Anthropic traced a month of Claude Code quality complaints to three overlapping shipped changes

Anthropic (Claude Code, Claude Agent SDK, and Claude Cowork users)·Claude CodeFixed

Anthropic shipped three separate changes to Claude Code's harness — lowering default reasoning effort, a context-clearing bug that repeated every turn instead of once, and a verbosity-reducing system prompt — each of which degraded coding quality, and their overlap made the cause hard to isolate.

Blast radius

A month of user reports of 'Claude getting dumber' across Sonnet 4.6, Opus 4.6 and Opus 4.7 in Claude Code, the Agent SDK, and Cowork; the API itself was not affected. Anthropic reset usage limits for all subscribers as redress.

Root cause

Three overlapping harness/prompt changes, each individually intended to cut latency or verbosity, combined to look like broad model degradation, and internal evals didn't reproduce the issue at first.

How it was caught

User reports accumulated over about a month before Anthropic could distinguish the pattern from normal feedback variance and trace it to the three changes.

What changed

All three changes were reverted or corrected by April 20, 2026 (Claude Code v2.1.116), and usage limits were reset for all subscribers.

Take-awayTreat agent-harness and system-prompt tweaks with the same regression testing as model releases — aggregate evals can miss quality regressions that show up only in real usage.