Langfuse

Langfuse is an observability platform for applications built on large language models. It records traces of model calls - the prompt, the response, the latency, the token count, the cost - so a team can see what an application actually did rather than inferring it from logs. The tracing model follows the shape of an LLM application. A trace is a single request; spans inside it represent retrieval, tool calls, and model invocations. Because the structure is explicit, a slow or expensive step is visible rather than buried in an aggregate. Beyond tracing, Langfuse provides evaluation and prompt management. A dataset of inputs can be run against a prompt and scored, either by a model or by a person, so a change to a prompt has a measured effect. The platform is published as open source and can be self-hosted, which matters for teams whose prompts contain sensitive data.