@dalias @Adrenochrome typing @SynthID in a Gemini message forces a SynthID tool call to execute before the LLM runs. The LLM receives the output from the SynthID checker (a very normal old-school program) + your message as context.
Of course the LLM could still randomly decide to lie about the result of the tool call, that's why I ran this and similar queries multiple times in different chat contexts.
To answer why I'm using an LLM to do this at all: There simply seems to be no other way to reliably use the SynthID checker (sometimes the Google reverse image search executes it and notifies you of a positive but sometimes it also doesn't) , this is the only supported way by Google (even tho I still consider it bad design).