Start here. This is the direct spoken answer to practice first.
Overview
Decomposition is useful when intermediate artifacts create a real validation or recovery boundary, not merely because multiple calls sound more advanced.
I keep a task in one call when the inputs, output, and quality criteria are simple. I split it when distinct stages need different context, schemas, models, validation, or retry behavior. For example, extracting facts and then drafting a response can be safer than asking one prompt to infer facts, decide policy, and write customer-facing text at once.