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The Power of TDD x Claude Code

Test-driven development (TDD) follows the cycle of “write a test, confirm it fails, implement, make the test pass, refactor.” Claude Code can autonomously run this entire cycle, dramatically lowering the barrier to practicing TDD.

Red-Green-Refactor in One Shot

Here’s how to delegate the entire TDD cycle to Claude Code at once.

> Implement a user registration feature with the following spec using TDD:
> - Email address validation
> - Password must be 8+ characters with upper/lowercase letters and numbers
> - Prevent duplicate email registration
>
> 1. Write the tests first
> 2. Confirm the tests fail
> 3. Implement and make the tests pass
> 4. Refactor

Example Tests Generated by Claude Code

import { describe, it, expect } from "vitest";
import { registerUser, ValidationError } from "./auth";

describe("registerUser", () => {
  it("registers successfully with valid email and password", async () => {
    const result = await registerUser({
      email: "[email protected]",
      password: "Passw0rd",
    });
    expect(result.success).toBe(true);
    expect(result.user.email).toBe("[email protected]");
  });

  it("throws error for invalid email address", async () => {
    await expect(
      registerUser({ email: "invalid", password: "Passw0rd" })
    ).rejects.toThrow(ValidationError);
  });

  it("throws error for short password", async () => {
    await expect(
      registerUser({ email: "[email protected]", password: "Pass1" })
    ).rejects.toThrow("at least 8 characters");
  });

  it("throws error for password without numbers", async () => {
    await expect(
      registerUser({ email: "[email protected]", password: "Password" })
    ).rejects.toThrow("must contain a number");
  });

  it("throws error for duplicate email address", async () => {
    await registerUser({
      email: "[email protected]",
      password: "Passw0rd",
    });
    await expect(
      registerUser({ email: "[email protected]", password: "Passw0rd" })
    ).rejects.toThrow("already registered");
  });
});

Test-First Prompt Patterns

Pattern 1: Generate Tests from Specifications

> Write tests based on the following API specification.
> Don't implement anything yet.

This two-stage approach lets you write tests first, review them, and then proceed to implementation.

Pattern 2: Add Tests to Existing Code Before Refactoring

> src/utils/formatter.ts has no tests.
> First write tests that guarantee the current behavior,
> then refactor the code.

This pattern is effective for improving legacy code. For details, see the Legacy Code Modernization Guide.

Pattern 3: Auto-Generate Boundary Value Tests

> Create comprehensive boundary value tests for the calculatePrice function.
> Cover 0, negative numbers, maximum values, and decimal rounding.

describe("calculatePrice", () => {
  it("returns 0 for quantity of 0", () => {
    expect(calculatePrice(100, 0)).toBe(0);
  });

  it("throws error for negative quantity", () => {
    expect(() => calculatePrice(100, -1)).toThrow();
  });

  it("rounds decimal places correctly", () => {
    expect(calculatePrice(33, 3)).toBe(99);
  });

  it("applies discount for bulk orders", () => {
    expect(calculatePrice(100, 1000)).toBe(90000); // 10% discount
  });
});

Automate the TDD Cycle with Hooks

Using Claude Code’s hooks feature, you can automatically run tests every time a file changes.

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "command": "npx vitest related \"$CLAUDE_FILE_PATH\" --run 2>&1 | tail -20"
      }
    ]
  }
}

For details on hooks, see the Hooks Feature Guide.

Enforce TDD Rules with CLAUDE.md

By writing rules in your project’s CLAUDE.md, Claude Code will always follow a TDD style.

## Development Style
- All new features must be implemented using TDD
- Write tests first and confirm they fail before implementing
- Maintain test coverage above 80%
- Place test files in the __tests__ directory

Combining with Test Strategy

Beyond writing unit tests with TDD, it’s important to build a comprehensive test strategy that includes integration tests and E2E tests. For overall test design, see the Complete Testing Strategy Guide.

Summary

The combination of Claude Code and TDD is a powerful approach that eliminates the tedium of writing tests while maintaining code quality. Start by trying the TDD style with small functions.

For more on writing tests and configuration, refer to the official Anthropic documentation and the Vitest official site.