LLM Application Tracing

LLM applications use increasingly complex abstractions, such as chains, agents with tools, and advanced prompts. The nested traces in Langfuse help to understand what is happening and identify the root cause of problems.

Example trace of our public demo

Why Use Tracing for an LLM Application?

  • Capture the full context of the execution, including API calls, context, prompts, parallelism, and more
  • Track model usage and cost
  • Collect user feedback
  • Identify low-quality outputs
  • Build fine-tuning and testing datasets

Why Langfuse?

  • Open-source
  • Low performance overhead
  • SDKs for Python and JavaScript
  • Integrated with popular frameworks: OpenAI SDK (Python), Langchain (Python, JS), LlamaIndex (Python)
  • Public API for custom integrations
  • Suite of tools for the whole LLM application development lifecycle

Introduction to Traces in Langfuse

A trace in Langfuse consists of the following objects:

  • A trace typically represents a single request or operation. It contains the overall input and output of the function, as well as metadata about the request, such as the user, the session, and tags.
  • Each trace can contain multiple observations to log the individual steps of the execution.
    • Observations are of different types:
      • Events are the basic building blocks. They are used to track discrete events in a trace.
      • Spans represent durations of units of work in a trace.
      • Generations are spans used to log generations of AI models. They contain additional attributes about the model, the prompt, and the completion. For generations, token usage and costs are automatically calculated.
    • Observations can be nested.

Hierarchical structure of traces in Langfuse

Example trace in Langfuse UI

Trace in Langfuse UI

Get Started

Follow the quickstart to add Langfuse tracing to your LLM app.

Advanced usage

You can extend the tracing capabilities of Langfuse by using the following features:

Enable/disable tracing

All Langfuse SDKs and integrations are designed to be non-intrusive. You can add Langfuse tracing to your application while being able to enable it only in specific environments.

By default, the Langfuse Tracing is enabled if an API key is set. You can manually disable tracing via the enabled flag. See the documentation for the specific SDK or integration for more details.

Event queuing/batching

Langfuse's client SDKs and integrations are all designed to queue and batch requests in the background to optimize API calls and network time. Batches are determined by a combination of time and size (number of events and size of batch).


All integrations have a sensible default configuration, but you can customise the batching behaviour to suit your needs.

Option (Python) [SDK constructor, Environment]Option (JS)Description
flush_at, LANGFUSE_FLUSH_ATflushAtThe maximum number of events to batch up before sending.
flush_interval, LANGFUSE_FLUSH_INTERVAL (s)flushInterval (ms)The maximum time to wait before sending a batch.

You can e.g. set flushAt=1 to send every event immediately, or flushInterval=1000 to send every second.

Manual flushing

This is especially relevant for short-lived applications like serverless functions. If you do not flush the client, you may lose events.

If you want to send a batch immediately, you can call the flush method on the client. In case of network issues, flush will log an error and retry the batch, it will never throw an exception.

# Decorator
from langfuse.decorators import langfuse_context
# low-level SDK

If you exit the application, use shutdown method to make sure all requests are flushed and pending requests are awaited before the process exits. On success of this function, no more events will be sent to Langfuse API.



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