Anthropic's flagship reasoning model, refined for longer context, cleaner code, and more reliable agentic workflows.
Claude Opus 4.5 (second generation) is Anthropic's most capable model family yet, tuned for the work developers and analysts actually do: multi-file refactors, 200K-token document synthesis, and agentic tool use that rarely goes off the rails. The refinement pass over the original Opus 4.5 cut hallucinations on long-context reasoning by a noticeable margin and made tool-calling far more consistent.
Who it's for: Engineers, researchers, and knowledge workers who push models to the limit with long documents, complex codebases, and multi-step agentic tasks.
Paste or index an entire repository and reason across it without losing the thread.
Call APIs, run code, and chain steps with far fewer mid-task detours than prior generations.
Parse charts, PDFs, and whiteboard photos into structured, citation-ready output.
Pairs with the Claude Code CLI for autonomous edits, tests, and PRs.
If you do serious reasoning work — long documents, big codebases, multi-step agents — Claude Opus 4.5 (2nd gen) is the benchmark to beat in 2026. The free tier is enough to test; Pro pays for itself fast.