Langfuse

Overview

Langfuse is the control room for anything you build on top of a language model. Your app sends a trace of every call it makes, and Langfuse shows you the full tree: the prompts that went out, the answers that came back, how long each step took, how many tokens it burned, and what it cost. When something goes wrong at 2am you can open the exact conversation that broke instead of guessing from logs.

Features

• Full traces of LLM calls, chains, agents, and tool use, nested step by step
• Token counts and cost per trace, per user, per model, on a live dashboard
• A prompt library with versioning, so you can change a prompt without a redeploy
• Datasets and experiments to compare two prompts or two models on the same inputs
• Evaluations, including LLM-as-a-judge scoring, plus manual annotation queues
• Multimodal traces: images, audio, and files attached to a trace and viewable in the UI
• Works with the OpenTelemetry standard and with the Python and JS SDKs, plus drop-in integrations for LangChain, LlamaIndex, OpenAI, and the Vercel AI SDK
• moose provisions and manages the database and cache Langfuse needs, so there is nothing to wire up

Setting up

The first account you register after install becomes the administrator, and it creates the first organization and project. Inside a project, open Settings then API keys to mint the public and secret key pair your application will send traces with.

Good to know

• Apps running somewhere else can send traces, and the app stays private. moose keeps this app owner-only, so the pages are behind the box login. The tracing and API paths are the exception: they answer without it, and the project API keys you mint are what guards them. Nothing to switch on, and no need to make the whole app public.
• Sign-up is open by default. The sign-up page sits behind the box login, so this only matters if you make the whole app public yourself. If you do, turn off new sign-ups from the Langfuse settings once your own account exists.
• This is a heavy app. It runs a web tier, a background worker, a ClickHouse analytics database, and an S3-compatible object store, alongside moose's managed database and cache. That is several gigabytes to download and a few gigabytes of memory to run comfortably, and trace history grows with use.
• Email is off until you connect a mail provider. Without one, user invitations, password resets, and notification emails are not sent. Everything else works, and you can still add members by having them register and then granting them access.
• Optional paid features are not included. Langfuse's enterprise features are locked behind a commercial licence key that this app does not ship. The full open-source product, which is everything above, runs without one.