Have you ever seen that scenario where the team leaves everything to test at the end and, when they finally run the complete system, critical bugs that no one expected show up? Even worse, bugs that go straight to production because the tests didn’t cover the right spots.
The dilemma is common: test only the APIs and trust that the interface will work? Or invest heavily in E2E tests and accept a pipeline that takes hours? The answer isn’t choosing a side. It’s understanding where each type of test fits and how to combine them intelligently.
Let’s break this down in a practical way in this blog.
What are API tests?
API tests validate the communication layer between systems. They verify if endpoints are responding correctly, if payloads are in the expected format, if status codes make sense, and if the business rules implemented in the backend work as they should.
The big advantage here is speed. API tests run fast because they don’t depend on graphical interfaces, browsers, or rendering visual elements. They’re also independent, meaning you can validate the system’s logic without needing the screen to be ready.
A practical example: imagine you need to test user registration. With an API test, you send a POST to the /users endpoint, pass the necessary data in the body, and validate if the response returns status 201 with the created user’s ID. Simple, straightforward, and in milliseconds.
What are E2E (end-to-end) tests?
End-to-End tests simulate the behavior of a real user navigating through the system. They cover the complete flow: interface + backend + integrations + database. Everything together.
They’re slower and more complex to maintain because they involve many parts. An E2E test needs to open the browser, load the page, fill out forms, click buttons, wait for responses, and validate if what appears on screen is correct.
Practical example: a complete purchase process in an e-commerce. The test starts on the homepage, searches for a product, adds it to cart, fills in payment details, and completes the purchase. Only then does it validate if the order was registered and if the user received confirmation.
When to use API tests
Use API tests when you need to validate isolated business rules. They’re ideal for verifying contracts between microservices, ensuring integrations work correctly, and testing scenarios with multiple data variations.
It’s also recommended when you’re developing a new feature and the interface doesn’t even exist yet. Testing the API first allows you to validate the logic before building the screen, speeding up feedback and avoiding rework.
A study published in the journal Electronics (2024) showed that API tests achieved 91.98% endpoint coverage in microservice systems, while E2E tests reached 45.42%. This makes it clear that APIs cover more ground with less effort.
TestBooster.ai is an ally. The platform uses AI to automate the execution of API tests, identifying which endpoints need to be tested and generating test cases intelligently. You save time on writing code and reduce manual maintenance work.
When to use E2E tests
Reserve E2E tests for critical business flows. Those that, if they break, directly affect revenue or user experience. Some examples are checkout in e-commerce, login and onboarding in SaaS, transfers in financial systems.
E2E tests are also essential when you need to validate visual aspects and interface behavior. Things that an API alone can’t capture: if a button is visible, if an error message appears in the right place, if the layout responds correctly on different devices.
Use E2E for regression testing on complex features, ensuring that code changes don’t break functionalities that were already working.
With TestBooster.ai, you don’t need to write endless manual scripts. The platform’s AI automates E2E tests, creating and maintaining test cases based on real user behavior. This drastically reduces time spent maintaining broken tests.
The testing pyramid, in practice
The testing pyramid is a classic concept that remains relevant. IBM research shows that bugs found in production cost 100 times more to fix than bugs found during development. The pyramid helps avoid this scenario.
At the base, you have many API tests (fast, cheap, and reliable). In the middle, integration tests that validate how different parts of the system communicate. At the top, a few strategic E2E tests focused on critical journeys.
Why does this work? Because it balances coverage with speed. Unit tests cost pennies to run and capture 70% of bugs, while integration tests cost a few dollars and catch another 20%. E2E tests cost hundreds of dollars and capture the final 10%.
If you flip the pyramid and focus only on E2E, your pipeline becomes slow, tests become unstable (flaky), and maintenance turns into an expensive nightmare.
Common mistakes during the testing process
- Only doing E2E: the pipeline takes hours to run, tests break with any visual change, and the team spends more time fixing tests than developing features.
- Only doing API: interface bugs slip through. According to GitLab’s 2023 Global DevSecOps report, 71% of security professionals state that at least a quarter of all security vulnerabilities are discovered by developers during coding. So, testing at lower layers is fundamental.
- Testing everything manually: overloaded team, delayed deliveries, and zero confidence that what worked yesterday still works today.
- Not documenting what each test covers: no one knows what’s being tested, duplicate tests appear, and coverage gaps go unnoticed.
How to combine both types
The strategy is simple: use API for logic and rules, use E2E for user journeys.
For example, in a hotel reservation system, you test via API if the price calculation correctly considers discounts, fees, and availability. Then, you use an E2E test to ensure the user can search for a hotel, select dates, add extras, and complete the reservation seeing the final price on screen.
Prioritize like this: start testing the base (APIs and integrations). Then, choose the 3 to 5 most critical flows in the system and cover them with E2E.
TestBooster.ai: your right tool for testing
TestBooster.ai was designed to solve this: automate both API and E2E tests with artificial intelligence, without you having to choose between speed and coverage.
The platform automates what needs to be tested and keeps everything updated as the system evolves. It integrates directly with CI/CD pipelines, allowing you to run tests with every commit without delaying deliveries.
This means less time configuring tests, fewer manual scripts breaking, and more confidence that your code is working at all layers.
Get to know TestBooster.ai to see in practice how intelligent automation can transform your testing strategy.
