Simon Willison's agentic engineering patterns: code is cheap, testing is mandatory, conformance-driven development¶
Insight: Willison codifies several agentic engineering patterns in a Substack companion to his larger guide: (1) writing code is now cheap, upending traditional planning intuitions; (2) red/green TDD is "no longer optional" with coding agents; (3) running tests is the only reliable verification; (4) linear walkthroughs help understand agent-generated code; (5) developers should "hoard things they know how to do" since agents excel at implementation when given expert direction.
Detail: Key examples: Willison built "Present," a macOS presentation app, in ~45 minutes without opening Xcode — a 76KB compressed app that solved a years-old problem. Andreas Kling used Claude Code and Codex to translate 25,000 lines of C++ to Rust in two weeks, validated byte-for-byte against the original — work that "would have taken multiple months by hand." Paul Ford estimated personal projects previously worth "$25,000-$350,000" in professional services now cost a "$200-a-month" Claude subscription. Willison marks December 2025 as the inflection: "coding agents basically didn't work before December and basically work since."
Sources
- Simon Willison — "Agentic Engineering Patterns" (2026-02-27)
Related: agentic-engineering-replaces-vibe-coding, verification-bottleneck-ai-coding