The system prompt should be considered a starting point only if you want reusable intelligence and focus. It should not be a grab bag of tools and PR style guides, etc...
Grab your agent and inspect its prompt.
I think the limiting factor is often the human's ability to articulate the goal and hold the agent accountable.
The working memory of technical people and the way they communicate often seems to prioritize how they're doing a task and not why they're doing it. So when they talk to you about their problem, they give you the human equivalent of modem noises and stack traces. This is such a common problem on tech support and Q&A forums like StackOverflow that it spawned its own terminology and website: https://xyproblem.info/
The agents don't respond to XY problems any better than a colleague does, and usually much worse.
An effective colleague asks why, and we need a similar relationship between the agent and its human supervisor.
Testing makes this especially clear: “use fuzzing” turns into generating random bytes, and “use formal verification” into proving a property that isn’t particularly useful. Formally, the technique is applied, but its actual purpose is lost.
This may also explain why different testing techniques produce similar results: the problem may be less about knowing a particular technique and more about the agent’s ability to keep its actual goal as the primary constraint.