Anthropic’s release yesterday of Claude Fable 5 and Mythos 5 has drawn comments from the industry both praising the models and discussing how best to secure them for the AI era.
Fable 5 is Anthropic’s frontier model with security safeguards, while Mythos 5 has some guardrails removed.
Good for autonomous testing work
In a statement, CodeRabbit wrote: “We saw that across multiple coding projects we used to test the model’s capabilities. We could give Fable 5 vague prompts and still get complete projects rather than prototype shells. It also found solution paths that felt less obvious, including approaches that earlier model reviews struggled to reach without more hand-holding.”
CodeRabbit noted, though, that that kind of behavior shows up as a cost, as it found that Fable 5 kept working until the harness stopped it. So the model feels capable, but is expensive and slow in agent workflows that do not have strong harnesses to cut them off.
It further recommends that you not switch everything to Fable 5, but to use it to explore, plan and build — especially where autonomy is the product — but keep the current reviewer in place.
Models pair innovation, resilience
Anthony Grieco, SVP, chief security and trust officer at Cisco, said organizations struggle with security cycles that don’t keep pace with changes in new models, those that “pair innovation with resilience” will be best positioned for success. AI-generated code.
Cisco — an early tester of both Anthropic and OpenAi models — said Anthropic’s releases yesterday align with its mission of giving enterprises the AI tools to get faster responses and improved resilience, along with the strategy and infrastructure to leverage those tools.
“The pace of frontier AI development is changing the security landscape in real time, and defenders cannot afford to wait for the dust to settle,” Grieco said. “Whether the model is Claude Mythos 5, Claude Fable 5, GPT-5.5-Cyber, or the next breakthrough, the challenge is no longer just access to advanced AI, but how organizations operationalize it with the right harness, infrastructure, and agentic logic to turn speed into clarity and action.
“That means continuing to invest in the fundamentals that never go out of date: patching, MFA, segmentation, and Zero Trust,” he added. “AI will raise the ceiling for what defenders can do, but security resilience remains the foundation that determines whether those gains translate into real protection.”
No downsides to public release
While many in the industry bemoaned the fact that Anthropic took a limited rollout approach to the models, Roger Grimes, CISO advisor at cybersecurity company KnowBe4, said there are no downsides to making Fable 5 publicly available. “The sooner the band-aid is ripped off, the sooner the defender lifecycle kicks in and helps us,” he wrote in a statement.
“Regarding whether cybercriminals will get access to these tools faster: no, not really,” he said in the statement. “Criminals have been using AI to find vulnerabilities, code exploits, and code malware since last year. Certainly, learning about Mythos put a renewed, more intense push on using AI to find vulnerabilities and exploit them, but it wasn’t like it hasn’t been what the elite cybercriminals haven’t been doing for a year already … Heck, I saw similar non-AI versions of Mythos being used by nation-states and large red teams over a decade ago. They were pretty good then, but now AI-enabled, they are supercharged. The only thing Mythos substantially changed was how quickly the defenders would get these tools. Sure, it accelerated and helped attackers, but they didn’t need the push. Defenders needed the bigger wake-up call.”
Grimes went on to say that he expects to see a spike in found and exploited vulnerabilities over the next 2-3 years, but then applications will become more secure.
He has three things that CIOs and CTOs need to be aware of:
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Vulnerabilities and zero days will explode over the next few years and be exploited faster and more successfully
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Defenders need to run the same AI-methods to find and fix vulnerabilities before the attackers do
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Patching needs to be done faster…with defenders perhaps re-examining their current risk acceptance, and potentially patching faster without testing.
Security by Design, not a ‘security armageddon’
Meanwhile, Charles Guillemet, CTO at blockchain security company Ledger, said “security by design is the only layer that makes infrastructure resistant to cyber vulnerability. That includes formal verification, using hardware based secure enclaves.”
In a LinkedIn post, Guilllemet cautioned against fear that exploited vulnerabilities in big numbers will lead to a security armageddon. “Mythos is, at its core, Opus 4.xx with reinforcement learning specialized on offensive security,” he wrote. “Attackers have had functionally equivalent capability for months. The proof is in the telemetry: a tidal wave of in-the-wild exploitation, and the price of stolen access on dark markets has never been lower. We’re barely scratching the surface. Nothing is secure anymore and that won’t change anytime soon.
That’s not changing. At the same time individuals and organizations remain slow to update their software stacks. Security used to be a cat and mouse game. Now, everyone can be a cat.”
He added his belief that security is largely absent from the broader AI agent conversation, and organizations are slow to update their software stacks. “Security,” he said, “used to be a cat and mouse game. Now, everyone can be a cat.”
