Anthropic Mythos 5 — The Frontier Tier Above Opus, Explained

Overview

Anthropic's Claude Mythos 5, launched June 9, 2026, is a frontier-tier AI model, sharing the same powerful core as Fable 5 but with safeguards lifted, making it Anthropic's most capable system. It offers unparalleled cybersecurity capabilities, leading to its deployment through Project Glasswing with the US government and a temporary export control suspension. Mythos 5 exemplifies a two-tier model release, offering a public, safeguarded version and a gated, unlocked version for vetted partners.

In this video

• Anthropic revealed Claude Fable 5 and its twin, Claude Mythos 5, on June 9, 2026, with Mythos 5 being the more capable, tightly controlled system.
• The Mythos-class models sit a full step above the Opus class, demonstrating state-of-the-art performance on nearly every tested benchmark, especially for complex tasks.
• Mythos 5 and Fable 5 share the same underlying engine, featuring a one-million-token context window and up to one-hundred-twenty-eight-thousand output tokens.
• Both models include persistent file-based memory, making them about three times better than Opus 4.8, and scored 80.3% on the SWE-Bench Pro coding benchmark.
• The primary difference between Mythos 5 and Fable 5 lies in guardrails, as Mythos 5 has safeguards lifted in areas where Fable 5 would defer to a more conservative model like Claude Opus 4.8.
• Mythos 5 did not launch as a public product but was first deployed narrowly through Project Glasswing, in collaboration with the United States government.
• Anthropic states that Mythos 5 possesses the strongest cybersecurity capabilities of any model globally, capable of reading and reasoning about code and system weaknesses at unmatched scale and speed.
• Due to its dual-use nature and sensitive cybersecurity capabilities, Mythos 5's unlocked version is gated and not sold to everyone, unlike the safeguarded Fable 5.
• On June 12, just three days after launch, Mythos 5 and Fable 5 were suspended for roughly two-and-a-half weeks due to US government export controls, before being restored on July 1.
• The publicly measured Fable 5 already ranks near the top of leaderboards, around number two overall with a composite score near 95, with Mythos 5 performing even higher on unrestricted tasks.

Frequently asked questions

What is Anthropic Claude Mythos 5?

Claude Mythos 5 is Anthropic's most capable and tightly controlled frontier-tier AI model, launched June 9, 2026. It shares the same powerful core engine as Claude Fable 5 but operates with some of its safeguards lifted.

How does Mythos 5 differ from Fable 5?

While Mythos 5 and Fable 5 share the identical underlying engine and raw capability, the difference lies in their guardrails. Mythos 5 has certain safeguards lifted, allowing it to answer directly in sensitive areas where Fable 5 would defer to a more conservative model.

What are the key capabilities of Mythos 5?

Mythos 5 is state-of-the-art, featuring a one-million-token context window and up to one-hundred-twenty-eight-thousand output tokens. It is about three times better than Opus 4.8 with persistent file-based memory and scored 80.3% on the SWE-Bench Pro coding benchmark.

Why was Mythos 5 not launched as a public product?

Mythos 5 was first deployed narrowly through Project Glasswing in collaboration with the United States government due to its extremely powerful and dual-use cybersecurity capabilities. Unrestricted access to its unlocked version poses a genuine risk.

What happened with export controls for Mythos 5?

Three days after its June 9 launch, Mythos 5, along with Fable 5, was suspended on June 12 due to US government export controls. Access was restored globally on July 1, after roughly two-and-a-half weeks of being offline.