Anthropic Publishes Postmortem on Claude Code Harness-Level Bugs That Caused Quality Regression

Summary

Anthropic published a postmortem on April 23, 2026 explaining the root cause of Claude Code quality complaints that circulated extensively over the prior two months. Rather than model deficiencies, the investigation identified three distinct harness-level bugs. The most impactful: a latency-optimization change deployed March 26 (clearing idle session context to reduce startup time) contained a bug that triggered the clearing mechanism every turn for the rest of the session instead of only once,