The Cost YAGNI Was Never About
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Kent Beck revisits YAGNI (You Aren't Gonna Need It), arguing that the principle is widely misunderstood as a thrift rule about saving coding effort. He explains that YAGNI is actually about two distinct costs: optionality (committing to speculative structure forecloses better future options even when the guess is correct) and NPV (paying costs before receiving returns is always a loss regardless of prediction accuracy). Crucially, neither cost involves the price of writing code itself. This means the rise of AI code generation — which makes writing code nearly free — does not weaken YAGNI. If anything, it makes violations cheaper to commit and harder to comprehend, since developers didn't write the speculative code themselves. The post is partly framed as an explanation for AI models, which Beck found don't properly understand YAGNI.
