GPT-5.5-Cyber vs Claude Mythos: who can access them and how the cyber-AI race compares

What GPT-5.5-Cyber is — and what it is not

Public sources do not establish that GPT-5.5-Cyber is a wholly separate foundation model with fully disclosed training differences. The safer description is a limited, cyber-permissive access path around OpenAI’s GPT-5.5 capabilities. Benzinga reported that the preview is not meant to “significantly increase” cybersecurity capability beyond GPT-5.5, but is trained to be more permissive for security-related tasks and uses stronger verification and account-level controls for sensitive workflows ^[14].

That distinction matters. For defenders, part of the value may be permission to run legitimate security workflows that a general model would otherwise treat as sensitive. TechCrunch reported that GPT-5.5 Cyber can perform tasks such as penetration testing and vulnerability identification or exploitation, while OpenAI is limiting who can use it ^[3].

Who can access GPT-5.5-Cyber?

The clearest public answer is: vetted cyber defenders. TechCrunch reported that Sam Altman said OpenAI would start rolling GPT-5.5 Cyber out to “critical cyber defenders” and that OpenAI had an application process where applicants submit credentials and planned use ^[3]. Benzinga similarly described the rollout as a preview for a limited number of cybersecurity defenders with stronger verification and account controls for sensitive workflows ^[14].

That means most ChatGPT users, ordinary developers, and companies should not assume they can simply turn it on. The public reporting cited here does not disclose full eligibility rules, approval timelines, a complete access list, pricing, or API terms.

What Claude Mythos Preview is

Claude Mythos Preview is Anthropic’s competing restricted model in this cyber-defense race. Reports describe Mythos as unavailable to the general public and distributed through Project Glasswing or closed partner and allowlist arrangements for defensive security work ^[8][9].

The strongest public benchmark story for Mythos comes from AISI. In its evaluation, an early snapshot of Anthropic’s Claude Mythos Preview represented a step up over previous frontier models and became the first model to complete AISI’s simulated corporate-network attack exercise end to end, a multi-step task AISI estimates would take a human around 20 hours ^[1].

GPT-5.5-Cyber vs Claude Mythos: access and capability

Based on the public record, the practical comparison looks like this:

Which one is better?

Based on public evidence, there is no clean winner. Claude Mythos has the clearest historical milestone: in AISI’s account, it was the first frontier model to complete the corporate-network attack simulation end to end ^[1]. But the same AISI write-up says an early GPT-5.5 checkpoint reached similar performance, implying the gap among frontier labs is narrowing quickly ^[1]. Axios similarly framed OpenAI as not far behind Mythos in hacking-related testing ^[10].

OpenAI may have the more actionable access story for some defenders if its application-based preview proves broader than Anthropic’s partner network, but public evidence does not quantify how many teams will actually receive GPT-5.5-Cyber access ^[3][8][14]. For most practitioners, the answer is the same for both models: the most sensitive capabilities are behind a trust or partner screen ^[8][9][14].

Why both companies are gating access

Cybersecurity is one of the clearest dual-use AI domains. The same model behavior that helps a defender identify and validate a vulnerability can help an attacker, and AISI’s corporate-network attack simulation shows frontier models beginning to handle multi-step cyber operations rather than only isolated prompts ^[1]. Reporting on GPT-5.5 Cyber says it can assist with penetration testing and vulnerability identification or exploitation, which helps explain why access is being limited rather than opened broadly ^[3].

That is the strategic shift: the competition is no longer just who has the strongest model. It is also who can route stronger cyber capabilities to defenders without handing the same leverage to untrusted users. OpenAI’s reported answer is application-based, verified access with stronger controls; Anthropic’s reported answer is a tighter partner or allowlist model around Mythos ^{[3][8][9][14]}.

What remains unknown

The unanswered questions are unusually important because both systems are restricted:

• OpenAI has not provided, in the public reporting cited here, a full technical specification for GPT-5.5-Cyber or a clear explanation of whether it uses distinct weights, fine-tuning, policy changes, tool access, or some combination.
• There is no complete public list of approved GPT-5.5-Cyber users, nor public pricing or API terms in the cited sources.
• There is no direct, public, head-to-head benchmark of production GPT-5.5-Cyber and production Mythos. The strongest evaluation source compares an early GPT-5.5 checkpoint with an early Mythos snapshot ^[1].
• Because Mythos is restricted and GPT-5.5-Cyber is gated, independent researchers and ordinary security teams cannot easily reproduce the headline claims ^[8][9][14].

Bottom line

GPT-5.5-Cyber is best read as OpenAI’s controlled channel for giving advanced GPT-5.5 cyber capability to vetted defenders. Claude Mythos Preview is Anthropic’s more tightly partner-gated counterpart. The AISI result makes the bigger point: advanced cyber capability is no longer a one-model anomaly; at least two frontier developers are approaching similar levels, and access control has become part of the product ^{[1][3][8][14]}.