diff --git a/packages/core/src/architecture.test.ts b/packages/core/src/architecture.test.ts
new file mode 100644
index 0000000000..7a1b5e0199
--- /dev/null
+++ b/packages/core/src/architecture.test.ts
@@ -0,0 +1,52 @@
+import { describe, expect, test } from 'bun:test'
+import { readdirSync, readFileSync } from 'node:fs'
+import { join, relative, resolve } from 'node:path'
+
+/**
+ * Layer rule (AGENTS.md): core is pure logic — no Three.js, no rendering.
+ * A runtime `three`/`@react-three/*` import in core evaluates R3F (and thus
+ * React client context) in every consumer of the barrel, which crashes
+ * Next.js route handlers under the RSC server condition (capture uploads
+ * 500'd this way once). Type-only imports are erased at build and allowed.
+ */
+const SRC = resolve(import.meta.dir)
+
+function sourceFiles(dir: string): string[] {
+  return readdirSync(dir, { withFileTypes: true }).flatMap((entry) => {
+    const full = join(dir, entry.name)
+    if (entry.isDirectory()) return sourceFiles(full)
+    if (/\.test\.tsx?$/.test(entry.name)) return []
+    return /\.tsx?$/.test(entry.name) ? [full] : []
+  })
+}
+
+const BANNED_SPEC = String.raw`(?:three(?:\/[^'"]*)?|@react-three\/[^'"]*)`
+// `import`/`export … from 'three…'` — group 1 captures a whole-clause `type`
+// qualifier, the only form guaranteed to be erased by the compiler.
+const FROM_RE = new RegExp(
+  String.raw`(?:import|export)\s+(type\s)?[\w*{}\s,$]*?from\s*['"]${BANNED_SPEC}['"]`,
+  'g',
+)
+// Bare side-effect form: `import 'three…'` — always a runtime import.
+const SIDE_EFFECT_RE = new RegExp(String.raw`import\s*['"]${BANNED_SPEC}['"]`, 'g')
+
+describe('architecture', () => {
+  test('core has no runtime three/@react-three imports', () => {
+    const files = sourceFiles(SRC)
+    const offenders: string[] = []
+
+    for (const file of files) {
+      const src = readFileSync(file, 'utf8')
+      for (const match of src.matchAll(FROM_RE)) {
+        if (!match[1]) offenders.push(`${relative(SRC, file)}: ${match[0].replaceAll('\n', ' ')}`)
+      }
+      for (const match of src.matchAll(SIDE_EFFECT_RE)) {
+        offenders.push(`${relative(SRC, file)}: ${match[0]}`)
+      }
+    }
+
+    expect(offenders).toEqual([])
+    // Guard against the walk passing vacuously.
+    expect(files.length).toBeGreaterThan(100)
+  })
+})
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index 72ff0972fa..4839bd50dc 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -281,6 +281,10 @@ export type {
   FloorPlacedFootprintsResolver,
 } from './registry'
 export * from './registry'
+// Exported here rather than from the registry barrel: that barrel is
+// reachable from server-safe graphs (schema → spatial grid → registry)
+// and must stay free of React imports.
+export { useRegistryVersion } from './registry/use-registry-version'
 export * from './schema'
 export * from './services'
 export { isMovable, movePlanToward, moveToward, resolveMovable } from './services/movement'
@@ -361,7 +365,6 @@ export {
   stepElevatorRuntimeState,
   stepElevatorRuntimes,
 } from './systems/elevator/elevator-runtime'
-export { ElevatorRuntimeSystem } from './systems/elevator/elevator-runtime-system'
 export {
   type ElevatorLevelEntry,
   resolveElevatorBuildingLevels,
diff --git a/packages/core/src/registry/index.ts b/packages/core/src/registry/index.ts
index da45fac0a6..83a1a7305c 100644
--- a/packages/core/src/registry/index.ts
+++ b/packages/core/src/registry/index.ts
@@ -162,4 +162,3 @@ export type {
   ToolHintChip,
   Vec2,
 } from './types'
-export { useRegistryVersion } from './use-registry-version'
diff --git a/packages/core/src/systems/elevator/elevator-runtime-system.tsx b/packages/core/src/systems/elevator/elevator-runtime-system.tsx
deleted file mode 100644
index 91c897a23a..0000000000
--- a/packages/core/src/systems/elevator/elevator-runtime-system.tsx
+++ /dev/null
@@ -1,10 +0,0 @@
-import { useFrame } from '@react-three/fiber'
-import { stepElevatorRuntimes } from './elevator-runtime'
-
-export function ElevatorRuntimeSystem() {
-  useFrame(({ clock }, delta) => {
-    stepElevatorRuntimes(clock.getElapsedTime() * 1000, delta)
-  }, 2)
-
-  return null
-}
diff --git a/packages/nodes/src/elevator/system.tsx b/packages/nodes/src/elevator/system.tsx
index 1a799b2514..e45853b982 100644
--- a/packages/nodes/src/elevator/system.tsx
+++ b/packages/nodes/src/elevator/system.tsx
@@ -1,7 +1,18 @@
 'use client'
 
-import { ElevatorOpeningSystem, ElevatorRuntimeSystem } from '@pascal-app/core'
+import { ElevatorOpeningSystem, stepElevatorRuntimes } from '@pascal-app/core'
 import { ElevatorInteractionSystem } from '@pascal-app/viewer'
+import { useFrame } from '@react-three/fiber'
+
+/** Cab travel + door state machine, stepped once per frame. Lives here rather
+ * than in core so the core barrel stays free of runtime R3F imports. */
+function ElevatorRuntimeSystem() {
+  useFrame(({ clock }, delta) => {
+    stepElevatorRuntimes(clock.getElapsedTime() * 1000, delta)
+  }, 2)
+
+  return null
+}
 
 /**
  * Composite system for elevator — bundles three per-frame systems: