Evaluation answers 'is the agent right'; observability answers 'what did the agent do'. The four credible 2026 agent-observability platforms (Langfuse, Arize, Helicone, LangSmith) split cleanly on a single structural axis: open-source-first vs SaaS-first. Helicone has been in maintenance mode since 3 March 2026 (founders joined Mintlify) and should not be selected for greenfield 2026 deployments. Production deployments need both eval and observability; the procurement decisions are different and conflating them produces SLA architecture that fails its first incident.

Companion piece to AM-122. Verified primary sources: Langfuse v3.172.1 release (1 May 2026, MIT-licensed, 26.5k stars, four-tier pricing Hobby $0/Core $29/Pro $199/Enterprise $2,499 with US/EU/Japan/HIPAA-US data residency); Arize AX docs and pricing (Phoenix free OSS plus AX Free $0/Pro $50/Enterprise custom with US/EU/CA residency); Helicone joining-Mintlify announcement (3 Mar 2026, services in maintenance mode, no migration timeline); LangSmith pricing repeated from AM-122; OpenTelemetry GenAI semantic conventions (currently in 'Development' status, OTEL_SEMCONV_STABILITY_OPT_IN required for experimental adoption). Editorial finding: Helicone maintenance-mode status is the 2026 procurement-material fact most catalogue-vendor-list articles miss; piece names this clearly and recommends migration within 6-12 months for existing customers. 60-day review cadence; trigger conditions include OTel GenAI spec stabilising or further M&A in the open-source observability layer.