Langfuse

Langfuse is an open-source LLM engineering platform for tracing, evaluating, and improving AI applications. It is one of the most practical starting points for a technical founder who needs to know what an agent did, what it cost, which prompt version ran, and whether a change helped or hurt.

The product is useful because it covers the boring production surfaces in one place: traces, prompts, datasets, scores, and dashboards. If an agent calls the wrong tool, retries too much, or gets more expensive after a prompt change, Langfuse gives you a trail to inspect instead of a vague chat transcript.

For a solo builder, the fit is strong because there is a free cloud plan, a lower-cost Core plan, and a self-hosted route. The trade is setup. This is not a no-code business tool; you wire it into your app and keep it clean. Use it once your agent is doing repeated work where quality, cost, or traceability matter.