W&B Weave vs Lunary
Both are observability & tracing tools. Here is how they actually differ on price, billing model and deployment.
W&B Weave
Weights & Biases' LLM tracing and evaluation product, now owned by CoreWeave after a reported $1.7B acquisition. Billed on GB of trace data ingested rather than spans or requests, which is a genuinely different cost model to everything else in the category.
Lunary
A lightweight Apache-2.0 observability platform aimed at RAG pipelines and chatbots. The fastest path from nothing to basic tracing, with a free tier metered daily rather than monthly - which catches people out.
| W&B Weave | Lunary | |
|---|---|---|
| Category | Observability & Tracing | Observability & Tracing |
| Our rating | 4/5 | 3/5 |
| Starting price | Per-seat plus usage | Around $20-30/mo |
| Billing meter | gb-ingested | event |
| Free plan | Yes | Yes |
| Free self-hosting | No or paid tier only | Yes, free |
| Best for | Teams already running Weights & Biases for model training and experiment tracking, who want LLM traces and evals in the same platform as their fine-tuning runs and are comfortable with usage-based ingestion billing. | Small teams shipping RAG pipelines or chatbots who want basic tracing working this afternoon, and who will either stay small or self-host before volume becomes expensive. |
Our verdict on W&B Weave
W&B Weave is the strongest option in the category for one specific team - the one already on Weights & Biases. The lineage story is real and unmatched - a model fine-tuned in Sweeps and the eval run testing it appear in the same interface, which no AI-native competitor can offer because none of them do training. The instrumentation API is excellent, the custom scorers are plain Python, and the integration coverage is among the broadest available. Two things temper it. The billing metric is GB of trace data ingested, which is genuinely harder to forecast than per-span or per-seat pricing because it scales with prompt and context size, not just traffic. And CoreWeave now owns it following a reported $1.7B acquisition, which has come with interoperability pledges but leaves an open question about long-term direction. If you are not already a W&B customer, the bundled per-seat maths works against you and there are cheaper focused tools.
Full W&B Weave review →Our verdict on Lunary
Lunary is a good starter tool that knows what it is. It is optimised for RAG pipelines and chatbots rather than trying to be a complete AI engineering platform, it is Apache 2.0 and self-hostable, and it is among the fastest things in the category to get running. Radar - which buckets responses against criteria you define - is a genuinely useful idea for spotting patterns that individual traces hide. Two caveats matter. The free tier is 1,000 events per day rather than per month, which is a meaningfully tighter constraint than the headline suggests and will not survive a real chatbot for long. And published pricing is inconsistent across sources in a way we could not resolve, with figures ranging from around $20 to $200 a month depending on where you look. Against Langfuse it is lighter on nearly every axis, and Langfuse is the better default for most teams. Lunary earns its place when speed to first trace matters more than depth.
Full Lunary review →Frequently Asked Questions
What is the main difference between W&B Weave and Lunary?
W&B Weave: Teams already running Weights & Biases for model training and experiment tracking, who want LLM traces and evals in the same platform as their fine-tuning runs and are comfortable with usage-based ingestion billing. Lunary: Small teams shipping RAG pipelines or chatbots who want basic tracing working this afternoon, and who will either stay small or self-host before volume becomes expensive. Both sit in Observability & Tracing, so the decision usually comes down to billing model and deployment rather than raw capability.
Which is cheaper, W&B Weave or Lunary?
It depends entirely on your workload shape, because they meter differently - W&B Weave bills on gb-ingested and Lunary bills on event. Published starting prices are Per-seat plus usage and Around $20-30/mo respectively, but those numbers are not comparable until you apply them to the same traffic. Use our cost calculator to model both against your own request volume and span count.
Can I self-host W&B Weave or Lunary?
W&B Weave: No or paid tier only. Lunary: Yes, free. Free self-hosting means no licence fee, not no cost - you still own the infrastructure, upgrades and on-call.