fix(diff): count dropped lines correctly in truncated diff snippet

src/tools/FileEditTool/utils.test.ts

@@ -1,5 +1,9 @@
 import { describe, expect, test } from 'bun:test'
-import { findWhitespaceAgnosticMatch, adjustNewStringIndentation } from './utils.js'
+import {
+  findWhitespaceAgnosticMatch,
+  adjustNewStringIndentation,
+  getSnippetForTwoFileDiff,
+} from './utils.js'
 
 describe('findWhitespaceAgnosticMatch', () => {
   test('returns exact match for simple string', () => {
@@ -180,3 +184,29 @@ describe('adjustNewStringIndentation', () => {
     expect(adjustNewStringIndentation(oldString, fileMatch, newString)).toBeNull()
   })
 })
+
+describe('getSnippetForTwoFileDiff truncation notice', () => {
+  test('reports the exact number of dropped lines when cutting at a line boundary', () => {
+    // A full-replacement diff of an empty file with a large file yields a single
+    // hunk whose line-numbered snippet far exceeds DIFF_SNIPPET_MAX_BYTES (8192),
+    // forcing truncation at a line boundary.
+    const bigFile =
+      Array.from({ length: 800 }, (_, i) => `line ${i + 1} content here`).join(
+        '\n',
+      ) + '\n'
+    const snippet = getSnippetForTwoFileDiff('', bigFile)
+
+    const match = snippet.match(/\.\.\. \[(\d+) lines truncated\] \.\.\./)
+    expect(match).not.toBeNull()
+    const reportedTruncated = Number(match![1])
+
+    // The snippet body before the marker holds the kept lines. Together with the
+    // reported truncated count they must sum to the total lines the untruncated
+    // snippet would have shown — here, the 800 added lines. Before the fix the
+    // boundary newline was double-counted and this reported 801.
+    const keptBody = snippet.slice(0, snippet.indexOf('\n\n... ['))
+    const keptLines = keptBody.split('\n').length
+
+    expect(keptLines + reportedTruncated).toBe(800)
+  })
+})

src/tools/FileEditTool/utils.ts

@@ -399,9 +399,22 @@ export function getSnippetForTwoFileDiff(
   // Truncate at the last line boundary that fits within the cap.
   // Marker format matches BashTool/utils.ts.
   const cutoff = full.lastIndexOf('\n', DIFF_SNIPPET_MAX_BYTES)
-  const kept =
-    cutoff > 0 ? full.slice(0, cutoff) : full.slice(0, DIFF_SNIPPET_MAX_BYTES)
-  const remaining = countCharInString(full, '\n', kept.length) + 1
+  let kept: string
+  let remaining: number
+  if (cutoff > 0) {
+    kept = full.slice(0, cutoff)
+    // `full[cutoff]` is the newline that terminates the last kept line, so
+    // counting newlines from `cutoff` onward counts that boundary newline plus
+    // every later one — exactly the number of dropped lines. No `+1`: the
+    // boundary newline is not itself a dropped line, it stands in for the
+    // missing trailing newline of the final dropped line.
+    remaining = countCharInString(full, '\n', cutoff)
+  } else {
+    kept = full.slice(0, DIFF_SNIPPET_MAX_BYTES)
+    // Mid-line cut (no newline within the cap): the partial tail line and every
+    // following line are dropped, so add 1 for that partial line.
+    remaining = countCharInString(full, '\n', kept.length) + 1
+  }
   return `${kept}\n\n... [${remaining} lines truncated] ...`
 }