Braintrust Open Source Alternative? LLM Evaluation Platform Comparison
Dive into the difference between Braintrust and Phoenix open source LLM evaluation and tracing
Braintrust is an evaluation platform that serves as an alternative to Arize Phoenix. Both platforms support core AI application needs, such as: evaluating AI applications, prompt management, tracing executions, and experimentation. However, there are a few major differences.
Why is Arize Phoenix a popular open source alternative to Braintrust?
Braintrust is a proprietary LLM-observability platform that often hits road-blocks when AI engineers need open code, friction-free self-hosting, or things like agent tracing or online evaluation. Arize Phoenix is a fully open-source alternative that fills those gaps while remaining free to run anywhere.
Top Differences (TL;DR)
Open source
OSS
Closed source
LLM Evaluation Library
OSS Pipeline Library and UI
UI Centric Workflows
BrainTrust versus Arize Phoenix Versus Arize AX: Feature Comparison
Open source
✅
–
❌
1-command self-host
✅
✅
❌
Free
✅
Free Tier
Free Tier
AI-powered search & analytics
❌
✅
❌
Key Differences
Complete Ownership vs. Vendor Lock-In
Phoenix:
100% open source
Free self-hosting forever - no feature gates, no restrictions
Deploy with a single Docker container - truly "batteries included"
Your data stays on your infrastructure from day one
Braintrust:
Proprietary closed-source platform
Self-hosting locked behind paid Enterprise tier (custom pricing)
Free tier severely limited: 14-day retention, 5 users max, 1GB storage
$249/month minimum for meaningful usage ($1.50 per 1,000 scores beyond limit)
Developer-First Experience
Phoenix:
Framework agnostic - works with LangChain, LlamaIndex, DSPy, custom agents, anything
Built on OpenTelemetry/OpenInference standard - no proprietary lock-in
Auto-instrumentation that just works across ecosystems
Deploy anywhere: Docker, Kubernetes, AWS, your laptop - your choice
Braintrust:
Platform-dependent approach
Requires learning their specific APIs and workflows
Limited deployment flexibility on free/Pro tiers
Forces you into their ecosystem and pricing model
Evaluation & Observability
Phoenix:
Unlimited evaluations - run as many as you need
Pre-built evaluators: hallucination detection, toxicity, relevance, Q&A correctness
Custom evaluators with code or natural language
Human annotation capabilities built-in
Real-time tracing with full visibility into LLM applications
Braintrust:
10,000 scores on free tier ($1.50 per 1,000 additional)
50,000 scores on Pro ($249/month) - can get expensive fast
Good evaluation features, but pay-per-use model creates cost anxiety
Enterprise features locked behind custom pricing
Self-Hosting — Ease & Cost
Phoenix deploys with one Docker command and is free/unlimited to run on-prem or in the cloud. Braintrust’s self-hosting is reserved for paid enterprise plans and uses a hybrid model: the control plane (UI, metadata DB) stays in Braintrust’s cloud while you run API and storage services (Brainstore) yourself, plus extra infra wiring (note: you still pay seat / eval / retention fees, with the free tier capped at 1M spans, 10K scores, 14 days retention).
Instrumentation & Agent Tracing
Phoenix ships OpenInference—an OTel-compatible auto-instrumentation layer that captures every prompt, tool call and agent step with sub-second latency. Braintrust has 5 instrumentation options supported versus Arize Ax & Phoenix who have 50+ instrumentations.
Arize AX and Phoenix are the leaders in agent tracing solutions. Brainstrust does not trace agents today. Braintrust accepts OTel spans but has no auto-instrumentors or semantic conventions; most teams embed an SDK or proxy into their code, adding dev effort and potential latency.
Evaluation (Offline & Online)
Phoenix offers built-in and custom evaluators, “golden” datasets, and high-scale evaluation scoring (millions/day) with sampling, logs and failure debugging. Braintrust’s UI is great for prompt trials but lacks benchmarking on labeled data and has weaker online-eval debugging.
The Phoenix Evaluation library is tested against public datasets and is community supported. It is an open source tried and tested library, with millions of downloads. It has been running in production for over two years by tens of thousands of top enterprise organizations.
Human-in-the-Loop
Phoenix and Arize AX include annotation queues that let reviewers label any trace or dataset and auto-recompute metrics. Braintrust lacks queues; “Review” mode is manual and disconnected from evals
Agent Evaluation
Phoenix and AX have released extensive Agent evaluation including path evaluations, convergence evaluations and session level evaluations. The investment in research, material and technology spans over a year of work from the Arize team. Arize is the leading company thinking and working on Agent evaluation.
Open Source vs. Proprietary
One of the most fundamental differences is Phoenix’s open-source nature versus Braintrust’s proprietary approach. Phoenix is fully open source, meaning teams can inspect the code, customize the platform, and self-host it on their own infrastructure without licensing fees. This openness provides transparency and control that many organizations value. In contrast, Braintrust is a closed-source platform, which limits users’ ability to customize or extend it.
Moreover, Phoenix is built on open standards like OpenTelemetry and OpenInference for trace instrumentation. From day one, Phoenix and Arize AX have embraced open standards and open standards, ensuring compatibility with a wide range of tools and preventing vendor lock-in. Braintrust relies on its own SDK/proxy approach for logging, and does not offer the same degree of open extensibility. Its proprietary design means that while it can be integrated into apps, it ties you into Braintrust’s way of operating (and can introduce an LLM proxy layer for logging that some teams see as a potential point of latency or risk).
Teams that prioritize transparency, community-driven development, and long-term flexibility often prefer an open solution like Phoenix.
How to Choose
Prototype & iterate fast? → Phoenix (open, free, unlimited instrumentation & evals).
Scale, governance, compliance? → Arize AX (also free to start, petabyte storage, 99.9 % SLA, HIPAA, RBAC, AI-powered analytics).
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