Langfuse vs Arize (2026): LLM Observability Compared — and Where Kosmoy Fits

Three shapes on the same ten axes. Langfuse (orange) and Arize (violet) both peak on Observability & FinOps and reach high on Testing & Evals — the category's two defining axes. Arize pulls ahead on Guardrails & Runtime Safety, because its Guards can block or regenerate responses, while Langfuse delegates runtime blocking to third-party libraries. Kosmoy (blue) trades raw tracing and eval depth for reach across inventory, gateway, compliance and agent containment. Read it as area: the two observability tools compete on one spoke; the suite covers the web.

Genuinely open-source core. Langfuse's core is MIT-licensed (OSI-approved) and unlimited when self-hosted, running the exact same codebase as its cloud with no scalability limits. Arize's Phoenix is under the Elastic License 2.0, which is not OSI-approved open source and restricts offering it as a managed service.

Air-gap and ownership. Langfuse's networking docs state it does not require internet access, with VPC/on-prem guidance and graceful offline degradation — an explicit air-gap story that Arize does not document.

Cost, adoption and EU heritage. Self-hosting is free under MIT; ~31k GitHub stars; a Berlin-based company with an EU-default cloud region — attractive to budget-constrained and EU-data-residency teams.

Runtime guardrails that act. Arize Guards inspect user input and LLM output and can block, substitute a default response, or regenerate. Langfuse's own docs delegate runtime blocking (prompt injection, PII, toxicity) to third-party libraries and evaluate their effectiveness after the fact — it observes rather than enforces.

Eval governance and modality breadth. The Evaluator Hub versions evaluators at commit level, Signal reviews production traces continuously, and Arize ships native voice-agent observability with audio session replay — depth Langfuse's strong-but-leaner eval suite does not match on those fronts.

Enterprise polish. GA role-based access control, the Alyx AI engineering agent, and named enterprise customers (Uber, Duolingo, PepsiCo, Booking.com) reflect an enterprise track record; AX Enterprise also self-hosts Kubernetes-first.

Both Langfuse and Arize answer “how do we trace, evaluate and improve our LLM apps and agents?” Neither answers “what AI are we running across the organization, is it compliant, and what happens when an agent misbehaves?” Those are different questions, and in a regulated enterprise they arrive together.

Kosmoy delivers the same operational observability both products do — usage, latency, cost and quality across every AI interaction in an Insights Dashboard — but wraps it in the layers an observability tool leaves out: a risk-tiered inventory of every model, MCP server and agent; one OpenAI-compatible gateway that enforces guardrails, RBAC and budgets in the request path across all traffic; EU AI Act, ISO 42001 (aligned) and NIST AI RMF evidence; and kernel-enforced Action Capsule containment for agents that act.

The honest framing is not “Kosmoy beats Langfuse and Arize at observability or evals” — it does not. Kosmoy has no dataset, LLM-as-judge or experiment tooling, and its evals score is a 4; the specialists own that spoke and should keep it. It is that observability is one spoke. If the requirement is the whole web — inventory, gateway, compliance and containment in one self-hosted platform — that is a suite decision, not an observability decision.

For a team whose problem genuinely is tracing and evaluation, pick on the axis that matters: Langfuse for an MIT-core platform you fully own and can air-gap, Arize for eval rigor with runtime guards and a Phoenix on-ramp. Both instrument via SDK and OpenTelemetry, so either can run alongside a gateway or a broader platform.

For an enterprise that has to prove control over all of its AI — not just observe and evaluate it — the choice is not between these two tools but between a point tool and a suite. Kosmoy sits comfortably next to either: keep Langfuse or Arize for eval depth while Kosmoy holds the inventory, gateway enforcement, compliance evidence and containment for what reaches production.