Changes / Anthropic / Intermittent error spikes for Claude Mythos 5.1 and Claude Fable 5.1

Anthropic: Major incident: Intermittent error spikes for Claude Mythos 5.1 and Claude Fable 5.1

Summary

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.

Machine-written from the diff below (Sol; type: incident degradation). Check the diff before relying on it.

Major incidentLabel: degradation (model (Jev), 52%)Changed Detected Source: stspg.io

What changed

AddedTitle#1
Before: not present
After: Intermittent error spikes for Claude Mythos 5.1 and Claude Fable 5.1
AddedImpact#2
Before: not present
After: major
AddedStatus#3
Before: not present
After: resolved
AddedStart time#4
Before: not present
After: 2026-09-15 09:48 UTC
AddedResolved time#5
Before: not present
After: 2026-09-15 11:14 UTC
AddedAffected components#6
Before: not present
After: claude.ai, Claude API (api.anthropic.com), Claude Code, Claude Cowork
AddedUpdateinvestigating#7
Before: not present
After: We are investigating elevated errors on requests to Claude Mythos 5.1 and Claude Fable 5.1. We will provide an update as soon as possible.
AddedUpdateinvestigating#8
Before: not present
After: We are investigating elevated errors on requests to Claude Mythos 5.1 and Claude Fable 5.1. We will provide an update as soon as possible.
AddedUpdatemonitoring#9
Before: not present
After: A fix has been deployed for Claude Mythos 5.1 and Claude Fable 5.1 and we are monitoring to ensure it has resolved the issue.
AddedUpdateresolved#10
Before: not present
After: A fix has been deployed for Claude Mythos 5.1 and Claude Fable 5.1 and we are monitoring to ensure it has resolved the issue.

First version of this incident archived by Rewindex. View the live incident.

History of this incident

When (UTC)ChangeKind
Major incident: Intermittent error spikes for Claude Mythos 5.1 and Claude Fable 5.1 (this change)
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.
Major incident

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