Claude Code quality issues traced to harness bugs, not model
Anthropic's postmortem reveals that recent complaints about Claude Code's declining quality were caused by three separate bugs in its harness, not the underlying models. A notable bug on March 26 cleared Claude's older thinking from idle sessions but erroneously repeated every turn, causing forgetfulness and repetition. Simon Willison, who frequently uses long-running sessions, highlighted the impact. The postmortem underscores the complexity of debugging agentic systems beyond model non-determinism.
Key facts
- High volume of complaints about Claude Code quality over past two months were grounded in real problems.
- Three separate issues in the Claude Code harness caused material problems.
- A bug on March 26 cleared Claude's older thinking from sessions idle over an hour, but repeated every turn instead of once.
- The bug made Claude seem forgetful and repetitive.
- Simon Willison reported having 11 stale Claude Code sessions at time of writing.
- Willison estimates he spends more time prompting in stale sessions than fresh ones.
- The postmortem was linked by Simon Willison on April 24, 2026.
- The article is described as a link post by Simon Willison.
Entities
Institutions
- Anthropic
- Claude Code