Right now, IntelliJ’s ACP integration doesn’t support selecting different reasoning levels for models like GPT-5.4. The default is set to medium reasoning, and there’s currently no way to override that from the IntelliJ side.
This comes down to how IntelliJ’s ACP client handles model selection — it uses a mode that collapses models down to their default variant rather than exposing the full set of parameters. It’s a known limitation we’re tracking internally.
In the meantime, if you need a specific reasoning level (e.g. high or extra-high), the Cursor editor itself does support reasoning level selection — so that’s the best option for now when you need more control.