LLM Observability Tools, Honestly Compared

LLM observability tools fall into four types, and picking the right type matters more than picking the brand. This page groups well-known options by type, says what each type is good at, and ends with a selection checklist.

No affiliate links and no paid placements, per this site's methodology. Features in this category change monthly, so treat the descriptions below as a starting map and verify specifics on each vendor's current docs. For what to capture in the first place, start with the LLM observability guide.

Open-source tracing and evaluation

• Langfuse: open-source LLM engineering platform covering tracing, prompt management, and evaluations. Self-hostable.
• Arize Phoenix: self-hostable tracing and evaluation from Arize. Arize calls it open source; the license is the Elastic License 2.0, so check its terms if that matters to you.
• OpenLLMetry (Traceloop): OpenTelemetry-based instrumentation that sends LLM traces to backends you may already run.

Good for: keeping data in your own infrastructure, controlling cost, and avoiding lock-in. You pay in hosting and upkeep.

Hosted platforms with evaluation built in

• LangSmith: LangChain's hosted tracing and evaluation platform, usable outside LangChain apps too.
• Braintrust: an evaluation-first platform with production logging.
• Weights & Biases Weave: tracing and evaluation inside the W&B ecosystem.

Good for: teams that want managed dashboards, dataset management, and human review workflows without building them.

Gateways and proxy loggers

• Helicone: open-source logging that typically sits in the request path as a proxy. Helicone was acquired by Mintlify and says the service is now in maintenance mode, so weigh that before adopting it for a new project.
• AI gateways in general centralize routing, keys, and usage tracking across providers. Jev itself is served through Vercel's AI Gateway per ecosystem documentation, and gateway-level usage data is often the first cost view teams get.

Good for: the fastest possible setup and cross-provider cost tracking. Depth of quality evaluation varies a lot between products.

APM suites with LLM modules

• Datadog Agent Observability (formerly marketed as LLM Observability): tracing, monitoring, and evaluation of LLM apps and agents inside Datadog. The natural pick if Datadog already runs your infrastructure monitoring.
• Other APM vendors have added similar modules, so check your existing contract before buying something new.

Good for: one pane of glass with infrastructure metrics, and procurement that's already done.

How to choose

Answer these in order:

1. Where must the data live? Self-hosted requirements narrow the list fast.
2. Traces only, or evaluations too? If quality scoring matters, favor eval-first tools.
3. Framework ties? Some tools are smoothest inside one framework.
4. Existing APM contract? It may already cover you.
5. Can it log judge pipelines properly? Probabilities, question versions, and escalation outcomes, not just text. See the verdict logging spec.
6. Does the pricing model survive your volume? Model it against your cost metering before signing.

Frequently asked questions

What's the best LLM observability tool?

The one whose type fits your constraints. Start from where your data must live and whether you need evaluations, then compare brands within that type.

Is there a free LLM observability tool?

Several open-source tools are self-hostable at no license cost. Hosting, storage, and maintenance are the real price.

Do I need a separate tool if I already use Datadog?

Not necessarily. Its LLM module may cover tracing and monitoring; compare its evaluation features against your needs.

Can these tools monitor Jev pipelines?

Any tool that logs arbitrary model calls can capture verdict calls. Log the probabilities and question versions, not only the text. The model is covered in what Jev is.

Does this site earn money from these vendors?

No. No affiliations and no paid placements.