Modal splits sandboxes into same-host sidecar containers, letting agent tool execution run apart from the main sandbox in public alpha

A sandbox no longer has to be a single container: sidecars let additional containers run in parallel with the main Sandbox on the same host, communicating over a low-latency internal network. Previously a sandbox workload had to live inside one container, leaving no same-host parallel slot for network proxies, multi-container combinations, or separating an agent harness from its tool execution. The public-alpha example first creates a main sandbox running sleep 600, then starts a sidecar named web with python -m http.server 8080; the main sandbox then requests http://web:8080 and prints status code 200.

Sidecars are created through an experimental interface on the sandbox object, with the command, image and name supplied at call time. A sidecar is not a standalone service but an attached container on an already-created sandbox, and the creation call happens after the main sandbox is established. The example calls sb._experimental_sidecars.create with python, -m, http.server, 8080 and name=web after creating the main sandbox, and ends with sb.terminate() to shut down the whole sandbox.

Sidecars are in public alpha, created in the example through an underscore-prefixed experimental interface, and the sidecar service is reached over an internal network on the same host; this scope shows only that same-host sidecar containers can start and be called from the main sandbox.

Sidecars are still labeled public alpha and the creation interface carries an experimental prefix; the material does not state a stability commitment or whether the interface will change.