W&B Weave vs Braintrust
If you are shipping an LLM app and need to watch what it does in production and score it, both of these fit. W&B Weave is the LLM layer of Weights & Biases. It is an open-source SDK that traces your calls and runs evals, and it sits close to the ML tooling many teams already use. Braintrust is a commercial platform built around evals, a prompt playground, and logging, with a polished UI as the main workspace.
| Dimension | W&B Weave | Braintrust |
|---|---|---|
| What it is | W&B's LLM tracing and eval SDK | A purpose-built eval and observability platform |
| License | Open-source SDK with a hosted backend | Commercial and hosted |
| Feels like | Code-first tracing, W&B style | A UI-forward eval workspace |
| You write evals in | Python or TypeScript | Python or TypeScript, plus in the UI |
| Tracing and observability | Strong, auto-logs LLM calls | Strong, logging and evals linked |
| Prompt iteration | Through code and UI | The playground is a core strength |
| Ecosystem fit | Best if you already use W&B | Standalone, assumes no prior tooling |
| Best for | Teams in the W&B world who want LLM tracing | Teams who want evals as the main product |
You already live in Weights & Biases, or you want an open-source SDK so your tracing is not locked behind one vendor. Weave gives you call-level tracing and evals that sit next to the rest of your ML work. Getting started costs nothing but an import.
Evals and prompt iteration are the daily job and you want a real product built around them. Braintrust's playground, dataset handling, and logging are tight and quick to adopt, which helps a team that has no prior observability stack to fit into.
If your team already uses W&B, Weave is the obvious call, and the open-source SDK is a real plus. If you are starting clean and evals are the point, Braintrust's UI-first workflow tends to get people running experiments faster, and the playground is genuinely good. The common mistake is picking on the feature checklist. Both trace, both eval, both log. Choose on what you already run and how your team likes to work, code-first or UI-first. And remember the tool is worthless until you write real eval cases from actual failures. That is the part people skip.