fix(langfuse): propagate trace_metadata as trace-level attributes in v4

v2 wrote trace(metadata=...) onto the trace object. In v4 the trace only
carries what the observations propagate, so a continuation request with
update_trace_keys=["trace_metadata"] updated the generation's metadata
while the trace kept its stale values. Coerce each entry to the SDK's
string limit and hand it to propagate_attributes(metadata=...).

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

litellm/integrations/langfuse/langfuse.py

@@ -210,6 +210,14 @@ def _coerce_propagated_value(value: object) -> str | Sequence[str]:
     return str(value)[:_PROPAGATED_VALUE_MAX_CHARS]
 
 
+def _propagated_trace_metadata(value: object) -> Mapping[str, str] | None:
+    """v2's ``trace(metadata=...)`` took any JSON; v4 propagates one flat string per key."""
+    entries: Final = _object_mapping(value)
+    if not entries:
+        return None
+    return MappingProxyType({str(key): str(item)[:_PROPAGATED_VALUE_MAX_CHARS] for key, item in entries.items()})
+
+
 def _trace_attributes_for_propagation(trace_params: Mapping[str, object]) -> Mapping[str, object]:
     """Trace-level fields in v4 are propagated onto the observations, not set on a trace object.
 
@@ -217,11 +225,15 @@ def _trace_attributes_for_propagation(trace_params: Mapping[str, object]) -> Map
     warning, and a dropped ``version`` would vanish from the generation too,
     because ``_generation_attributes`` already stripped it as propagated.
     """
+    trace_metadata: Final = _propagated_trace_metadata(trace_params.get("metadata"))
     return MappingProxyType(
         {
-            propagated: _coerce_propagated_value(trace_params[key])
-            for key, propagated in _PROPAGATED_TRACE_KEYS.items()
-            if trace_params.get(key) is not None
+            **{
+                propagated: _coerce_propagated_value(trace_params[key])
+                for key, propagated in _PROPAGATED_TRACE_KEYS.items()
+                if trace_params.get(key) is not None
+            },
+            **({"metadata": trace_metadata} if trace_metadata is not None else {}),
         }
     )
 

tests/logging_callback_tests/langfuse_expected_request_body/completion_with_langfuse_metadata.json

@@ -35,6 +35,7 @@
             "cache_read_input_tokens": 0
         },
         "langfuse.release": "test_trace_release",
+        "langfuse.trace.metadata.test_key": "test_value",
         "langfuse.trace.name": "test_trace_name",
         "langfuse.trace.tags": [
             "test_tag",

tests/test_litellm/integrations/test_langfuse.py

@@ -1879,6 +1879,34 @@ def test_update_trace_keys_from_the_request_body_list_applies_when_enabled():
     assert span.attributes["langfuse.release"] == "v1.2.3"
 
 
+def test_update_trace_keys_trace_metadata_reaches_the_trace_not_just_the_generation():
+    """v2 updated the trace object's metadata; v4 has to propagate it as a trace attribute."""
+    rig = _steering_logger()
+
+    with patch.object(litellm, "langfuse_enable_update_trace_keys", True):
+        _, _, span = _emit(
+            rig,
+            metadata={
+                "existing_trace_id": "trace-1",
+                "parent_observation_id": "b" * 16,
+                "update_trace_keys": ["trace_metadata"],
+                "trace_metadata": {"step": 2, "note": "x" * 300},
+            },
+        )
+
+    assert span.attributes["langfuse.trace.metadata.step"] == "2"
+    assert span.attributes["langfuse.trace.metadata.note"] == "x" * 200
+    assert span.attributes["langfuse.observation.metadata.step"] == 2
+
+
+def test_trace_metadata_is_not_propagated_when_absent():
+    rig = _steering_logger()
+
+    _, _, span = _emit(rig, metadata={"trace_name": "plain"})
+
+    assert not any(key.startswith("langfuse.trace.metadata.") for key in span.attributes or {})
+
+
 def test_update_trace_keys_matches_whole_keys_not_substrings():
     rig = _steering_logger()