Ah OK sorry, I misinterpreted. But yes, I double checked one case and I am indeed setting the parameter explicitly (defaulting to medium effort). But no luck. It feels like the model ignores what I'm telling it.

For example, I pass it a list of database collections and tools to search through them, ask a question that can very obviously be answered with them, and it responds with "I can’t tell yet from your current records" (just tested with GPT 5.4-mini).

But I've prodded it a bit more now, and maybe the model doesn't want to answer unless it can be very very confident of the answer it produces. So it's sort of a "soft refusal".