Vercel Sandbox adds fork() so agents branch from a warm snapshot.

Vercel shipped it July 28: a fork starts from the source sandbox's current snapshot, inherits config and env vars, and supports per-field overrides. Forking takes about the same time as creating a sandbox with equivalent limits and falls back to fresh creation when no snapshot exists. The gotcha in the docs: it copies the latest saved state, not live in-memory state of a running sandbox. Named use cases are branching agents off shared setup, per-tenant template copies, and simultaneous setup variations.