Ask Claude to Write Tests for Existing Code That Has No Test Coverage

Retrofitting tests onto untested code is a chore nobody volunteers for. Claude reads the existing implementation, understands what it does, and writes tests that cover everything — including edge cases you might have missed when writing the code.

> read app/Services/PricingService.php and write a comprehensive
> test suite for it — cover the happy path, edge cases, and
> error conditions

Claude traces every code path — conditional branches, early returns, exception throws, null checks — and generates a test for each one. It doesn't just test that the method runs; it verifies the actual behavior at each branch.

// Claude generates tests like these from reading your code:
it('calculates subtotal from line item prices and quantities')
it('applies percentage discount before tax')
it('applies fixed discount after percentage discount')
it('throws when discount exceeds subtotal')
it('returns zero tax for tax-exempt customers')
it('rounds totals to two decimal places')
it('handles empty cart gracefully')

Notice how Claude found behaviors you might not think to test — the order discounts are applied, the rounding behavior, and the empty cart edge case — because it read the actual implementation and saw each path.

You can target specific areas:

> write tests for all the public methods in the OrderController
> — mock the dependencies and test the HTTP responses

> add test coverage for the email notification service — test
> what happens when the mail server is down

> write tests for the user registration flow end-to-end —
> from form submission through to the welcome email

After generating tests, have Claude run them to verify they pass:

> run the new tests and fix any that fail — the tests should
> match the current behavior, not an idealized version

This is crucial: the tests should document what the code does, not what you wish it did. If the tests reveal bugs, Claude flags them — but keeps the test matching current behavior so you can fix the bug separately.

Untested code is a liability. Let Claude write the safety net so you can change it with confidence.

via Claude Code