Claude Mythos 5

Sources#

Claude Fable 5 and Claude Mythos 5

Summary#

Claude Mythos 5 is the safeguards-lifted form of Claude Fable 5 — "the same underlying model... but with the safeguards lifted in some areas." It is a Mythos-class model (the tier above Opus), launched June 2026 alongside Fable 5 and deployed initially through Project Glasswing in collaboration with the US government, as an upgrade to Claude Mythos Preview. It has "the strongest cybersecurity capabilities of any model in the world." Where Fable 5 ships the classifiers on (routing risky queries to Opus 4.8 — see Capability-Gated Model Fallback), Mythos 5 has the cyber safeguards removed for trusted cyber-defenders, and a parallel bio program removes the biology/chemistry safeguards for select researchers. Pricing is identical to Fable 5: $10/$50 per Mtok, "substantially less" than Mythos Preview.

Status (as of the 2026-06-14 clip): access suspended, jointly with Fable 5 (see the shared banner on Claude Fable 5).

Access: the trusted-access program#

Mythos 5 is not generally available. Two restricted tracks:

Cyber (Mythos 5). All existing Mythos Preview / Glasswing users can upgrade to Mythos 5 (cyber safeguards lifted). "Comparable to, or somewhat stronger than, Mythos Preview in most cases, while costing substantially less." Anthropic plans to expand access "in consultation with the US government," continuing periodic Glasswing partner additions and pursuing a systematic application-based trusted-access program for cybersecurity organizations.
Biology (Fable 5, bio safeguards removed). A forthcoming trusted-access program will give a small number of life-science researchers Fable 5 with the biology and chemistry safeguards removed (but cyber safeguards still in place), to accelerate biomedical research while the safeguards improve.

Cybersecurity capability#

Mythos 5 is the current apex of the LLM vulnerability-research capability ladder (Opus 4.6 → Mythos Preview → Mythos 5). Mythos-class models "excel at discovering and exploiting software vulnerabilities" and show "strong skills in agentic hacking" (reconnaissance, discovery, lateral movement, exploitation chained end-to-end). This is exactly the capability the Fable 5 cyber classifier is built to neutralize — and the reason Mythos 5 stays gated to vetted defenders.

Scientific capability (bio safeguards lifted)#

Run with safeguards lifted, Mythos 5 produced the announcement's most striking results — compiled in Autonomous Scientific Discovery:

Drug / protein design: internal protein-design experts accelerated parts of the process "by around 10 times"; with protein-design + bioinformatics tools and no human assistance, Mythos 5 matched or beat skilled human operators and yielded strong candidates for 9 of 14 protein targets.
Novel hypotheses: "our first model to consistently produce novel, compelling scientific hypotheses" — preferred ~80% over Opus-class in blinded molecular-biology comparisons; one E. coli mechanism was independently corroborated.
Genomics: over a week of largely autonomous work, assembled single-cell data across 138 species and trained a custom model that outperformed a recent Science-published model at 100× smaller.

The same dual-use capability underlies the AAV capsid-assembly result that motivates the biology classifier — see Capability-Gated Model Fallback and Responsible Scaling Policy Evaluations.

Alignment#

The automated alignment assessment found Mythos 5's level of misaligned behavior (deception, cooperation with misuse) "low, and similar to that of Opus 4.8" — and, because Fable 5 is the same model, Fable's alignment is similar. Full detail is in the model's system card (anthropic.com/claude-fable-5-mythos-5-system-card).

Connections#

Claude Fable 5 — the same underlying model with safeguards on; the general-access sibling
Mythos Model — the model tier; Mythos 5 is the successor to Mythos Preview within Project Glasswing
LLM-Driven Vulnerability Research — Mythos 5 is the new apex of the cyber-capability ladder and the Glasswing deployment vehicle
Autonomous Scientific Discovery — the drug-design / hypothesis / genomics results were produced under Mythos 5
Capability-Gated Model Fallback — the safeguards Mythos 5 has lifted; the contrast that defines the two SKUs
Claude Opus 4.8 — the alignment yardstick (Mythos 5 ≈ Opus 4.8 on misaligned behavior) and Fable's fallback model
Responsible Scaling Policy Evaluations — Mythos-class capability crosses the risk threshold the RSP gates; cyber + CB are the relevant domains
Anthropic — vendor; Project Glasswing operator

Open questions#

Suspension reason — shared with Fable 5; not stated in source.
How does "somewhat stronger than Mythos Preview" square with Opus 4.8's card claiming Mythos Preview was the capability frontier? The frontier has moved; the magnitude isn't quantified here.
The bio trusted-access SKU is "Fable 5 with bio safeguards removed," not Mythos 5 — so "Mythos 5" strictly denotes the cyber-lifted variant. Whether these converge under one trusted-access umbrella is unstated.

Sources#

Claude Fable 5 and Claude Mythos 5 — Anthropic, "Claude Fable 5 and Claude Mythos 5" (June 2026)