Anthropic investigates elevated errors across five Claude models

On October 25, 2026, Anthropic opened an investigation into elevated request errors affecting Claude Mythos 5.1, Fable 5.1, Opus 5.5, Sonnet 5.5 and Haiku 5.5. claude.ai, Claude API, Claude Code and Claude Cowork were all marked Partial Outage, with 90-day uptime of 99.57–99.64%.

• Five models affected: Mythos 5.1, Fable 5.1, Opus 5.5, Sonnet 5.5, Haiku 5.5
• Partial outage on claude.ai, Claude API, Claude Code and Claude Cowork
• 90-day uptime: 99.58% for claude.ai and Claude Code, 99.64% for API
• Three error incidents for Fable, Opus and Sonnet resolved on October 24