Inside GPT-5.5-Cyber: Capabilities, Refusals, and Federal Briefings Explained

Originally published on NextFuture OpenAI shipped GPT-5.5-Cyber to Trusted Access for Cyber (TAC) program participants in late April 2026 — exactly one week after Anthropic announced Mythos. Unlike standard GPT-5.5, this variant is fine-tuned on offensive and defensive security workflows, hardened against system prompt injection, and gated behind a roughly 40-org allowlist. If you're evaluating a TAC application, building defensive tooling, or just trying to understand what independent evals act