LLM Observability Stack Selection

The 2026 observability market splits into two categories. Development platforms (LangSmith, Langfuse, Comet Opik) bundle monitoring with evals, prompt management, session replays. Gateway/instrumentation tools (Helicone, SigNoz, OpenLLMetry, Phoenix) focus on telemetry. Langfuse is MIT-licensed core with strong OSS balance (50K events/month free cloud). Phoenix is OpenTelemetry-native under Elastic License 2.0 — excellent for drift/RAG visualization, not a persistent production backend. Arize AX uses zero-copy Iceberg/Parquet integration claiming 100x cheaper than monolithic observability. LangSmith leads for LangChain/LangGraph, $39/user/mo, self-host in Enterprise only. Helicone is proxy-based with 15-30 min setup, 100K req/mo free, but less depth on agent traces. Common production pattern: Gateway (Helicone/Portkey) + eval platform (Phoenix/TruLens) glued by OpenTelemetry.

Learn 1 hr Python (stdlib, toy trace-sampling simulator)

Prerequisites: Phase 17 · 08 (Inference Metrics), Phase 14 (Agent Engineering)

#Learning Objectives

Distinguish development platforms (bundled: evals + prompts + sessions) from gateway/telemetry tools (traces + metrics only).
Map six major tools (Langfuse, LangSmith, Phoenix, Arize AX, Helicone, Opik) to their licensing, pricing, and sweet-spot use cases.
Explain the OpenTelemetry-glue pattern that lets you combine a gateway tool with a separate eval platform.
Name the 2026 cost differentiator (Arize AX's zero-copy approach vs monolithic ingest) and state the rough 100x multiplier.

#Learning Objectives

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