Best Free LLM Observability Tools

Langfuse is an open-source LLM observability and engineering platform providing tracing, prompt management, evaluations, and dataset management for production AI applications.

Braintrust is an evals-first LLM observability platform combining production tracing, prompt playgrounds, autoevals, and Topics-based pattern discovery for teams shipping AI in production.

AIMon (officially AIMon Labs) is a Bessemer Venture Partners-backed LLM evaluation and monitoring product focused on the hard problems that show up the moment an AI app reaches real users: hallucinations, instruction-following drift, completeness gaps, conciseness regressions, and toxicity or PII leakage. The team's bet is that generic LLM-as-judge approaches are too slow and too expensive for production guardrails — so AIMon ships fine-tuned small-model detectors (the HDM-2 family of hallucinat

Open-source LLM observability and AI gateway — logs every prompt, response, cost, and latency across 20+ providers with a one-line proxy or async SDK, plus caching, retries, and prompt experiments.