Model comparison
Claude Fable 5.1 vs Claude Mythos 5.1: who can use which
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.
· Prices as of September 26, 2026
Claude Fable 5.1
Anthropic · Released September 1, 2026
Anthropic's most capable generally available model, aimed at demanding reasoning and long-horizon agentic coding. Anthropic suggests it when Opus-tier results fall short.
Claude Fable 5.1 facts and comparisonsClaude Mythos 5.1
Anthropic · Released September 1, 2026 · Restricted access
The same model as Claude Fable 5.1 with more permissive cybersecurity and life-sciences safeguards, offered only to vetted organizations.
Claude Mythos 5.1 facts and comparisons
The short answer
Claude Mythos 5.1 is the same model as Claude Fable 5.1, with more permissive safeguards and access by invitation only. Their prices are identical, so the example agentic coding session costs $10.50 on either. Choose Mythos 5.1 only if your organization has been approved for it and your work needs those safeguards; everyone else uses Fable 5.1.
Choose Claude Fable 5.1 if
- You want to start today: Fable 5.1 is generally available and needs no invitation.
- You work in Cursor, OpenRouter, or GitHub Copilot, which list Fable 5.1 but not Mythos 5.1.
- Your coding work fits within Anthropic's standard safeguards.
Choose Claude Mythos 5.1 if
- Your organization has been approved through Project Glasswing or one of Anthropic's trusted-access programs.
- Your cybersecurity or life-sciences work needs the more permissive safeguards Mythos 5.1 offers.
- You reach Claude through the Claude API, Amazon Bedrock, Google Cloud, or Microsoft Foundry, where approved organizations can use Mythos 5.1.
Side by side
Specs and prices
| Fact | Claude Fable 5.1 | Claude Mythos 5.1 |
|---|---|---|
| Maker | Anthropic | Anthropic |
| API model id | claude-fable-5-1 | claude-mythos-5-1 |
| Released | September 1, 2026 | September 1, 2026 |
| Status | Current | Restricted access |
| Context window | 1M tokens | 1M tokens |
| Max output | 128K tokens | 128K tokens |
| Open weights | No | No |
| Input, per 1M tokens | $10 | $10 |
| Cache hit, per 1M | $0.25 | $0.25 |
| Cache write, per 1M | $12.50 (5-minute), $20 (1-hour) | $12.50 (5-minute), $20 (1-hour) |
| Output, per 1M tokens | $50 | $50 |
| Runs in | Claude Code, Cursor, OpenCode, OpenRouter, and GitHub Copilot | Its maker's API |
Standard API rates in US dollars, as published by each maker on September 26, 2026. Batch and priority tiers, taxes, and subscription plans are not included. Claude Fable 5.1: The full 1M context window is billed at standard rates. Claude Mythos 5.1: The full 1M context window is billed at standard rates.
Cost
What the same work costs
The same token counts, priced at each model’s published rates. Cached tokens are billed at each maker’s cache prices, so the session shows what caching is worth on each model.
Real sessions differ: the two models count the same code as different numbers of tokens, and reasoning settings change how much each one writes. Your own history is the real test.
| Workload | Claude Fable 5.1 | Claude Mythos 5.1 |
|---|---|---|
| Agentic coding session, 100K input, 400K written to cache, 2M read from cache, 50K output | $10.50 | $10.50 |
| Large one-off review, 150K input with no cache hits, 10K output | $2.00 | $2.00 |
| Output-heavy generation, 30K input, 80K output | $4.30 | $4.30 |
| A month of sessions, 110 sessions: 5 a day, 22 working days | $1,155.00 | $1,155.00 |
| Where the session’s cost goes | ||
| Cache writes | $6.50 | $6.50 |
| Cache reads | $0.50 | $0.50 |
| Uncached input | $1.00 | $1.00 |
| Output | $2.50 | $2.50 |
- caching saves on the session with Claude Fable 5.1 (62%)
- $17.00
- caching saves on the session with Claude Mythos 5.1 (62%)
- $17.00
Is Claude Mythos 5.1 the same model as Claude Fable 5.1?
Yes, by Anthropic's own description: "Claude Mythos 5.1 is identical to Fable 5.1, but it offers more permissive safeguards for vetted individuals and organizations." The safeguards in question cover cybersecurity and the life sciences. The other difference is access: Claude Mythos 5.1 is offered only to vetted organizations, while Claude Fable 5.1 is generally available.
Nothing else on the spec sheet separates them. Both launched on September 1, 2026, both take 1M tokens of context billed at standard rates, and both write up to 128K tokens of output. Both use Anthropic's newer tokenizer, which counts about 30% more tokens than earlier Claude models for the same text, so a given prompt is the same size on either.
Do Fable 5.1 and Mythos 5.1 cost the same?
They do, line for line. Input costs $10 per million tokens and output $50. A 5-minute cache write costs $12.50 and a 1-hour write costs $20. Those are the 1.25x and 2x multiples Anthropic applies across its Claude models. A cache hit costs $0.25, because Anthropic bills hits on these two models at 0.025x the input price rather than the 0.1x that applies to most Claude models.
On the example agentic coding session that works out to $10.50 on each. Cache writes are $6.50 of it, or 62%, and output is $2.50. Reading 2M tokens from the cache costs only $0.50, which is why caching saves $17.00 per session, 62% of what the same tokens would cost without it.
Uncached work shows the same symmetry: $2.00 for the large one-off review and $4.30 for the output-heavy generation, on either model. At 110 sessions a month the projection is $1,155.00 for both. Since price can't separate them, the decision rests on access and safeguards alone.
How do you get access to Claude Mythos 5.1?
Mythos 5.1 is invitation only. Anthropic offers it through Project Glasswing and its trusted-access programs, currently to a set of US organizations. Approved organizations can call it on the Claude API, Amazon Bedrock, Google Cloud, and Microsoft Foundry. Cursor, OpenRouter, and GitHub Copilot do not list it.
Fable 5.1 needs no approval. It is listed in Claude Code, Cursor, OpenRouter, OpenCode, and GitHub Copilot. In Claude Code you pick it with /model fable, since it is not the default on any plan. Fable 5.1 requires 30-day data retention, so an organization that runs under zero data retention cannot use it.
In Claude Code, the cache lifetime for Fable 5.1 depends on how you sign in: the main conversation uses the 1-hour cache on a Claude subscription and the 5-minute cache with an API key. Anthropic's rule of thumb is that a 5-minute write pays for itself after one cache read and a 1-hour write after two, and at $0.25 per hit each read saves most of the $10 input price.
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
Your own numbers
See what Claude Fable 5.1 and Claude Mythos 5.1 really cost you.
everyaitoken reads your Claude Code, Cursor, OpenCode, and OpenRouter history on your Mac and prices every request at API rates, with what caching saved or cost. $9 once.
FAQ
Questions
Is Claude Mythos 5.1 more capable than Claude Fable 5.1?
Anthropic says the two are identical. Mythos 5.1 differs in its more permissive safeguards and in who can get access, not in the underlying model.
Does Claude Mythos 5.1 cost more than Claude Fable 5.1?
No. Both cost $10 per million input tokens, $50 per million output tokens, and $0.25 per million for a cache hit. The example session costs $10.50 on either.
Can I use Claude Mythos 5.1 in Cursor, OpenRouter, or GitHub Copilot?
No. None of the three lists it. Approved organizations reach Mythos 5.1 through the Claude API, Amazon Bedrock, Google Cloud, or Microsoft Foundry.
How do I see what Claude Fable 5.1 costs me in practice?
EveryToken is a $9 one-time Mac app that reads your local Claude Code history and prices every request at API rates, with cache writes and hits shown separately. That shows how much of your Fable 5.1 spend the cheap cache hits are saving.
Sources
- Anthropic: Pricing
- Anthropic docs: Claude Fable 5.1
- Anthropic: Claude Fable 5.1 and Claude Mythos 5.1
- Claude Code docs: Model configuration
- Cursor docs: Claude Fable 5.1
- OpenRouter: Claude Fable 5.1
- OpenCode docs: Zen
- GitHub Docs: Supported AI models in Copilot
- Anthropic docs: Claude Mythos 5.1
- Anthropic: Claude Mythos
- Anthropic: Prompt caching