The LLM Observability & Eval Index (2026)

Once an agent hits production you need to see what it did and score whether it was any good. Here is a neutral index of the LLM observability + evaluation tools, by focus (tracing / evaluation / monitoring), hosting, and license. No prices — the tool is rarely the dominant cost.

The matrix

Tool	Focus	Hosting	License	Best for
LangSmith	All-in-one	Managed (enterprise self-host)	Proprietary	Teams building on LangChain / LangGraph — native graphs and replay
Langfuse	All-in-one	Both (self-host or cloud)	Open-source	Open-source, framework-agnostic tracing + eval with full data ownership (OTel)
Arize Phoenix	Tracing + evaluation	Both	Open-source	OTel-native tracing with rigorous, ML-grade evaluation primitives
Braintrust	Evaluation + tracing	Managed	Proprietary	Eval-first workflows — datasets, prompt iteration, and scoring
Confident AI (DeepEval)	Evaluation	Both	Open-source (DeepEval)	Pytest-style LLM evals and regression tests in CI
Weights & Biases Weave	Tracing + evaluation	Managed (enterprise self-host)	Proprietary	Teams already in the Weights & Biases ML ecosystem
Comet Opik	Tracing + evaluation	Both	Open-source	Open-source tracing + eval, optionally inside the Comet platform
Helicone	Tracing + monitoring	Both	Open-source	Drop-in proxy logging, cost tracking, and caching with minimal code
Langtrace	Tracing	Both	Open-source	Vendor-neutral, OpenTelemetry-native tracing
MLflow Tracing	Tracing + evaluation	Both	Open-source	Teams standardised on MLflow for the ML lifecycle
Latitude	Evaluation	Both	Open-source	Open-source prompt engineering with built-in evals
Maxim AI	All-in-one	Managed	Proprietary	End-to-end eval + observability across the agent lifecycle

Quick picks

• You build on LangChain / LangGraph → LangSmith
• You want open-source and full data ownership → Langfuse
• You want OTel-native tracing with rigorous evals → Arize Phoenix
• You want eval-first iteration (datasets, scoring) → Braintrust or Confident AI
• You want CI regression tests for prompts → Confident AI (DeepEval)
• You want drop-in proxy logging + cost control → Helicone
• You already use W&B, MLflow, or Comet → Weave, MLflow, or Opik
• You want vendor-neutral OpenTelemetry tracing → Langtrace

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📚 More from The 2026 AI Stack Index: Automation Tools · Agent Frameworks · Vector Databases · LLM Observability · LLM Gateways

This is a neutral, no-affiliate reference — no prices (they go stale), no rankings-for-pay. The full, always-updated interactive version with FAQs and the rest of the AI-stack indexes lives at aiprosol.com/llm-observability. Disclosure: I run Aiprosol, an automation consultancy — the index doesn't favour anyone.