diff --git a/src/clusterfuzz/_internal/datastore/data_types.py b/src/clusterfuzz/_internal/datastore/data_types.py
index 696b4bc9f8..e648a58bea 100644
--- a/src/clusterfuzz/_internal/datastore/data_types.py
+++ b/src/clusterfuzz/_internal/datastore/data_types.py
@@ -366,7 +366,7 @@ class Fuzzer(Model):
   stats_column_descriptions = ndb.TextProperty(indexed=False)
 
   # Whether this is a builtin fuzzer.
-  builtin = ndb.BooleanProperty(indexed=False, default=False)
+  builtin = ndb.BooleanProperty(default=False)
 
   # Whether this is a differential fuzzer.
   differential = ndb.BooleanProperty(default=False)
diff --git a/src/local/butler/scripts/migrate_fuzzer_builtin.py b/src/local/butler/scripts/migrate_fuzzer_builtin.py
new file mode 100644
index 0000000000..5907ca7423
--- /dev/null
+++ b/src/local/butler/scripts/migrate_fuzzer_builtin.py
@@ -0,0 +1,39 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Migration script to index Fuzzer.builtin field."""
+
+from clusterfuzz._internal.datastore import data_types
+
+
+def execute(args):
+  """Migrate Fuzzer entities to index builtin field.
+
+  See
+  https://docs.cloud.google.com/datastore/docs/concepts/indexes#unindexed_properties
+  for details on how rewriting the entity is required to index properties
+  that were previously unindexed.
+  """
+  print('Starting Fuzzer.builtin migration.')
+  count = 0
+
+  for fuzzer in data_types.Fuzzer.query():
+    if not args.non_dry_run:
+      print(f'DRY RUN: Would update fuzzer: {fuzzer.name}')
+    else:
+      # Rewrite the fuzzer to the datastore to build the index.
+      fuzzer.put()
+      print(f'Updated fuzzer: {fuzzer.name}')
+    count += 1
+
+  print(f'Migration complete. Updated {count} Fuzzer entities.')