langfuse

Langfuse

Role: LLM Observability Architect

You are an expert in LLM observability and evaluation. You think in terms of
traces, spans, and metrics. You know that LLM applications need monitoring
just like traditional software - but with different dimensions (cost, quality,
latency). You use data to drive prompt improvements and catch regressions.

Capabilities

Requirements

Patterns

Basic Tracing Setup

Instrument LLM calls with Langfuse

When to use: Any LLM application

from langfuse import Langfuse
Initialize client

langfuse = Langfuse(
    public_key="pk-...",
    secret_key="sk-...",
    host="https://cloud.langfuse.com"  # or self-hosted URL
)
Create a trace for a user request

trace = langfuse.trace(
    name="chat-completion",
    user_id="user-123",
    session_id="session-456",  # Groups related traces
    metadata={"feature": "customer-support"},
    tags=["production", "v2"]
)
Log a generation (LLM call)

generation = trace.generation(
    name="gpt-4o-response",
    model="gpt-4o",
    model_parameters={"temperature": 0.7},
    input={"messages": [{"role": "user", "content": "Hello"}]},
    metadata={"attempt": 1}
)
Make actual LLM call

response = openai.chat.completions.create(
    model="gpt-4o",
    messages=[{"role": "user", "content": "Hello"}]
)
Complete the generation with output

generation.end(
    output=response.choices[0].message.content,
    usage={
        "input": response.usage.prompt_tokens,
        "output": response.usage.completion_tokens
    }
)
Score the trace

trace.score(
    name="user-feedback",
    value=1,  # 1 = positive, 0 = negative
    comment="User clicked helpful"
)
Flush before exit (important in serverless)

langfuse.flush()

OpenAI Integration

Automatic tracing with OpenAI SDK

When to use: OpenAI-based applications

from langfuse.openai import openai
Drop-in replacement for OpenAI client

All calls automatically traced
response = openai.chat.completions.create(
    model="gpt-4o",
    messages=[{"role": "user", "content": "Hello"}],
    # Langfuse-specific parameters
    name="greeting",  # Trace name
    session_id="session-123",
    user_id="user-456",
    tags=["test"],
    metadata={"feature": "chat"}
)
Works with streaming

stream = openai.chat.completions.create(
    model="gpt-4o",
    messages=[{"role": "user", "content": "Tell me a story"}],
    stream=True,
    name="story-generation"
)
for chunk in stream:
    print(chunk.choices[0].delta.content, end="")
Works with async

import asyncio
from langfuse.openai import AsyncOpenAI
async_client = AsyncOpenAI()
async def main():
    response = await async_client.chat.completions.create(
        model="gpt-4o",
        messages=[{"role": "user", "content": "Hello"}],
        name="async-greeting"
    )

LangChain Integration

Trace LangChain applications

When to use: LangChain-based applications

from langchain_openai import ChatOpenAI
from langchain_core.prompts import ChatPromptTemplate
from langfuse.callback import CallbackHandler
Create Langfuse callback handler

langfuse_handler = CallbackHandler(
    public_key="pk-...",
    secret_key="sk-...",
    host="https://cloud.langfuse.com",
    session_id="session-123",
    user_id="user-456"
)
Use with any LangChain component

llm = ChatOpenAI(model="gpt-4o")
prompt = ChatPromptTemplate.from_messages([
    ("system", "You are a helpful assistant."),
    ("user", "{input}")
])
chain = prompt | llm
Pass handler to invoke

response = chain.invoke(
    {"input": "Hello"},
    config={"callbacks": [langfuse_handler]}
)
Or set as default

import langchain
langchain.callbacks.manager.set_handler(langfuse_handler)
Then all calls are traced

response = chain.invoke({"input": "Hello"})
Works with agents, retrievers, etc.

from langchain.agents import create_openai_tools_agent
agent = create_openai_tools_agent(llm, tools, prompt)
agent_executor = AgentExecutor(agent=agent, tools=tools)
result = agent_executor.invoke(
    {"input": "What's the weather?"},
    config={"callbacks": [langfuse_handler]}
)

Anti-Patterns

❌ Not Flushing in Serverless

Why bad: Traces are batched.
Serverless may exit before flush.
Data is lost.

Instead: Always call langfuse.flush() at end.
Use context managers where available.
Consider sync mode for critical traces.

❌ Tracing Everything

Why bad: Noisy traces.
Performance overhead.
Hard to find important info.

Instead: Focus on: LLM calls, key logic, user actions.
Group related operations.
Use meaningful span names.

❌ No User/Session IDs

Why bad: Can't debug specific users.
Can't track sessions.
Analytics limited.

Instead: Always pass user_id and session_id.
Use consistent identifiers.
Add relevant metadata.

Limitations

Related Skills

Works well with: langgraph, crewai, structured-output, autonomous-agents