Observability is more than request logs
LLM applications fail in several layers: the model may produce a poor answer, retrieval may supply the wrong context, a tool call may fail, or cost and latency may drift without an obvious error. Langfuse, Phoenix and Helicone all sit in this observability space, but they emphasize different entry points. A useful comparison therefore includes traces, evaluation data, prompt or gateway configuration, and the effort required to connect evidence back to a user-visible outcome.
Relative strengths and suitable jobs
Langfuse is relatively strong for teams that want a product-style workspace spanning traces, prompts, datasets and usage analysis. Phoenix is relatively strong when the team wants an open-source tracing and evaluation workflow close to experiments in LLM and agent quality. Helicone is relatively strong when routing model traffic through a gateway is the simplest way to add request visibility, cost reporting and provider comparisons. These are relative fits, not a claim that one system wins every workload.
Boundaries and responsibility
Telemetry can contain prompts, retrieved documents, tool arguments and user identifiers. Define redaction and retention before enabling collection, then check where data is stored and who can read it. A trace can show that a request was slow without proving why the user disliked the answer. Keep a small human-reviewed evaluation set and record model, prompt, retrieval and tool versions. Also account for the fact that gateway-based visibility and in-application tracing expose different portions of the request path.
A comparison experiment to run
Instrument one small agent with the same five tasks in each setup: a direct answer, a retrieval answer, a tool failure, a refusal and a long-running request. Compare setup time, trace completeness, redaction controls, evaluator workflow, cost attribution and the number of manual steps needed to move from a bad output to a reproducible fix. Have two people inspect the same trace and write the suspected cause independently. Agreement is more useful than a colorful dashboard.