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When you ask an LLM to fix a bug, you expect a focused diff. What you often get instead is a sweeping “improvement” that touches comments, reformats whitespace, adds type hints, upgrades validation logic, and restructures boolean returns — none of which you asked for. Surgical Changes enforces a simple discipline: every changed line must trace directly to the user’s request, nothing more.

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Two rules

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Don’t touch what you shouldn’t

When editing existing code:
  • Don’t “improve” adjacent code, comments, or formatting
  • Don’t refactor things that aren’t broken
  • Match existing style, even if you’d do it differently
  • If you notice unrelated dead code, mention it — don’t delete it

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Clean up your own orphans

When your changes create unused artifacts:
  • Remove imports, variables, and functions that your changes made unused
  • Don’t remove pre-existing dead code unless explicitly asked

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The test

Every changed line should trace directly to the user’s request. If you can’t answer “why did this line change?” with a direct reference to the task, the change shouldn’t be there.

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Drive-by refactoring example

Request: “Fix the bug where empty emails crash the validator”
What LLMs do wrong — “fixing” adjacent username logic, adding a docstring, improving email validation beyond the bug
  def validate_user(user_data):
-     # Check email format
-     if not user_data.get('email'):
+     """Validate user data."""
+     email = user_data.get('email', '').strip()
+
+     # Validate email
+     if not email:
          raise ValueError("Email required")
-
-     # Basic email validation
-     if '@' not in user_data['email']:
+     if '@' not in email or '.' not in email.split('@')[1]:
          raise ValueError("Invalid email")

-     # Check username
-     if not user_data.get('username'):
+     # Validate username
+     username = user_data.get('username', '').strip()
+     if not username:
          raise ValueError("Username required")
+     if len(username) < 3:
+         raise ValueError("Username too short")
+     if not username.isalnum():
+         raise ValueError("Username must be alphanumeric")

      return True
Unasked-for changes: improved email validation, added username length and alphanumeric checks, changed comments, added a docstring.
The correct approach — only the lines that fix the empty email crash
  def validate_user(user_data):
      # Check email format
-     if not user_data.get('email'):
+     email = user_data.get('email', '')
+     if not email or not email.strip():
          raise ValueError("Email required")

      # Basic email validation
-     if '@' not in user_data['email']:
+     if '@' not in email:
          raise ValueError("Invalid email")

      # Check username
      if not user_data.get('username'):
          raise ValueError("Username required")

      return True
Only changed: the specific lines that fix empty email handling.

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Style drift example

Request: “Add logging to the upload function”
What LLMs do wrong — changing quote style, adding type hints, adding a docstring, reformatting whitespace
- def upload_file(file_path, destination):
+ def upload_file(file_path: str, destination: str) -> bool:
+     """Upload file to destination with logging."""
+     logger.info(f"Uploading {file_path} to {destination}")
+
      try:
-         with open(file_path, 'rb') as f:
-             data = f.read()
-
-         response = requests.post(destination, files={'file': data})
-
-         if response.status_code == 200:
-             return True
-         else:
-             return False
+         with open(file_path, "rb") as f:
+             data = f.read()
+         response = requests.post(destination, files={"file": data})
+         success = response.status_code == 200
+
+         if success:
+             logger.info(f"Upload successful: {file_path}")
+         else:
+             logger.error(f"Upload failed: {file_path}, status={response.status_code}")
+
+         return success
      except Exception as e:
-         print(f"Error: {e}")
+         logger.exception(f"Upload error for {file_path}: {e}")
          return False
Unasked-for changes: type hints, docstring, quote style changed from ' to ", whitespace reformatted, boolean return logic restructured.
The correct approach — match existing style, add only the logging lines
+ import logging
+
+ logger = logging.getLogger(__name__)
+
  def upload_file(file_path, destination):
+     logger.info(f'Starting upload: {file_path}')
      try:
          with open(file_path, 'rb') as f:
              data = f.read()

          response = requests.post(destination, files={'file': data})

          if response.status_code == 200:
+             logger.info(f'Upload successful: {file_path}')
              return True
          else:
+             logger.error(f'Upload failed: {file_path}, status={response.status_code}')
              return False
      except Exception as e:
-         print(f"Error: {e}")
+         logger.exception(f'Upload error: {file_path}')
          return False
Matched: single quotes, no type hints, existing boolean pattern, original spacing style.

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When this principle is working

  • Diffs are small and focused — only requested changes appear
  • PRs are clean with no drive-by refactoring or “improvements”
  • Code style is consistent within each file, even across contributors
  • Every line in the diff has an obvious reason for changing