---
title: sanderling replay
---

# sanderling replay

Local web UI for exploring runs produced by `sanderling test`. Reads `runs/<id>/meta.json` and `runs/<id>/trace.jsonl` from disk.

```
sanderling replay [run-or-runs-dir] [--port N] [--no-open] [--dev]
```

The positional argument can be a runs directory or a single run directory (auto-detected by `meta.json`). Defaults to `./runs`.

![sanderling replay](../../_assets/replay-ui.png)

## Panels

| Panel | What it shows |
|---|---|
| Screenshot | Device screenshot for the focused step, with the dispatched action's target spotlit. |
| ActionList | Ordered list of steps with the verb and target for each action. |
| Timeline | Per-property lane chart over the run; cells coloured by violated, pending, or holds. |
| ViolationsPanel | Property statuses at the focused step with residual formulas for any that failed. |
| HierarchyPanel | Filterable table of every UI element captured at the focused step; mirrors the selectors documented in [Spec language reference](../spec-language/). |
| SnapshotTable | Flattened `snapshots` map for the focused step, with diffs against the previous step. Nothing fills `snapshots` today, so it reads "no snapshots" on every step. |
| MetricsChart | Heap and other host-side metrics sampled per step. |
| ExceptionsPanel | Uncaught exceptions captured during the run, with a jump-to-first control. |

## Keyboard shortcuts

| Key | Action |
|---|---|
| `j`, `Right` | Next step |
| `k`, `Left` | Previous step |
| `Shift+j`, `Shift+Right` | Jump 10 forward |
| `Shift+k`, `Shift+Left` | Jump 10 back |
| `g` / `G` | First / last step |
| `.` | Next step with a violation |

Arrow keys inside a tablist or listbox yield to those widgets. Use `j`/`k` when focus is on one.

## Deep links

`/runs/:id/steps/:n` links to a specific step. Use it in issues or PRs when pointing at a violation.

The run index auto-refreshes over SSE as new runs land, so `sanderling replay` and `sanderling test` can run side by side.

## Development

Two-process loop while iterating on the UI:

```
make web-dev     # bun + vite on http://127.0.0.1:5173
make replay-dev  # sanderling replay --dev, proxies non-API to 5173
```

Single binary with the bundle embedded:

```
make sanderling
```