--- a/taskcluster/taskgraph/transforms/tests.py
+++ b/taskcluster/taskgraph/transforms/tests.py
@@ -1444,16 +1444,20 @@ def set_test_manifests(config, tasks):
     loader_cls = manifest_loaders[config.params['test_manifest_loader']]
     loader = loader_cls(config.params)
 
     for task in tasks:
         if task['suite'] in CHUNK_SUITES_BLACKLIST:
             yield task
             continue
 
+        # Set 'tests_grouped' to "1", so we can differentiate between suites that are
+        # chunked at the test runtime and those that are chunked in the taskgraph.
+        task.setdefault("tags", {})["tests_grouped"] = "1"
+
         if taskgraph.fast:
             # We want to avoid evaluating manifests when taskgraph.fast is set. But
             # manifests are required for dynamic chunking. Just set the number of
             # chunks to one in this case.
             if task['chunks'] == 'dynamic':
                 task['chunks'] = 1
             yield task
             continue