Simon Willison’s Weblog

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Monday, 23rd February 2026

Agentic Engineering Patterns >

Red/green TDD

"Use red/green TDD" is a pleasingly succinct way to get better results out of a coding agent.

TDD stands for Test Driven Development. It's a programming style where you ensure every piece of code you write is accompanied by automated tests that demonstrate the code works.

The most disciplined form of TDD is test-first development. You write the automated tests first, confirm that they fail, then iterate on the implementation until the tests pass. [... 348 words]

# 7:12 am / testing, tdd, coding-agents, ai-assisted-programming

Sunday, 22nd February 2026

2026 » February

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