This is/was a great trick for improving accuracy of small model + structured output. Kind of an old-fashoined Chain of Thought type of thing. Eg: I used this before with structured outputs in Gemini Flash 2.0 to significantly improve the quality of answers. Not sure if 2.5 Flash requires it, but for 2.0 Flash you could use the propertyOrdering field to force a specific ordering of JSONSchema response items, and force it to output things like "plan", "rationale", "reasoning", etc as the first item, then simply discard it.