Arize Phoenix vs Braintrust: LLM Observability 2026
Quick Comparison
| Arize Phoenix | Braintrust | |
|---|---|---|
| Best For | Teams running a mix of traditional ML and LLM-powered applications who want a single, free, open-source tool for RAG evaluation and OTel-native tracing. | Product teams that want evaluation results to gate deploys directly, rather than teams that just want to observe production traffic after the fact. |
| Pricing | Free tier available | Free tier available / $249/mo starting (Pro) |
| Winner | Our Pick |
Tool Breakdown
Arize Phoenix
For most teams evaluating LLM and RAG applications, Phoenix's zero-cost, self-hosted, OpenTelemetry-native tooling removes the pricing risk entirely — Braintrust only pulls ahead once a team specifically needs its CI/CD-gated eval workflow and is ready to pay $249/mo flat for it.
- Free to self-host indefinitely with no usage caps — no tier to outgrow or upgrade path to budget for
- OpenInference/OpenTelemetry-native instrumentation makes it vendor- and framework-agnostic, easy to combine with an existing OTel collector
- Built-in LLM-graded evaluators purpose-built for RAG quality work: retrieval relevance, hallucination detection, QA correctness
- Licensed under Elastic License 2.0, not a permissive OSI license like MIT or Apache — restricts using Phoenix as the basis for a competing hosted service
- No native CI/CD gating workflow built into the core product — teams wanting evals to block a deploy pipeline need to wire that up themselves or use Arize's separate managed product, Arize AX
Braintrust
Braintrust is an LLM observability and evaluation platform built around a trace-to-test pipeline, with self-hostable SDKs and an Apache-2.0 AI proxy, designed for teams that want eval results to actively drive CI/CD decisions.
- Generous free tier for evaluation: 1M trace spans/mo, 1GB processed data, 10K scores, 14-day retention, unlimited users, no credit card required
- Open-source tracing/eval SDKs (Python, JS/TS under Apache-2.0, plus Go, Java, Rust) with auto-instrumentation for OpenAI, Anthropic, LangChain, and LlamaIndex
- Pro tier is a flat $249/mo rather than usage-metered per-seat pricing, which simplifies budgeting for a growing team
- Core platform is proprietary, not open source — only the SDKs, the autoevals scoring library, and the AI proxy are open under permissive licenses
- Full self-hosting (a customer-run data plane alongside Braintrust's managed control plane) is gated to the Enterprise tier, which requires contacting sales for pricing
- No confirmed commission-based affiliate or referral program — Braintrust offers startup credits, not a payout program
Frequently Asked Questions
Is Arize Phoenix or Braintrust actually free to use? +
Both have a free tier, but they work differently. Arize Phoenix is fully open source and free to self-host indefinitely with no usage caps on the core product. Braintrust's free tier is a hosted plan with defined limits — 1M trace spans/mo, 1GB processed data, 10K scores, and 14-day retention — after which you need the $249/mo Pro plan or a custom Enterprise plan. If your priority is avoiding any recurring cost at scale, Phoenix's self-hosted model has no ceiling to hit; Braintrust's free tier is generous for evaluation but has real limits once you're in production.
Which tool is better for gating CI/CD pipelines with eval results? +
Braintrust. Its trace-to-test pipeline is specifically built so evaluation results can drive automated decisions, like blocking a deploy when a model's outputs fail a quality threshold. Arize Phoenix has strong evaluation tooling for RAG quality (retrieval relevance, hallucination detection) but doesn't ship a native CI/CD gating workflow out of the box — teams wanting that with Phoenix typically build the integration themselves or move to Arize's separate managed product, Arize AX. If CI/CD-gated evaluation is your core requirement, Braintrust is the more direct fit.
Can I self-host either platform completely, with no vendor-managed component? +
Only Arize Phoenix offers full self-hosting for free — it's open source under the Elastic License 2.0 and runs entirely in your own infrastructure with no usage caps. Braintrust uses a hybrid model: even on its self-hosting option, a Braintrust-managed control plane (UI, metadata) stays in Braintrust's cloud while your infrastructure runs the data plane (deployed via Braintrust's official Terraform or Helm tooling). Full self-hosting with Braintrust is also gated to the Enterprise plan, which requires contacting sales. If complete infrastructure independence matters, Phoenix is the only one of the two that delivers it without an enterprise contract.