You read the adapters article. You understood defineAdapter . You went to wire up your next concern, opened the v5 docs, and got hit with a second word: definePlugin . The shapes look almost identical — both take a name , both can return middleware() , both can register DI tokens, both can contribute context decorators. Hovering the types in your editor only deepens the question: "I just need to register a service. defineAdapter or definePlugin ?" This article is the answer. Short version: the surfaces overlap because they were designed to. The difference is identity, lifecycle, and distribution — not capability. Once you internalize that, the choice becomes mechanical. What an adapter actually is An adapter is a piece of app-level infrastructure that owns long-lived resources and plugs into the bootstrap lifecycle.…