What I mean is as currently done it seems to be "try to identify if the request is asking for something we don't want to answer" rather than "verify the produce

What I mean is as currently done it seems to be "try to identify if the request is asking for something we don't want to answer" rather than "verify the produced output doesn't contain anything we don't want", the latter seems much more robust, and wouldn't need anything like the computation power of content generation. I'm sure some degree of input filtering would minimize wasted work, but a simple (at the level that's been trivially possible for more than a decade now) output check seems like it would be much more robust, cheap, and much harder to circumvent - the post check is not trying to interpret the request just blocking the output