## Summary
Adds `screencast_start` and `screencast_stop` MCP tools that allow
agents to record a video of a page using Puppeteer's `page.screencast()`
API.
Only enabled if the command line option `--experimental-screencast` is
provided
Closes#878.
## New Tools
### `screencast_start`
Starts recording a screencast (video) of the selected page in mp4
format.
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `path` | string (optional) | temp file | Output file path |
### `screencast_stop`
Stops the active recording and reports the saved file path.
## Design
- **Start/stop pair** following the `performance_start_trace` /
`performance_stop_trace` pattern
- **State management** via `getScreenRecorder()` / `setScreenRecorder()`
on the Context interface
- **Category**: `DEBUGGING` (alongside `take_screenshot`)
- **ffmpeg dependency**: Clear error message when ffmpeg is not
installed
- **Temp file fallback**: When no `path` is provided, creates a temp
file with the correct extension
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Co-authored-by: Alex Rudenko <alexrudenko@chromium.org>
Dobes Vandermeer