Anthropic
Claude Mythos 5.1: price, context window, and caching
The same model as Claude Fable 5.1 with more permissive cybersecurity and life-sciences safeguards, offered only to vetted organizations.
Released September 1, 2026 · Prices as of September 26, 2026
In Anthropic’s words
“Claude Mythos 5.1 is identical to Fable 5.1, but it offers more permissive safeguards for vetted individuals and organizations”
Facts
Specs and prices
| Fact | Claude Mythos 5.1 |
|---|---|
| Maker | Anthropic |
| API model id | claude-mythos-5-1 |
| Released | September 1, 2026 |
| Status | Restricted access |
| Context window | 1M tokens |
| Max output | 128K tokens |
| Open weights | No |
| Input, per 1M tokens | $10 |
| Cache hit, per 1M | $0.25 |
| Cache write, per 1M | $12.50 (5-minute), $20 (1-hour) |
| Output, per 1M tokens | $50 |
| Runs in | Its maker's API |
Standard API rates in US dollars, as published by the maker on September 26, 2026. Batch and priority tiers, taxes, and subscription plans are not included. Claude Mythos 5.1: The full 1M context window is billed at standard rates.
Good to know
- Like every Claude model from 4.7 on, it uses Anthropic's newer tokenizer, which counts about 30% more tokens than earlier Claude models for the same text.
- Invitation only, through Project Glasswing and Anthropic's trusted-access programs, currently for a set of US organizations.
- Available on the Claude API, Amazon Bedrock, Google Cloud, and Microsoft Foundry for approved organizations.
Cost
What typical work costs
Example token counts at Claude Mythos 5.1’s published rates. On the agentic session, caching saves $17.00 against billing every token as ordinary input.
| Agentic coding session, 100K input, 400K written to cache, 2M read from cache, 50K output | $10.50 |
|---|---|
| Large one-off review, 150K input with no cache hits, 10K output | $2.00 |
| Output-heavy generation, 30K input, 80K output | $4.30 |
| A month of sessions, 110 sessions: 5 a day, 22 working days | $1,155.00 |
Prompt caching
How Anthropic bills cached tokens
Anthropic
Claude caches a prompt prefix up to a breakpoint. One top-level cache_control field places the breakpoint automatically and moves it as the conversation grows, or you can mark up to 4 blocks yourself. Claude Code manages caching for you.
A 5-minute cache write costs 1.25x the input price and a 1-hour write costs 2x. A cache hit costs 0.1x input on most models, 0.05x on Claude Opus 5.5, and 0.025x on Claude Fable 5.1 and Claude Mythos 5.1. Every hit restarts the cache lifetime at no charge.
Anthropic's rule of thumb: a 5-minute write pays for itself after one cache read, and a 1-hour write after two.
In Claude Code, the main conversation uses the 1-hour cache on a Claude subscription and the 5-minute cache with an API key. Each model has its own cache, so switching models starts over.
Source: Anthropic: Prompt caching
Claude Mythos 5.1 compared
Claude Fable 5.1 vs Claude Mythos 5.1
Claude Mythos 5.1 is Claude Fable 5.1 with more permissive safeguards and invitation-only access. Prices match, down to $0.25 per million for a cache hit.
Your own numbers
See what Claude Mythos 5.1 really costs you.
everyaitoken reads your AI coding history on your Mac and prices every request at API rates, with what caching saved or cost. $9 once.