The AI industry moves fast, but the week of November 18, 2025, may go down as one of the most chaotic in its history. In a span of just seven days, the definition of “state-of-the-art” shifted three times. First, Google’s Gemini 3 Pro claimed the top spot on November 18. The very next day, OpenAI released GPT-5.1-Codex-Max, taking the crown. Then, on November 24, Anthropic released Claude Opus 4.5, its new flagship large language model (LLM), positioning it as the new “best model in the world” specifically for coding, agents, and computer use.
Under the hood: Claude Opus 4.5 architecture
Unfortunately, Claude models have the lowest transparency when it comes to model architecture. But from the system card (Anthropic doesn’t use the term “model card,” indicating that you’re not directly interacting with the LLM when you send requests to Claude), we know that Claude Opus 4.5 is a “hybrid reasoning” model, which means a single model trained for both direct inference and chain-of-thought (CoT) reasoning.
Similar to the architecture used since Sonnet 3.7, Opus 4.5 distinguishes between a default mode for rapid responses and an “extended thinking” mode. This allows the model to deliberate on complex problems before generating an output, effectively showing its work internally before presenting a solution.
