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Claude Fable 5

Claude Fable 5 — Technology. characteristic: Model whose output quality is capped by the clarity of the human's unknowns · category: Mythos-class foundation model, general purpose · role: Model (pre-release, Mythos class) that rewrote Bun in Rust; mechanical port + adversarial review · status: Replaced by Fable 5.1; serves as a reference for benchmark and cost comparisons · rate: $10 / $50 per M tokens (input/output) — 2× Opus 4.8 · pricing: $10/M input tokens, $50/M output tokens

Anthropic announced Claude Fable 5 on 9 June 2026: a Mythos-class model made safe for general use, at $10 per million input tokens and $50 output, under half the price of Mythos Preview. Its conservative classifieurs de sécurité answer with Opus 4.8 on certain topics, in fewer than 5% of sessions.

Jarred Sumner used a pre-release version to port Bun from Zig to Rust in 11 days: 6,502 commits, 60,624 existing TypeScript tests kept as the oracle, roughly $165,000 in tokens. Sumner credits the reliability to a second Claude that saw only the diff and was told to find why the code was wrong.

Thariq Shihipar of the Claude Code team described Fable 5 as the first model where the quality of his work was capped by his ability to clarify its unknowns. The bottleneck moved from the model to the human's clarity.

Price is why the model recurs outside engineering accounts. Didier Girard cites $10/$50 as 2× Opus 4.8, the rising ceiling in an inference market whose floor keeps dropping. On GDPval-AA, Fable 5 scored 1783 Elo against 1615 for Opus 4.8 and 1524 for GLM-5.2; on SWE-Bench Pro it held roughly 15 points over GPT-5.6 Sol, which cost about a third less per task on Terminal-Bench 2.1. Claude Fable 5.1 replaced it on 1 September 2026, and Fable 5 became the baseline its successor is measured against: 24.7% to 52.6% on Terminal-Bench-Science 0.1.

Type
Technology
characteristic
Model whose output quality is capped by the clarity of the human's unknowns
category
Mythos-class foundation model, general purpose
role
Model (pre-release, Mythos class) that rewrote Bun in Rust; mechanical port + adversarial review
status
Replaced by Fable 5.1; serves as a reference for benchmark and cost comparisons
rate
$10 / $50 per M tokens (input/output) — 2× Opus 4.8
pricing
$10/M input tokens, $50/M output tokens
relations
17
Cited in
6 fiches

Adoption measures

Neighborhood

Anthropic classe Mythos Claude Mythos 5 Claude Fable 5.1 Finding Your Unknowns Jarred Sumner classifieurs de sécu… GLM-5.2 agents autonomes sur… Opus 4.8

← created

Anthropic ORGANISATION high confidence stable Source ↗

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Anthropic ORGANISATION high confidence stable Source ↗

→ is an instance of

classe Mythos CONCEPT high confidence stable Source ↗

→ is a variant of

Claude Mythos 5 TECHNOLOGIE high confidence stable Source ↗

← replaces

Claude Fable 5.1 TECHNOLOGIE high confidence stable Source ↗

← applies to

Finding Your Unknowns METHODOLOGIE high confidence timeless Source ↗

← uses

Jarred Sumner PERSONNE high confidence stable Source ↗

→ uses

classifieurs de sécurité TECHNOLOGIE high confidence stable Source ↗
Opus 4.8 TECHNOLOGIE high confidence evolving Source ↗

→ outperforms

GLM-5.2 TECHNOLOGIE high confidence evolving Source ↗
Opus 4.8 TECHNOLOGIE high confidence stable Source ↗

→ enables

agents autonomes sur tâches à long horizon CONCEPT high confidence timeless Source ↗

Cited in (6)