diff --git a/tests/setup.ts b/tests/setup.ts
new file mode 100644
index 0000000..aa1ebdc
--- /dev/null
+++ b/tests/setup.ts
@@ -0,0 +1,24 @@
+/**
+ * Test setup — polyfill window for Obsidian plugin compatibility.
+ * Obsidian plugins use window.setTimeout/window.clearTimeout,
+ * but tests run in Node.js where window doesn't exist.
+ */
+
+if (typeof globalThis.window === 'undefined') {
+  // Create a proxy that always delegates to globalThis
+  // This ensures vi.useFakeTimers() works correctly
+  (globalThis as any).window = new Proxy(globalThis, {
+    get(target, prop) {
+      return (target as any)[prop];
+    }
+  });
+}
+
+if (typeof globalThis.activeDocument === 'undefined') {
+  (globalThis as any).activeDocument = {
+    querySelectorAll: () => [],
+    createElement: (tag: string) => ({ id: '', textContent: '', style: {} }),
+    head: { appendChild: () => {} },
+    getElementById: () => null,
+  };
+}
diff --git a/vitest.config.ts b/vitest.config.ts
index 7832f4e..afb9cef 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -5,6 +5,7 @@ export default defineConfig({
   test: {
     globals: true,
     environment: "node",
+    setupFiles: ["tests/setup.ts"],
     include: [
       "tests/**/*.test.ts",
       "tests/**/*.property.test.ts",