---
type: concept
title: "Test-Driven Development"
description: "Writing tests first so the agent has an objective target and a validation signal to code against."
tags: [tdd, testing, validation]
videos: [i-forced-claude-to-code-for-24-hours-nonstop-heres-what-happened]
created: 2026-07-21
updated: 2026-07-21
---

# Test-Driven Development

Test-driven development (TDD) flips the usual order: you define the tests, the success criteria, before writing any implementation, so the agent has a concrete, objective target to code against. Cole calls it "very powerful for AI coding": "It relies on the concept of testdriven development, which is very powerful for AI coding. We define the success criteria, all of our tests upfront before we do any of the actual coding" ([I Forced Claude to Code for 24 Hours Nonstop](../sources/i-forced-claude-to-code-for-24-hours-nonstop-heres-what-happened.md), 0:02:51).

Why it fits agentic coding so well: a pre-written test suite converts a vague prompt into an unambiguous, machine-checkable goal. The agent can implement, run the tests, see failures, and iterate autonomously without a human re-stating what "done" means. In the 24-hour nonstop run, completion was defined entirely by the suite: "This has the 200 or more test cases that need to pass for our application to be considered complete" (0:04:13). The tests are the spec and the finish line at once, which is precisely the objective signal an autonomous loop needs to grade itself against.

TDD is the up-front, deterministic base of a broader validation strategy. It gives the agent a signal to iterate on, but Cole pairs it with checks that go past mocked unit tests, so the running system, not just the assertions, is proven.

## Part of

- [Validation](./validation.md) - TDD is the objective, up-front signal at the base of the validation strategy.

## Builds on

- [AI Coding Harness](./ai-coding-harness.md) - the harness runs the test suite and loops the agent until it passes.

## Related

- [End-to-End Validation](./end-to-end-validation.md) - TDD's unit-level tests are complemented by exercising the whole system as a user would.

## Tools

- [Claude Code](../entities/tools/claude-code.md) - codes against the pre-written test suite and iterates until all cases pass.
- [Puppeteer](../entities/tools/puppeteer.md) - Browser-automation MCP server that lets a coding agent load pages, click, and screenshot to visually validate that a feature actually works before marking it passed.

## Sources

- [I Forced Claude to Code for 24 Hours Nonstop, Here's What Happened](../sources/i-forced-claude-to-code-for-24-hours-nonstop-heres-what-happened.md) - "[0:02:51] It relies on the concept of testdriven development, which is very powerful for AI coding. We define the success criteria, all of our tests upfront before we do any of the actual coding."
- [I Forced Claude to Code for 24 Hours Nonstop, Here's What Happened](../sources/i-forced-claude-to-code-for-24-hours-nonstop-heres-what-happened.md) - "[0:04:13] This has the 200 or more test cases that need to pass for our application to be considered complete."
