diff --git a/__tests__/platform/platformGuards.test.ts b/__tests__/platform/platformGuards.test.ts
new file mode 100644
index 00000000..7e63fd07
--- /dev/null
+++ b/__tests__/platform/platformGuards.test.ts
@@ -0,0 +1,57 @@
+import type { PlatformScope } from '../../src/platform'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+
+// The two arms of platform() that runState.test.ts never reaches: the throw
+// when nothing has installed a platform, and the fall-back to the process-wide
+// value when an installed scope has no platform for the current work (an
+// adapter's AsyncLocalStorage outside any run). Each case imports src/platform
+// fresh so that neither the empty slot nor the installed scope leaks into the
+// platform setup.ts gives every other test.
+
+beforeEach(() => vi.resetModules())
+afterEach(() => vi.resetModules())
+
+async function freshPlatformModule() {
+  return import('../../src/platform')
+}
+
+describe('platform()', () => {
+  it('throws, naming both installers, when no platform has been installed', async () => {
+    const { platform } = await freshPlatformModule()
+
+    expect(() => platform()).toThrow(new Error('no platform installed: the entry point must call setPlatform() or runWithPlatform() before dispatching an event'))
+  })
+
+  it('falls back to the process-wide platform when the installed scope has none for the current work', async () => {
+    const { actionsPlatform } = await import('../../src/adapters/actions')
+    const { installPlatformScope, platform, setPlatform } = await freshPlatformModule()
+    const processWide = actionsPlatform()
+    const scoped = actionsPlatform()
+    let inRun = false
+    const scope: PlatformScope = {
+      get: () => (inRun ? scoped : undefined),
+      run: (_value, fn) => {
+        inRun = true
+        try {
+          return fn()
+        }
+        finally {
+          inRun = false
+        }
+      },
+    }
+    installPlatformScope(scope)
+    setPlatform(processWide)
+
+    expect(platform()).toBe(processWide)
+    expect(scope.run(scoped, () => platform())).toBe(scoped)
+    expect(platform()).toBe(processWide)
+  })
+
+  it('still throws under an installed scope when neither it nor the process has a platform', async () => {
+    const { installPlatformScope, platform } = await freshPlatformModule()
+    installPlatformScope({ get: () => undefined, run: (_value, fn) => fn() })
+
+    expect(() => platform()).toThrow('no platform installed')
+  })
+})