AI Regression Test Generator
Build regression tests that catch bugs before they ship
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How many bugs has your team fixed twice? Not similar bugs, the same one, returning six months later because nothing was ever written to stop it?
A regression test is the difference between a fix and a fix that holds. It is also the part of the work everyone skips, because the bug is gone and the ticket is closed. The AI Regression Test Generator writes those tests from the code you just changed.
Short answer: The AI Regression Test Generator is a free AIToolsay tool that writes tests to stop fixed behaviour breaking again. Paste the code or describe the function, choose your language, test framework and coverage level, and it returns tests that pin the behaviour you care about.
What is AI Regression Test Generator?
A regression test does one job: it fails if behaviour that used to be correct stops being correct. It is not there to explore, it is there to hold a line.
The prompt box asks you to paste the code or describe the function to test. For regression work, add what changed and what it used to do wrong. That context is what turns a general test suite into tests aimed at the thing that actually broke.
Test Framework covers Jest, PyTest, JUnit, Mocha, PHPUnit, NUnit, Go test, RSpec and Vitest, so what comes back fits the suite you already have rather than needing translation.
Why Use AI Regression Test Generator?
The honest reason is timing. Regression tests are written at the end of a fix, when the interesting work is done and the pressure is to move on. Anything that makes them a two minute job rather than a twenty minute one changes whether they exist.
The second reason is aim. Hand written regression tests tend to test the fix. Good ones test the behaviour, which survives the next refactor. Describing what the code should do rather than what you changed is what produces the second kind.
| Test written against | Survives a refactor? | Why |
|---|---|---|
| The internal fix | No | It breaks when the implementation changes, even correctly |
| The public behaviour | Yes | It only fails when the outcome changes |
| A mocked call sequence | Rarely | It asserts how, not what |
| The original bug input | Yes | That exact input must keep working forever |
Who Should Use It?
- Developers closing a bug who want the fix to stay fixed
- Teams with a suite that grew unevenly, where some areas have no coverage at all
- Anyone about to refactor and needing a safety net before they start
- Maintainers of older code where the current behaviour is the specification
- Reviewers asking for a test alongside a fix and wanting to make that easy to provide
Note Include the input that triggered the original bug. That specific value is the most valuable test case you will ever have for this function, and it is the one nobody writes down before closing the ticket.
How Does AI Regression Test Generator Work?
Prompt box. Open the AI Regression Test Generator and paste the code or describe the function, along with what went wrong before.
Model selector. Set the engine, from MSB AI, OpenAI ChatGPT, Google Gemini, Anthropic Claude AI, xAI Grok AI, DeepSeek, Qwen, Meta AI, NVIDIA AI, OpenRouter AI and MiniMax.
Advanced options accordion. The panel holds ten controls: Language, Test Framework, Test Type and Coverage Level as dropdowns, four toggles, a Coverage Depth slider and a free text field.
Generate button. Code, model and settings run through the prompt engineering layer written for test generation, which is the instruction set that produces runnable tests rather than a list of things to test.
Output card. The tests appear below the button with a live word count, plus copy, listen, reuse, download and open in full view.
Export row. DOC, TXT and HTML. TXT for anything going straight into a test file.
Activity history. Session generations stay listed, which is how you build regression tests for several functions in one sitting and keep them consistent.
Key Features
Behaviour pinned, not implementation
Tests written against outcomes survive the next refactor instead of failing on it.
Your framework, your idioms
Jest, PyTest, JUnit, Mocha, PHPUnit, NUnit, Go test, RSpec or Vitest, matching the suite you already run.
Edge cases included
The empty, null and boundary inputs that caused the original bug get their own cases.
Mocks where they help
Include Mocks isolates the function from services you do not want a test to depend on.
Best Use Cases
- Closing a bug ticket, where the test is the proof it will not return
- Adding a net under an area you are about to refactor
- Older code with no tests, where the current behaviour is what you must preserve
- A function that has been fixed more than once already
- Pinning a business rule that lives only in an implementation
The order that works when you are adding tests after a fix:
- Write down what the function should do, in one sentence.
- Add the input that caused the bug and the output it should now produce.
- Paste the function and both of those into the prompt.
- Set Language and Test Framework to match your project exactly.
- Set Test Type to Unit and Coverage Level to Standard for a first pass.
- Generate, then delete any test that asserts how the function works internally.
- Run the suite. Then break the fix on purpose and check that a test fails.
- Commit the tests with the fix, in the same change.
Advanced Options Guide
| Option | What it controls | When to change it | Suggested start |
|---|---|---|---|
| Language | Auto Detect, Python, JavaScript, TypeScript, Java, C#, C++, Go, PHP or Ruby | Always set it, so the syntax and idioms match your codebase | Your project language |
| Test Framework | Auto, Jest, PyTest, JUnit, Mocha, PHPUnit, NUnit, Go test, RSpec or Vitest | Set it explicitly. Auto guesses, and a mismatched framework means rewriting everything | The framework your suite already uses |
| Test Type | Unit, Integration, Edge Cases, Happy Path, Negative, Performance, End to End or Mixed | Unit for a single function, Integration when the bug crossed a boundary | Unit |
| Coverage Level | Basic, Standard, Thorough or Exhaustive | Thorough for code that has broken more than once | Standard |
| Include Edge Cases | Adds empty, null, boundary and unusual inputs | Leave on. Regressions almost always live at a boundary | On |
| Include Mocks | Adds mocks for external dependencies | Turn on when the function calls a service, off for pure logic | Off for pure functions |
| Add Assertions | Adds explicit assertions rather than implicit checks | Leave on. A test with no clear assertion is a test that cannot fail usefully | On |
| Add Comments | Explains what each test is protecting | Keep on for regression tests, so the next person knows why the case is so specific | On |
| Coverage Depth | Slider from 1 to 100 setting how many cases are produced | Keep it moderate. A hundred tests nobody reads is worse than ten that are right | 55 |
| Custom Instructions | Free text up to 1000 characters over the settings | Name your fixtures, factories and the bug this is protecting against | "Use the order factory, ticket 4412: refund was one day too generous" |
Important A regression test that passes before you fix anything is not a regression test. Break the fix deliberately, run the suite, and confirm the right test fails. Without that check you have added confidence rather than coverage.
Tips & Common Mistakes
Tests worth keeping
- The exact input that caused the original bug
- Assertions on the outcome rather than the internals
- A comment naming the ticket the test protects
- Boundary cases either side of the value that failed
Tests worth deleting
- Anything asserting which private method was called
- Duplicates of cases the suite already covers
- Mocks so complete that the test only exercises the mocks
- Tests that pass whether or not the fix is present
- ✅ The original failing input included as a case
- ✅ Test Framework set explicitly, not left on Auto
- ✅ Assertions on behaviour, not on implementation
- ✅ The fix broken on purpose to prove a test fails
- ✅ Tests committed alongside the fix, not afterwards
| Situation | Test Type | Coverage Level |
|---|---|---|
| Closing a single bug | Unit | Standard |
| Before a large refactor | Mixed | Thorough |
| A function fixed three times | Edge Cases | Exhaustive |
| A bug that crossed a service boundary | Integration | Standard |
Pro tip Name every regression test after the ticket it came from. Six months later, a very specific test case with no explanation looks like something safe to delete, and a ticket number in the name is what stops that happening. For broader coverage of the same function, the AI Unit Test Generator and the AI Test Case Generator are the tools to pair with this one.
AIToolsay is a free AI tools platform made of dedicated workspaces rather than one general chat box under many names. Each tool has its own prompt engineering and its own options panel, which is why a testing tool asks about frameworks and coverage instead of tone and audience. All of the tools are free and none of them asks for an account. You also select the engine, choosing from MSB AI, OpenAI ChatGPT, Google Gemini, Anthropic Claude AI, xAI Grok AI, DeepSeek, Qwen, Meta AI, NVIDIA AI, OpenRouter AI and MiniMax, and different engines pick different edge cases, which is a genuine reason to run a second pass. Sitting beside the tools are an AI directory, an AI models directory, courses, prompts, guides and news, all reachable from the AIToolsay homepage.
Frequently Asked Questions
Is the AI Regression Test Generator free?
Yes. It is free to use, nothing is installed, and no account is needed to generate tests.
What makes a regression test different from a unit test?
Intent. A unit test checks that a function works. A regression test exists because something specific broke once, and it holds that particular line. In practice it is a unit test with a very deliberate case in it.
Should I test the fix or the behaviour?
The behaviour. Tests written against the fix break during the next refactor even when the refactor is correct, and a test that cries wolf gets deleted.
How do I know the test actually protects anything?
Break the fix on purpose and run the suite. If nothing fails, the test is not covering what you think it covers. This takes thirty seconds and it is the only real check.
Will it match my test framework?
Set Test Framework explicitly rather than leaving it on Auto. Mocking style and assertion syntax vary enough that a mismatch means rewriting most of what comes back.
How many tests should I keep?
Fewer than it generates. Keep the original failing input, the boundaries either side of it, and one happy path. Delete the rest unless they are testing something genuinely different.
Can I use it on code with no tests at all?
Yes, and it is a good starting point before a refactor. Generate tests against the current behaviour, confirm they pass, and you have a net before you change anything.
Fixing a bug is satisfying. Making sure it never comes back is what actually saves the time. Paste the function, include the input that broke it, and let the AI Regression Test Generator write the test you would otherwise have promised to add later.
Thanks for reading, and enjoy closing that ticket properly. If this becomes part of how you finish a fix, join the AIToolsay community, follow along on social media, turn on push notifications for new tools, and subscribe to the newsletter for the occasional round up.
Let AI Speak.
Sabir
Founder of AIToolsay and a passionate AI enthusiast dedicated to building practical, user-friendly AI tools that simplify everyday tasks.
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