Claude Fable 5

Sources#

Claude Fable 5 and Claude Mythos 5

Summary#

Claude Fable 5 is Anthropic's first generally-available Mythos-class model (launched June 2026) — a tier of Claude models that "sit above our Opus class in capability." It is described as state-of-the-art on nearly all tested capability benchmarks (software engineering, knowledge work, vision, scientific research), with the lead widening "the longer and more complex the task." Fable 5 is the same underlying model as Claude Mythos 5 — the two differ only in their safeguards (footnote: Fable from Latin fabula, "that which is told," akin to the Greek mythos). Fable ships the safeguards on; Mythos has them lifted in some areas. The safeguard architecture — classifiers that route risky queries to Opus 4.8 instead of refusing — is documented in Capability-Gated Model Fallback.

Status (as of the 2026-06-14 clip): access suspended. The source page now leads with a banner: "We are suspending access to Claude Fable 5 and Claude Mythos 5. We apologize for this disruption to our customers and are working to restore access as soon as possible." The launch announcement below is preserved; the suspension is the live state.

Pricing and identity#

$10 per million input tokens / $50 per million output tokens — explicitly "less than half the price of Claude Mythos Preview." Same pricing for Fable 5 and Mythos 5.
API model id: claude-fable-5 (via the Claude API / platform.claude.com).
"Fable 5's capabilities exceed those of any model we've ever made generally available."

Capability highlights#

The announcement leads with Fable 5 (general access); the science-heavy results run under Mythos 5 (bio safeguards lifted) and are compiled in Autonomous Scientific Discovery. Fable's headline demonstrations:

Software engineering. Stripe reported Fable 5 "compressed months of engineering into days" — a codebase-wide migration in a 50-million-line Ruby codebase done in a day that "would otherwise have taken a whole team over two months by hand." On Cognition's FrontierCode eval (passing hard coding tasks while meeting production-codebase standards), Fable 5 scores highest among frontier models "even at medium effort," and is more token-efficient than past Claude models.
Knowledge work. Highest score of any model on Hebbia's Finance Benchmark for senior-level reasoning (document reasoning, chart/table interpretation, problem solving); IMC reported Fable 5 "aced their trading-analysis evaluations nearly across the board."
Vision (new SOTA). Extracts precise numbers from detailed scientific figures; rebuilds a web app's source code from screenshots alone. Crucially, it "needs less scaffolding" — previous Claude models struggled to play Pokémon FireRed even with helper harnesses, but Fable 5 beat FireRed with a minimal, vision-only harness (raw game screenshots, no maps/navigation aids/game-state). This is a flagship case of Harness Shrinkage as Models Improve.
Memory and long-context. Stays focused "across millions of tokens" and improves outputs using its own notes. In Slay the Spire, file-based persistent memory improved Fable 5's performance 3× more than it improved Opus 4.8's, and Fable reached the final act 3× more often.
Autonomy. Fable 5 and Mythos 5 "can work autonomously for longer than any previous Claude models" — see Task Time-Horizon Scaling.

(The head-to-head benchmark table vs other leading models is published only as an image in the source and is not transcribed here.)

Safeguards (why Fable exists)#

Fable is the safeguarded SKU. Because Mythos-class capability "could be misused to cause serious damage" (especially cybersecurity, biology), Fable ships with classifiers covering cybersecurity, biology & chemistry, and distillation. When a classifier fires, the response is handled by Opus 4.8 rather than refused, and the user is told. Anthropic tuned the classifiers conservatively (they "sometimes catch harmless requests," triggering in "less than 5% of sessions"); more than 95% of Fable sessions involve no fallback at all, and for those Fable's performance "is effectively the same as that of Mythos 5." Full treatment: Capability-Gated Model Fallback.

Alignment: the automated alignment assessment found Mythos 5's misaligned behavior "low, and similar to that of Opus 4.8"; "given that they are the same underlying model, Fable 5's level of alignment will be similar." A 30-day data-retention requirement now applies to all Mythos-class traffic (safety use only).

Availability and rollout#

Fable 5 "available everywhere today"; fully available from launch on the Claude API and consumption-based Enterprise plans.
Staged rollout on subscription plans: free on Pro/Max/Team/seat-based Enterprise from launch through June 22; from June 23 it requires usage credits; Anthropic "aim[s] to restore Fable 5 as a standard part of subscription plans" once capacity allows.
Demand expected "very high, and difficult to predict" — the stated reason for staged access. (See the suspension note above for the live state.)

Connections#

Mythos Model — the model tier and its first member (Mythos Preview); Fable 5 is the first general-access Mythos-class model
Claude Mythos 5 — the same underlying model with safeguards lifted; the cyber/bio sibling
Claude Opus 4.8 — the fallback model: safeguarded Fable queries are answered by Opus 4.8 instead of refused
Capability-Gated Model Fallback — Fable's defining safeguard architecture (classifiers + fallback-not-refuse + 30-day retention)
Harness Shrinkage as Models Improve — Pokémon-FireRed-vision-only and Slay-the-Spire-memory are flagship harness-shrinkage demonstrations
Autonomous Scientific Discovery — the science results (run under Mythos 5, the bio-safeguards-lifted form of the same model)
Task Time-Horizon Scaling — "work autonomously for longer than any previous Claude models" pushes the autonomy-duration curve
Anthropic — vendor
Claude Code — the agentic runtime Mythos-class coding gains flow through

Open questions#

Why was access suspended after launch? The source banner gives no reason (capacity? a safety finding? the UK-AISI jailbreak progress noted in Capability-Gated Model Fallback?). Not in source.
Exact benchmark numbers vs GPT-5.x / Gemini are image-only in the source; not transcribed.
How much of Fable's general-access experience is actually Fable vs Opus-4.8 fallback for security-research-adjacent users whose queries trip the conservative classifiers?

Sources#

Claude Fable 5 and Claude Mythos 5 — Anthropic, "Claude Fable 5 and Claude Mythos 5" (June 2026; AAV edit June 9, 2026)