Use Case

Write Unit Tests for Existing Code

Add test coverage to code that has none, without spending a week writing boilerplate and mocks.

Difficulty:intermediate

Time saved:3-6 hours per module

Our pick

Best tool for this job

Cursor

AI-first code editor that writes, edits, and debugs with you.

Cursor can read the function, the files it imports and the tests you already have, so the new tests follow your conventions instead of inventing their own. Point it at one module, ask for the edge cases first, then have it write them. It's also good at the tedious part: setting up mocks and fixtures that match your real types.

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Pricing

Pricing changes often: check the official site for current plans.

Setup

easy

Coding required?

Yes

🥈 Runner-up

Also worth considering

GitHub Copilot

AI pair programmer for VS Code, JetBrains, and JetBrains Air.

Copilot is quick once you've written the first test. It picks up the pattern and fills in the next ones as you type. It sees less of the codebase though, so it's more likely to mock the wrong thing.

Visit GitHub Copilot See review

Getting Started

✓ Do this

→Start with pure functions. They're the easiest to test and you'll see how well the AI handles your style.
→Ask for a list of cases in plain English before any code, and cut the ones that don't matter.
→Show it one existing test as the template. This does more than any instruction about style.
→Run the tests right away, then break the code on purpose (flip a condition) to confirm a test actually fails.
→Ask it to explain any mock it wrote that you don't understand. A confusing mock is often a wrong one.
→Commit tests in small batches so a bad one is easy to revert.

✗ Avoid this

→Accepting tests that pass because they assert whatever the code currently returns, bugs included.
→Letting it test private details, so every refactor breaks a dozen tests.
→Generating 40 tests for one file and never reading them.
→Mocking so much that the test only checks the mocks.

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Getting Started

✓ Do this

→Start with pure functions. They're the easiest to test and you'll see how well the AI handles your style.
→Ask for a list of cases in plain English before any code, and cut the ones that don't matter.
→Show it one existing test as the template. This does more than any instruction about style.
→Run the tests right away, then break the code on purpose (flip a condition) to confirm a test actually fails.
→Ask it to explain any mock it wrote that you don't understand. A confusing mock is often a wrong one.
→Commit tests in small batches so a bad one is easy to revert.

✗ Avoid this

→Accepting tests that pass because they assert whatever the code currently returns, bugs included.
→Letting it test private details, so every refactor breaks a dozen tests.
→Generating 40 tests for one file and never reading them.
→Mocking so much that the test only checks the mocks.