Evals & observability
W&B Weave
Tracing and evaluation from Weights & Biases, aimed at teams already tracking experiments there.
- Category
- Evals & observability
- Pricing
- Free tier
- Runs
- Hosted
- Languages
- Python, TypeScript
- Interface
- SDK, Dashboard
- Source
- Open source
What W&B Weave is
Weave brings model application tracing and evaluation into Weights & Biases, so LLM work sits beside the experiment tracking an ML team already runs. Adding a decorator captures inputs, outputs, and cost, and comparing eval runs uses the same interface as comparing training runs.
Best for
Organisations already on Weights & Biases who want one place for all model work.
Consider something else if
Most of the value assumes you are in that ecosystem; standalone, other tools are more focused.
W&B Weave alternatives
The closest options in evals & observability, on the axes that actually separate them.
| Tool | Best for | Pricing | Runs |
|---|---|---|---|
| W&B Weaveopen source | Organisations already on Weights & Biases who want one place for all model work. | Free tier | Hosted |
| LangSmith | LangChain and LangGraph users, who get the deepest integration for the least work. | Free tier | Hosted |
| Braintrust | Teams treating prompt changes as changes that need tests, with evals wired into CI. | Free tier | Hosted |
| Langfuseopen source | Teams who need traces containing customer data to stay inside their own infrastructure. | Open source | Hosted or self-hosted |
| Heliconeopen source | Getting cost and latency visibility across a codebase quickly, especially one you do not want to instrument. | Free tier | Hosted or self-hosted |
| Phoenixopen source | Teams standardised on OpenTelemetry, and anyone debugging retrieval quality interactively. | Open source | Hosted or self-hosted |
Choosing within evals & observability
Tracing or evaluation first
Some tools lead with tracing and add evals; others are eval-first with tracing attached. If you are debugging today, start with tracing. If you are about to change a prompt that is already in production, start with evals.
How it gets your data
A proxy needs a one-line base-URL change and captures everything. An SDK needs instrumentation and captures more structure. OpenTelemetry-based tools slot into tracing you may already run.
Questions
What is W&B Weave?
Weave brings model application tracing and evaluation into Weights & Biases, so LLM work sits beside the experiment tracking an ML team already runs. Adding a decorator captures inputs, outputs, and cost, and comparing eval runs uses the same interface as comparing training runs. It is open source and hosted.
What are the alternatives to W&B Weave?
The closest alternatives are LangSmith, Braintrust, Langfuse, Helicone, Phoenix. They sit in the same category — evals & observability — and differ mainly on hosting model, pricing shape, and how much they abstract away.
Is W&B Weave the right choice?
Organisations already on Weights & Biases who want one place for all model work. The main caveat: Most of the value assumes you are in that ecosystem; standalone, other tools are more focused.
Whatever you build on, the model is the line item that scales. See what each one costs per million tokens, or price your own workload.