perf: stop building chat diff sections beyond preview limit

src/renderer/src/components/native-chat/native-chat-diff.test.ts

@@ -63,6 +63,27 @@ describe('diffFromToolCall', () => {
     ])
   })
 
+  it.each([31_999, 32_000, 32_001])(
+    'preserves multi-file truncation at %i characters before the next section',
+    (length) => {
+      const header = '--- a\n+++ a\n'
+      const diff = '@@ -1 +1 @@\n-old\n+new\n'
+      const first = diff + 'x'.repeat(length - header.length - diff.length)
+      const changes = [
+        { path: 'a', diff: first },
+        null,
+        { path: 'ignored' },
+        { path: 'b', kind: { move_path: 'c' }, diff: '@@ -1 +1 @@\n-b\n+c' }
+      ]
+      const text = `${header}${first}\n--- b\n+++ c\n@@ -1 +1 @@\n-b\n+c`
+      for (const maxLines of [2, 120, 40_000]) {
+        expect(diffFromToolCall('apply_patch', { changes }, maxLines)).toEqual(
+          diffFromText(text, maxLines)
+        )
+      }
+    }
+  )
+
   it('returns null when there is no old/new payload', () => {
     expect(diffFromToolCall('Edit', { file_path: '/x' })).toBeNull()
   })

src/shared/native-chat-diff.ts

@@ -100,13 +100,15 @@ function patchTextFromToolInput(value: Record<string, unknown>): string | null {
   if (!Array.isArray(value.changes)) {
     return null
   }
-  const sections = value.changes.flatMap((entry) => {
+  const sections: string[] = []
+  let length = 0
+  for (const entry of value.changes) {
     if (typeof entry !== 'object' || entry === null) {
-      return []
+      continue
     }
     const change = entry as Record<string, unknown>
     if (typeof change.diff !== 'string') {
-      return []
+      continue
     }
     const path = typeof change.path === 'string' ? change.path : 'file'
     const kind =
@@ -114,8 +116,14 @@ function patchTextFromToolInput(value: Record<string, unknown>): string | null {
         ? (change.kind as Record<string, unknown>)
         : null
     const nextPath = kind && typeof kind.move_path === 'string' ? kind.move_path : path
-    return [`--- ${path}\n+++ ${nextPath}\n${change.diff}`]
-  })
+    const section = `--- ${path}\n+++ ${nextPath}\n${change.diff}`
+    length += section.length + (sections.length > 0 ? 1 : 0)
+    sections.push(section)
+    // Keep the extra character that tells toLines the diff was truncated.
+    if (length > MAX_DIFF_CHARS) {
+      break
+    }
+  }
   return sections.length > 0 ? sections.join('\n') : null
 }