Birgitta Böckeler, writing about harness engineering at Thoughtworks, draws a distinction that turns out to be more useful than it first appears: a harness contains guides and sensors. Guides point the agent toward correct behavior up front. Sensors catch the agent after the fact, when it has produced something that does not meet the bar.
Most teams that adopt agents at scale eventually invest in one of these and neglect the other. The neglect produces predictable failure modes. Naming the two categories explicitly is the first step to balancing them.
Guides
What I've been calling "guidelines" in previous articles
A guide is anything in the harness that shapes what the agent does before it produces output. Rules files. Example code in context. Skills and slash commands. Style guides referenced in CLAUDE.md. The repository's own existing code, which the agent reads as a source of patterns. All of these are guides; they all do the same job, which is to load priors into the agent's session that bias it toward the behavior the team wants.
