Claude Opus 4.5 (November 2025) was already a step change in model quality and was when the reality of AI agents started to match the hype.

The idea of an even more capable being launched so soon afterwards is amazing progress and also a little scary.

https://fortune.com/2026/03/26/anthropic-says-testing-mythos-powerful-new-ai-model-after-data-leak-reveals-its-existence-step-change-in-capabilities/