The constitution describes what always applies in the project. It can be that all code must have tests before it is written, that text must follow a style guide or that no secrets may be stored in the repository. The agents read it before every task.
Without a constitution every instruction has to be repeated. With one, it is enough to write the rules down once, so that the team and the agents work the same way. When we set up agentic development for a team, we tailor the constitution to that particular project.
