A task brief is not a longer prompt. It is the specification that makes your intent executable and inspectable before Cowork does any work. A good brief has seven fields: objective, allowed sources, exclusions, output artifact, decision rules, review checkpoints, and stop conditions. Vague instruction plus broad access is how Cowork produces fluent, plausible, wrong output. The brief is also your verification standard: if the output does not match the brief, the work is not done.