fix(capture): keep $release_id on minimized flag-called events

.sampo/changesets/release-id-in-minimal-flag-called.md

@@ -0,0 +1,5 @@
+---
+pypi/posthog: patch
+---
+
+Keep `$release_id` on minimized `$feature_flag_called` events. The property is a static, low-cardinality runtime identity dimension, so the capture contract requires the minimal-event allowlist to retain it when the SDK stamped it from `POSTHOG_RELEASE_ID`.

posthog/client.py

@@ -337,6 +337,10 @@ def wrapper(self, *args, **kwargs):
         "$os_distro",
         "$python_runtime",
         "$python_version",
+        # Deploy-time runtime identity, stamped from POSTHOG_RELEASE_ID. Static and
+        # low-cardinality like the platform keys above, and the capture contract
+        # requires a minimized event to retain it when the SDK stamped it.
+        "$release_id",
     }
 )
 

posthog/test/test_feature_flag_called_minimization.py

@@ -8,12 +8,14 @@
 to the full legacy shape.
 """
 
+import os
 import unittest
 from unittest import mock
 
 from parameterized import parameterized
 
 from posthog.client import _MINIMAL_FLAG_CALLED_EVENT_PROPERTIES, Client
+from posthog.release_id import RELEASE_ID_ENV_VAR
 from posthog.request import GetResponse
 from posthog.test.test_utils import FAKE_TEST_API_KEY
 from posthog.utils import system_context
@@ -68,20 +70,28 @@ class _CapturedEventsMixin:
     ``before_send``, so tests assert the exact wire shape after every enrichment
     step (system context, super properties, $lib, ...)."""
 
-    def _make_client(self, **kwargs):
+    def _make_client(self, release_id=None, **kwargs):
         captured = []
 
         def before_send(msg):
             captured.append(msg)
             return msg
 
-        client = Client(
-            FAKE_TEST_API_KEY,
-            send=False,
-            before_send=before_send,
-            super_properties={"app_version": "1.2.3"},
-            **kwargs,
+        # The release id is read once at construction, so pin the environment here
+        # rather than inheriting whatever the test runner happens to export.
+        env = (
+            {RELEASE_ID_ENV_VAR: release_id}
+            if release_id is not None
+            else {RELEASE_ID_ENV_VAR: ""}
         )
+        with mock.patch.dict(os.environ, env, clear=False):
+            client = Client(
+                FAKE_TEST_API_KEY,
+                send=False,
+                before_send=before_send,
+                super_properties={"app_version": "1.2.3"},
+                **kwargs,
+            )
         return client, captured
 
     def _flag_called_properties(self, captured, index=0):
@@ -134,6 +144,19 @@ def test_gated_non_experiment_flag_sends_exactly_the_allowlist(self, patch_flags
         self.assertEqual(properties["$feature_flag_reason"], "Matched condition set 1")
         self.assertEqual(properties["$feature_flag_request_id"], "req-1")
         self.assertEqual(properties["$feature_flag_evaluated_at"], 1640995200000)
+        self.assertNotIn("$release_id", properties)
+
+    @mock.patch("posthog.client.flags")
+    def test_gated_non_experiment_flag_keeps_the_release_id(self, patch_flags):
+        patch_flags.return_value = _flags_response(has_experiment=False, gate=True)
+        client, captured = self._make_client(release_id="rel-abc123")
+
+        client.get_feature_flag_result("person-flag", "some-distinct-id")
+
+        properties = self._flag_called_properties(captured)
+        self.assertEqual(properties["$release_id"], "rel-abc123")
+        self.assertLessEqual(set(properties), _MINIMAL_FLAG_CALLED_EVENT_PROPERTIES)
+        self.assertNotIn("app_version", properties)
 
     @mock.patch("posthog.client.flags")
     def test_gated_non_experiment_flag_with_groups_keeps_group_context(

posthog/test/test_release_id.py

@@ -135,8 +135,30 @@ def test_release_id_is_read_once_at_client_init(self) -> None:
             client.capture("python test event", distinct_id="user-1")
         self.assertEqual(events[0]["properties"]["$release_id"], "at-init")
 
-    def test_minimal_flag_called_events_keep_their_strict_allowlist(self) -> None:
-        client, events = self._client("0198c1a2-release")
+    def test_minimal_flag_called_events_keep_the_release_id(self) -> None:
+        client, events = self._client(
+            "0198c1a2-release", super_properties={"app_version": "1.2.3"}
+        )
+        client._enqueue(
+            {
+                "event": "$feature_flag_called",
+                "distinct_id": "user-1",
+                "timestamp": None,
+                "properties": {"$feature_flag": "my-flag", "plan": "pro"},
+            },
+            None,
+            property_allowlist=_MINIMAL_FLAG_CALLED_EVENT_PROPERTIES,
+        )
+        properties = events[0]["properties"]
+        self.assertEqual(properties["$feature_flag"], "my-flag")
+        self.assertEqual(properties["$release_id"], "0198c1a2-release")
+        # Still a strict allowlist: nothing outside it survives minimization.
+        self.assertLessEqual(set(properties), _MINIMAL_FLAG_CALLED_EVENT_PROPERTIES)
+        self.assertNotIn("plan", properties)
+        self.assertNotIn("app_version", properties)
+
+    def test_minimal_flag_called_events_omit_an_unset_release_id(self) -> None:
+        client, events = self._client(None)
         client._enqueue(
             {
                 "event": "$feature_flag_called",
@@ -147,7 +169,6 @@ def test_minimal_flag_called_events_keep_their_strict_allowlist(self) -> None:
             None,
             property_allowlist=_MINIMAL_FLAG_CALLED_EVENT_PROPERTIES,
         )
-        self.assertEqual(events[0]["properties"]["$feature_flag"], "my-flag")
         self.assertNotIn("$release_id", events[0]["properties"])