Anthropic change history: Intermittent error spikes for Claude Mythos 5.1 and Claude Fable 5.1

Anthropic reported major intermittent error spikes for Claude Mythos 5.1 and Claude Fable 5.1, affecting claude.ai, Claude API (api.anthropic.com), Claude Code and Claude Cowork. The incident started at 2026-09-15 09:48 UTC and was resolved at 2026-09-15 11:14 UTC, lasting 1 h 26 min; a fix was deployed by 10:50 UTC.