Opik vs Arize Phoenix in 2026 - The License Decides It

Opik and Arize Phoenix are the two names that come up when a team says “we want open-source observability we can self-host.” They are both fast, both capable, both free to run yourself. So the choice is not really about features - it is about two specific things: the license you can live with, and whether RAG evaluation is your priority. Get those two right and the decision makes itself.

I have run both. Here is the honest breakdown.

The short version

	Opik	Arize Phoenix
License	Apache-2.0 (OSI permissive)	Server ELv2, client/eval Apache-2.0
Self-host	Full feature set, unlimited, no gates	Free, no usage caps, but ELv2 server
Managed cloud	$19/mo for 100k spans	Arize AX, contact-sales, pricing unconfirmed
OpenTelemetry	One ingestion path among 60+	Native architecture
Eval strength	LLM-judge, code metrics, Agent Optimizer	50+ metrics, best RAG eval in category
Backed by	Comet (founded 2017, ~$63M raised)	Arize ($70M Series C, Feb 2025)

Opik: the permissive license and the cheapest cloud

Opik is Comet’s open-source observability and eval platform, and its whole story is the terms. The OSS build is Apache-2.0 - the most permissive license of the major platforms - and Comet states the self-hosted version ships the full feature set with unlimited spans, members and retention, and no feature gates. That is the cleanest “open source that actually means open source” answer in the category. You keep every feature when you self-host, and there is no restriction on what you do with it.

If you want managed instead, Opik’s Pro cloud is $19/mo for 100k spans - the cheapest paid tier of the major eval platforms, with $5 per additional 100k. The market has noticed - it is the fastest-growing project of its peers at roughly 20.8k GitHub stars, zero to 12.5k in about eight months.

The honest gotcha is per-seat pricing at scale. The recurring complaint is that seat costs climb as the team grows - the $19 headline is the small-team configuration, not the large one. A few users also report occasional UI slowdown on very large projects. Neither undercuts the core value, but if you are a big team, model the seat cost before you commit to the cloud.

Arize Phoenix: OTel-native, and the best RAG eval

Phoenix is the OSS pick that starts fastest - a working trace UI on your laptop in under a minute. It is genuinely OpenTelemetry-native, built on OTel and Arize’s own OpenInference conventions, so it is framework-agnostic in a way many tools are not. And its 50+ pre-built eval metrics include what reviewers call the best RAG evaluation in the category - serious retrieval and answer scoring without writing your own judge prompts.

If your workload is RAG-heavy and you care about strict OTel-native architecture, this is the stronger tool. Arize is well-funded too - a $70M Series C in February 2025 - so it is not going anywhere.

The gotcha is the license, and it is a real one. Arize markets Phoenix as “fully open source, no feature gates,” but the main server repo is Elastic License 2.0 - source-available, not OSI-approved open source. Only the client and eval subpackages (phoenix-evals, phoenix-client, phoenix-otel) are Apache-2.0. ELv2 forbids offering Phoenix as a hosted service to third parties. For internal use it behaves like open source and the features are not gated - but that is not what “fully open source” implies, and it rules Phoenix out for exactly one group: anyone who wanted to resell it as a service. There are also reports of ingest lag before traces appear.

Where Langfuse sits between them

Worth naming the third default. Langfuse is MIT - genuinely OSI open source, cleaner on license than Phoenix, and it self-hosts free with only three features gated. It is not as RAG-deep as Phoenix, and its self-host is a heavier four-service stack (Postgres, ClickHouse, Redis, S3). If OSI open source is a hard requirement but you want a more mature managed-pricing story than Phoenix and broader adoption than Opik, Langfuse is the one to weigh. We break the pairings down in Opik vs Langfuse and Langfuse vs Arize Phoenix.

Opik vs Phoenix: which should you pick?

• You want a permissive license with zero asterisk and no reselling restriction - Opik. Apache-2.0 is the cleanest license in this pairing, full stop.
• You want the cheapest predictable managed cloud - Opik at $19/mo. Phoenix’s managed AX pricing is not even publicly confirmed.
• RAG evaluation is your center of gravity - Arize Phoenix. The RAG eval is the best in the category, and nothing here matches it.
• You are standardizing strictly on OpenTelemetry - Arize Phoenix, which is OTel-native rather than treating it as one path.
• You plan to offer the tool as a hosted service to customers - Opik, because Phoenix’s ELv2 forbids exactly that.
• You want the cleanest OSI license overall - look at Langfuse too, and read the open-source observability roundup.

Still narrowing it down? Opik alternatives and Arize Phoenix alternatives cover the wider field.

Every price and date here was read from each vendor’s own materials on 26 July 2026, except Arize AX pricing, which we could not confirm because the vendor page returned an HTTP 403. This category ships breaking changes monthly, so we re-verify every 30 days. The license difference is the one that decides most of these - read it before you build.