Opus 4.5 is so bad at instruction following (30% worse per benchmark shared above) that it requires a manual toggle for plan mode.

GPT 5.2 simply obeys instruction to assemble a plan and avoids the need to compensate for poor steerability that would require the user to manually manage modalities.

Opus has improved though so the plan mode is less necessary than it was before, but it is still far behind state of art steerability.