Claude Code's Feb–Mar 2026 Updates Quietly Broke Complex Engineering — Here's the Technical Deep-Dive

A community reverse-engineered the failure modes, and the fixes are surprisingly straightforward. The Problem In February and March 2026, Anthropic shipped three simultaneous changes to Claude Code that together created a perfect storm for anyone doing serious, multi-file engineering work: Opus 4.6 + Adaptive Thinking (Feb 9): The model now decides how long to think per turn, rather than using a fixed reasoning budget. effort=85 (medium) became the default (Mar 3): Without fanfare for many users, the default effort dropped from high to medium.