Custom templates are the documented way to carry user-level configuration into a
sandbox, but sbx v0.37.x silently drops every layer stacked on the base image
(docker/sbx-releases#366), so nothing baked into an image arrives. This installs
the same material into a stock sandbox after creation instead.
setup copies an allowlist of ~/.claude into the sandbox, imports skills into the
shared store, and adds each known marketplace before installing every enabled
plugin. Enablement survives here precisely because it happens after creation:
claude plugin install writes enabledPlugins itself, whereas sbx recreates
settings.json when the sandbox is created.
Tools come from mise, copied from the host because mise.jdx.dev is outside the
default network policy. Tools resolve through shims rather than mise activate,
which only fires for interactive shells and would leave the agent silently using
system versions.
sbx exec drains stdin, which truncated both install loops to their first entry,
and tab is an IFS whitespace character, which collapsed the empty repo field and
shifted the URL into it for git-sourced marketplaces. Both are handled.
mroberts
1fdbbff2e2