> ## Documentation Index > Fetch the complete documentation index at: https://docs.veval.dev/llms.txt > Use this file to discover all available pages before exploring further. # Snapshots > A snapshot is a stored known-good run that new runs are compared against. A snapshot stores every step of a known-good run, in order, with the input your agent sent to each step and the output it got back. Comparing a new run against it shows exactly what your agent now does differently. ## How snapshots fit in You save a snapshot under a name, from a trace ID or from a run you just executed. The snapshot is stored self-contained, and its source trace is pinned, so neither is removed by trace retention. Saving again under the same name replaces the baseline. You then compare runs against it: * In tests, with the `MatchesSnapshot` [assertion](/concepts/assertions). A missing baseline fails the assertion. * In production, with `CompareSnapshotAsync`. Each result is recorded in the dashboard, with expected and actual steps side by side. ## What a comparison reports | Change | Example | | - | - | | **Input changed** | The system prompt was edited, or a tool now gets different arguments. | | **Added** | A new step ran, or a known step ran again. | | **Removed** | A step stopped running. | | **Moved** | A step ran at a different point in the sequence. | | **Output changed** | Only when you turn on `CompareOutputs`. Use it for deterministic steps. | Every change comes with its position and, for input and output changes, a line diff. ## Snapshots and replay In a [replay](/concepts/replay), model outputs are recorded, so step inputs are where a code or prompt change shows up. A snapshot catches that change with no LLM call. Whether the change made outputs better or worse is a question for a live `Judge` run. ## Deep dives Save a baseline, check runs in tests, and monitor production. Run your agent against recorded traces.