Creating sbx kits

A kit is a spec.yaml, and it decides what a sandbox is configured and permitted to do. What is installed belongs in an image — reach for creating-sbx-templates when the answer is a Dockerfile.

Kits stack at run time, because --kit repeats. That turns N agents × M task types into N bases plus M mixins, so write a mixin unless you are defining a whole agent. Of 198 published specs, 145 are kind: mixin.

Start from a skeleton

assets/mixin-spec.yaml is a commented v2 mixin. assets/sandbox-spec.yaml defines a whole agent. Copy one and strip what you do not need.