Fm::Guide

Annotation for adding generation constraints to Generable fields.

These constraints map to JSON Schema validation keywords and are used by the on-device model to enforce output structure during guided generation.

struct Movie
  include JSON::Serializable
  include Fm::Generable

  getter title : String

  @[Fm::Guide(any_of: ["G", "PG", "PG-13", "R", "NC-17"])]
  getter rating : String

  @[Fm::Guide(minimum: 0, maximum: 10)]
  getter score : Int32

  @[Fm::Guide(pattern: "^[A-Z]")]
  getter director : String

  @[Fm::Guide(min_items: 1, max_items: 5)]
  getter genres : Array(String)
end

Supported constraints:

• any_of : Restrict string values to a set of choices (JSON Schema "enum")
• constant : Fix a field to a single constant value (JSON Schema "const")
• minimum : Minimum numeric value (JSON Schema "minimum")
• maximum : Maximum numeric value (JSON Schema "maximum")
• pattern : Regex pattern for string values (JSON Schema "pattern")
• min_items : Minimum array length (JSON Schema "minItems")
• max_items : Maximum array length (JSON Schema "maxItems")
• count : Exact array length (sets both minItems and maxItems)
• description : Human-readable field description (JSON Schema "description")