test(compliance): expose the legacy payload accessor to the harness

sdk_compliance_adapter/README.md

@@ -25,6 +25,7 @@ The adapter implements the standard SDK adapter interface defined in the [test h
 - `POST /flush` - Flush pending events
 - `GET /state` - Return internal state
 - `POST /get_feature_flag` - Evaluate a flag locally or remotely
+- `POST /get_feature_flag_payload` - Read a flag payload remotely with the legacy payload accessor
 - `POST /reload_feature_flag_definitions` - Fresh, bounded definitions readiness barrier
 - `POST /reset` - Reset SDK state
 

sdk_compliance_adapter/adapter.py

@@ -313,6 +313,8 @@ def health():
     capabilities.append("feature_flags_local_evaluation_v1")
     # Remote /flags requests send ?v=4 and read the v4 flag record.
     capabilities.append("flags_response_v4")
+    # The legacy payload accessor is exposed on /get_feature_flag_payload.
+    capabilities.append("feature_flag_payloads")
     return jsonify(
         {
             "sdk_name": "posthog-python",
@@ -710,6 +712,32 @@ def get_feature_flag():
         return jsonify({"error": str(e)}), 500
 
 
+@app.route("/get_feature_flag_payload", methods=["POST"])
+def get_feature_flag_payload():
+    """Read a flag's payload with the legacy payload accessor (remote evaluation)."""
+    try:
+        if not state.client:
+            return jsonify({"error": "SDK not initialized"}), 400
+        data = request.json or {}
+        if not data.get("key") or not data.get("distinct_id"):
+            return jsonify({"error": "key and distinct_id are required"}), 400
+        client = state.remote_client or state.client
+        payload = client.get_feature_flag_payload(
+            data["key"],
+            data["distinct_id"],
+            person_properties=data.get("person_properties"),
+            groups=data.get("groups"),
+            group_properties=data.get("group_properties"),
+            disable_geoip=data.get("disable_geoip"),
+            only_evaluate_locally=False,
+        )
+        client.flush()
+        return jsonify({"success": True, "value": payload})
+    except Exception as e:
+        logger.exception("Error reading feature flag payload")
+        return jsonify({"error": str(e)}), 500
+
+
 @app.route("/reset", methods=["POST"])
 def reset():
     """Reset SDK state"""

sdk_compliance_adapter/test_adapter.py

@@ -87,6 +87,7 @@ def test_health_opts_into_local_evaluation_without_losing_capture(
     capabilities = adapter.app.test_client().get("/health").json["capabilities"]
     assert "feature_flags_local_evaluation_v1" in capabilities
     assert "flags_response_v4" in capabilities
+    assert "feature_flag_payloads" in capabilities
     assert capability in capabilities
     assert "capture_ai_v0" in capabilities
 
@@ -181,6 +182,30 @@ def test_force_remote_bypasses_loaded_local_definitions(adapter, monkeypatch):
     remote.assert_called_once()
 
 
+def test_payload_route_reads_remotely_and_returns_the_parsed_payload(
+    adapter, monkeypatch
+):
+    client = initialize(adapter)
+    remote = Mock(
+        return_value=posthog.client.normalize_flags_response(
+            {
+                "featureFlags": {"flag": True},
+                "featureFlagPayloads": {"flag": '{"columns": 2}'},
+            }
+        )
+    )
+    monkeypatch.setattr(adapter.state.remote_client, "_get_flags_decision", remote)
+    response = client.post(
+        "/get_feature_flag_payload", json={"key": "flag", "distinct_id": "user"}
+    )
+    assert response.json == {"success": True, "value": {"columns": 2}}
+    remote.assert_called_once()
+    assert (
+        client.post("/get_feature_flag_payload", json={"key": "flag"}).status_code
+        == 400
+    )
+
+
 def test_rejects_conflicting_evaluation_modes(adapter):
     client = initialize(adapter)
     response = client.post(