You’ve been over-engineering your Swift models. “Should this model be Decodable or Codable? Am I doing this wrong?” — Here's how to pick the right one.

Swift doesn’t provide a single “JSON protocol.” It gives you three, each with a specific responsibility

Most networking code only needs Decodable

When your app fetches data and displays it, data flows one way — from the server into your models. The Decodable conformance is a signal to anyone reading this code: this model is read-only by design. You don’t need Codable here, just in case.

struct Article : Decodable { let id : Int let title : String let publishedAt : Date let author : Author }

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Use Codable when data moves in both directions

Two situations make Codable the right call:

You’re sending data back to a server

A request body going out as JSON, a form payload, a PUT with updated fields — anything where your Swift type needs to become JSON.

struct UpdateProfileRequest : Encodable { let displayName :…