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AnthropicClaude 5 familyCurrentReasoning modelReleased 1 Sep 2026

Claude Mythos 5.1

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.

Model idclaude-mythos-5-1

Price$10 in, $50 out per 1M tokens

Checked9 November 2026

Knows up to June 2026

1,795events after its cutoff

81 historic, 409 major. 136 days of news.

It launched on 1 September 2026, already missing 191 of them.

Five things it has never heard of

How to call it

Where to use Claude Mythos 5.1, with its model id.

Providers, model ids, endpoints and docs for Claude Mythos 5.1
ProviderModel idEndpointDocs
Anthropic API (Claude API) claude-mythos-5-1 https://api.anthropic.com/v1/messages platform.claude.com

Pricing

Prices for Claude Mythos 5.1
Input$10 per 1M tokens
Output$50 per 1M tokens
Cache read$0.25 per 1M tokens

As published: “per 1M tokens (USD); Batch API 50% off”.

Source: platform.claude.com, checked 9 November 2026.

Specs

Input
Text, images and PDF
Output
Text
Context window
1,000,000 tokens
Max output
128,000 tokens
Open weights
No
Licence
Proprietary
Training cutoff
June 2026
Released
1 September 2026
Status
Current
Type
Reasoning model

What stands out

What its maker marketed at launch, and what people found later, each with its source.

  • 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: anthropic.com

  • Scientific discovery

    Shares Fable 5.1’s launch results, e.g. protein designs with about 10x higher binding affinity than competition winners.

    Source: anthropic.com

Four assistants, one scale

Claude Mythos 5.1 is added as a fifth row. Each row runs from what the model knows (graphite) through the months between its cutoff and its launch (hatched) to today (pink).

  1. GPT-6 Astra 271 at launch, 1,863 today
  2. Claude Opus 5.5 372 at launch, 1,795 today
  3. Gemini 4 Hydrogen 1,863 at launch, 1,889 today
  4. Grok 4.7 387 at launch, 1,829 today
  5. Claude Mythos 5.1 191 at launch, 1,795 today
Show as a table
ModelCutoffReleasedMissing at launchMissing today
GPT-6 AstraApril 20263 September 20262711,863
Claude Opus 5.5June 202622 September 20263721,795
Gemini 4 HydrogenMarch 202612 November 20261,8631,889
Grok 4.7May 202621 September 20263871,829
Claude Mythos 5.1June 20261 September 20261911,795

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).

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: