Opus 4.5

Opus 4.5 is an AI model Anthropic launched in November 2025. It works as the "brain" inside Claude Code, the tool that writes code on a user's behalf, actually carrying out the work.

With this model, Claude Code can now keep working on its own for hours without stopping, hold far more information in mind, and even organize its own team of smaller helper agents to split up the parts of a large program and assign work to them. It's as if an employee who used to get tired after a few hours suddenly turned into a manager who works through the night with full focus and even assembles their own team.

Anthropic has a hiring test that gives engineering candidates an extremely difficult take-home assignment. The company said Opus 4.5 scored higher on this test than any human candidate ever had. The development team that used it daily noticed the change gradually, but from the outside, it looked as though the model suddenly crossed a wall at some point.

The article explains that "with Opus 4.5, Claude Code can now run continuously for hours... and assemble a team of multiple subagents." It's important not to confuse Opus 4.5 with Claude Code itself — Opus 4.5 is the model that powers the tool, not the tool.

Give Claude or Claude Code a complex, multi-step task (for example, asking it to add a new feature to existing code and then run tests on it), and watch how long it can keep working on its own without stopping partway through.