Claude Fable 5 (and Claude Mythos 5)

What it is

Claude Fable 5 is a large language model made by Anthropic and announced on 9 June 2026. Anthropic described it as a Mythos-class model for general use, and as the most capable model the company had made generally available up to that point. ^[1] Claude Mythos 5 is the same underlying model with some safeguards lifted for vetted users. ^[1] The API model ID is claude-fable-5. ^[1]

Background

Fable 5 belongs to the Mythos line of models. Mythos 5 was first deployed through Project Glasswing, carried out in collaboration with the US government, as an upgrade from Mythos Preview. ^[1] Both Fable 5 and Mythos 5 are priced at less than half the price of Mythos Preview. ^[1][2]

Main features

Price and context. Both models cost $10 per million input tokens and $50 per million output tokens. Like other Claude models from version 4.6 onward, they include the full 1M-token context window at standard pricing, so processing a long document or a large codebase carries no extra surcharge. ^[1][2]

Capability. Anthropic said Fable 5 was state-of-the-art on nearly all of the capability benchmarks it tested. As one example from outside the company, Stripe reported a 50-million-line Ruby migration completed in a day rather than over two months. ^[1]

Safeguards. Requests in restricted areas, namely cybersecurity, biology and chemistry, and distillation, fall back to Claude Opus 4.8, and the user is told with a notice. Anthropic said these classifiers trigger in under 5% of sessions on average. An external bug bounty ran for more than 1,000 hours without finding a universal jailbreak, which is a single method that gets past the protections in every case. ^[1]

Trusted access. A trusted-access programme gives select biology researchers Fable 5 with the biology safeguards removed. ^[1]

Data and access. Business customer data on Mythos-class models is kept for 30 days and is not used for training. ^[1] Fable 5 was included in subscription plans at no extra cost until 22 June 2026. After that date, subscription users needed usage credits, while the model stayed fully available through the API. ^[1]

Science. Mythos 5 accelerated parts of drug design by about ten times and produced strong candidates for 9 of 14 protein targets. ^[1]

Position in the timeline

On 12 June 2026, access to both models was suspended. Wikipedia says Anthropic acted to comply with a directive from the US Commerce Department. ^[1][5] CNBC reported on 30 June that the administration had lifted the export controls, and Anthropic redeployed both models on 1 July. ^[1][4][5]

Training data ran to January 2026, according to Wikipedia. ^[5] Fable 5 was succeeded by Claude Fable 5.1 and Mythos 5.1 on 1 September 2026, and its retirement is not scheduled before 9 June 2027. ^[3]