The Design Patterns and Legacy Code Books Are More Valuable Than Ever

June 03, 2026

The Gang of Four book and Working Effectively With Legacy Code are more valuable after agents than before.

Legacy Code because of the proliferation of massive codebases of questionable quality. Before this, it was rare to have codebases like this. That's why Facebook used to recommend this to every new hire.

And Design Patterns? If you can identify where there's a clean seam, where there's a clean pattern to use. Say the strategy pattern. Then you can point an agent powered by the smallest LLM in the world at that code and have it generate a new strategy. A 0.8B Qwen 3.5 can do it in 3 seconds.

Clean patterns don't get recommended by LLMs in my experience. I'm not sure why. I think it takes a human eye and judgement. I sat with my daughter while she and her friends created a video game with Claude. It had dozens of powerups and items the girls dreamed up and asked Claude to implement.

Claude happily created a multi-thousand line CSS/HTML/JS abomination. There was a single switch statement for each of the color, movement speed boost, and item stats. So claude repeated:

const mainColor = isWhite ? '#FAFAFA' : (isGray ? '#808080' : (isGolden ? '#FFD700' : (isBlack ? '#0a0a0a' : (isTuxedo ? '#1a1a1a' : '#FF8C00'))));

Like a bad dream. Human judgement is needed.

Regardless of where engineers are in their careers when the LLMs got good, there is more leverage now to be gained from these books in 2026 than in 2024.