fix(langchain): propagate trace name metadata

langfuse/langchain/CallbackHandler.py

@@ -287,6 +287,11 @@ def _parse_langfuse_trace_attributes(
         ):
             attributes["user_id"] = metadata["langfuse_user_id"]
 
+        if "langfuse_trace_name" in metadata and isinstance(
+            metadata["langfuse_trace_name"], str
+        ):
+            attributes["trace_name"] = metadata["langfuse_trace_name"]
+
         if tags is not None or (
             "langfuse_tags" in metadata and isinstance(metadata["langfuse_tags"], list)
         ):
@@ -369,6 +374,7 @@ def on_chain_start(
                     session_id=parsed_trace_attributes.get("session_id", None),
                     tags=parsed_trace_attributes.get("tags", None),
                     metadata=parsed_trace_attributes.get("metadata", None),
+                    trace_name=parsed_trace_attributes.get("trace_name", None),
                 )
 
                 self._propagation_context_manager.__enter__()
@@ -1403,6 +1409,7 @@ def _strip_langfuse_keys_from_dict(
         "langfuse_session_id",
         "langfuse_user_id",
         "langfuse_tags",
+        "langfuse_trace_name",
     ]
 
     metadata_copy = metadata.copy()

tests/unit/test_langchain.py

@@ -1,4 +1,6 @@
+from contextvars import copy_context
 from unittest.mock import patch
+from uuid import uuid4
 
 import pytest
 from langchain.messages import HumanMessage
@@ -166,3 +168,84 @@ def test_chat_model_error_marks_generation_error(langfuse_memory_client, get_spa
     assert (
         "boom" in span.attributes[LangfuseOtelSpanAttributes.OBSERVATION_STATUS_MESSAGE]
     )
+
+
+def test_root_chain_metadata_propagates_trace_name(
+    langfuse_memory_client, get_span, find_spans
+):
+    response = ChatResult(
+        generations=[
+            ChatGeneration(
+                message=AIMessage(content="knock knock"),
+                text="knock knock",
+            )
+        ],
+        llm_output={
+            "token_usage": {
+                "prompt_tokens": 4,
+                "completion_tokens": 2,
+                "total_tokens": 6,
+            },
+            "model_name": "gpt-4o-mini",
+        },
+    )
+
+    with patch.object(ChatOpenAI, "_generate", return_value=response):
+        handler = CallbackHandler()
+        prompt = ChatPromptTemplate.from_template("tell me a joke about {topic}")
+        chain = prompt | ChatOpenAI(api_key="test", temperature=0) | StrOutputParser()
+
+        result = chain.invoke(
+            {"topic": "otters"},
+            config={
+                "callbacks": [handler],
+                "metadata": {"langfuse_trace_name": "langchain-trace-name"},
+            },
+        )
+
+    assert result == "knock knock"
+
+    langfuse_memory_client.flush()
+    root_span = get_span("RunnableSequence")
+    generation_span = get_span("ChatOpenAI")
+
+    assert (
+        root_span.attributes[LangfuseOtelSpanAttributes.TRACE_NAME]
+        == "langchain-trace-name"
+    )
+    assert (
+        generation_span.attributes[LangfuseOtelSpanAttributes.TRACE_NAME]
+        == "langchain-trace-name"
+    )
+    assert (
+        f"{LangfuseOtelSpanAttributes.OBSERVATION_METADATA}.langfuse_trace_name"
+        not in root_span.attributes
+    )
+    assert len(find_spans("ChatOpenAI")) == 1
+
+
+def test_root_chain_exports_when_end_runs_in_copied_context(
+    langfuse_memory_client, get_span
+):
+    handler = CallbackHandler()
+    run_id = uuid4()
+
+    handler.on_chain_start(
+        {"id": ["RunnableSequence"]},
+        {"topic": "otters"},
+        run_id=run_id,
+        metadata={"langfuse_trace_name": "async-root-trace"},
+    )
+
+    copy_context().run(
+        handler.on_chain_end,
+        {"output": "knock knock"},
+        run_id=run_id,
+    )
+
+    langfuse_memory_client.flush()
+    root_span = get_span("RunnableSequence")
+
+    assert root_span.attributes[LangfuseOtelSpanAttributes.TRACE_NAME] == (
+        "async-root-trace"
+    )