Anthropic’s Claude Code regression postmortem is unusually specific. In an April 23 engineering note, the company said three vendor-side changes made Claude Code feel worse for users, while the API and inference layer were not impacted .

That distinction matters. This was not Anthropic saying the underlying model weights were broadly downgraded. It was saying that defaults, cache behavior, and system-prompt instructions inside a hosted product changed the model’s effective intelligence anyway.

What Anthropic admitted changed in Claude Code regression

Anthropic’s postmortem names three issues, with dates, affected products, and reversions.…