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

# Dependency direction

> What foundation and support may depend on.

`packages/fapost-foundation` and `packages/fapost-support` **do not depend on
Core**. The arrow points one way, always.

## Not allowed

* `use App\…` inside `foundation` or `support`
* References from either package to concrete Core domain classes
* Moving Core business logic into `support`

## Why it matters

Extensions are built against `foundation`. If `foundation` referenced `App\…`,
then installing an extension would pull Core's internals into its dependency
graph, and every Core refactor would become a breaking change for every extension.

The one-way arrow is what buys Core the freedom to reorganise its own domains
without coordinating with anyone. That freedom is the whole reason the package
exists.

`support` is stricter still: it holds primitives with no domain semantics at all.
The moment Core logic moves there "to share it", it stops being a primitive and
starts being Core in a package that Core cannot see.

## Deciding where something goes

| The code is…                                       | It belongs in        |
| -------------------------------------------------- | -------------------- |
| A contract needed by external Solutions or Plugins | `foundation`         |
| A pure reusable primitive with no Core dependency  | `support`            |
| Used within one domain and carrying its meaning    | Core, in that domain |

The middle row is the one most often got wrong. "Several domains use it" is not
sufficient — the question is whether it carries domain meaning. A date helper is a
primitive. A "resolve the assistant for this contact" helper is Core logic no
matter how many domains call it.

## How it is checked

PHPat rules, run through PHPStan:

```bash theme={"theme":"one-dark-pro"}
composer run test:arch
```

The rules live in `tests/Architecture`. The default PHPUnit run covers them
through `tests/Unit/Architecture/MigrationTest.php`.

## If the contract you need is missing

Adding to `foundation` is a deliberate act — it is a public promise, and it is
much harder to withdraw than to make. That is the right conversation to have, and
it is still cheaper than an extension that reaches into `App\…` and breaks
silently later. See [Stability policy](/extending/stability-policy).