> For the complete documentation index, see [llms.txt](https://docs.layerlens.ai/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.layerlens.ai/more-in-this-section-5/investigate-failing-trace.md).

# Investigate a failing trace

Guide — investigate a failing trace.

1. Open the trace in **Premium → Traces**
2. Read the span tree top-down — first failure span is your starting point
3. Inspect inputs and outputs at every span boundary
4. Check span-level errors, latencies, costs for outliers
5. Run an ad-hoc judge against a single span to test hypotheses

## See also

* [Concept: Traces and spans](/6.-build-wire-your-code/traces-and-spans.md)
* [Workflow: Observe](/7.-observe-see-whats-happening/workflow.md)