Arize AX vs MLflow
Both are observability & tracing tools. Here is how they actually differ on price, billing model and deployment.
Arize AX
The commercial cloud product from the makers of Phoenix. The upgrade from free Phoenix is not a tier change - it is a repricing onto per-span billing, and online monitoring is the feature that forces it.
MLflow
The open-source ML platform that grew a serious GenAI half. MLflow 3 adds OpenTelemetry-compatible tracing, LLM judges and review apps - free and self-hostable, with Databricks selling the managed version. The best zero-cost option if you can run infrastructure.
| Arize AX | MLflow | |
|---|---|---|
| Category | Observability & Tracing | Observability & Tracing |
| Our rating | 4/5 | 4/5 |
| Starting price | $50/mo | $0 (open source) |
| Billing meter | span | No usage metering |
| Free plan | Yes | Yes |
| Free self-hosting | No or paid tier only | Yes, free |
| Best for | Teams that outgrew self-hosted Phoenix and want the managed version without operating it, mixed ML and GenAI shops that need computer vision and model observability in the same platform, and regulated teams needing HIPAA. | Teams that already run MLflow for classical ML, Databricks customers, and anyone who wants a genuinely free and complete self-hosted platform and has the operational capacity to run it. |
Our verdict on Arize AX
Arize AX is the managed, production-grade half of a genuinely good two-product strategy, and the honest way to evaluate it is to understand what forces the move from free Phoenix. It is not scale, and it is not support. It is online monitoring. The open-source Phoenix build does not include online monitoring or online evaluations as part of its observability stack, so the moment you need to evaluate live production traffic rather than debug locally, you are buying AX. That is a defensible commercial line, and Arize is more upfront about it than most, but it means Phoenix is best understood as an excellent development and debugging tool rather than a complete production platform. AX itself is strong - genuinely mature ML and computer vision observability inherited from Arize's older business, online evals, and pricing that scales on spans rather than seats, which is unusually favourable for large teams. The catch is that per-span billing punishes agent workloads, and the jump from $50 a month to a reported median Enterprise spend near $60,000 a year is a cliff with nothing published in between.
Full Arize AX review →Our verdict on MLflow
MLflow is the strongest zero-cost option in the category, with the caveat that free software is not free to operate. MLflow 3 turned what was an experiment-tracking tool into a real GenAI platform - OpenTelemetry-compatible tracing from a single line of code, built-in and custom LLM judges, review apps that collect expert feedback and align automated judges against it, and evaluation datasets built directly from production traces. It is Apache 2.0 and the open-source build is complete rather than a gated teaser, which is more than can be said for several commercial competitors advertising self-hosting. Two honest caveats. The UI is functional rather than pleasant, and it shows its lineage as a tool built for ML engineers rather than application developers. And the genuinely best-governed experience - Unity Catalog trace storage in OTel Delta tables, no storage cap, SQL queryable - is available on Databricks, which is where the commercial gravity sits. If you already run MLflow or Databricks, this is close to automatic.
Full MLflow review →Frequently Asked Questions
What is the main difference between Arize AX and MLflow?
Arize AX: Teams that outgrew self-hosted Phoenix and want the managed version without operating it, mixed ML and GenAI shops that need computer vision and model observability in the same platform, and regulated teams needing HIPAA. MLflow: Teams that already run MLflow for classical ML, Databricks customers, and anyone who wants a genuinely free and complete self-hosted platform and has the operational capacity to run it. Both sit in Observability & Tracing, so the decision usually comes down to billing model and deployment rather than raw capability.
Which is cheaper, Arize AX or MLflow?
It depends entirely on your workload shape, because they meter differently - Arize AX bills on span and MLflow bills on no usage metering. Published starting prices are $50/mo and $0 (open source) respectively, but those numbers are not comparable until you apply them to the same traffic. Use our cost calculator to model both against your own request volume and span count.
Can I self-host Arize AX or MLflow?
Arize AX: No or paid tier only. MLflow: Yes, free. Free self-hosting means no licence fee, not no cost - you still own the infrastructure, upgrades and on-call.