From 0883af2d182f19dde4ac72558dc2fd9501bdb7fa Mon Sep 17 00:00:00 2001
From: Brennan Benson <79079362+brennanb2025@users.noreply.github.com>
Date: Tue, 3 Nov 2026 14:25:27 -0800
Subject: [PATCH] Avoid copying chat history on preview-only updates (#37922)

* Avoid copying chat history on preview-only updates

* Use fixed timestamps in preview input regression test
---
 ...eChatResolvedView.streaming-input.test.tsx | 163 ++++++++++++++++++
 .../native-chat/NativeChatResolvedView.tsx    |  17 +-
 2 files changed, 172 insertions(+), 8 deletions(-)
 create mode 100644 src/renderer/src/components/native-chat/NativeChatResolvedView.streaming-input.test.tsx

diff --git a/src/renderer/src/components/native-chat/NativeChatResolvedView.streaming-input.test.tsx b/src/renderer/src/components/native-chat/NativeChatResolvedView.streaming-input.test.tsx
new file mode 100644
index 00000000000..1fd302d51f8
--- /dev/null
+++ b/src/renderer/src/components/native-chat/NativeChatResolvedView.streaming-input.test.tsx
@@ -0,0 +1,163 @@
+// @vitest-environment happy-dom
+
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { act, cleanup, fireEvent, render, screen } from '@testing-library/react'
+import type { NativeChatMessage } from '../../../../shared/native-chat/native-chat-types'
+import type { NativeChatLiveSession } from './use-native-chat-live-session'
+import type { NativeChatComposerProps } from './native-chat-composer-types'
+import type { AgentStatusPayload } from '../../../../shared/agent-status-types'
+import type * as StreamingModule from '../../../../shared/native-chat/native-chat-streaming'
+import { clearPendingSendCacheForTests } from './native-chat-pending'
+
+const probe = vi.hoisted<{
+  session: NativeChatLiveSession | null
+  inputs: (readonly NativeChatMessage[])[]
+  shown: NativeChatMessage[]
+}>(() => ({ session: null, inputs: [], shown: [] }))
+vi.mock('./use-native-chat-retained-session', () => ({
+  useNativeChatRetainedSession: () => probe.session
+}))
+vi.mock('./NativeChatMessageList', () => ({
+  NativeChatMessageList: ({ session }: { session: NativeChatLiveSession }) => {
+    probe.shown = session.messages
+    return null
+  }
+}))
+vi.mock('./NativeChatComposer', () => ({
+  NativeChatComposer: ({ onOptimisticSend }: NativeChatComposerProps) => (
+    <button onClick={() => onOptimisticSend?.('Follow up')}>send</button>
+  )
+}))
+vi.mock('../../../../shared/native-chat/native-chat-streaming', async (importOriginal) => {
+  const original = await importOriginal<typeof StreamingModule>()
+  return {
+    ...original,
+    deriveNativeChatStreamingText: (
+      args: Parameters<typeof original.deriveNativeChatStreamingText>[0]
+    ) => {
+      probe.inputs.push(args.messages)
+      return original.deriveNativeChatStreamingText(args)
+    }
+  }
+})
+
+const { NativeChatResolvedView } = await import('./NativeChatResolvedView')
+const { TooltipProvider } = await import('@/components/ui/tooltip')
+const { useAppStore } = await import('../../store')
+const paneKey = 'preview-perf:leaf'
+const message = (id: string, role: 'user' | 'assistant', text: string): NativeChatMessage => ({
+  id,
+  role,
+  blocks: [{ type: 'text', text }],
+  timestamp: 1,
+  source: 'transcript'
+})
+
+function setPreview(text: string, extra: Partial<AgentStatusPayload> = {}): void {
+  useAppStore.getState().setAgentStatus(paneKey, {
+    prompt: 'Prompt',
+    agentType: 'claude',
+    state: 'working',
+    lastAssistantMessage: text,
+    ...extra
+  })
+}
+
+function previewTicks(expected: readonly NativeChatMessage[], extra = {}): void {
+  probe.inputs = []
+  for (let tick = 0; tick < 20; tick += 1) {
+    act(() => setPreview(`Current reply ${tick}`, extra))
+  }
+  expect(probe.inputs.length).toBeGreaterThanOrEqual(20)
+  expect(new Set(probe.inputs).size).toBe(1)
+  expect(probe.inputs[0]).toBe(expected)
+}
+
+beforeEach(() => {
+  vi.useFakeTimers({ toFake: ['Date'] })
+  vi.setSystemTime(1_000)
+  clearPendingSendCacheForTests()
+  probe.inputs = []
+  probe.shown = []
+  useAppStore.setState({ agentStatusByPaneKey: {}, nativeChatLaunchPromptByTabId: {} })
+})
+afterEach(() => {
+  cleanup()
+  vi.useRealTimers()
+  clearPendingSendCacheForTests()
+  useAppStore.setState({ agentStatusByPaneKey: {}, nativeChatLaunchPromptByTabId: {} })
+})
+
+describe('preview input allocation at realistic history sizes', () => {
+  it.each([1_000, 10_000])('reuses input across ticks with %i loaded messages', (size) => {
+    const transcript = Array.from({ length: size }, (_, index) =>
+      message(`history-${index}`, index % 2 ? 'assistant' : 'user', `History ${index}`)
+    )
+    probe.session = {
+      messages: transcript,
+      status: 'working',
+      sessionId: 'preview-perf',
+      agent: 'claude',
+      hasMore: false,
+      loadingEarlier: false,
+      olderHistoryGeneration: 0,
+      loadEarlier: vi.fn(),
+      readPhase: 'ready'
+    }
+    setPreview('Current reply')
+    render(
+      <TooltipProvider>
+        <NativeChatResolvedView
+          paneKey={paneKey}
+          agent="claude"
+          sessionId="preview-perf"
+          transcriptPath={null}
+          isVisible
+          isFocusedGroup
+          targetPtyId="preview-perf-pty"
+          terminalTabId="preview-perf"
+          ownsTabWideLaunchDraft={false}
+        />
+      </TooltipProvider>
+    )
+    previewTicks(transcript)
+    act(() => fireEvent.click(screen.getByRole('button', { name: 'send' })))
+    const queued = probe.inputs.at(-1)!
+    expect(queued).toHaveLength(size + 1)
+    expect(queued.at(-1)?.pendingSend).toBe('queued')
+    previewTicks(queued)
+    expect(probe.shown.at(-2)?.id).toBe('streaming')
+    expect(probe.shown.at(-1)?.pendingSend).toBe('queued')
+    previewTicks(queued, { lastAssistantMessageIsToolOutput: true })
+
+    const caughtUp = [...transcript, message('reply', 'assistant', 'Current reply 19')]
+    act(() => {
+      probe.session = { ...probe.session!, messages: caughtUp }
+      setPreview('Current reply 19')
+    })
+    const updated = probe.inputs.at(-1)!
+    expect(updated).not.toBe(queued)
+    expect(updated.at(-2)?.id).toBe('reply')
+    expect(updated.at(-1)?.pendingSend).toBe('queued')
+    previewTicks(updated)
+
+    act(() => {
+      probe.session = { ...probe.session!, status: 'ready' }
+      setPreview('Idle', { state: 'done' })
+    })
+    const idle = probe.inputs.at(-1)!
+    expect(idle).not.toBe(updated)
+    previewTicks(idle, { state: 'done' })
+
+    const retired = [
+      ...caughtUp,
+      { ...message('follow-up', 'user', 'Follow up'), timestamp: 1_001 },
+      { ...message('next-reply', 'assistant', 'Done'), timestamp: 1_002 }
+    ]
+    act(() => {
+      probe.session = { ...probe.session!, messages: retired }
+      setPreview('Done', { state: 'done' })
+    })
+    previewTicks(retired, { state: 'done' })
+  })
+})
diff --git a/src/renderer/src/components/native-chat/NativeChatResolvedView.tsx b/src/renderer/src/components/native-chat/NativeChatResolvedView.tsx
index 0184136f664..53c0547c0ca 100644
--- a/src/renderer/src/components/native-chat/NativeChatResolvedView.tsx
+++ b/src/renderer/src/components/native-chat/NativeChatResolvedView.tsx
@@ -222,20 +222,21 @@ export function NativeChatResolvedView({
     () => pendingSendsAsMessages(pending, sessionAfterCommandBoundaries.messages, sendWorkingEpoch),
     [pending, sessionAfterCommandBoundaries.messages, sendWorkingEpoch]
   )
+  const messagesWithPending = useMemo(
+    () =>
+      pendingMessages.length === 0
+        ? sessionAfterCommandBoundaries.messages
+        : [...sessionAfterCommandBoundaries.messages, ...pendingMessages],
+    [sessionAfterCommandBoundaries.messages, pendingMessages]
+  )
   const streamingText = useMemo(() => {
     return deriveNativeChatStreamingText({
-      messages: [...sessionAfterCommandBoundaries.messages, ...pendingMessages],
+      messages: messagesWithPending,
       previewText: hookPreview,
       working: liveWorking,
       previewIsToolOutput: hookPreviewIsToolOutput
     })
-  }, [
-    sessionAfterCommandBoundaries.messages,
-    pendingMessages,
-    hookPreview,
-    liveWorking,
-    hookPreviewIsToolOutput
-  ])
+  }, [messagesWithPending, hookPreview, liveWorking, hookPreviewIsToolOutput])
   const sessionWithPending = useMemo<typeof session>(() => {
     if (pending.length === 0 && commandMarkers.length === 0 && !streamingText) {
       return sessionAfterCommandBoundaries