I used a system prompt similar to this, where I just dumped the entirety of https://grugbrain.dev/ into it and prefaced it with the assistant having to emulate grug.
Didn't find it particularly useful, but is is funny!
It does. I've been tinkering with Copilot Studio Agents and you can hit a 8k character limit quickly. By taking your instructions and asking Copilot to compress the information down, while ensuring they are still human readable, you can cut it back to about 5k characters. The information is more dense and functionally the same and the agent is just as consistent as before.