Claude Fable 5.2 and Mythos 5.2 are available

What changed

Fable 5.2 and Mythos 5.2 are one model. Fable keeps strong cybersecurity and biology safeguards and is available broadly. Mythos is for organizations Anthropic verifies through the Life Sciences and Cyber Verification Programs.

Anthropic says testers found the models communicate more clearly than earlier Fable and Mythos models, and that they generate outputs up to 30% faster than the 5.1 versions. List price is $8 per million input tokens and $40 per million output tokens, down from $10 and $50. An optional fast mode speeds token generation by up to 2.5 times, at $16 and $80 per million tokens.

On the Claude Platform, both models use a 1 million token context window, 128,000 max output tokens, and always-on adaptive thinking. The model IDs are claude-fable-5-2 and claude-mythos-5-2. Fable 5.2 is on the Claude API, Claude Platform on AWS, Claude in Amazon Bedrock, Claude Models on Google Cloud (Anthropic managed and Google managed), and Claude in Microsoft Foundry. Fast mode for Fable 5.2 on the Messages API is speed: "fast" with no beta header. Mythos fast mode is limited to verification-program organizations. Fast mode is also on Claude Code and the clouds Anthropic lists in the launch post.

Anthropic's launch post says these models are for the most ambitious, open-ended technical work. For most coding and knowledge tasks it says to start with Claude Opus 5.5, then move up to Fable or Mythos, or down to Sonnet 5.5 or Haiku 5.5. Medium is the default effort in the Claude apps and Claude Code. Fable 5.2 can also be an Advisor that smaller models consult, or an Orchestrator that hands simpler tasks to them.

API behavior that will break a naive swap, from the November 9 platform notes:

• The prefix check applies to every account. If your code edits the system prompt, tools, or an earlier message and then sends back a Fable 5.2 or Mythos 5.2 thinking block, the request returns a 400.
• On the Claude API and on Google-managed Claude Models on Google Cloud, moving onto these models keeps reasoning from Opus 5.5, Sonnet 5.5, and Haiku 5.5. Moving off them drops the thinking blocks they produced. The request still succeeds.
• Computer use goes through computer_toolset_20260801. The older computer_20251124 tool returns a 400 everywhere except Amazon Bedrock.
• With the advisor tool, still in beta, a Fable 5.2 or Mythos 5.2 executor accepts only one of those two models as advisor. Any executor model can use Fable 5.2 as its advisor.
• Both models require 30-day data retention. They are not available under zero data retention unless Anthropic expressly authorizes it. Enterprise Frontier Safeguards, which runs safety monitoring on data stored in the customer's infrastructure, is in private beta.
• Thinking blocks work only in the account that produced them, or an account linked to it.

This week, Anthropic says eligible paid plans that include Fable get one to three extra Fable usage resets based on how long they have subscribed. On Pro, and on standard Team seats, Fable is included for up to 50% of weekly usage limits.

Claude Enterprise customers can use Claude Security, which now includes Mythos 5.2, without joining the Cyber Verification Program. Mythos itself still requires that program, or the life sciences program.

Why it matters

Do not point every workflow at Fable because it is the newest model. Anthropic's own guidance is to keep Opus 5.5 as the daily driver and call Fable when the task is long, ambiguous, or expensive to get wrong.

If you do switch an API integration, check four things before the first production request: whether your client rewrites the prompt prefix, which computer-use tool you declare, which model you set as advisor, and whether the account is allowed to keep the required 30-day retention. A 400 on a thinking block is the prefix check, not a random outage.

The lower list price is real ($8 / $40 versus $10 / $50). Fast mode is a separate, higher price. Usage caps still apply in the apps. Pro and standard Team seats can spend at most half of the weekly limit on Fable during this launch window.

Frequently asked questions

Are Fable 5.2 and Mythos 5.2 different models?

Anthropic says they are the same underlying model. Fable ships with cybersecurity and biology safeguards and is broadly available. Mythos is for organizations in the Life Sciences or Cyber Verification Programs.

What does Fable 5.2 cost on the API?

$8 per million input tokens and $40 per million output tokens. Fast mode is $16 and $80. The 5.1 models were $10 and $50. Context is 1 million tokens, and max output is 128,000.

Should we replace Opus 5.5 with Fable 5.2?

Anthropic says no for most coding and knowledge work. Start with Opus 5.5, then move up to Fable or Mythos when the task needs it, or down to Sonnet 5.5 or Haiku 5.5.

Sources

• Claude on X: Introducing Claude Fable 5.2 and Mythos 5.2 (Nov 9, 2026)
• Anthropic on X: Fable 5.2 and Mythos 5.2 are available now
• Claude Fable 5.2 and Mythos 5.2
• Claude Platform release notes, November 9, 2026