Claude Mythos 5.1 API Pricing
Claude Mythos 5.1 is the restricted-access twin of Claude Fable 5.1: the same model with safeguards lifted in some areas, released September 1, 2026 for vetted cybersecurity and life-sciences organisations. Anthropic publishes the full rate card - $10/M input, $50/M output, $0.25/M cache hits - even though there is no self-serve sign-up. It is priced on the same table as every public model, which is why we can list it at all.
Run the numbers.
The rate card is public even though access is not, so the arithmetic here is as real as on any other page - it just applies to an account you have to be approved for. Same rates as Fable 5.1.
Real-world presets.
Codebase vulnerability sweep
Literature synthesis pass
Incident triage
Scientific hypothesis pass
Paste text. See tokens. See cost.
This is a chars-per-token approximation, not a real tokenizer. Actual tokens vary by language, code density, and tool-call overhead — counts are typically ±10–20% off for English prose, more for code or non-Latin scripts. For exact billing, use the vendor's official tokenizer.
| Model | Input /M | Output /M | Effective blended | Context | Best for |
|---|---|---|---|---|---|
| Claude Mythos 5.1 Current | $10.00 cache $0.25 | $50.00 | $6.26 restricted access | 1M | Glasswing cyber / bio partners |
| Claude Fable 5.1 | $10.00 cache $0.25 | $50.00 | $6.26 identical card, public access | 1M | The version anyone can call |
| Claude Mythos 5 | $10.00 cache $1.00 | $50.00 | $6.82 same base, 4x the cache read | 1M | The row this one replaces |
| Claude Opus 5 | $5.00 cache $0.50 | $25.00 | $3.41 half the card, public | 1M | Anthropic's general frontier tier |
| Claude Sonnet 5 | $2.00 cache $0.20 | $10.00 | $1.36 speed / intelligence balance | 1M | Most production workloads |
Frequently asked.
Who can actually call it, why a model with no public sign-up has a public price, and what separates it from Fable 5.1.
Q · 01 What is Claude Mythos 5.1 priced at? +
$10/M base input, $12.50/M and $20/M cache writes, $0.25/M cache hits, and $50/M output, with batch at $5/$25. Under this site's 92/8 agentic blend at an 82% cache-hit rate that is $6.2586/M effective - identical to Fable 5.1.Q · 02 Can I sign up for it? +
Q · 03 Then why does this page exist? +
Q · 04 What changed from Claude Mythos 5? +
$1/M to $0.25/M - a 0.025x multiplier on base input where every other Claude model uses 0.1x. Base input, both cache-write tiers, output and batch are unchanged. See the Fable 5.1 page for what that cut is worth on a real workload; the short version is 8.3% on our standard blend, and more when reads far outnumber writes.