diff --git a/frontend/src/lib/shared/api.server.spec.ts b/frontend/src/lib/shared/api.server.spec.ts
new file mode 100644
index 00000000..d53a0675
--- /dev/null
+++ b/frontend/src/lib/shared/api.server.spec.ts
@@ -0,0 +1,20 @@
+import { describe, it, expect } from 'vitest';
+import { extractErrorCode } from './api.server';
+
+describe('extractErrorCode', () => {
+	it('returns the code string when error has a code property', () => {
+		expect(extractErrorCode({ code: 'DOCUMENT_NOT_FOUND' })).toBe('DOCUMENT_NOT_FOUND');
+	});
+	it('returns undefined when error is undefined', () => {
+		expect(extractErrorCode(undefined)).toBeUndefined();
+	});
+	it('returns undefined when error is null', () => {
+		expect(extractErrorCode(null)).toBeUndefined();
+	});
+	it('returns undefined when error is a plain string', () => {
+		expect(extractErrorCode('oops')).toBeUndefined();
+	});
+	it('returns undefined when error object has no code property', () => {
+		expect(extractErrorCode({ message: 'fail' })).toBeUndefined();
+	});
+});