diff --git a/src/lib/shops.ts b/src/lib/shops.ts
new file mode 100644
index 0000000..4ac2070
--- /dev/null
+++ b/src/lib/shops.ts
@@ -0,0 +1,17 @@
+export type Shop = 'amazon' | 'idealo' | 'geizhals'
+
+const PATTERNS: Array<{ shop: Shop; hostMatch: RegExp }> = [
+  { shop: 'amazon', hostMatch: /(^|\.)amazon\.(de|com|co\.uk|fr|it|es|nl)$/i },
+  { shop: 'idealo', hostMatch: /(^|\.)idealo\.(de|com|at|fr|it|es|co\.uk)$/i },
+  { shop: 'geizhals', hostMatch: /(^|\.)geizhals\.(de|at|eu)$/i },
+]
+
+export function detectShop(input: string): Shop | null {
+  let host: string
+  try {
+    host = new URL(input).hostname
+  } catch {
+    return null
+  }
+  return PATTERNS.find((p) => p.hostMatch.test(host))?.shop ?? null
+}
diff --git a/tests/shops.test.ts b/tests/shops.test.ts
new file mode 100644
index 0000000..c4aea17
--- /dev/null
+++ b/tests/shops.test.ts
@@ -0,0 +1,22 @@
+import { describe, it, expect } from 'vitest'
+import { detectShop } from '@/lib/shops'
+
+describe('detectShop', () => {
+  it.each([
+    ['https://www.amazon.de/dp/B0C5BMMJTL', 'amazon'],
+    ['https://amazon.com/gp/product/B0C5BMMJTL', 'amazon'],
+    ['https://www.idealo.de/preisvergleich/OffersOfProduct/123456_-foo.html', 'idealo'],
+    ['https://geizhals.de/sony-playstation-5-a2362829.html', 'geizhals'],
+    ['https://www.geizhals.eu/foo', 'geizhals'],
+  ])('detects %s as %s', (url, expected) => {
+    expect(detectShop(url)).toBe(expected)
+  })
+
+  it('returns null for unknown shop', () => {
+    expect(detectShop('https://example.com/foo')).toBeNull()
+  })
+
+  it('returns null for invalid url', () => {
+    expect(detectShop('not a url')).toBeNull()
+  })
+})
diff --git a/vitest.config.ts b/vitest.config.ts
new file mode 100644
index 0000000..c967e0c
--- /dev/null
+++ b/vitest.config.ts
@@ -0,0 +1,10 @@
+import { defineConfig } from 'vitest/config'
+import path from 'node:path'
+
+export default defineConfig({
+  test: {
+    environment: 'node',
+    include: ['tests/**/*.test.ts'],
+  },
+  resolve: { alias: { '@': path.resolve(__dirname, './src') } },
+})