Felix Rieseberg of Anthropic described, as quoted by Simon Willison, a reworked architecture for Claude's Cowork agent tool.
In the earlier version, model inference ran in the cloud but tool calls executed inside an Anthropic-provided VM that was shipped to the user's own computer. Rieseberg said this local VM was added for capability, safety and security reasons, mapping in only the data a user explicitly added to their session. He said users liked what they could do with Claude through it but disliked the disk, battery and performance cost of running the VM locally — and disliked that closing the laptop stopped the work.
In the new version, both model inference and the VM run in the cloud. Each session gets its own isolated sandbox that does not share state with other sessions. When the VM needs something from the user's device, such as a file, the desktop app is now responsible for handling that specific file-access tool call rather than running the whole VM locally.
Rieseberg said the change is intended to address complaints about using Cowork from a phone, about work stopping when a device closes, and about losing battery life to a locally running VM. Anthropic has also published a help page with further detail, referenced in the original post.