessionUpdates([fromDb], [duplicate])
+    expect(result).toHaveLength(1)
+    expect(result[0].sequenceNumber).toBe(1)
+  })
+
+  it('should preserve order of non-duplicate updates', () => {
+    const u1 = makeUpdate({
+      sequenceNumber: 1,
+      update: {
+        sessionId: 's1',
+        update: { sessionUpdate: 'text_delta', text: 'a' }
+      } as StoredSessionUpdate['update']
+    })
+    const u2 = makeUpdate({
+      sequenceNumber: 2,
+      update: {
+        sessionId: 's1',
+        update: { sessionUpdate: 'text_delta', text: 'b' }
+      } as StoredSessionUpdate['update']
+    })
+    const u3 = makeUpdate({
+      sequenceNumber: 3,
+      update: {
+        sessionId: 's1',
+        update: { sessionUpdate: 'end_turn' }
+      } as StoredSessionUpdate['update']
+    })
+
+    const result = mergeSessionUpdates([u1], [u2, u3])
+    expect(result).toHaveLength(3)
+    expect(result[0].sequenceNumber).toBe(1)
+    expect(result[1].sequenceNumber).toBe(2)
+    expect(result[2].sequenceNumber).toBe(3)
+  })
+})