The launch marks a new "Mythos-class" era of model intelligence, moving past the previous Opus 4.8 frontier.The essential details from the official release include:1. The Core Split: Public vs. Gated
The launch marks a new "Mythos-class" era of model intelligence, moving past the previous Opus 4.8 frontier.
The essential details from the official release include:
1. The Core Split: Public vs. Gated Cyberdefense
Fable 5 and Mythos 5 are the exact same underlying model. The only differentiator between them is the restriction level of the safety layer:
Claude Fable 5 (Public GA): The first public-facing Mythos-class model. It features strict, highly conservative guardrails. If a user inputs a high-risk prompt (such as advanced malware analysis, biological scripting, or chemical formulas), Fable 5 will refuse the query and silently fall back to Claude Opus 4.8 to prevent misuse.
Claude Mythos 5 (Restricted Access): This variation has those security safeguards completely lifted. Because of its elite offensive and defensive cybersecurity capabilities, it is strictly restricted to vetted defense teams and infrastructure providers under the banner of Project Glasswing (conducted in collaboration with the US government).
2. Performance Metrics & Real-World Velocity
Anthropic notes that the model family excels at long-running, asynchronous execution and large-scale complex coding tasks:
Codebase Migrations: In early partner testing with Stripe, Fable 5 completed a comprehensive codebase migration across a 50-million-line Ruby repository in a single day—a task Stripe estimated would take a full engineering team over two months by hand.
Complex Data Analytics: It is the first frontier model to break 90% on core analytics benchmarks, jumping 10 points above Opus 4.8 on multi-step reasoning.
Bioinformatics Acceleration: In internal pharmaceutical trials, Mythos 5 accelerated aspects of automated drug and protein design workflows by 10x, matching or beating skilled human operators without intervention.
3. API Pricing & Mandatory Data Changes
Token Tariffs: Both models cost exactly double the price of Opus 4.8. Input tokens are priced at $10 per million, and output tokens cost $50 per million.
30-Day Data Retention Guardrail: For security reasons, Anthropic is mandating a strict 30-day data retention policy for all traffic hitting Mythos-class endpoints to track exploitation patterns. You cannot opt out of this into zero-retention parameters on Bedrock or AWS.
4. Immediate Dashboard Rollout Schedule
Anthropic is using a staged capacity-testing window to introduce Fable 5 to retail users:
Through June 22, 2026: Fable 5 is included natively for all Pro, Max, Team, and Enterprise seat tiers at zero extra cost.
On June 23, 2026: Fable 5 will be pulled from basic subscription models. Beyond this date, retail web users will need to buy developer usage credits to access it, until Anthropic scales out server infrastructure to support permanent integration.