Fable 5

Anthropic's most intelligent model (ID claude-fable-5), released 2026-06-09 — a new tier above Opus. A Mythos-class model with safeguards active, generally available.

Mythos 5

The same underlying model with safeguards lifted in specific domains. Restricted to vetted partners; see Mythos, explained.

Mythos-class

Anthropic's term for this capability tier — powerful enough that public versions ship with active classifier safeguards rather than training-time restrictions alone.

Adaptive thinking

thinking: {"type": "adaptive"} — the model decides when and how deeply to reason per request. The only thinking mode on Fable; fixed budget_tokens is gone.

Effort

output_config.effort — how hard the model works: low, medium, high (default), xhigh, max. Scales thinking depth, tool-call consolidation, and verbosity together.

Thinking display

Whether reasoning text appears in responses. Defaults to omitted on Fable; set display: "summarized" to show users a summary while it thinks.

Task budget

A beta feature giving the model a visible token allowance for a whole agentic run, which it paces itself against. Distinct from max_tokens, the hard per-response cap it can't see. Recipe 06.

Structured outputs

Schema-guaranteed JSON via output_config.format or messages.parse(). The replacement for assistant prefills, which now return 400. Recipe 03.

Prompt caching

Reusing the unchanged front of your prompt at ~10% of input price. Prefix-match by exact bytes; Fable's minimum cacheable prefix is 2,048 tokens. Recipe 04.

Classifier safeguards

Runtime filters watching cyber, bio/chem, and distillation topics — triggering on <5% of sessions. Distinct from refusals trained into the model itself.

Fallback (Opus 4.8)

When a safeguard trips, the response is handled by Claude Opus 4.8 instead of Fable. You still get an answer — from the safer sibling.

Distillation

Training a competing model on another model's outputs. Fable's third classifier watches for extraction attempts.

Project Glasswing

The partner program whose members get Mythos 5 with cyber safeguards lifted, for security research.

Subscription window

Fable included free on claude.ai paid plans 2026-06-09 → 06-22; usage credits required from 06-23. Details.

Agents First

Joshua Baer's framework (agentsfirst.dev): build for the agent that decides as well as the human who pays. It's why this site has /llms.txt and typed /api/ twins.