GPT-5.5-Cyber vs Claude Mythos vs GPT-5.5 — what's the difference?

Three different access models for security-grade frontier AI. (1) GPT-5.5-Cyber (OpenAI, limited preview May 7, 2026) is a specialized variant of GPT-5.5 with more permissive behavior for defensive cybersecurity tasks — vulnerability identification, patch validation, malware analysis, binary reverse engineering, detection engineering. Access is gated through OpenAI's Trusted Access for Cyber (TAC) program; only vetted security professionals at qualifying organizations get access. (2) Claude Mythos Preview (Anthropic, research preview since April 8, 2026) is Anthropic's most capable frontier model — strong at long-horizon agentic work (METR clocked a 50% time-horizon of ~16 hours), used inside Project Glasswing to find zero-days at scale. Access via application to Anthropic's frontier program. (3) Standard GPT-5.5 (OpenAI, GA) is the general-purpose model with strong cyber capability on benchmarks like CyberGym, but it refuses many offensive-leaning prompts even for defensive scenarios.

Which model should defensive security teams pick in May 2026?

Match access tier to your work. (1) Independent researcher or small infosec team — standard GPT-5.5 plus careful prompting handles most CTF, triage, and tooling needs without TAC paperwork. (2) Enterprise security team at a critical-infrastructure org (energy, finance, healthcare, telco) — apply to TAC for GPT-5.5-Cyber; the more permissive defensive behavior is the difference between 'can analyze this malware sample' and 'I can't help with that.' (3) Long-horizon agentic security work (continuous codebase audit, sustained adversary emulation, multi-day investigations) — Claude Mythos Preview's 16-hour time horizon and Project Glasswing partnership history make it the strongest agentic option, but access is gated even harder than TAC. Most teams in May 2026 use GPT-5.5 by default and apply for TAC only for the workloads that hit refusals.

What's the catch with Claude Mythos Preview for cyber work?

Three catches. (1) Access is harder than TAC — Mythos Preview is research preview, not a productized tier; Anthropic vets requests and prioritizes Project Glasswing coalition members (AWS, Apple, Google, Microsoft, NVIDIA, JPMorgan, Palo Alto Networks, CrowdStrike, Cisco, Linux Foundation, Broadcom). (2) Cost is unclear at production scale — Mythos Preview pricing has been sticker-shock-inducing for early users; budget 3-5x your Opus 4.7 spend. (3) Capability cuts both ways — METR explicitly flagged that Mythos can identify and exploit software vulnerabilities at human-expert level, which is why Project Glasswing exists. For defensive work this is a feature; for risk and compliance teams it's a new attack surface to govern. Combine Mythos cyber work with strong audit logging and human-in-the-loop review for any high-blast-radius actions.

Quick Answer

GPT-5.5-Cyber vs Claude Mythos vs GPT-5.5 (May 2026)

Q: Is GPT-5.5-Cyber actually different from GPT-5.5, or just a relabeled product?

It's a meaningfully different deployment, not just rebranding. OpenAI describes GPT-5.5-Cyber as a specialized variant with adjusted refusal behavior, designed to assist with vulnerability identification and triage, patch validation, malware analysis, binary reverse engineering, detection engineering, and authorized red teaming. The underlying capability comes from GPT-5.5 (codename 'Spud,' released April 23, 2026), but the safety policies are tuned for verified defenders. Access requires application and vetting through Trusted Access for Cyber. UK AISI's evaluation of GPT-5.5-Cyber capabilities is the public benchmark of record. The closest precedent is Anthropic's gated access to Mythos Preview for Project Glasswing partners — same pattern: most-capable cyber work is going behind verified-identity tiers, not into the public API.

Published: May 10, 2026

GPT-5.5-Cyber vs Claude Mythos vs GPT-5.5 (May 2026)

OpenAI rolled out GPT-5.5-Cyber to Trusted Access for Cyber participants on May 7, 2026 — a more permissive variant for verified defenders, arriving roughly a month after Anthropic’s Mythos Preview began driving Project Glasswing zero-day work. Three frontier-tier security AI options, three very different access stories. Here’s how to think about them.

Last verified: May 10, 2026

The three at a glance

Capability	GPT-5.5-Cyber	Claude Mythos Preview	Standard GPT-5.5
Provider	OpenAI	Anthropic	OpenAI
Status (May 2026)	Limited preview	Research preview	GA
Released	May 7, 2026	April 8, 2026	April 23, 2026
Access	TAC application	Anthropic vetting	Open API
Specialty	Defensive cyber tasks	Long-horizon agentic	General-purpose
Refusal behavior	More permissive (defensive)	Standard (Mythos guardrails)	Standard (consumer-safe)
Time horizon	Hours	~16 hours (METR)	Hours
Best for	Vetted SOC, IR, vuln research	Long autonomous campaigns	General security tooling

What each model actually is

GPT-5.5-Cyber: gated permissive model for verified defenders

OpenAI’s Trusted Access for Cyber (TAC) program launched the GPT-5.5-Cyber variant in limited preview on May 7, 2026. The model itself derives from GPT-5.5 (codename “Spud,” released April 23, 2026), but the safety policies are tuned for verified cybersecurity work.

What “more permissive” means in practice:

Vulnerability identification and triage — analyze CVE candidates, prioritize patching, model attack chains.
Patch validation — read vendor patches, confirm they actually fix the issue, identify regression risk.
Malware analysis — static and dynamic analysis assistance, IOC extraction, family classification.
Binary reverse engineering — disassembly assistance, decompilation cleanup, control-flow recovery.
Detection engineering — Sigma/YARA/Suricata rule writing tuned to specific TTPs.
Authorized red teaming and pen testing — supports approved offensive work for verified defenders.

Access requires application and identity verification. OpenAI’s stated bar is “verified cybersecurity experts and organizations responsible for protecting critical infrastructure.” UK AISI published an evaluation of GPT-5.5-Cyber’s capabilities at launch — that’s the public reference benchmark.

Claude Mythos Preview: long-horizon agent for sustained security work

Anthropic released Mythos Preview on April 8, 2026 (codename “Capybara”). METR’s evaluation found a 50% time horizon of at least 16 hours on software task benchmarks — the longest of any frontier model evaluated. METR also flagged that 16 hours is at the upper limit of what their current evaluation suite can reliably measure.

Mythos Preview is the engine inside Project Glasswing — Anthropic’s $100M defensive coalition (April 2026) with AWS, Apple, Broadcom, Cisco, CrowdStrike, Google, JPMorgan Chase, Linux Foundation, Microsoft, NVIDIA, and Palo Alto Networks. The coalition uses Mythos to find zero-day vulnerabilities in critical software before adversaries do.

What that means for a security team: if you can get access (Project Glasswing membership helps; otherwise apply directly), Mythos is the strongest option for sustained agentic work — continuous codebase audit, multi-week red-team campaigns, autonomous IR triage that actually completes the investigation instead of stopping after the easy steps.

Standard GPT-5.5: the workhorse

GPT-5.5 itself is a strong cyber model on public benchmarks like CyberGym. The catch is consumer-safety refusal behavior. For most defensive work this is fine — the model helps. For specific tasks (analyzing in-the-wild malware, walking through exploit primitives in a CTF, deep binary reverse engineering on hostile samples) it will refuse, even in legitimate defender contexts.

Decision tree: which one for my team?

You’re a solo security researcher or small SOC. → Standard GPT-5.5 covers 80%+ of needs. Use Claude Opus 4.7 as a second opinion for harder reverse-engineering work. Skip TAC paperwork unless you hit refusals on real work.

You’re a critical-infrastructure operator or large enterprise SOC. → Apply for TAC. The permissive defensive behavior of GPT-5.5-Cyber is the unlock for malware analysis and authorized red teaming. Pair with Snyk + Claude or Opsera AI SDLC governance for shift-left.

You’re running long autonomous security workflows (continuous audit, multi-day campaigns, agentic IR). → Apply for Claude Mythos Preview. The 16-hour time horizon is the differentiator. Budget 3-5x your standard frontier model spend.

You’re a vendor building an AI security product. → Build on multiple. GPT-5.5 for default capability, GPT-5.5-Cyber via TAC partnership for gated workflows, Mythos for the agentic high-end. Avoid single-vendor lock-in.

What changed in May 2026

May 7: GPT-5.5-Cyber TAC preview opens. Axios + SiliconANGLE first reports.
May 7: AISI publishes its GPT-5.5-Cyber capability evaluation.
May 8: TechRadar frames the launch as “OpenAI’s response to Anthropic Mythos.”
April 7-16: Project Glasswing publicly announced; Mythos Preview public on April 8.
April 23: GPT-5.5 (Spud) launches.

What to watch next

TAC program expansion — does OpenAI open the program beyond critical-infrastructure orgs? When does GPT-5.5-Cyber leave preview?
Mythos GA — when does Mythos Preview become generally available, and at what price?
AISI benchmarks — AISI publishes capability evaluations for both frontier providers; their next round will tell us how the cyber gap is evolving.
Project Glasswing zero-day disclosures — public coordinated disclosures from the coalition will reveal real-world Mythos performance.

Last verified: May 10, 2026 — sources: OpenAI Trusted Access for Cyber announcement, AISI GPT-5.5-Cyber evaluation, Anthropic Mythos Preview release notes, METR time-horizons report, Project Glasswing coalition page, SiliconANGLE, TechRadar, Cybernews, Axios.

GPT-5.5-Cyber vs Claude Mythos vs GPT-5.5 (May 2026)

The three at a glance

What each model actually is

GPT-5.5-Cyber: gated permissive model for verified defenders

Claude Mythos Preview: long-horizon agent for sustained security work

Standard GPT-5.5: the workhorse

Decision tree: which one for my team?

What changed in May 2026

What to watch next

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