# Cooperation

**Plugins never call each other.** They meet through state the engine owns.

That is what makes them removable: if hunger called into rest directly, deleting rest would
break hunger. Instead they meet in three places, none of which require either to exist.

## 1. The field bag is flat

Any plugin can read any declared field:

```js
$.field('satiety')
```

No namespacing, no imports. Which means you must **ask before you rely**:

```js
$.hasPlugin('hunger') && $.field('satiety') < 20
```

Without that guard, a game with no hunger plugin reads `null` and your rule behaves as
though everyone is permanently starving — or never hungry, depending which way your
comparison points. See [Missing values](/docs/missing-values).

## 2. Engine events

One plugin's `$.damage()` fires `onDamage`, which another plugin's hook catches. Neither
knows the other exists; they both know the engine.

## 3. Declared events — offering a say

An event is what your plugin offers *other* plugins a veto over.

The rest plugin publishes `rest:sleep`. Hunger answers it:

```js
// hook on rest:sleep
if ($.field('satiety') <= 0) {
  $.veto("they are too hollow with hunger to settle")
}
```

**Hooking an event no installed plugin declares is inert, not broken.** So a rule that
answers an optional system simply does nothing when that system is absent — which is
exactly the behaviour that lets both plugins ship independently.

Use [`veto`](/docs/steps), not `refuse`, when answering an event. `refuse` stops the
player's *action*; using it here tells a player standing still that hunger clawed them out
of a sleep they never started.

## The test

**Delete either side of any pairing and the other still works.**

Hunger without rest: still drains, still starves, never vetoes a sleep because nothing
proposes one. Rest without hunger: still restores, never gets vetoed. Both coherent.

If deleting one side leaves the other broken or nonsensical, they are not cooperating —
one is depending on the other, and you have built one plugin in two pieces.

## Next

[Formulas and dials](/docs/formulas) — sharing tuning numbers correctly.