As far as my understanding goes. It is not a breakthrough model itself but finetuned model with right tools and skills. Fairly similiar to today's coding agents with difference that they are made for software engineering not cyber security.
It has considerably more parameters than most frontier models of today. Which gives it a lot more oomph per token.
Is it a "breakthrough" as in "something novel and unexpected"? No. Is it a "breakthrough" as in "something we know works, but made to work on a greater scale"? Very much so.
Supposedly 10T scale. Literally the next big thing. A bit like what OpenAI tried with GPT-4.5 - but Anthropic actually made it work with MoE, reasoning, tool use, RLVR, etc.
It matters because the "g factor" of today's LLMs is at least in part a function of raw scale. Larger models are just smarter - assuming you can handle the training and inference at this increased scale.