Pydantic Logfire vs Arize Phoenix
Both are observability & tracing tools. Here is how they actually differ on price, billing model and deployment.
Pydantic Logfire
OpenTelemetry-native observability from the Pydantic team, and the cheapest credible option in the category at $2 per million records with 10M free every month. The catch is that metrics count as records, and common integrations emit them silently.
Arize Phoenix
The default open-source choice for LLM tracing and eval - runs locally in under a minute, built on OpenTelemetry. The catch is the license - the server is Elastic License 2.0, source-available, not OSI open source.
| Pydantic Logfire | Arize Phoenix | |
|---|---|---|
| Category | Observability & Tracing | Observability & Tracing |
| Our rating | 5/5 | 4/5 |
| Starting price | $49/mo | $0 |
| Billing meter | record | No usage metering |
| Free plan | Yes | Yes |
| Free self-hosting | No or paid tier only | Yes, free |
| Best for | Teams that want one observability tool for their whole application rather than an LLM-specific silo, are comfortable on OpenTelemetry, and care about predictable, genuinely low cost. Especially strong for Python shops already using Pydantic or Pydantic AI. | Teams that want fast, framework-agnostic OSS tracing and strong RAG eval, and who are not reselling Phoenix as a hosted service |
Our verdict on Pydantic Logfire
Logfire is the best value in this category and it is not close. Ten million free records a month, then $2 per million, with no metering on seats, hosts or projects. The free tier hard-caps rather than billing you, which means a runaway instrumentation loop pauses ingestion instead of generating a five-figure invoice - a design decision almost nobody else in observability makes. It is full-stack OpenTelemetry rather than an LLM silo, so the database query that made your agent slow appears in the same trace as the LLM call, and you can query all of it with PostgreSQL-compatible SQL. The one real trap is the billing unit. Logfire bills records, and records include metrics, and integrations like httpx, SQLAlchemy and FastAPI emit aggregate metrics under the hood whether you asked for them or not. Teams get surprised by this. Turn off auto-emitted metrics you do not need and the economics are excellent.
Full Pydantic Logfire review →Our verdict on Arize Phoenix
The default open-source pick for good reasons - it starts in under a minute, the RAG eval is the best around, and it is built on OpenTelemetry so you are not locked to one framework. Just read the license before you build a business on it. The server is Elastic License 2.0, source-available rather than OSI open source, which only bites if you plan to offer Phoenix as a service - but it is not what "fully open source" implies.
Full Arize Phoenix review →Frequently Asked Questions
What is the main difference between Pydantic Logfire and Arize Phoenix?
Pydantic Logfire: Teams that want one observability tool for their whole application rather than an LLM-specific silo, are comfortable on OpenTelemetry, and care about predictable, genuinely low cost. Especially strong for Python shops already using Pydantic or Pydantic AI. Arize Phoenix: Teams that want fast, framework-agnostic OSS tracing and strong RAG eval, and who are not reselling Phoenix as a hosted service Both sit in Observability & Tracing, so the decision usually comes down to billing model and deployment rather than raw capability.
Which is cheaper, Pydantic Logfire or Arize Phoenix?
It depends entirely on your workload shape, because they meter differently - Pydantic Logfire bills on record and Arize Phoenix bills on no usage metering. Published starting prices are $49/mo and $0 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 Pydantic Logfire or Arize Phoenix?
Pydantic Logfire: No or paid tier only. Arize Phoenix: Yes, free. Free self-hosting means no licence fee, not no cost - you still own the infrastructure, upgrades and on-call.