From ddbec7b9b069f2bf5b3218db790b3e66ac8add62 Mon Sep 17 00:00:00 2001
From: Brennan Benson <79079362+brennanb2025@users.noreply.github.com>
Date: Wed, 12 Aug 2026 19:43:14 -0700
Subject: [PATCH] fix(native-chat): preserve image caption whitespace

---
 ...tive-chat-image-transcript-markers.test.ts | 15 ++++++++++--
 .../native-chat-image-transcript-markers.ts   | 23 +++++++++++++------
 2 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/src/shared/native-chat-image-transcript-markers.test.ts b/src/shared/native-chat-image-transcript-markers.test.ts
index 0308c44951d..cd9e7d3b8d6 100644
--- a/src/shared/native-chat-image-transcript-markers.test.ts
+++ b/src/shared/native-chat-image-transcript-markers.test.ts
@@ -43,14 +43,25 @@ describe('normalizeImageTranscriptMessages', () => {
 
   it.each([
     ['[Image #1] describe this', 'describe this'],
+    ['[Image #1]\t  describe this', 'describe this'],
     ['describe this [Image #1]', 'describe this'],
-    ['describe [Image #1] this', 'describe this'],
+    ['describe this  \t[Image #1]', 'describe this'],
+    ['describe [Image #1] this', 'describe  this'],
+    ['describe  [Image #1]\t this', 'describe  \t this'],
+    ['describe[Image #1]\t  this', 'describe\t  this'],
+    ['describe\n[Image #1]\nthis', 'describe\n\nthis'],
     ['com[Image #1]pare this', 'compare this'],
-    ['[Image #1] [Image #2]', '']
+    ['[Image #1] [Image #2]', ''],
+    ['literal [Image #x] text', 'literal [Image #x] text']
   ])('strips image prompt markers anywhere in text', (text, expected) => {
     expect(stripImagePromptMarker(text)).toBe(expected)
   })
 
+  it('returns long marker-free whitespace without regex backtracking', () => {
+    const text = ' '.repeat(50_000)
+    expect(stripImagePromptMarker(text)).toBe(text)
+  })
+
   it('converts a lone [Image: source] turn (no prompt) into an image-ref instead of raw text', () => {
     const out = normalizeImageTranscriptMessages([
       userText('a', '[Image: source: /Users/me/Pictures/hero-image-2.png]')
diff --git a/src/shared/native-chat-image-transcript-markers.ts b/src/shared/native-chat-image-transcript-markers.ts
index a14f395ea2d..6523688018b 100644
--- a/src/shared/native-chat-image-transcript-markers.ts
+++ b/src/shared/native-chat-image-transcript-markers.ts
@@ -2,7 +2,11 @@ import { isTextBlock, type NativeChatBlock, type NativeChatMessage } from './nat
 
 const IMAGE_SOURCE_MARKER = /^\[Image:\s*source:\s*(.+?)\]\s*$/
 const IMAGE_PROMPT_MARKER = /\[Image #\d+\]/
-const IMAGE_PROMPT_MARKER_RUN = /[^\S\r\n]*(?:\[Image #\d+\][^\S\r\n]*)+/g
+const IMAGE_PROMPT_MARKERS = /\[Image #\d+\]/g
+const IMAGE_PROMPT_MARKER_AT_START = /^\[Image #\d+\]/
+const IMAGE_PROMPT_MARKER_AT_END = /\[Image #\d+\]$/
+const HORIZONTAL_WHITESPACE_START = /^[^\S\r\n]+/
+const HORIZONTAL_WHITESPACE_END = /[^\S\r\n]+$/
 
 function soleText(message: NativeChatMessage): string | null {
   return message.blocks.length === 1 && isTextBlock(message.blocks[0])
@@ -15,12 +19,17 @@ export function imageSourcePathFromText(text: string): string | null {
 }
 
 export function stripImagePromptMarker(text: string): string {
-  return text.replace(IMAGE_PROMPT_MARKER_RUN, (run, offset: number, source: string) => {
-    const hasTextBefore = offset > 0
-    const hasTextAfter = offset + run.length < source.length
-    const hadSurroundingWhitespace = run.replace(/\[Image #\d+\]/g, '').length > 0
-    return hasTextBefore && hasTextAfter && hadSurroundingWhitespace ? ' ' : ''
-  })
+  const stripped = text.replace(IMAGE_PROMPT_MARKERS, '')
+  if (stripped === text) {
+    return text
+  }
+  let result = IMAGE_PROMPT_MARKER_AT_START.test(text)
+    ? stripped.replace(HORIZONTAL_WHITESPACE_START, '')
+    : stripped
+  if (IMAGE_PROMPT_MARKER_AT_END.test(text)) {
+    result = result.replace(HORIZONTAL_WHITESPACE_END, '')
+  }
+  return result
 }
 
 function stripImagePromptMarkersFromFirstText(