How do API tests complement a Selenium suite?

Most defects can be caught below the browser. API tests are faster, less flaky and easier to diagnose. Selenium is expensive, so a healthy suite pushes validation down and keeps a smaller set of end-to-end journeys for integration confidence.

String token = given()
    .contentType(ContentType.JSON)
    .body(Map.of("email", "asha@example.com", "password", "correct-horse"))
    .when()
    .post("/api/login")
    .then()
    .statusCode(200)
    .extract()
    .path("token");

given()
    .header("Authorization", "Bearer " + token)
    .contentType(ContentType.JSON)
    .body(Map.of("sku", "SKU-1", "quantity", 2))
    .when()
    .post("/api/cart/items")
    .then()
    .statusCode(201);

• Test validation, permissions and calculations at the API layer
• Create users, orders and fixtures through APIs instead of slow UI setup
• Use Selenium for rendering, navigation, uploads and genuine user flows
• Share authentication state between API and UI layers when the product supports it
• Assert response status, schema and important business fields, not the whole payload
• Keep API tests independent of the UI so a frontend change does not break them

The boundary matters. An API test cannot prove that a button is visible or that a form submits correctly, and a UI test is a poor way to exhaustively test validation rules. Use each layer for what it observes best, and keep a small overlap where the integration risk justifies it.