Proposed reform to Choice structure in TypeScript syntax, with JSON examples for StartingEquipment and a Multiattack action.

choice_structure.ts

interface Choice {
  desc: string
  choose: number
  type: string
  from: OptionSet
}

type OptionSet = {
  option_set_type: 'resource_list' | 'equipment_category' | 'options_array'
} & (ResourceListOptionSet | EquipmentCategoryOptionSet | ArrayOptionSet)

/* Choose from a ResourceList by it's URL */
interface ResourceListOptionSet {
  option_set_type: 'resource_list'
  resource_list_url: string // url of the resource list endpoint
}

/* Choose an Equipment item from an EquipmentCategory */
interface EquipmentCategoryOptionSet {
  option_set_type: 'equipment_category'
  equipment_category: APIReference
}

/* Choose from an array of complex Options */
interface ArrayOptionSet {
  option_set_type: 'options_array'
  options: Option[]
}

type Option = {
  option_type: 'reference' | 'action' | 'multiple' | 'choice'
  desc?: string
  prerequisites?: Prerequisite[]
} & (ReferenceOption | ActionOption | MultipleOption | ChoiceOption)

/* A single kind of object of the `type` defined in the root Choice */
interface ReferenceOption {
  option_type: 'reference'
  item: CountedReference
}

/* A named action, for Multiattack actions */
interface ActionOption {
  option_type: 'action'
  action_name: string
  count: number
  type: "melee" | "ranged"
}

/* An array of options. The "chooser" gets all of these. */
interface MultipleOption {
  option_type: 'multiple'
  items: Option[]
}

/* A nested choice. The `choice.type` should equal the root `type`. */
interface ChoiceOption {
  option_type: 'choice'
  choice: Choice
}

/* Some auxilliary structures */

interface CountedReference {
  count: number
  of: APIReference
}

interface APIReference {
  index: string,
  name: string,
  url: string
}

interface Prerequisite {
  type: "proficiency" | "feature" | "spell" | "level"
}

interface ProficiencyPrerequisite extends Prerequisite {
  type: "proficiency"
  proficiency: APIReference
}
/* <other proficiency types here> */

multiattack.json

{
  "name": "Multiattack",
  "desc": "The captain makes three melee attacks: two with its scimitar and one with its dagger. Or the captain makes two ranged attacks with its daggers.",
  "options": {
    "desc": "(a) two with its scimitar and one with its dagger or (b) two ranged attacks with its daggers.",
    "choose": 1,
    "type": "action",
    "from": {
      "option_set_type": "options_array",
      "options": [
        {
          "desc": "two with its scimitar and one with its dagger",
          "option_type": "multiple",
          "items": [
            {
              "option_type": "action",
              "action_name": "Scimitar",
              "count": 2,
              "type": "melee"
            },
            {
              "option_type": "action",
              "action_name": "Dagger",
              "count": 1,
              "type": "melee"
            }
          ]
        },
        {
          "desc": "two ranged attacks with its daggers",
          "option_type": "action",
          "action_name": "Dagger",
          "count": 2,
          "type": "ranged"
        }
      ]
    }
  }
}

starting_equipment_choices.json

[
  {
    "desc": "(a) chain mail or (b) leather armor, longbow, and 20 arrows",
    "type": "equipment",
    "choose": 1,
    "from": {
      "option_set_type": "options_array",
      "options": [
        {
          "option_type": "reference",
          "item": {
            "count": 1,
            "of": {
              "name": "Chain Mail",
              "index": "chain-mail",
              "url": "/api/equipment/chain-mail"
            }
          }
        },
        {
          "option_type": "multiple",
          "items": [
            {
              "option_type": "reference",
              "item": {
                "count": 1,
                "of": {
                  "name": "Leather",
                  "index": "leather",
                  "url": "/api/equipment/leather"
                }
              }
            },
            {
              "option_type": "reference",
              "item": {
                "count": 1,
                "of": {
                  "name": "Longbow",
                  "index": "longbow",
                  "url": "/api/equipment/longbow"
                }
              }
            },
            {
              "option_type": "reference",
              "item": {
                "count": 20,
                "of": {
                  "name": "Arrow",
                  "index": "arrow",
                  "url": "/api/equipment/arrow"
                }
              }
            }
          ]
        }
      ]
    }
  },
  {
    "desc": "(a) a martial weapon and a shield or (b) two martial weapons",
    "type": "equipment",
    "choose": 1,
    "from": {
      "option_set_type": "options_array",
      "options": [
        {
          "option_type": "multiple",
          "items": [
            {
              "option_type": "choice",
              "choice": {
                "desc": "a martial weapon",
                "type": "equipment",
                "choose": 1,
                "from": {
                  "option_set_type": "equipment_category",
                  "equipment_category": {
                    "index": "martial-weapons",
                    "name": "Martial Weapons",
                    "url": "/api/equipment-categories/martial-weapons"
                  }
                }
              }
            },
            {
              "option_type": "reference",
              "item": {
                "count": 1,
                "of": {
                  "name": "Shield",
                  "index": "shield",
                  "url": "/api/equipment/shield"
                }
              }
            }
          ]
        },
        {
          "option_type": "choice",
          "choice": {
            "desc": "a martial weapon",
            "type": "equipment",
            "choose": 2,
            "from": {
              "option_set_type": "equipment_category",
              "equipment_category": {
                "index": "martial-weapons",
                "name": "Martial Weapons",
                "url": "/api/equipment-categories/martial-weapons"
              }
            }
          }
        }
      ]
    }
  },
  {
    "desc": "(a) a light crossbow and 20 bolts or (b) two handaxes",
    "type": "equipment",
    "choose": 1,
    "from": {
      "option_set_type": "options_array",
      "options": [
        {
          "option_type": "multiple",
          "items": [
            {
              "option_type": "reference",
              "item": {
                "count": 1,
                "of": {
                  "name": "Crossbow, Light",
                  "index": "crossbow-light",
                  "url": "/api/equipment/crossbow-light"
                }
              }
            },
            {
              "option_type": "reference",
              "item": {
                "count": 20,
                "of": {
                  "name": "Crossbow bolt",
                  "index": "crossbow-bolt",
                  "url": "/api/equipment/crossbow-bolt"
                }
              }
            }
          ]
        },
        {
          "option_type": "reference",
          "item": {
            "count": 2,
            "of": {
              "name": "Handaxe",
              "index": "handaxe",
              "url": "/api/equipment/handaxe"
            }
          }
        }
      ]
    }
  },
  {
    "desc": "(a) a dungeoneer's pack or (b) an explorer's pack",
    "type": "equipment",
    "choose": 1,
    "from": {
      "option_set_type": "options_array",
      "options": [
        {
          "option_type": "reference",
          "item": {
            "count": 1,
            "of": {
              "name": "Dungeoneer's Pack",
              "index": "dungeoneers-pack",
              "url": "/api/equipment/dungeoneers-pack"
            }
          }
        },
        {
          "option_type": "reference",
          "item": {
            "count": 1,
            "of": {
              "name": "Explorer's Pack",
              "index": "explorers-pack",
              "url": "/api/equipment/explorers-pack"
            }
          }
        }
      ]
    }
  }
]