> ## Documentation Index
> Fetch the complete documentation index at: https://developer.ubiquex.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Cross-Stack References

> A pinned import against a neighbor's ledger, never a live pointer.

A stack sometimes needs to reference something another stack owns — a
VPC ID, a shared subnet. `ubx` calls this a cross-stack reference, and
the defining choice behind it is what it explicitly is *not*: it is
never a live pointer into a neighbor's current state. It's a pin,
resolved once, against the neighbor's [ledger](/concepts/ledger) at a
specific point.

## How it resolves

A `$cross` marker names `{stack, ledger_dir, path}` — an explicit path to
the neighbor stack's own ledger directory, the same convention
`--ledger-dir` already uses locally. The [resolver](/concepts/resolver)
opens that ledger, folds its current state, and reads the referenced
value the same way it would resolve an intra-stack reference to an
already-applied resource.

The critical part: `pinned_head` — the neighbor ledger's `Head()` at
resolve time — gets recorded into the resolved delta. This is what turns
a cross-stack reference into something [staleness](/concepts/staleness)
can actually detect. If the neighbor's head has advanced by the time this
proposal is up for acceptance, re-verifying `pinned_head` against the
neighbor's current head catches it — the identical "resolved-time truth
vs. accept-time reality" check already used for live cloud state, just
one level up, applied to a ledger instead of a resource.

## Almost always concrete, honestly not always

A cross-stack reference resolves to a concrete value almost every time.
The one real exception: if the neighbor's own folded state at that exact
path is itself a `$computed` marker — the neighbor has its own accepted
but not-yet-shipped change whose create used `$computed` there — the
marker propagates forward into this stack's resolved delta unresolved.
This is deliberate honesty, not a bug: a resolver that pretended to know
a value it doesn't have would break the trust chain's own first
invariant.

Full detail, including the `stack`-by-name alternative to an explicit
`ledger_dir` path: [`docs/resolver.md`](https://github.com/Ubiquex/ubiquex/blob/main/docs/resolver.md)'s
"Cross-stack refs" section in `ubiquex`.