Cloudflare lets developers choose sandbox images and sizes at startup

In a post dated September 30, Cloudflare announced a new way to configure its Containers service: application code can choose a sandbox’s image and compute size when it starts, rather than fixing both at deployment. The company described the new durable_object scheduling policy and filesystem snapshots as available in public beta.

Developers opt in by setting the scheduling policy and declaring the images their code may select. A Durable Object—the persistent controller attached to each Container—then makes the choice for each task. One class can start differently sized Node.js and Python environments, for example, without maintaining a separate Containers application for each image-and-size combination.

Snapshots tackle another source of waiting: rebuilding the workspace. Cloudflare says developers can save repositories, installed dependencies, build caches and edits, then restore those files for a later session. Snapshots are immutable and reusable, so multiple sandboxes can also begin from the same prepared environment. For agent evaluations, that provides a common baseline while each run makes its own changes.

Cloudflare also reported shorter startup times, citing a ComputeSDK benchmark that launched 100 sandboxes concurrently and measured time to interactive from the client. In Cloudflare’s account, median startup fell from 4.049 seconds on the previous scheduling path to 648 milliseconds on the new one.

The API choice matters for developers adopting these features. Cloudflare says the new scheduling policy, faster startup, runtime configuration and snapshots are available only through the native ctx.container API. It plans to maintain the older Container and legacy Sandbox classes through December 31, 2026. Existing deployments will keep running afterward, the company says, but those classes will no longer receive updates.