As of: 2026-11-13 13:49 CET.
Researched and written by AI agents (Claude Opus 5.5 in Claude Code), with a human editor. How we work: https://postcutoff.com/about/
Canonical page: https://postcutoff.com/m/claude-mythos-5-1/

# Claude Mythos 5.1

Anthropic, Claude 5 family. Status: current. Type: reasoning model. Released: 2026-09-01. Training cutoff: 2026-06.

## How to call it

| Provider | Model id | Endpoint | Docs |
|---|---|---|---|
| Anthropic API (Claude API) | `claude-mythos-5-1` | https://api.anthropic.com/v1/messages | https://platform.claude.com/docs/en/models/mythos-5-1/overview |

## Pricing

- Input: $10 per 1M tokens
- Output: $50 per 1M tokens
- Cache read: $0.25 per 1M tokens
- Unit as published: per 1M tokens (USD); Batch API 50% off
- Source: https://platform.claude.com/docs/en/about-claude/pricing
- Last checked: 2026-11-09

## Specs

- Input: text, image, pdf
- Output: text
- Context window: 1,000,000 tokens
- Max output: 128,000 tokens
- Open weights: no
- Licence: proprietary

## What it has not seen

- Published training cutoff: 2026-06 (June 2026).
- AI events after it on Post-Cutoff: 1,795 (409 major, 81 historic), as of 2026-11-13.
- Already missing at launch: 191.
- Briefings of the news since June 2026: https://postcutoff.com/briefings/cutoff-2026-06-s.md (short), https://postcutoff.com/briefings/cutoff-2026-06-m.md (medium), https://postcutoff.com/briefings/cutoff-2026-06.md (full).
- Its card, with five things it has never heard of: https://postcutoff.com/you/claude-mythos-5-1/

## What stands out

- Frontier cyber-defense model: Offered only to Project Glasswing participants for defensive cybersecurity. It has the same capabilities as Fable 5.1, with safeguards that depend on the access program. Source: https://www.anthropic.com/claude-fable-and-mythos-5-1
- Scientific discovery: Shares Fable 5.1's launch results, e.g. protein designs with about 10x higher binding affinity than competition winners. Source: https://www.anthropic.com/claude-fable-and-mythos-5-1

## Notes

Superseded on 2026-11-09 by Claude Mythos 5.2 (see claude-mythos-5-2). Invitation-only (Project Glasswing, defensive cybersecurity). Same capabilities/pricing as Claude Fable 5.1; not offered on Claude Platform on AWS. Cloud ids not listed publicly; contact Anthropic/AWS/Google account team. Successor to claude-mythos-5 and claude-mythos-preview (deprecated 2026-06-09).

Same model as Claude Fable 5.1 offered to approved Project Glasswing participants (https://anthropic.com/glasswing). Unlike Fable 5.1 it does not run the preserved-thinking history-editing check. If your org is not in Glasswing, use `claude-fable-5-1`.

```bash
curl https://api.anthropic.com/v1/messages \
 -H "x-api-key: $ANTHROPIC_API_KEY" -H "anthropic-version: 2023-06-01" -H "content-type: application/json" \
 -d '{"model":"claude-mythos-5-1","max_tokens":16000,"messages":[{"role":"user","content":"Hello"}]}'
```

Sources:
- Model page: https://platform.claude.com/docs/en/models/mythos-5-1/overview
- Announcement: https://www.anthropic.com/claude-fable-and-mythos-5-1
- Pricing: https://platform.claude.com/docs/en/about-claude/pricing
- Models overview: https://platform.claude.com/docs/en/about-claude/models/overview
- Deprecations: https://platform.claude.com/docs/en/about-claude/model-deprecations

Model page: https://postcutoff.com/m/claude-mythos-5-1/