diff --git a/bin/check_autogenerated_edits.py b/bin/check_autogenerated_edits.py
index 732adcd1c..816e17d21 100755
--- a/bin/check_autogenerated_edits.py
+++ b/bin/check_autogenerated_edits.py
@@ -4,7 +4,9 @@ This script checks for unauthorized modifications in autogenerated sections of m
 It is designed to be used in a GitHub Actions workflow or a local pre-commit hook.
 
 Features:
-- Detects markdown files changed between two commits (default last commit).
+- Detects markdown files changed between a commit and one of its ancestors. Default is to
+  check the last commit only. When triggered on a pull request it should typically compare the
+  pull request branch head and its merge base - the commit on the main branch before it diverged.
 - Identifies modified autogenerated sections marked by specific comments.
 - Reports violations using GitHub Actions error messages.
 - Exits with a nonzero status code if unauthorized changes are found.
@@ -24,14 +26,14 @@ def get_changed_files(base, head):
     """
     Retrieve a list of markdown files that were changed between the base and head commits.
     """
-    files = run_git(["diff", "--name-only", base, head]).splitlines()
+    files = run_git(["diff", "--name-only", f"{base}...{head}"]).splitlines()
     return [f for f in files if f.endswith(".md")]
 
 def get_diff(file, base, head):
     """
     Get the diff of a given file between the base and head commits.
     """
-    return run_git(["diff", "-U0", base, head, "--", file]).splitlines()
+    return run_git(["diff", "-U0", f"{base}...{head}", "--", file]).splitlines()
 
 def get_file_content(ref, file):
     """