Stay free if you only need full observability library — tracing, evaluation, experimentation and self-hosted with user-managed infrastructure. Upgrade if you need custom span limits and ingestion volume and configurable data retention. Most solo builders can start free.
Why it matters: AX Pro cloud pricing based on span volume ($10/million additional) can become costly for high-throughput production applications
Available from: AX Pro
Why it matters: Self-hosted open-source deployment requires managing PostgreSQL, storage, and compute infrastructure
Available from: AX Pro
Why it matters: Steeper learning curve than simpler logging solutions — requires understanding of tracing concepts, spans, and evaluation methodologies
Available from: AX Pro
Why it matters: AX Free tier limited to 25K spans/month and 7-day retention — may be too constrained for even moderate production workloads
Available from: AX Pro
Why it matters: Store more data and handle larger projects. Prevents hitting usage walls.
Available from: AX Pro
Why it matters: Store more data and handle larger projects. Prevents hitting usage walls.
Available from: AX Pro
Phoenix provides LLM-specific metrics — hallucination detection, prompt drift, semantic similarity, retrieval quality — that general monitoring tools don't support. It understands AI-specific concepts like tokens, embeddings, and evaluation scores while Datadog focuses on infrastructure metrics. Phoenix's experiment playground for prompt iteration has no equivalent in traditional monitoring.
Yes. While Phoenix provides automatic instrumentation for 20+ popular frameworks, it also supports custom instrumentation via Python SDK, JavaScript SDK, and OpenTelemetry-compatible spans for monitoring any LLM application or custom agent implementation.
Phoenix is the open-source library with full tracing, evaluation, and experimentation features — self-hosted and free. Arize AX is the managed cloud platform that adds hosted infrastructure, online evaluations, the Alyx AI assistant, product monitoring, compliance certifications (SOC 2, HIPAA), and enterprise features like SSO and RBAC.
Both. Phoenix supports real-time trace collection with low-latency ingestion, plus offline batch evaluation for deep analysis. AX adds online evaluations that score production traces continuously and trigger alerts on quality degradation or safety violations.
AX Free includes 25K spans/month and 1 GB ingestion. AX Pro is $50/month with 50K spans and 100 GB, with overages at $10 per million spans and $3 per GB. Enterprise pricing is custom based on volume, retention, and compliance requirements.
Start with the free plan — upgrade when you need more.
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Last verified March 2026