What is Arize Phoenix?

AI Observability and Evaluation

Phoenix helps you understand and improve AI applications by giving you a workflow for debugging and iteration. You can send detailed logging information, known as traces, from your app to see exactly what happened during a run, score outputs using evaluation tests to identify failures and regressions, iterate on your prompts using real production examples, and optimize your app with experiments that compare changes on the same inputs. Together, these tools help you move from inspecting individual runs to improving quality with evidence.

Phoenix is built by Arize AI and the open-source community. It is built on top of OpenTelemetry and is powered by OpenInference instrumentation. See Integrations for details.

Features

Tracing

Tracing in Phoenix

Tracing lets you see what happened during a single run of your AI application, step by step. A trace captures model calls, retrieval, tool use, and custom logic so you can debug behavior and understand where time is spent.

Phoenix accepts traces over OpenTelemetry (OTLP) and provides auto-instrumentation for popular frameworks (LlamaIndex, LangChain, DSPy, Mastra, Vercel AI SDK), providers (OpenAI, Bedrock, Anthropic), and languages (Python, TypeScript, Java).

Evaluation

Evals in the Phoenix UI

Evaluations help you measure the output quality of your application. You can score traces & spans with LLM-based evaluators, code-based checks, or human labels so you can track performance and identify failures consistently.

• LLM-based evaluations — Run pre-built or custom evaluators on your data
• Evaluator integrations — Use Phoenix evals, or bring your own from Ragas, Deepeval, or Cleanlab
• Human annotations — Attach ground truth labels directly in the UI

Prompt Engineering

Phoenix Prompt Playground

Phoenix helps you iterate on prompts using real examples from your application. You can version prompts, test prompt variants across datasets, and replay calls to see how changes affect outputs before rolling them out.

• Prompt Management — Version, store, and deploy prompts
• Prompt Playground — Experiment with prompts and models side-by-side
• Span Replay — Debug by replaying LLM calls with different inputs
• Prompts in Code — Sync prompts across environments via SDK

Datasets & Experiments

Experiments in Phoenix

Datasets & Experiments help you test changes systematically using the same inputs. You can group traces into datasets, rerun them through different versions of your application, and compare evaluation results to confirm whether a change actually improved performance.

• Run Experiments — Compare different versions of your application
• Create Datasets — Collect traces or upload from code/CSV
• Test at Scale — Run datasets through Playground or export for fine-tuning

Quick Starts

Running Phoenix for the first time? Select a quick start below.

Send Traces From Your App

See what’s happening inside your LLM application with distributed tracing

Measure Performance with Evaluations

Measure quality with LLM-as-a-judge and custom evaluators

Iterate on Your Prompts

Experiment with prompts, compare models, and version your work

Optimize Your App with Experiments

Test your application systematically and track performance over time

Next Steps

The best next step is to start using Phoenix. Start with a quickstart to send data into Phoenix, then build from there. See the Quickstart Overview for more information about what you’ll build.

Other Resources

Integrations

Add instrumentation for OpenAI, LangChain, LlamaIndex, and more

Self-Host

Deploy Phoenix on Docker, Kubernetes, or your cloud of choice

Cookbooks

Example notebooks for tracing, evals, RAG analysis, and more

Community

Join the Phoenix Slack to ask questions and connect with developers