Langfuse
Open source observability and evaluation platform for model-based applications, built on OpenTelemetry: traces of every call and every agent step, costs, scores set by a human or a judge model, prompt version management. The core is MIT licensed; only the `ee/` folders fall under an enterprise license. Two paths: Langfuse Cloud, or self-hosting with Docker Compose on one machine, Kubernetes (Helm) in production. ClickHouse acquired Langfuse in January 2026; the announcement says no licensing change is planned and the cloud keeps running as is.
Best for
- Tracing a bad production answer back to the agent step that produced it
- Centralizing traces from several agents once the second or third goes live
- Keeping traces in-house through self-hosting with no core capability removed