# 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, sol)_
- Kind: Major incident · Label: degradation (model (Jev))
- Changed: 2026-09-15 09:48 UTC · Detected: 2026-10-17 18:21 UTC
- [Source: https://stspg.io/x1qpycpg6ln7](https://stspg.io/x1qpycpg6ln7)
- [Company: Anthropic](https://rewindex.com/company/anthropic.md)
- Versions: before (none: first archived) → after incident\_version:1/6304r9jjhj34@87fc835897f7
- Change 1 of 1 for this incident

## What changed

**1. Added: Title**

```diff
+ Intermittent error spikes for Claude Mythos 5.1 and Claude Fable 5.1
```

**2. Added: Impact**

```diff
+ major
```

**3. Added: Status**

```diff
+ resolved
```

**4. Added: Start time**

```diff
+ 2026-09-15 09:48 UTC
```

**5. Added: Resolved time**

```diff
+ 2026-09-15 11:14 UTC
```

**6. Added: Affected components**

```diff
+ claude.ai, Claude API (api.anthropic.com), Claude Code, Claude Cowork
```

**7. Added: Update** (investigating) · 2026-09-15 09:48 UTC

```diff
+ 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.
```

**8. Added: Update** (investigating) · 2026-09-15 09:52 UTC

```diff
+ 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.
```

**9. Added: Update** (monitoring) · 2026-09-15 10:50 UTC

```diff
+ 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.
```

**10. Added: Update** (resolved) · 2026-09-15 11:14 UTC

```diff
+ 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.
```

## History of this incident

| When (UTC) | Change | Label |
| --- | --- | --- |
| 2026-09-15 09:48 UTC | Major incident: Intermittent error spikes for Claude Mythos 5.1 and Claude Fable 5.1 (this change) | degradation |

All Anthropic changes: [timeline](https://rewindex.com/company/anthropic/changes.md)
_Diff excerpts are shown on normalized text: dates, counters and tokens appear as ‹date›, ‹count›, ‹tok›; noise-only edits are hidden. Rewindex shows the changed passages, not a copy of the page._


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Canonical: https://rewindex.com/change/i554f36e70a2387 · Markdown: https://rewindex.com/change/i554f36e70a2387.md · JSON: https://rewindex.com/api/v1/changes/i554f36e70a2387 · Generated 2026-10-17T23:59:07.908Z