diff --git a/.github/scripts/gitutils.py b/.github/scripts/gitutils.py
index aa64fe15387e..f97c2f6c4403 100644
--- a/.github/scripts/gitutils.py
+++ b/.github/scripts/gitutils.py
@@ -273,6 +273,11 @@ class GitRepo:
     def amend_commit_message(self, msg: str) -> None:
         self._run_git("commit", "--amend", "-m", msg)
 
+    def diff(self, from_ref: str, to_ref: Optional[str] = None) -> str:
+        if to_ref is None:
+            return self._run_git("diff", f"{from_ref}^!")
+        return self._run_git("diff", f"{from_ref}..{to_ref}")
+
 
 def clone_repo(username: str, password: str, org: str, project: str) -> GitRepo:
     path = tempfile.mkdtemp()
@@ -331,3 +336,18 @@ def patterns_to_regex(allowed_patterns: List[str]) -> Any:
                 rc += c
     rc += ")"
     return re.compile(rc)
+
+def _shasum(value: str) -> str:
+    import hashlib
+    m = hashlib.sha256()
+    m.update(value.encode("utf-8"))
+    return m.hexdigest()
+
+
+def are_ghstack_branches_in_sync(repo: GitRepo, head_ref: str) -> bool:
+    """ Checks that diff between base and head is the same as diff between orig and its parent """
+    orig_ref = re.sub(r'/head$', '/orig', head_ref)
+    base_ref = re.sub(r'/head$', '/base', head_ref)
+    orig_diff_sha = _shasum(repo.diff(f"{repo.remote}/{orig_ref}"))
+    head_diff_sha = _shasum(repo.diff(f"{repo.remote}/{base_ref}", f"{repo.remote}/{head_ref}"))
+    return orig_diff_sha == head_diff_sha
diff --git a/.github/scripts/test_gitutils.py b/.github/scripts/test_gitutils.py
index 78696771d993..9987cdea9781 100644
--- a/.github/scripts/test_gitutils.py
+++ b/.github/scripts/test_gitutils.py
@@ -1,6 +1,11 @@
 #!/usr/bin/env python3
-from gitutils import PeekableIterator, patterns_to_regex
-from unittest import TestCase, main
+from gitutils import PeekableIterator, patterns_to_regex, GitRepo, are_ghstack_branches_in_sync, _shasum
+from unittest import TestCase, main, SkipTest
+from pathlib import Path
+
+
+BASE_DIR = Path(__file__).parent
+
 
 class TestPeekableIterator(TestCase):
     def test_iterator(self, input_: str = "abcdef") -> None:
@@ -35,5 +40,34 @@ class TestPattern(TestCase):
             self.assertTrue(patterns_re.match(filename))
 
 
+class TestGitRepo(TestCase):
+    def setUp(self) -> None:
+        repo_dir = BASE_DIR.parent.parent.absolute()
+        if not (repo_dir / ".git").is_dir():
+            raise SkipTest("Can't find git directory, make sure to run this test on real repo checkout")
+        self.repo = GitRepo(str(repo_dir))
+
+    def _skip_if_ref_does_not_exist(self, ref: str) -> None:
+        """ Skip test if ref is missing as stale branches are deleted with time """
+        try:
+            self.repo.show_ref(ref)
+        except RuntimeError as e:
+            raise SkipTest(f"Can't find head ref {ref} due to {str(e)}") from e
+
+    def test_compute_diff(self) -> None:
+        diff = self.repo.diff("HEAD")
+        sha = _shasum(diff)
+        self.assertEqual(len(sha), 64)
+
+    def test_ghstack_branches_in_sync(self) -> None:
+        head_ref = "gh/SS-JIA/206/head"
+        self._skip_if_ref_does_not_exist(head_ref)
+        self.assertTrue(are_ghstack_branches_in_sync(self.repo, head_ref))
+
+    def test_ghstack_branches_not_in_sync(self) -> None:
+        head_ref = "gh/clee2000/1/head"
+        self._skip_if_ref_does_not_exist(head_ref)
+        self.assertFalse(are_ghstack_branches_in_sync(self.repo, head_ref))
+
 if __name__ == '__main__':
     main()
diff --git a/.github/scripts/trymerge.py b/.github/scripts/trymerge.py
index 182d39e0f5de..f8a59d905c76 100755
--- a/.github/scripts/trymerge.py
+++ b/.github/scripts/trymerge.py
@@ -29,6 +29,7 @@ from pathlib import Path
 
 from gitutils import (
     GitRepo,
+    are_ghstack_branches_in_sync,
     get_git_remote_name,
     get_git_repo_dir,
     patterns_to_regex,
@@ -619,6 +620,7 @@ def can_skip_internal_checks(pr: "GitHubPR", comment_id: Optional[int] = None) -
         return False
     return comment.author_login == "facebook-github-bot"
 
+
 def get_ghstack_prs(repo: GitRepo, pr: "GitHubPR") -> List[Tuple["GitHubPR", str]]:
     '''
     Get the open PRs in the stack that are below this PR.  Throws error if any of the PRs are out of sync.
@@ -646,9 +648,7 @@ def get_ghstack_prs(repo: GitRepo, pr: "GitHubPR") -> List[Tuple["GitHubPR", str
             entire_stack.append((pr, rev))
 
     for stacked_pr, rev in entire_stack:
-        commit_sha = stacked_pr.last_commit()['oid']
-        tree_sha = repo._run_git("rev-parse", commit_sha + "^{tree}")
-        if tree_sha not in repo.commit_message(rev):
+        if not are_ghstack_branches_in_sync(repo, stacked_pr.head_ref()):
             raise RuntimeError(
                 f"PR {stacked_pr.pr_num} is out of sync with the corresponding revision {rev} on " +
                 f"branch {orig_ref} that would be merged into master.  " +