How to set trace_id when using langchain.CallbackHandler in Langfuse v3.99.0?

[fer-prieto]

Describe your question

How can I set the trace_id parameter using the langfuse.create_trace_id function?

from langfuse import get_client
langfuse = get_client()

trace_id_msg = langfuse.create_trace_id(seed=f"{idConversacion}-{current_time.isoformat()}")

response_text = await ChatbotViewSet._full_chain.ainvoke({
    "pregunta_usuario": pregunta_usuario},
config={"callbacks": [langfuse_handler],
	"metadata": {
	    "langfuse_trace_name" : "FullChain ChatBot",
	    "langfuse_user_id": idUser,
	    "langfuse_session_id": idConversation,
	    "langfuse_trace_id" : trace_id_msg
	    }
	}
)

I've tried using "langfuse_trace_id" but it is not working :(

Langfuse Cloud or Self-Hosted?

Self-Hosted

If Self-Hosted

3.99.0

If Langfuse Cloud

No response

SDK and integration versions

No response

Pre-Submission Checklist

• I have checked for existing issues/discussions and consulted Langfuse AI.

[inkeep-ai-assistant[bot]]

There is no langfuse_trace_id metadata field — metadata only supports langfuse_user_id, langfuse_session_id, and langfuse_tags for setting trace attributes dynamically (1). To use a custom trace ID, wrap the LangChain execution in a span that sets the predefined trace ID via trace_context (1):

from langfuse import get_client, Langfuse
from langfuse.langchain import CallbackHandler

langfuse = get_client()

# Generate deterministic trace ID from external system
external_request_id = "req_12345"
predefined_trace_id = Langfuse.create_trace_id(seed=external_request_id)

langfuse_handler = CallbackHandler()

# Use the predefined trace ID with trace_context
with langfuse.start_as_current_observation(
    as_type="span",
    name="langchain-request",
    trace_context={"trace_id": predefined_trace_id}
) as span:
    span.update(input={"person": "Ada Lovelace"})

    with propagate_attributes(
        user_id="user_123",
    ):
        # LangChain execution will be part of this trace
        response = chain.invoke(
            {"person": "Ada Lovelace"},
            config={"callbacks": [langfuse_handler]}
        )

    span.update(output={"response": response})

print(f"Trace ID: {predefined_trace_id}")  # Use this for scoring later
print(f"Trace ID: {langfuse_handler.last_trace_id}")  # Care needed in concurrent environments where handler is reused

(1)

Note that Langfuse.create_trace_id(seed=...) is called on the Langfuse class here (1).

You can also pass trace_context directly to the handler (2):

import uuid
from langfuse.langchain import CallbackHandler

trace_id = str(uuid.uuid4()).replace("-", "")
handler = CallbackHandler(trace_context={"trace_id": trace_id})

(2)

Keep in mind this approach comes from a community discussion rather than the reference docs, and reusing a single shared handler instance across concurrent requests means you cannot set a per-request trace ID that way (2).

For your user_id and session_id, the metadata approach you're already using is correct (1).

📚 Sources:

1. LangChain Tracing & LangGraph Integration
2. how to customize trace id when using callback handler in langchain integration

---

Have another question? Just tag @inkeep.