Software engineering has always relied heavily on unit testing, and as agentic development gains popularity, it is becoming more and more important to depend on well-structured tests to guarantee system accuracy.
n The pace of implementation has grown dramatically as development teams begin integrating technologies such as Copilot, Claude, and other AI-based coding assistants into their processes. Functions that previously needed detailed preparation and execution may now be created in a matter of seconds. Nevertheless, this productivity boost introduces a new problem: manually reviewing each line of generated code is no longer feasible.
Unit testing has proven significantly more reliable than closely examining the produced implementations in our experience. As we began using AI-assisted development daily, we came to understand that the organization of our unit tests was equally as important as the tests themselves.
n It was challenging to verify if all behavioral scenarios had been addressed when assessments were not well structured. However, it became much simpler to confidently rework old logic and evaluate AI-generated code when tests were organized appropriately. To make unit tests more comprehensible and reliable throughout development, we eventually settled on a straightforward, consistent methodology. The method is explained in this article.
Why AI-Generated Code Needs a Unit Test Structure
Particularly good at producing small and medium-sized functions that appear proper at first glance are AI coding tools. However, thorough validation is still needed to make sure that all execution pathways are handled appropriately. n In reality, we discovered that going over implementations line by line was unreliable and time-consuming. A better strategy was to use well-specified unit tests to validate behavior.
n Well-designed testing enabled us to:
- Regenerate implementations with confidence
- Refactor existing logic safely
- Validate AI-generated code quickly
- Confirm that edge cases were handled correctly
This approach was made considerably more challenging by poorly designed tests. It was frequently unclear which execution pathways were being verified, even when coverage statistics showed high percentages.
n New logic was introduced to a function in several instances, but the existing tests did not make it clear where additional situations needed to be verified. This raised the likelihood of missing edge cases and made code reviews more difficult.
We had to reconsider the way we organized our unit tests as a result.
Consider the following function:
function fetchAccounts(payload) {
if (!payload) {
return;
}
try {
http.get(payload).subscribe();
} catch (e) {
log.error(e);
}
}
Despite its short size, this function has several execution paths:
- No payload is provided
- A payload is provided and the request succeeds
- A payload is provided and the request fails
Despite being easy to recognize, these situations are not often adequately represented in the test suite’s architecture. n This disparity between implementation and test structure worsens as functions grow in size.
A typical test file might look like this:
describe('fetchAccounts', () => {
it('should return if payload is missing', () => {
fetchAccounts(null);
expect(http.get).not.toHaveBeenCalled();
});
it('should call http.get when payload exists', () => {
fetchAccounts('/accounts');
expect(http.get).toHaveBeenCalled();
});
it('should log error if request fails', () => {
http.get.mockImplementation(() => {
throw new Error('error');
});
fetchAccounts('/accounts');
expect(log.error).toHaveBeenCalled();
});
});
Although this structure may provide complete coverage, it is not a clear representation of the various scenarios that the function handles. When AI tools produce identical implementations, this approach makes it harder to confirm that all execution paths have been adequately vetted. It is not always clear from the test file what the behavioral situations are, even when the coverage reports appear accurate.
Identifying Execution Paths: Before writing tests, we found it useful to identify the execution paths explicitly.
For the example above, the control flow can be represented as:
Payload provided?
├── no → return
└── yes
├── request succeeds
└── request fails
Every branch represents a distinct behavior that must be verified. n It is possible to map branches directly into the test structure once they have been discovered. n Before any tests are developed, this phase alone frequently identifies missing situations.
Organizing Tests Based on Context
We began classifying tests using nested contexts that reflect the function’s decision structure rather than informally grouping them.
describe('fetchAccounts', () => {
describe('when payload is not provided', () => {
it('should not call http.get', () => {
fetchAccounts(null);
expect(http.get).not.toHaveBeenCalled();
});
});
describe('when payload is provided', () => {
describe('when the request succeeds', () => {
it('should call http.get with the payload', () => {
http.get.mockReturnValue({ subscribe: jest.fn() });
fetchAccounts('/accounts');
expect(http.get).toHaveBeenCalledWith('/accounts');
});
});
describe('when the request fails', () => {
it('should log the error', () => {
http.get.mockImplementation(() => {
throw new Error('error');
});
fetchAccounts('/accounts');
expect(log.error).toHaveBeenCalled();
});
});
});
});
This structure directly reflects the function’s behavior. n
What is being verified is easily understood since the test hierarchy mirrors the execution pathways. n Because new situations inherently transfer into new contexts, it becomes easier to identify missing scenarios. n This framework provides implementations of AI coding tools with a clear behavioral reference against which they may be verified.
Consistent Context Naming: Another detail that proved useful was maintaining consistent context naming.
Using a consistent pattern such as below keeps the test hierarchy predictable.
describe('when payload is provided')
describe('when the request succeeds')
describe('when the request fails')
Mixing patterns, such as when the payload exists, a request fails, or invalid input is provided, can complicate the structure, particularly when test files are larger. Maintaining readability over time is facilitated by consistent nomenclature, which also makes test output easier to scan. (Biagiola et al., 2024)
There were other useful advantages to using this system in daily growth. n
- Extensive Coverage of Behavior: It is easier to identify missing instances when execution paths are explicitly accessible in the test file.
- Simpler Verification of AI-Created Code: It is easy to evaluate generated implementations against well-defined behavioral situations.
- Refactoring with Safety in Mind: It becomes simpler to determine which behaviors need to stay the same when a function is modified.
- Improved readability: By looking at the test hierarchy, even developers unfamiliar with the implementation can understand the intended behavior.
In conclusion
Reliable unit tests are becoming increasingly crucial as AI-assisted development becomes more widespread. n It is easier to validate the code produced and maintain trust during refactoring when tests are well-structured. n Organizing tests so that nested contexts represent the control flow of the function being evaluated is a useful strategy to increase dependability. As the code evolved over time, this straightforward method made our test suites more reliable and easier to understand.
Reference(s):
- https://arxiv.org/abs/2304.10778
