# Anthropic: Major incident: Elevated errors for Claude Mythos 5.1 and Claude Fable 5.1

**Summary:** Anthropic reported major elevated errors for Claude Mythos 5.1 and Claude Fable 5.1 across claude.ai, Claude API (api.anthropic.com), Claude Code and Claude Cowork. The incident started 2026-09-11 14:19 UTC, lasted 17 min and was resolved after a fix was deployed; Anthropic separately reported impact from 13:57 to 14:20 UTC. _(machine-written, sol)_
- Kind: Major incident · Label: outage (model (Jev))
- Changed: 2026-09-11 14:19 UTC · Detected: 2026-10-17 18:21 UTC
- [Source: https://stspg.io/n9n8kfcxtkfm](https://stspg.io/n9n8kfcxtkfm)
- [Company: Anthropic](https://rewindex.com/company/anthropic.md)
- Versions: before (none: first archived) → after incident\_version:1/t33dncr5ydvl@3df08b02e9cd
- Change 1 of 1 for this incident

## What changed

**1. Added: Title**

```diff
+ Elevated errors 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
+ 2026-09-11 14:19 UTC
```

**5. Added: Resolved time**

```diff
+ 2026-09-11 14:37 UTC
```

**6. Added: Affected components**

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

**7. Added: Update** (investigating) · 2026-09-11 14:19 UTC

```diff
+ We are investigating elevated errors on requests to Claude Fable 5.1 and Claude Mythos 5.1. We will provide an update as soon as possible.
```

**8. Added: Update** (identified) · 2026-09-11 14:21 UTC

```diff
+ We have identified the cause of elevated errors on requests to Claude Mythos 5.1 and Claude Fable 5.1 and are working on a fix. We will provide an update as soon as possible.
```

**9. Added: Update** (monitoring) · 2026-09-11 14:28 UTC

```diff
+ A fix has been deployed for Claude Mythos 5.1 and Claude Fable 5.1 and we are monitoring for recovery.
```

**10. Added: Update** (monitoring) · 2026-09-11 14:28 UTC

```diff
+ A fix has been deployed for the issue affecting Claude services and we are monitoring for recovery.
```

**11. Added: Update** (resolved) · 2026-09-11 14:37 UTC

```diff
+ The issue affecting Claude services has been resolved. Impact occurred from 13:57 to 14:20 UTC
```

## History of this incident

| When (UTC) | Change | Label |
| --- | --- | --- |
| 2026-09-11 14:19 UTC | Major incident: Elevated errors for Claude Mythos 5.1 and Claude Fable 5.1 (this change) | outage |

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/i055b3888ce3932 · Markdown: https://rewindex.com/change/i055b3888ce3932.md · JSON: https://rewindex.com/api/v1/changes/i055b3888ce3932 · Generated 2026-10-18T00:04:36.162Z