Describe the Behavior First and Let Claude Do TDD — Test, Then Implement

Most developers write code first and tests second — or skip tests entirely when they're in a rush. Claude Code makes TDD practical by handling the tedious part: writing the test from a behavior description, then implementing just enough code to pass it.

I need a function that calculates shipping cost based on weight and destination. 
Write the test first — cover standard rates, free shipping over a threshold, 
international surcharges, and invalid inputs. Then implement the function 
to make all tests pass.

Claude writes the tests first, defining the contract:

// Tests written first — these define the behavior
it('calculates domestic shipping by weight', () => {
  expect(calculateShipping(2, 'US')).toBe(5.99);
});
it('applies free shipping for orders over threshold', () => {
  expect(calculateShipping(10, 'US', { subtotal: 100 })).toBe(0);
});
it('adds international surcharge', () => {
  expect(calculateShipping(2, 'UK')).toBe(15.99);
});
it('throws on negative weight', () => {
  expect(() => calculateShipping(-1, 'US')).toThrow();
});

Then runs them (all fail), implements the function, and runs again until green.

This workflow is powerful for:

# New feature from a spec
Here's the business requirement for discount codes. Write tests 
that cover every rule, then implement the discount engine to pass them.

# Bug fix with regression prevention
This bug lets users checkout with expired coupons. Write a test that 
reproduces the bug first, then fix the code to make it pass.

# API endpoint design
I need a POST /api/bookings endpoint. Write the feature tests first — 
validation, authentication, success response, error cases — then 
implement the controller and route to satisfy them.

The key prompt pattern for TDD with Claude:

Write the tests FIRST. Do not write any implementation code until the 
tests are complete and running (they should all fail). Then implement 
the minimum code to make each test pass, one at a time.

TDD is hard to start but easy to maintain — let Claude handle the discipline of writing tests first so you get the safety net without the friction.

via Claude Code