feat: we should capture which properties were added as tags (#304)

Author: daibhin

CHANGELOG.md

@@ -1,3 +1,7 @@
+# 6.5.0 - 2025-08-08
+
+- feat: Add `$context_tags` to an event to know which properties were included as tags
+
 # 6.4.1 - 2025-08-06
 
 - fix: Always pass project API key in `remote_config` requests for deterministic project routing

posthog/client.py

@@ -87,6 +87,7 @@ def add_context_tags(properties):
     current_context = _get_current_context()
     if current_context:
         context_tags = current_context.collect_tags()
+        properties["$context_tags"] = set(context_tags.keys())
         # We want explicitly passed properties to override context tags
         context_tags.update(properties)
         properties = context_tags

posthog/test/test_client.py

@@ -13,6 +13,7 @@
 from posthog.test.test_utils import FAKE_TEST_API_KEY
 from posthog.types import FeatureFlag, LegacyFlagMetadata
 from posthog.version import VERSION
+from posthog.contexts import tag
 
 
 class TestClient(unittest.TestCase):
@@ -2354,3 +2355,15 @@ def test_get_all_flags_and_payloads_with_empty_string(self, patch_batch_post):
         self.assertEqual(
             result["featureFlagPayloads"]["normal-payload-flag"], "normal payload"
         )
+
+    def test_context_tags_added(self):
+        with mock.patch("posthog.client.batch_post") as mock_post:
+            client = Client(FAKE_TEST_API_KEY, on_error=self.set_fail, sync_mode=True)
+
+            with new_context():
+                tag("random_tag", 12345)
+                client.capture("python test event", distinct_id="distinct_id")
+
+            batch_data = mock_post.call_args[1]["batch"]
+            msg = batch_data[0]
+            self.assertEqual(msg["properties"]["$context_tags"], ["random_tag"])

posthog/version.py

@@ -1,4 +1,4 @@
-VERSION = "6.4.1"
+VERSION = "6.5.0"
 
 if __name__ == "__main__":
     print(VERSION, end="")  # noqa: T201