In this engineering post-mortem, Anthropic documents three distinct incidents that degraded the perceived quality of Claude Code, the Claude Agent SDK, and Claude Cowork between March and April 2026, while specifying that the underlying API was never affected.
The first incident (March 4 - April 7) involved a configuration change to the default reasoning level, switched from "high" to "medium" to resolve interface freezing issues caused by extended thinking in high mode. Internal tests showed that medium mode offered "slightly lower intelligence with significantly reduced latency." However, users quickly reported that Claude seemed "less intelligent." Despite several design iterations (notifications, effort selector), users retained the medium default. Anthropic ultimately reversed its decision by switching to "xhigh" level for Opus 4.7 and "high" for the other models.
The second incident (March 26 - April 10) is the most technical and the most damaging. A prompt caching optimization intended to clean up old thinking sections from sessions inactive for more than an hour contained an implementation flaw. The API header clear_thinking_20251015 with the keep:1 parameter was meant to run only once but triggered on every subsequent turn, progressively erasing Claude's reasoning context. This caused cascading cache misses, making Claude "forgetful and repetitive" and depleting usage quotas faster. The bug proved difficult to detect because unrelated internal experiments masked the issue. Notably, it was Opus 4.7's Code Review tool, fed with the full repository context, that identified the bug retrospectively — Opus 4.6 had not been able to.
The third incident (April 16-20) resulted from an instruction added to the system prompt limiting verbosity (text between tool calls capped at 25 words, final responses at 100 words). Internal tests had detected no regression, but broader ablation tests revealed a 3% intelligence drop for both Opus 4.6 and Opus 4.7.
All issues were resolved by April 20 with version 2.1.116. Anthropic reset usage limits for all subscribers on April 23. The company announced several process improvements: increased internal use of public builds, per-model evaluations, systematic ablation testing, stabilization periods, phased rollouts, and the creation of the @ClaudeDevs account on X for more detailed product communication.