Anthropic publishes postmortem on April Claude Code outages

Anthropic published a postmortem attributing weeks of degraded Claude Code output quality in April 2026 to three separate, unrelated changes rather than a single fault. A 4 March change had lowered the default reasoning effort from “high” to “medium” to cut latency, a trade-off Anthropic later judged mistaken and reversed on 7 April. A 26 March caching optimisation, intended to clear stale reasoning only from sessions idle for over an hour, instead ran on every turn for the rest of a session, causing Claude to progressively lose context — fixed on 10 April, after Opus 4.7’s code-review tooling caught the bug that Opus 4.6 had missed. A verbosity-limiting system prompt added around the same time cut response quality by a measured 3% in evaluations and was reverted on 20 April. Anthropic said all three issues were resolved by 23 April and reset affected subscribers’ usage limits.