fix: preserve explicit parent for child events

langfuse/_client/span.py

@@ -1125,9 +1125,10 @@ def start_observation(
         """
         if as_type == "event":
             timestamp = time_ns()
-            event_span = self._langfuse_client._otel_tracer.start_span(
-                name=name, start_time=timestamp
-            )
+            with otel_trace_api.use_span(self._otel_span):
+                event_span = self._langfuse_client._otel_tracer.start_span(
+                    name=name, start_time=timestamp
+                )
             return cast(
                 LangfuseEvent,
                 LangfuseEvent(

tests/unit/test_otel.py

@@ -1,5 +1,6 @@
 import json
 import logging
+from contextlib import ExitStack
 from datetime import datetime
 from hashlib import sha256
 from typing import List, Sequence
@@ -72,6 +73,76 @@ def generate_span_id(self) -> int:
         return self._span_ids.pop(0)
 
 
+@pytest.mark.parametrize("active_context", ["none", "parent", "unrelated"])
+@pytest.mark.parametrize("creation_method", ["event", "create_event", "span"])
+def test_child_observation_uses_explicit_parent(
+    langfuse_memory_client, memory_exporter, active_context, creation_method
+):
+    client = langfuse_memory_client
+    with ExitStack() as stack:
+        active = (
+            stack.enter_context(client.start_as_current_observation(name="active"))
+            if active_context != "none"
+            else None
+        )
+        parent = (
+            active
+            if active_context == "parent"
+            else client.start_observation(name="manual-parent")
+        )
+        current_id = client.get_current_observation_id()
+        attributes = {
+            "name": "child",
+            "input": {"step": "started"},
+            "output": {"step": "finished"},
+            "metadata": {"source": "manual"},
+            "version": "v1",
+            "level": "WARNING",
+            "status_message": "test-status",
+        }
+        if creation_method == "create_event":
+            child = parent.create_event(**attributes)
+        else:
+            child = parent.start_observation(as_type=creation_method, **attributes)
+            if creation_method == "span":
+                child.end()
+
+        assert client.get_current_observation_id() == current_id
+        if parent is not active:
+            parent.end()
+
+    client.flush()
+    spans = memory_exporter.get_finished_spans()
+    child_spans = [span for span in spans if span.name == "child"]
+    assert len(child_spans) == 1
+    exported = child_spans[0]
+    assert exported.parent is not None
+    assert format(exported.parent.span_id, "016x") == parent.id
+    assert format(exported.context.trace_id, "032x") == parent.trace_id
+    assert format(exported.context.span_id, "016x") == child.id
+    expected_type = "span" if creation_method == "span" else "event"
+    assert (
+        exported.attributes[LangfuseOtelSpanAttributes.OBSERVATION_TYPE]
+        == expected_type
+    )
+    assert json.loads(
+        exported.attributes[LangfuseOtelSpanAttributes.OBSERVATION_INPUT]
+    ) == {"step": "started"}
+    assert json.loads(
+        exported.attributes[LangfuseOtelSpanAttributes.OBSERVATION_OUTPUT]
+    ) == {"step": "finished"}
+    assert exported.attributes["langfuse.observation.metadata.source"] == '"manual"'
+    assert exported.attributes[LangfuseOtelSpanAttributes.VERSION] == "v1"
+    assert (
+        exported.attributes[LangfuseOtelSpanAttributes.OBSERVATION_LEVEL] == "WARNING"
+    )
+    assert exported.attributes[
+        LangfuseOtelSpanAttributes.OBSERVATION_STATUS_MESSAGE
+    ] == ("test-status")
+    if creation_method != "span":
+        assert exported.start_time == exported.end_time
+
+
 class TestOTelBase:
     """Base class for OTEL tests with common fixtures and helper methods."""