LLM observability · 2026
A practical map of where each LLM observability tool wins and where it does not. Drop-in proxy versus SDK-first. Open source versus closed. Self-hostable versus cloud-only. Below are the five most common alternatives teams compare Spanlens to, plus migration guides if you already have data in one of them.
| Tool | Install | License | Self-host |
|---|---|---|---|
| Spanlens | 1-line baseURL swap or SDK | MIT (entire repo) | Yes, one Docker command |
| Langfuse | SDK wrap or OTel exporter | MIT core + EE folder | Yes, Docker Compose |
| Helicone | 1-line baseURL swap | Apache 2.0 | Yes, Docker Compose |
| LangSmith | LangChain callback or SDK | Closed source | Paid plan only |
| Braintrust | SDK wrap | Closed source | No (cloud only) |
| Arize Phoenix | OTel exporter or SDK | ELv2 (source-available) | Yes, Docker |
The most mature OSS observability tool. We diverge on instrumentation model (proxy vs SDK), license boundary (full MIT vs OSS plus EE folder), and built-in eval shape.
Teams pick Spanlens over Langfuse when they want a 1-line baseURL swap instead of wrapping every call, full MIT with no EE folder gating SCIM or audit logs, and statistical Prompt A/B (Welch t-test) built in.
The closest architectural match. Both are proxy-based, though Helicone entered maintenance mode after its 2026 Mintlify acquisition.
Spanlens adds Critical Path agent tracing, Prompt A/B with Welch t-test, judge-to-human correlation tracking, and a ClickHouse fallback-replay safety net that survives transient backend outages without losing rows.
LangChain's commercial offering. Excellent if you live inside LangChain, locked-in if you don't.
Spanlens is framework-agnostic. The proxy works whether you use LangChain, LangGraph, Vercel AI SDK, or plain HTTP from any language. No callback handler required for off-chain code paths.
Eval-first, closed-source SaaS. Strong eval UX.
Spanlens bundles eval into a full observability platform with proxy-based logging, agent tracing, cost optimization, and Prompt A/B. You can self-host the whole thing with one Docker command instead of staying on a hosted-only product.
Source-available (ELv2) observability from Arize. Python-first, ML-engineer-leaning.
Spanlens is built for application developers shipping LLM features. JS/TS gets equal-class SDK support, the install is a baseURL swap rather than an OTel pipeline, and the license is full MIT instead of ELv2.
Step-by-step guides with code diffs, env var mapping, and a dual-run cutover plan so you can switch without losing history.
Try Spanlens free for 50K requests a month. No credit card. Drop the SDK in, see every LLM call in your dashboard within 60 seconds.
Free tier · No credit card · Self-host with Docker