Arize Phoenix

Arize Phoenix is an open-source, OpenTelemetry-based observability and evaluation platform for LLM, RAG, and agent applications that self-hosts in one command. It provides tracing, LLM-as-a-judge evaluations, and drift visualization for free, and sits beneath Arize's managed AX platform. In August 2026, Arize agreed to be acquired by Dynatrace for roughly $915 million.

Phoenix is Arize's open-source tool for tracing, evaluating, and debugging LLM and agent applications. It captures spans and traces using OpenTelemetry standards, so you can see each step of an agent or RAG pipeline, run LLM-as-a-judge and other evaluations, and visualize issues like hallucinations or embedding drift. Because it self-hosts in a single command and runs locally, it is popular for development-time debugging and for teams that want observability without shipping data to a vendor. Phoenix is the free, source-available layer beneath Arize's commercial managed platform, Arize AX, which adds hosted storage, longer retention, collaboration, monitoring, and enterprise features with tiered pricing (a free tier plus paid plans). This gives a clear split: use Phoenix for free observability you host yourself, or adopt Arize AX for a managed, production-scale experience. A major 2026 development is corporate: in August 2026, Dynatrace announced an agreement to acquire Arize for roughly $915 million, positioning AI observability inside a larger enterprise-observability company. For Phoenix users this signals strong backing but also potential roadmap and productization changes over time, so it is worth watching how the open-source project is governed post-acquisition.