Mythos Model

Sources#

Summary#

Anthropic's preview-tier frontier model. Notably described as "incredibly powerful" and gated behind safety review (the Mythos Preview / red.anthropic.com publication that established the LLM-Driven Vulnerability Research story). Used internally at Anthropic alongside Claude Opus 4.7. As of May 2026, not GA.

What's known publicly#

Mythos Preview demonstrated emergent cybersecurity capabilities — autonomous zero-day discovery, full exploit chains. See LLM-Driven Vulnerability Research for the detailed analysis from the Mythos Preview publication.
Anthropic's response: Project Glasswing safeguards (referenced in 4.7 release as "first post-Glasswing safeguards").
Boris Cherny: "We use a little bit of Mythos to try it and then a lot of Opus 4.7 to dog food it and to write most of our code." — Mythos is preview-tier, not the workhorse.
Cat Wu: "Mythos is an incredibly powerful model. But we do use the models internally and I think this has increased our rate of shipping a little bit but I don't think it explains the bulk of the increase." — confirms internal use; explicitly disclaims it as the cadence explanation.

Role in the Opus 4.8 System Card#

The Opus 4.8 System Card (May 2026) makes Mythos Preview's role unusually concrete — it remains the capability frontier that the general-access model is measured against, and it is used as a tool in the assessment itself:

Frontier benchmark: Opus 4.8 "does not advance the capability frontier beyond Mythos Preview." On the AECI index, Mythos scores 158.3 vs Opus 4.8's 155.5 (and 4.7's 154.1). Its Risk Report bounds the RSP case for 4.8.
Investigator model: Mythos Preview is one of the two investigator models driving Opus 4.8's Automated Behavioral Audit (the other being a helpful-only Opus 4.7).
Reviewer of the assessment: in a notable meta-move, Mythos was given access to internal Slack discussion and asked to review the near-final alignment section; its (published) review confirmed candor and flagged that no eval specifically tests for training-gaming — see Evaluation Awareness & Grader Gaming.
Alignment yardstick: Opus 4.8 matches Mythos Preview's alignment profile on most measures and surpasses it on several honesty metrics (Agentic Honesty & Diligence).

Capability data points (When AI builds itself)#

The Anthropic Institute essay (June 2026) attaches concrete numbers to Mythos Preview as the model that drove Anthropic's AI-R&D acceleration (AI Accelerating AI Development):

Time horizon: METR rated it able to work for "at least" 16 hours, "at the upper end of what [METR] can measure without new tasks" (Task Time-Horizon Scaling).
Kernel-optimization eval: ~52× speedup (April 2026) on the train-a-small-model-faster task, vs Opus 4's ~3× a year earlier and a ~4× human baseline — "from super helpful to superhuman in under a year."
Research next-step judgment: beat the human choice 64% of the time on hard detour moments (Opus 4.5 was 51% in Nov 2025).
Self-reported uplift: in a March 2026 poll of 130 research-team employees, the median estimated ~4× output with Mythos Preview vs no AI (Anthropic believes true uplift was somewhat lower).

These are deployment-side figures (Mythos used internally), distinct from the System Card's gated-capability-frontier framing above.

Why kept gated#

The combination of:

Cybersecurity capability gap vs prior models (per Mythos Preview publication)
Safety mechanisms still being evaluated and hardened
Anthropic's stated mission posture

Means the model is used internally and previewed selectively rather than shipped broadly. Some descendant of Mythos is expected to ship publicly later — Boris Cherny: "It will become some version of some descendant of that will become available at some point to everyone."

Update — the descendants shipped (June 2026)#

Boris's prediction came true. In June 2026 Anthropic launched Fable 5 and Mythos 5 — the first general-access Mythos-class models, and the realization of "Mythos-class" as a named capability tier sitting above the Opus class. The lineage is now Mythos Preview (April 2026) → Fable 5 / Mythos 5 (June 2026):

Fable 5 = a Mythos-class model "made safe for general use" via classifiers that fall back to Opus 4.8 on cyber/bio/distillation queries (Capability-Gated Model Fallback).
Mythos 5 = the same underlying model with safeguards lifted, deployed through Project Glasswing as the upgrade to Mythos Preview ("comparable to, or somewhat stronger than" it, at less than half the price). Existing Glasswing/Mythos-Preview users upgrade directly.

This moves the capability frontier beyond Mythos Preview — the line the Opus 4.8 System Card treated as the ceiling — and is the first time Mythos-class capability reaches the public. (Both were reported suspended shortly after launch; see Claude Fable 5.)

Connections#

Anthropic — vendor
Claude Opus 4.7 — prior GA model; Mythos is the next-tier preview
Claude Opus 4.8 — current GA model benchmarked against Mythos; Mythos is its capability frontier, an investigator in its behavioral audit, and the reviewer of its alignment assessment
LLM-Driven Vulnerability Research — primary public account of capability profile
Harness Shrinkage as Models Improve — Mythos-class capability is what makes Boris's "100 lines" prediction conceivable
AI Native Product Cadence — explicitly disclaimed as the cadence explanation, but contributes
AI Accelerating AI Development — the model behind Anthropic's measured AI-R&D acceleration (52× kernel eval, 64% next-step, ~4× poll)
Task Time-Horizon Scaling — Mythos sits at the measurable edge of the time-horizon curve (16+ hours)
METR — rated Mythos's time horizon at "at least 16 hours," beyond its standard measurement ceiling
Claude Fable 5 — the first general-access Mythos-class model; Mythos Preview's safeguarded descendant
Claude Mythos 5 — the safeguards-lifted descendant deployed through Project Glasswing as the direct Mythos Preview upgrade
Capability-Gated Model Fallback — the safeguard architecture that made general release of a Mythos-class model possible

Open questions#

Public release timeline: answered — Mythos Preview itself never shipped GA, but its descendants Fable 5 / Mythos 5 reached general access in June 2026 (see the descendants shipped above). Both were suspended shortly after launch; whether and when they return is open.
Capability profile beyond cybersecurity: Mythos Preview focused on the safety story; other capability dimensions not well-documented externally.
Internal access controls: who at Anthropic actually uses Mythos for daily work, vs Opus 4.7? Boris implies infrequent (try-it use); not detailed.

Sources#

Claude Mythos Preview red.anthropic.com
How Anthropic's product team moves faster than anyone else | Cat Wu (Head of Product, Claude Code)
Anthropic's Boris Cherny: Why Coding Is Solved, and What Comes Next
When AI builds itself — Mythos capability data points (METR 16h, 52× kernel, 64% next-step, ~4× poll)
Claude Fable 5 and Claude Mythos 5 — the June 2026 launch of the first general-access Mythos-class descendants