Before it
The Opus 4.x line had run through five point releases.
The problem
Agentic evaluation was moving to longer, multi-tool tasks.
What it introduced
The final Opus 4.x release, leading on long-horizon agent benchmarks.
Why it matters
Closes the 4.x generation before the Claude 5 line.
What modern models inherit
The end of a generation's iteration cycle.
What came next
Claude Sonnet 5 and Opus 5.