Only when MAX mode is enabled will the subagent inherit the main agent’s model. Otherwise, it falls back to composer1.5.
There is also a known bug where subagents that spawn additional subagents (level 2) always fall back to composer1.5, even when MAX mode is enabled.
See the reported bug:
https://forum.cursor.com/t/nested-subagents-dont-inherit-model-in-max-mode/152680