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 actually show about this model, here's the full picture. Why this matters now OpenAI spent most of April 2026 publicly criticizing Anthropic for locking Mythos behind an allowlist. On April 30, OpenAI did exactly the same thing with GPT-5.5-Cyber — restricting access to TAC participants only. In parallel, OpenAI briefed US federal agencies, state governments, and Five Eyes allies on the model's capabilities, as BensBites sources reported .…