Artifact guidance now warns that browser storage in pages is per-device and never syncs or is visible to Claude
Guidance text used when Claude builds artifact pages (in the artifacts/Claude Design prompt material) now explains browser storage mechanisms — localStorage, sessionStorage, and IndexedDB. It states that these are:
- per-origin (tied to that specific page's address)
- persistent across republishes of the same artifact
- never synced across different viewers, devices, or back to Claude
- prone to failing in private/incognito windows or during thumbnail capture
The guidance says this storage should only be used for small per-viewer conveniences, like a remembered tab, filter, collapsed section, or unsent draft, wrapped in try/catch, and never for state that must persist reliably or be shared or read back by Claude.
This steers Claude away from relying on browser storage for anything important in an artifact page, since it can silently fail or vanish for different viewers, and toward using it only for minor convenience features.