Why You Need to Perform API Testing Along with Functional Testing

If you’ve spent any time in the world of software testing, you’ve probably heard someone say, “Don’t forget to test the APIs!” And if you’re anything like I was early in my QA career, you might have thought, “Sure, but isn’t functional testing enough? I mean, if the app works for the user, what else matters?”

Over the years—and after working on more projects than I can count—I’ve learned just how critical API testing is. In fact, I’d argue that API testing is every bit as important as functional testing, and skipping it is kind of like checking that your car looks good from the outside but never bothering to look under the hood. Everything might seem fine… until you hit the highway.

Let me walk you through why both are essential, and how embracing API testing has helped my teams deliver better, more robust software.

Let’s start with what we all know best: functional testing. This is the bread-and-butter of QA—walking through the application like a user would, clicking buttons, filling out forms, making sure features work as intended. If you’re a tester, you probably know the drill:

• Open the login page, enter credentials, verify you’re taken to the dashboard.
• Try to reset your password, make sure the email shows up.
• Add an item to your cart, check out, get a confirmation.

Functional testing is great because it’s what the end user sees. It’s the layer that gets the most attention during demos and sprint reviews, and it’s where most people start their testing journey. You’re validating that, from the user’s perspective, everything works.

But here’s the thing: modern applications are built on layers. There’s the user interface (UI), which everyone sees, but under the surface, there are services talking to each other, APIs fetching data, business logic firing off, and integrations happening behind the scenes.

That’s where API testing comes in.

An API (Application Programming Interface) is, essentially, how different parts of your application—and often, other applications—communicate. In today’s world of microservices, mobile apps, and cloud platforms, APIs are everywhere.

I’ll never forget the first time I ran into an “invisible bug” that could only be found at the API layer. We had a beautiful new dashboard, and all the UI tests passed. Users could log in, click through their reports, and see the right numbers. But something felt off—some users started reporting that the data wasn’t up-to-date.

Turns out, there was an issue with one of the APIs. It was caching old data under certain conditions, so the UI showed stale information even though the functional flow “worked.” Our functional tests didn’t catch it because they only looked at what appeared on the surface. API testing would have revealed the problem in seconds.

That was a lightbulb moment for me. Ever since, I make sure we never skip API testing—and here’s why you shouldn’t, either.

1. APIs Are the Backbone of Modern Apps

Think about your favorite apps—banking, shopping, social media. The UI is just the tip of the iceberg. When you click a button, it often triggers an API call:

• Fetching your account balance
• Pulling in recent transactions
• Posting a new photo
• Syncing your shopping cart across devices

If the API fails, the app fails, even if the UI looks great. Functional testing alone can’t catch data issues, authorization errors, or edge cases that happen at the integration level. API testing goes straight to the heart of the application.

2. Catching Bugs Early (and Fast)

Here’s something I’ve noticed again and again: API bugs are often cheaper and faster to find early.

Imagine you’re testing a new feature. The developers release the backend APIs before the UI is finished. If you start writing API tests right away, you can catch problems with logic, validation, or performance—before the UI is even built.

On one project, we found a major authentication bug during API testing—weeks before the frontend team even started hooking up their pages. If we’d waited for functional testing, that same bug would have blocked everyone and slowed the whole release down.

API testing lets you be proactive, not just reactive.

3. Better Test Coverage (and More Control)

With functional testing, you’re limited by the UI:

• You can only test flows that have screens
• Some edge cases are tough (or impossible) to trigger via the interface
• UI tests can be slow and flaky, especially as apps grow more complex

API testing is more direct. Want to test what happens if a user submits a 10,000-character username? Easy—just hit the API with that data. Need to simulate a service outage or a specific error response? Mock it at the API level.

This control means you can cover more scenarios—and cover them faster.

4. Decoupling Frontend and Backend

A big lesson from working with distributed teams: the frontend and backend often move at different speeds.

Sometimes the backend is ready before the UI; sometimes, the UI is waiting on APIs. API testing allows both teams to move independently. Frontend testers can use API mocks or stubs. Backend testers can validate business logic directly, without needing the full UI in place.

This separation makes development and testing more efficient, especially in agile and DevOps environments.

5. Security and Compliance

Let’s not forget about security. Many vulnerabilities (like broken authentication, data exposure, or injection attacks) happen at the API layer.

I’ve seen apps with flawless UIs, but their APIs were wide open—allowing users to access data they shouldn’t, or perform actions that should be blocked.

API testing (and related security testing) helps ensure your application is secure, not just pretty. In industries with regulatory requirements, it’s also a must for compliance.

6. Performance and Reliability

Ever heard of “it works on my machine”?

Functional tests can tell you the app “works,” but only API testing can tell you if it works well.

• How does the API handle 1000 simultaneous requests?
• What happens if the database is slow?
• Does the API gracefully handle errors or go down in flames?

API tests are perfect for performance and reliability checks—often long before users ever notice a problem.

So, is API testing a replacement for functional testing? Absolutely not. You need both.

Functional testing validates the user experience—the clicks, the flows, the business rules as seen by the end user.

API testing validates the logic, integration, data, and security that power the app.

The best teams I’ve worked with treat these as complementary. Functional tests are the “what,” API tests are the “how.” By layering both, you catch more bugs, deliver higher-quality software, and have confidence that your app works inside and out.

I’ll end with a quick story. On a recent project, our team launched a new payment feature. We had plenty of UI tests, and everything looked good during demos. But a handful of API tests uncovered a race condition that caused payments to double-charge users under heavy load. It was subtle, and users might never have reported it—but fixing it before launch saved our client from a nightmare of support calls and refunds.

That’s the power of API testing.

If you’re not already performing API testing alongside your functional tests, now’s the time to start. It’s not just “extra work”—it’s a smart investment that pays off in faster releases, fewer bugs, and happier users.

In my experience, the teams that embrace both approaches are the ones who sleep better at night. After all, wouldn’t you rather look under the hood and make sure everything’s running smoothly, instead of hoping for the best?

So next time you write a test plan, ask yourself:

“Did I check the APIs too?”

Your future self—and your users—will thank you