fix(langfuse): do not claim trace root when continuing an existing trace

Langfuse derives a trace's name and I/O from any observation flagged
langfuse.internal.as_root, so a request carrying existing_trace_id
renamed the trace to the generation name and replaced the trace input
and output on every continuation. v2 only updated the keys listed in
update_trace_keys. Continuations now export as plain children of the
remote parent and keep the explicit langfuse.trace.* attributes for the
fields they do want changed.

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

litellm/integrations/langfuse/langfuse.py

@@ -976,6 +976,7 @@ class LangFuseLogger:
                     client=self.Langfuse,
                     trace_id=resolved_trace_id,
                     parent_observation_id=resolve_observation_id(parent_observation_id),  # pyright: ignore[reportArgumentType]  # metadata value, str or None at runtime
+                    existing_trace=existing_trace_id is not None,
                 )
                 log_provider_specific_information_as_span(
                     client=self.Langfuse,

litellm/integrations/langfuse/langfuse_sdk.py

@@ -88,6 +88,7 @@ def open_trace_context(
     client: Langfuse,
     trace_id: str,
     parent_observation_id: str | None,
+    existing_trace: bool = False,
 ) -> tuple[Context, bool]:
     """Build the OTel context that places new observations inside ``trace_id``.
 
@@ -96,11 +97,16 @@ def open_trace_context(
     observation is a child of something that will never be exported; the public
     SDK path compensates by marking the span as root and this path must do the
     same.
+
+    ``existing_trace`` is the v2 ``existing_trace_id`` contract: the trace is
+    appended to, never rewritten. The server takes a root observation's name and
+    I/O as the trace's, so a continuation must not claim root; trace fields it
+    does want changed travel as explicit ``langfuse.trace.*`` attributes.
     """
     remote_parent: Final = client._create_remote_parent_span(  # pyright: ignore[reportPrivateUsage]  # no public equivalent in v4
         trace_id=trace_id, parent_span_id=parent_observation_id
     )
-    return otel_trace.set_span_in_context(remote_parent), parent_observation_id is None
+    return otel_trace.set_span_in_context(remote_parent), parent_observation_id is None and not existing_trace
 
 
 def start_generation(

tests/test_litellm/integrations/test_langfuse.py

@@ -1923,6 +1923,28 @@ def test_update_trace_keys_input_output_reach_the_trace_even_under_a_parent():
     assert "the-output" in str(span.attributes["langfuse.trace.output"])
 
 
+def test_existing_trace_id_appends_without_claiming_trace_root():
+    """Langfuse copies a root observation's name and I/O onto the trace, so a
+    continuation that claimed root would rename the trace after every request;
+    v2 only ever touched the keys in ``update_trace_keys``."""
+    rig = _steering_logger()
+
+    _, _, span = _emit(rig, metadata={"existing_trace_id": "trace-1", "trace_name": "second-call"})
+
+    assert span.parent is not None
+    assert span.attributes.get("langfuse.internal.as_root") is None
+    assert "langfuse.trace.name" not in (span.attributes or {})
+
+
+def test_a_fresh_trace_still_claims_root_so_its_generation_names_it():
+    rig = _steering_logger()
+
+    _, _, span = _emit(rig, metadata={"trace_id": "a" * 32, "trace_name": "first-call"})
+
+    assert span.attributes.get("langfuse.internal.as_root") is True
+    assert span.attributes["langfuse.trace.name"] == "first-call"
+
+
 def test_trace_io_is_not_stamped_when_update_trace_keys_does_not_ask():
     rig = _steering_logger()