The outer batch passes a command.batch step on as written, so the inner
steps' `$step` references point at the inner batch's steps. The docs say
that `$step` is a reserved key in step params.
Inside command.batch, `{"$step": n}` anywhere in a step's params is
replaced by the `id` in step n's result before the step runs
(`{"$step": n, "field": "..."}` picks another field). A nested tree,
such as a frame with children, now takes one batch and one undo step,
not one round-trip per level with ids copied between calls by hand.
Malformed references (forward or self, negative or fractional index,
missing or non-string field, extra keys) fail the batch with BadParams
naming the step, and the existing group machinery rolls it back. The
params walk is depth-capped. Steps are journalled with resolved params.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
bflatastic