diff --git a/git_config.py b/git_config.py
index 9b3188f1e..888ae888c 100644
--- a/git_config.py
+++ b/git_config.py
@@ -40,7 +40,7 @@ from repo_trace import Trace
 # that is saved in the config.
 SYNC_STATE_PREFIX = "repo.syncstate."
 
-ID_RE = re.compile(r"^[0-9a-f]{40}$")
+ID_RE = re.compile(r"^[0-9a-f]{40,64}$")
 
 REVIEW_CACHE = {}
 
@@ -49,8 +49,8 @@ def IsChange(rev):
     return rev.startswith(R_CHANGES)
 
 
-def IsId(rev):
-    return ID_RE.match(rev)
+def IsId(rev: str) -> bool:
+    return bool(ID_RE.match(rev))
 
 
 def IsTag(rev):
diff --git a/project.py b/project.py
index 0ad558446..0b336b272 100644
--- a/project.py
+++ b/project.py
@@ -2709,7 +2709,7 @@ class Project:
         if depth:
             current_branch_only = True
 
-        is_sha1 = bool(IsId(self.revisionExpr))
+        is_sha1 = IsId(self.revisionExpr)
 
         if current_branch_only:
             if self.revisionExpr.startswith(R_TAGS):
diff --git a/tests/test_git_config.py b/tests/test_git_config.py
index 3bc140522..9583f3e9f 100644
--- a/tests/test_git_config.py
+++ b/tests/test_git_config.py
@@ -244,3 +244,27 @@ def test_remote_save_with_push_url_without_projectname(
     assert (
         written_config.GetString("remote.origin.pushurl") == "ssh://example.com"
     )
+
+
+@pytest.mark.parametrize(
+    "rev, expected",
+    (
+        ("a" * 40, True),
+        ("0" * 40, True),
+        ("f" * 40, True),
+        ("a" * 64, True),
+        ("0" * 64, True),
+        ("f" * 64, True),
+        ("a" * 39, False),
+        ("a" * 41, True),
+        ("a" * 63, True),
+        ("a" * 65, False),
+        ("g" * 40, False),
+        ("g" * 64, False),
+        ("refs/heads/master", False),
+        ("refs/tags/v1.0", False),
+    ),
+)
+def test_is_id(rev: str, expected: bool) -> None:
+    """Test IsId identifies both SHA-1 and SHA-256 hashes."""
+    assert git_config.IsId(rev) == expected