diff --git a/frontend/src/__tests__/payment.spec.ts b/frontend/src/__tests__/payment.spec.ts
new file mode 100644
index 0000000..fd1cd25
--- /dev/null
+++ b/frontend/src/__tests__/payment.spec.ts
@@ -0,0 +1,52 @@
+import { describe, it, expect } from 'vitest'
+import { buildSwishPaymentUrl } from '@/api/payment'
+
+describe('buildSwishPaymentUrl', () => {
+  it('normalises Swedish national format to international', () => {
+    expect(buildSwishPaymentUrl('0701234567', 49, 'test')).toContain(
+      'sw=46701234567',
+    )
+  })
+
+  it('strips a leading + from international format', () => {
+    const url = buildSwishPaymentUrl('+46701234567', 49, 'test')
+    expect(url).toContain('sw=46701234567')
+    expect(url).not.toContain('sw=%2B')
+    expect(url).not.toContain('sw=+')
+  })
+
+  it('leaves already-international numbers unchanged', () => {
+    expect(buildSwishPaymentUrl('46701234567', 49, 'test')).toContain(
+      'sw=46701234567',
+    )
+  })
+
+  it('leaves Swish Business numbers (123…) unchanged', () => {
+    expect(buildSwishPaymentUrl('1234567890', 49, 'test')).toContain(
+      'sw=1234567890',
+    )
+  })
+
+  it('strips whitespace from the number', () => {
+    expect(buildSwishPaymentUrl('070 123 45 67', 49, 'test')).toContain(
+      'sw=46701234567',
+    )
+  })
+
+  it('includes the amount with two decimal places in amt', () => {
+    expect(buildSwishPaymentUrl('0701234567', 49, 'test')).toContain(
+      'amt=49.00',
+    )
+  })
+
+  it('URL-encodes the message in the msg parameter', () => {
+    const url = buildSwishPaymentUrl('0701234567', 49, 'ABC 123')
+    expect(url).toContain('msg=ABC+123')
+  })
+
+  it('uses the correct Swish C2B base URL', () => {
+    expect(buildSwishPaymentUrl('0701234567', 49, 'test')).toContain(
+      'https://app.swish.nu/1/p/sw/?',
+    )
+  })
+})