fix(decorators): merge trace tags and metadata on updates

langfuse/decorators/langfuse_decorator.py

@@ -792,6 +792,18 @@ def update_current_trace(
             if v is not None
         }
 
+        # metadata and tags are merged server side. Send separate update event to avoid merging them SDK side
+        server_merged_attributes = ["metadata", "tags"]
+        if any(attribute in params_to_update for attribute in server_merged_attributes):
+            self.client_instance.trace(
+                id=trace_id,
+                **{
+                    k: v
+                    for k, v in params_to_update.items()
+                    if k in server_merged_attributes
+                },
+            )
+
         _observation_params_context.get()[trace_id].update(params_to_update)
 
     def update_current_observation(

tests/test_decorators.py

@@ -1541,3 +1541,31 @@ def main():
     assert parsed_reference_string["content_type"] == "application/pdf"
     assert parsed_reference_string["media_id"] is not None
     assert parsed_reference_string["source"] == "bytes"
+
+
+def test_merge_metadata_and_tags():
+    mock_trace_id = create_uuid()
+
+    @observe
+    def nested():
+        langfuse_context.update_current_trace(
+            metadata={"key2": "value2"}, tags=["tag2"]
+        )
+
+    @observe
+    def main():
+        langfuse_context.update_current_trace(
+            metadata={"key1": "value1"}, tags=["tag1"]
+        )
+
+        nested()
+
+    main(langfuse_observation_id=mock_trace_id)
+
+    langfuse_context.flush()
+
+    trace_data = get_api().trace.get(mock_trace_id)
+
+    assert trace_data.metadata == {"key1": "value1", "key2": "value2"}
+
+    assert trace_data.tags == ["tag1", "tag2"]