How to approach APIs for exploratory Testing?

Top API Testing Tools for 2025

1. Postman
2. Bruno
3. Insomnia
4. Swagger

Why API's Exploratory Testing is required?

Early adoption and detection are one of the most used concepts for Agile Software Development Lifecycle. Hence APIs exploration and testing gives us the testers part of that cycle. This can help in early detection of bugs and reduce the overall cost of fixes required in case of major bug impacts.

Before diving deep into how and what should we explore in an API. Let's first understand what's an API?

An API is an interface by which an application can use the product programmatically. Now this integration depends upon various factors like environments, code compatibility, request & responses etc.

Who is the end-user for APIs?

Usually the developers, but even if its not then the product team should be able to understand the use cases easily by following the documentation.

What is Exploratory Testing?

It is a process of discovery. We tend to use the software and explore it how it works, performs and functions. This exploration does not depend on GUI and can conducted on APIs or any other form of applications like IOTs.

Exploring an API at early stages using programming/non-programming (automation/manual) approaches can yield impressive findings.

How to conduct Exploratory Testing?

1) Initial Steps

• Explore the API documentation
• Use the API and find out how it works (the happy path)
• Use tools like BurpSuite/Fiddler/ProxyMan to play with APIs request body and hit the endpoints.

2) Finding how it works?

• What happens when different inputs are provided, what's the output?
• When does exception occurs?
• What are the error codes and messages received
• What happens if a API call fails due to poor network?

3) Dependency

• If your APIs work using different environments then exploring it with those can provide useful results.
• Remove one of the dependencies and check how the API responds

4) Value Generation

• Why does this API exists?
• Does it serve any purpose, which needs to be a separate program interface?

5) Updates

Software applications are bounded to be maintained and updated with time. If not then those applications are dead.

So usually APIs version are updated:

• Does the latest API version work seamlessly with older versions?
• What changes are required to incorporate the newer versions by the customers?
• What if the APIs are removed -> How would that affect the functionality?

6) Explore the documentation

Many tools in the market like Postman, Swagger etc. generate the documentations for the APIs once setup is done. Although these documentation are good for initial setup but they do not provide much knowledge about the system.

Hence we should improve it but trying our explorations, testing the functionality and knowing the customer expectations out the product (in this case its an API).

• Finding client expectations
• Documenting what and why?
• Clean up the auto-generated documents using above.

7) Repetition leads to finding patterns

Repeating similar steps again and again helps to analyse the pattern.

These patterns help to understand the behaviour. Once we are aware then move on and explore other parts.

8) Automation

Automate only those tests which are necessary for every iteration/release and leave those which are not redundant. The key expect in mind should always be how you are going to maintain these tests before automating.

Summary

• Look for documentation
• Understand the purpose of product (API)
• Understand the endpoints, request methods, response codes/messages/body
• Understand how system responds if network failure occurs
• Understand how system responds when unexpected failures like system reboot happens
• Understand the key expectations and functionalities for product (API)
• Identify potential risks and vulnerabilities (security threats) using varying inputs, chained requests, concurrent requests, error handling etc.
• Observe the responses, console logs
• Document what's important for customers

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