diff --git a/packages/scheduler/test/compile-in-system.integration.test.ts b/packages/scheduler/test/compile-in-system.integration.test.ts
new file mode 100644
index 0000000..a717d07
--- /dev/null
+++ b/packages/scheduler/test/compile-in-system.integration.test.ts
@@ -0,0 +1,133 @@
+// compile() inside a system, driven by the scheduler. `query.compile` is a call-ONCE API (it codegens
+// per-archetype runners), but a SystemDef has only a per-frame `run` and no init hook — so the idiomatic
+// pattern is to lazily build the runner on first frame and cache it in the system's closure. This locks in
+// that pattern end-to-end: the cached compiled runner integrates correctly across frames under
+// scheduler.update(), matches an equivalent `.each` system byte-for-byte, and a `.changed()` consumer sees
+// the compiled writes (compile preserves reactivity where bindColumns/eachChunk would not).
+
+import { describe, expect, test } from 'vitest'
+import { createWorld, defineComponent, read, write } from '@ecsia/core'
+import type { EntityHandle } from '@ecsia/core'
+import { createScheduler, defineSystem } from '@ecsia/scheduler'
+
+type CompileQuery = {
+  compile<Ctx>(body: (e: { position: { x: number; y: number }; velocity: { dx: number; dy: number } }, ctx: Ctx) => void): (
+    ctx: Ctx,
+  ) => void
+}
+
+function kit(n: number) {
+  const Position = defineComponent({ x: 'f32', y: 'f32' }, { name: 'position' })
+  const Velocity = defineComponent({ dx: 'f32', dy: 'f32' }, { name: 'velocity' })
+  const world = createWorld({ components: [Position, Velocity], maxEntities: 1 << 14 })
+  const handles: EntityHandle[] = []
+  for (let i = 0; i < n; i++) {
+    const h = world.spawnWith(Position, Velocity)
+    const v = world.entity(h).write(Velocity) as { dx: number; dy: number }
+    v.dx = i + 1
+    v.dy = -(i + 1)
+    handles.push(h)
+  }
+  return { world, Position, Velocity, handles }
+}
+
+describe('compile() inside a scheduler-driven system', () => {
+  test('a lazily-cached compiled runner integrates correctly across frames', () => {
+    const N = 40
+    const { world, Position, Velocity, handles } = kit(N)
+    let run: ((ctx: { dt: number }) => void) | null = null
+    let builds = 0
+    const Movement = defineSystem({
+      name: 'Movement',
+      read: [Velocity],
+      write: [Position],
+      run({ query, dt }) {
+        // Build once, reuse every frame — the call-once API in a per-frame body.
+        if (run === null) {
+          builds++
+          run = (query(read(Velocity), write(Position)) as unknown as CompileQuery).compile<{ dt: number }>((e, ctx) => {
+            e.position.x += e.velocity.dx * ctx.dt
+            e.position.y += e.velocity.dy * ctx.dt
+          })
+        }
+        run({ dt })
+      },
+    })
+    const scheduler = createScheduler(world, [Movement])
+    const FRAMES = 5
+    for (let f = 0; f < FRAMES; f++) scheduler.update(1)
+
+    expect(builds).toBe(1) // built exactly once, not per frame
+    for (let i = 0; i < N; i++) {
+      const p = world.entity(handles[i] as EntityHandle).read(Position) as { x: number; y: number }
+      expect(p.x).toBe((i + 1) * FRAMES)
+      expect(p.y).toBe(-(i + 1) * FRAMES)
+    }
+  })
+
+  test('matches an equivalent .each system byte-for-byte', () => {
+    const N = 33
+    const buildWorld = (compiled: boolean) => {
+      const { world, Position, Velocity, handles } = kit(N)
+      let run: ((ctx: { dt: number }) => void) | null = null
+      const sys = defineSystem({
+        name: 'Move',
+        read: [Velocity],
+        write: [Position],
+        run({ query, dt }) {
+          const q = query(read(Velocity), write(Position))
+          if (compiled) {
+            run ??= (q as unknown as CompileQuery).compile<{ dt: number }>((e, ctx) => {
+              e.position.x += e.velocity.dx * ctx.dt
+              e.position.y += e.velocity.dy * ctx.dt
+            })
+            run({ dt })
+          } else {
+            q.each((e) => {
+              const el = e as unknown as { position: { x: number; y: number }; velocity: { dx: number; dy: number } }
+              el.position.x += el.velocity.dx * dt
+              el.position.y += el.velocity.dy * dt
+            })
+          }
+        },
+      })
+      const scheduler = createScheduler(world, [sys])
+      for (let f = 0; f < 7; f++) scheduler.update(1 / 60)
+      // Extract plain values per-resolve — the pooled accessor view is re-pointed on the next resolve.
+      return handles.map((h) => {
+        const p = world.entity(h).read(Position) as { x: number; y: number }
+        return { x: p.x, y: p.y }
+      })
+    }
+    expect(buildWorld(true)).toEqual(buildWorld(false))
+  })
+
+  test('a .changed(Position) consumer sees the compiled writes each frame', () => {
+    const N = 16
+    const { world, Position, Velocity } = kit(N)
+    let run: ((ctx: { dt: number }) => void) | null = null
+    let lastChanged = -1
+    const changed = world.query(read(Position)).changed(Position) as unknown as {
+      eachChanged(fn: () => void): void
+    }
+    const Movement = defineSystem({
+      name: 'Movement',
+      read: [Velocity],
+      write: [Position],
+      run({ query, dt }) {
+        run ??= (query(read(Velocity), write(Position)) as unknown as CompileQuery).compile<{ dt: number }>((e, ctx) => {
+          e.position.x += e.velocity.dx * ctx.dt
+        })
+        run({ dt })
+      },
+    })
+    const scheduler = createScheduler(world, [Movement])
+    for (let f = 0; f < 3; f++) {
+      scheduler.update(1)
+      let n = 0
+      changed.eachChanged(() => n++)
+      lastChanged = n
+    }
+    expect(lastChanged).toBe(N) // every entity's Position write is observed
+  })
+})
diff --git a/website/guide/performance.md b/website/guide/performance.md
index 8419de7..fa63ada 100644
--- a/website/guide/performance.md
+++ b/website/guide/performance.md
@@ -206,6 +206,34 @@ Two things set it apart from `bindColumns`:
 Call `compile` once and reuse the returned `run` every frame, the same as `bindColumns`. Structural
 changes during `run()` follow the same collect-first, mutate-after rule as every other loop.
 
+### Inside a system
+
+A `defineSystem` has only a per-frame `run` — no separate setup step — so build the runner on the first
+frame and cache it in the system's closure (the same pattern applies to `bindColumns`). The query you
+need is the one `run` receives, so the lazy build is the natural place for it:
+
+```ts no-check
+let move: ((ctx: { dt: number }) => void) | null = null
+
+const Movement = defineSystem({
+  name: 'Movement',
+  read: [Velocity],
+  write: [Position],
+  run({ query, dt }) {
+    move ??= query(read(Velocity), write(Position)).compile<{ dt: number }>((e, ctx) => {
+      e.position.x += e.velocity.dx * ctx.dt
+      e.position.y += e.velocity.dy * ctx.dt
+    })
+    move({ dt })
+  },
+})
+```
+
+The runner is built once, on the first frame, then reused — and because `compile` preserves the write
+log, a `.changed()`/observer system later in the schedule still sees the writes. (A worker-eligible
+system runs its separately-authored kernel on worker threads; `compile` is a main-thread `run`-body
+tool, exactly like `each` and `bindColumns`.)
+
 ## Reproduce
 
 ```bash