diff --git a/client/engine/spin-lock.d.ts b/client/engine/spin-lock.d.ts
new file mode 100644
index 0000000..aaecf0d
--- /dev/null
+++ b/client/engine/spin-lock.d.ts
@@ -0,0 +1,29 @@
+// Type declarations for the plain-JS spin lock (spin-lock.js). The browser
+// loads the JS as an IIFE; the Vitest suite imports its CommonJS export, so
+// this declaration gives the typecheck the shape of that export.
+
+export type SpinPhase = "idle" | "starting" | "animating";
+
+export const PHASE: {
+  readonly IDLE: "idle";
+  readonly STARTING: "starting";
+  readonly ANIMATING: "animating";
+};
+
+export interface SpinLock {
+  readonly PHASE: typeof PHASE;
+  /** Current lifecycle phase. */
+  readonly phase: SpinPhase;
+  /** True while a spin occupies any non-idle phase. */
+  isLocked(): boolean;
+  /** True only once the result is committed and the reels are animating. */
+  isAnimating(): boolean;
+  /** Attempt to begin a spin; true if this caller now owns it. */
+  tryAcquire(): boolean;
+  /** Promote the in-flight spin to the visible animation phase. */
+  markAnimating(): void;
+  /** Release the lock. Safe from any phase. */
+  release(): void;
+}
+
+export function createSpinLock(): SpinLock;