From 8cbe8513391bb36770827a8b0132ad80d6d246f2 Mon Sep 17 00:00:00 2001
From: Andrew Garrett <andrewgarrett@google.com>
Date: Mon, 9 Feb 2026 17:37:53 +1100
Subject: [PATCH] Fix newline insertion bug in replace tool (#18595)

---
 packages/core/src/tools/edit.test.ts | 37 ++++++++++++++++++++++++++++
 packages/core/src/tools/edit.ts      |  4 +--
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/packages/core/src/tools/edit.test.ts b/packages/core/src/tools/edit.test.ts
index 445e048202..56dc2cb2c4 100644
--- a/packages/core/src/tools/edit.test.ts
+++ b/packages/core/src/tools/edit.test.ts
@@ -372,6 +372,43 @@ describe('EditTool', () => {
       expect(result.newContent).toBe(expectedContent);
       expect(result.occurrences).toBe(1);
     });
+
+    it('should NOT insert extra newlines when replacing a block preceded by a blank line (regression)', async () => {
+      const content = '\n  function oldFunc() {\n    // some code\n  }';
+      const result = await calculateReplacement(mockConfig, {
+        params: {
+          file_path: 'test.js',
+          instruction: 'test',
+          old_string: 'function  oldFunc() {\n    // some code\n  }', // Two spaces after function to trigger regex
+          new_string: 'function newFunc() {\n  // new code\n}', // Unindented
+        },
+        currentContent: content,
+        abortSignal,
+      });
+
+      // The blank line at the start should be preserved as-is,
+      // and the discovered indentation (2 spaces) should be applied to each line.
+      const expectedContent = '\n  function newFunc() {\n    // new code\n  }';
+      expect(result.newContent).toBe(expectedContent);
+    });
+
+    it('should NOT insert extra newlines in flexible replacement when old_string starts with a blank line (regression)', async () => {
+      const content = '  // some comment\n\n  function oldFunc() {}';
+      const result = await calculateReplacement(mockConfig, {
+        params: {
+          file_path: 'test.js',
+          instruction: 'test',
+          old_string: '\nfunction oldFunc() {}',
+          new_string: '\n  function newFunc() {}', // Include desired indentation
+        },
+        currentContent: content,
+        abortSignal,
+      });
+
+      // The blank line at the start is preserved, and the new block is inserted.
+      const expectedContent = '  // some comment\n\n  function newFunc() {}';
+      expect(result.newContent).toBe(expectedContent);
+    });
   });
 
   describe('validateToolParams', () => {
diff --git a/packages/core/src/tools/edit.ts b/packages/core/src/tools/edit.ts
index 40ae914f50..d7c8973a91 100644
--- a/packages/core/src/tools/edit.ts
+++ b/packages/core/src/tools/edit.ts
@@ -167,7 +167,7 @@ async function calculateFlexibleReplacement(
     if (isMatch) {
       flexibleOccurrences++;
       const firstLineInMatch = window[0];
-      const indentationMatch = firstLineInMatch.match(/^(\s*)/);
+      const indentationMatch = firstLineInMatch.match(/^([ \t]*)/);
       const indentation = indentationMatch ? indentationMatch[1] : '';
       const newBlockWithIndent = replaceLines.map(
         (line: string) => `${indentation}${line}`,
@@ -229,7 +229,7 @@ async function calculateRegexReplacement(
 
   // The final pattern captures leading whitespace (indentation) and then matches the token pattern.
   // 'm' flag enables multi-line mode, so '^' matches the start of any line.
-  const finalPattern = `^(\\s*)${pattern}`;
+  const finalPattern = `^([ \t]*)${pattern}`;
   const flexibleRegex = new RegExp(finalPattern, 'm');
 
   const match = flexibleRegex.exec(currentContent);