JSON struct tags for encoding/decoding control

Without struct tags, Go exports struct fields using their exact name (PascalCase), which doesn't match the snake_case or camelCase conventions expected by JSON APIs.

Use json struct tags to control field names and behaviour:

type User struct {
    ID        int    `json:"id"`
    FirstName string `json:"first_name"`
    LastName  string `json:"last_name"`
    Password  string `json:"-"`           // never marshaled
    CreatedAt time.Time `json:"created_at,omitempty"` // omit if zero
    Internal  string `json:"internal,omitempty"`
}

// Marshal
data, err := json.Marshal(user)
// Unmarshal
err = json.Unmarshal(data, &user)

// Use json.NewDecoder for streams (HTTP bodies)
err = json.NewDecoder(r.Body).Decode(&user)

The encoding/json package reads struct tags at runtime via reflection. The tag syntax is json:"name,options" where options include omitempty and string.