Custom Tags

Built-in Tags

Sentinel extracts these tags by default:

Registering Custom Tags

Register additional tags before extraction:

// Register at init or before first Inspect/Scan
sentinel.Tag("graphql")
sentinel.Tag("proto")
sentinel.Tag("xml")

Tags can be registered at any time, but only affect future extractions:

sentinel.Inspect[User]() // "custom" tag NOT extracted

sentinel.Tag("custom")

sentinel.Inspect[Product]() // "custom" tag IS extracted
sentinel.Inspect[User]()    // Still cached without "custom"

Accessing Tags

Tags are stored as a map on each field:

type User struct {
    Email string `json:"email" validate:"required,email" custom:"pii"`
}

metadata := sentinel.Inspect[User]()
field := metadata.Fields[0]

field.Tags["json"]     // "email"
field.Tags["validate"] // "required,email"
field.Tags["custom"]   // "pii"

Tag Parsing

Sentinel preserves the raw tag value. Parsing tag syntax is your responsibility:

// Tag value: "required,email,max=100"
rules := field.Tags["validate"]

// Parse as needed for your validation library
parts := strings.Split(rules, ",")

Use Cases

Validation Rules

type Form struct {
    Email    string `validate:"required,email"`
    Password string `validate:"required,min=8"`
}

for _, field := range metadata.Fields {
    if rules, ok := field.Tags["validate"]; ok {
        // Apply validation logic
    }
}

Database Schema

type Model struct {
    ID   string `db:"id,primarykey"`
    Name string `db:"name,index"`
}

for _, field := range metadata.Fields {
    if col, ok := field.Tags["db"]; ok {
        // Generate DDL or migrations
    }
}

API Documentation

type Request struct {
    UserID string `json:"user_id" example:"usr_123" desc:"The user identifier"`
}

for _, field := range metadata.Fields {
    example := field.Tags["example"]
    desc := field.Tags["desc"]
    // Generate OpenAPI spec
}

Next Steps

• Scanning — recursive type discovery
• Testing — testing tag extraction
• API Reference — Tag() function documentation