Skills are tested code, not just text. Same input, same output. Calculations, reports, and documents are produced reproducibly and verifiably, instead of being estimated by a language model. Where a plain chatbot would plausibly guess a number, a skill runs the code behind it and returns a deterministic, traceable result. Each skill runs in its own isolated workspace, separated from other agents and data. You install skills from the catalog, bring existing Claude Code or Codex skills, or write your own in a folder with a skill.md. Because the format is open, there is no proprietary lock-in. The decisive difference: for anything that has to be exactly right, such as tariffs, deadlines, or reports, you rely on tested code instead of a guess.