From a1afa2acbb01c7d1e641c3ac18ec99194a49d3d7 Mon Sep 17 00:00:00 2001
From: Neil <neil@stably.ai>
Date: Fri, 11 Sep 2026 23:56:15 -0700
Subject: [PATCH] perf: scan chat activity lines from the end

---
 .../native-chat-activity-tail-line.test.ts    | 87 +++++++++++++++++++
 src/shared/native-chat-turn-activity.ts       | 19 ++--
 2 files changed, 100 insertions(+), 6 deletions(-)
 create mode 100644 src/shared/native-chat-activity-tail-line.test.ts

diff --git a/src/shared/native-chat-activity-tail-line.test.ts b/src/shared/native-chat-activity-tail-line.test.ts
new file mode 100644
index 00000000000..e441127a03a
--- /dev/null
+++ b/src/shared/native-chat-activity-tail-line.test.ts
@@ -0,0 +1,87 @@
+import { expect, it, vi } from 'vitest'
+import { normalizePromptField } from './agent-status-field-normalization'
+import type { AgentJournalRenderItem } from './agent-session-journal-types'
+import { selectStructuredAgentTurnActivity } from './native-chat-turn-activity'
+
+function status(text: string): AgentJournalRenderItem {
+  return {
+    itemId: 'status',
+    revision: 1,
+    sequence: 1,
+    observedAt: 1,
+    body: { kind: 'status', text }
+  }
+}
+
+it('does not trim every preceding status line to select the final activity', () => {
+  const text = `${'Previous activity\n'.repeat(500)}Preparing the answer`
+  const trim = vi.spyOn(String.prototype, 'trim')
+  try {
+    expect(selectStructuredAgentTurnActivity([status(text)], 'turn')).toEqual({
+      kind: 'description',
+      text: 'Preparing the answer'
+    })
+    expect(trim.mock.calls.length).toBeLessThan(10)
+  } finally {
+    trim.mockRestore()
+  }
+})
+
+it('preserves the last nonempty LF-delimited line before prompt normalization', () => {
+  const texts = [
+    '',
+    '\n',
+    '\n\n',
+    ' \r\n\t',
+    'first\rsecond',
+    '\nfirst\r\nsecond\r\n',
+    'first\n\u00a0\u2003\n',
+    'first\n\u2028second\u2029',
+    'a\n😀',
+    'a\n\ud800',
+    'a\n\udc00',
+    `a\n${'x'.repeat(199)}😀`,
+    `first\n${' '.repeat(3000)}last`,
+    'first\n\0\n',
+    'first\n\ufeff\n'
+  ]
+  const alphabet = [
+    'a',
+    ' ',
+    '\n',
+    '\r',
+    '\t',
+    '\u00a0',
+    '\u2003',
+    '\u2028',
+    '\ufeff',
+    '😀',
+    '\ud800',
+    '\udc00'
+  ]
+  let seed = 57
+  const next = () => {
+    seed = (Math.imul(seed, 1664525) + 1013904223) >>> 0
+    return seed
+  }
+  for (let index = 0; index < 1000; index++) {
+    let text = ''
+    const length = next() % 300
+    for (let offset = 0; offset < length; offset++) {
+      text += alphabet[next() % alphabet.length]
+    }
+    texts.push(text)
+  }
+  for (const text of texts) {
+    const line = text
+      .split('\n')
+      .map((part) => part.trim())
+      .findLast((part) => part.length > 0)
+    const normalized = line ? normalizePromptField(line) : ''
+    const expected = normalized ? { kind: 'description', text: normalized } : null
+    expect(selectStructuredAgentTurnActivity([status(text)], 'turn')).toEqual(expected)
+    expect(selectStructuredAgentTurnActivity([], 'turn', { turnId: 'turn', text })).toEqual(
+      expected
+    )
+  }
+})
diff --git a/src/shared/native-chat-turn-activity.ts b/src/shared/native-chat-turn-activity.ts
index fbc1756529b..cac19796334 100644
--- a/src/shared/native-chat-turn-activity.ts
+++ b/src/shared/native-chat-turn-activity.ts
@@ -7,12 +7,19 @@ import { describeActiveToolCall, formatActiveToolLabel } from './native-chat-too
 export type NativeChatTurnActivity = { kind: 'description'; text: string }
 
 function activityLine(text: string): string | null {
-  const lines = text
-    .split('\n')
-    .map((line) => line.trim())
-    .filter(Boolean)
-  const latest = lines.at(-1)
-  return latest ? normalizePromptField(latest) || null : null
+  let end = text.length
+  while (end > 0) {
+    const start = text.lastIndexOf('\n', end - 1) + 1
+    const latest = text.slice(start, end).trim()
+    if (latest) {
+      return normalizePromptField(latest) || null
+    }
+    if (start === 0) {
+      break
+    }
+    end = start - 1
+  }
+  return null
 }
 
 function recentToolActivityLabels(items: readonly AgentJournalRenderItem[]): Set<string> {