Intel / article

Quoting Felix Rieseberg

Source: simonwillison.net
Date: Published Oct 5, 2026

From the Daily Brief

In a statement quoted by Simon Willison, Anthropic's Felix Rieseberg describes how Cowork's architecture changed. The earlier version ran model inference in the cloud but executed tool calls inside an Anthropic-provided VM shipped to the user's computer, mapping in only the data a user explicitly added to a session. Rieseberg says users liked the results but not the disk, battery and performance cost of the local VM, or that closing a laptop stopped the work. The new version runs both inference and the VM in the cloud. Each session gets its own sandbox that shares no state with other sessions, and when the VM needs something from the user's device, such as a file, the desktop app handles that file access tool call. Anthropic expects this to allow Cowork from a phone and work that keeps running when the laptop closes. For anyone designing agent sandboxes, it is a clear example of splitting execution from local data access: isolation lives in the cloud, and the device only answers narrow requests.

Read the 2026-10-06 Brief →

Context

Anthropic has changed where its Cowork feature, which has Claude carry out tasks for a user, runs its virtual machine, according to a quotation from Anthropic's Felix Rieseberg that Simon Willison posted on October 5, 2026. In the older design, model ran in the cloud but tool calls executed in a virtual machine (a software-emulated computer) that Anthropic shipped to the user's computer. It was added for capability, safety and security reasons, mapping in only the data the user explicitly added. Rieseberg says people liked the results but not the disk, battery and performance cost of running it locally, nor that closing a laptop stopped the work.

In the newer design, Rieseberg says, both inference and the virtual machine run in the cloud, each session gets its own that shares no state with others, and the desktop app handles any tool call that needs something on the user's device, such as a file. Rieseberg says Anthropic thinks this addresses using Cowork from a phone, keeping work running, and battery loss to the virtual machine. That is the company's account of intent, and the quote is excerpted. Anthropic's help article adds that local files and folders still need the desktop app open and connected, and that cloud sessions keep running but cannot reach the computer if it is closed.

Key takeaways · AI-distilled

Rieseberg says the earlier Cowork already ran inference in the cloud, but executed tool calls inside an Anthropic-provided VM shipped to the user's computer, mapping in only the data explicitly added to a session.
Anthropic added that local VM for capability, safety and security, but users disliked its disk, battery and performance cost, and the fact that closing the laptop stopped the work.
In the new design each session gets its own cloud sandbox that shares no state with other sessions; when the VM needs something on the device, such as a file, the desktop app handles that file-access tool call.

Terms in this piece · Glossary

inference — Running a trained model to get answers — the phase where AI is actually used, as opposed to trained.
sandbox — An isolated environment where AI-generated code or agent actions run without being able to touch anything real.

Read the source simonwillison.net