Struct BuiltinField 

pub struct BuiltinField {

    pub name: &'static str,
    pub field_type: FieldType,
    pub widget: Widget,
    pub required: bool,
    pub default_json: Option<&'static str>,
    pub format: Option<&'static str>,
    pub enum_values: Option<&'static [&'static str]>,
    pub items_type: Option<FieldType>,
    pub one_of_members: Option<&'static [BuiltinField]>,
    pub description: &'static str,
    pub label: Option<&'static str>,
    pub label_key: &'static str,
    pub score: u8,
    pub skip_schema: bool,
    pub group: &'static str,
    pub file_kinds: Option<&'static [ExtKind]>,
}

A builtin frontmatter field definition.

Each entry describes a field that moss recognizes in markdown frontmatter. The schema::builtin_schema() function reads this table to produce the ContentSchema returned to the editor and validation engine.

Fields

name: &'static str

Field name as it appears in YAML frontmatter.

field_type: FieldType

Data type of the field.

widget: Widget

UI widget hint for the editor form.

required: bool

Whether the field is required.

default_json: Option<&'static str>

Default value as a JSON literal (e.g. "true", "\"list\"", "1").

format: Option<&'static str>

Format hint (e.g. "date" for YYYY-MM-DD validation).

enum_values: Option<&'static [&'static str]>

Allowed values for select/enum fields.

items_type: Option<FieldType>

Item type for array fields (e.g. FieldType::String for tags: [...]).

one_of_members: Option<&'static [BuiltinField]>

Member variants for a OneOf union field. Each member is itself a BuiltinField (scalar field_type/widget — const-legal). Set only for union fields (children, series); builtin_schema() recursively materializes these into the owned FieldDefinition::one_of.

description: &'static str

Human-readable description shown in the editor form.

label: Option<&'static str>

Optional human-readable label for the chip bar. When None, the frontend falls back to using the field key. Useful for fields with unfriendly internal names (e.g. children_depth → “Depth”).

label_key: &'static str

i18n key for the chip bar label, resolved by the TypeScript registry. Format: “chip..label”. Empty string → frontend falls back to field name. The existing label field is deprecated in favour of this key.

score: u8

Display score for chip bar ordering and add-property search list ordering. Lower values appear first / sort higher in the list. Formula: score = 100 - (Frequency6 + Importance4) where Frequency (0–5) = real usage frequency, Importance (0–5) = first-principles importance. 0 means unset (skip-schema fields). Typical range: 0 (title) to 100 (draft/listed/cascade).

skip_schema: bool

If true, the field exists in the FrontMatter struct but is NOT exposed in the editor schema or validation. Used for site-level config, auto-generated fields, and fields migrating to plugin-contributed schemas.

The field name IS surfaced to the frontend via FrontmatterSchema::internal_fields (populated by builtin_schema()), so the chip bar can filter these out of its render list without a hand-maintained denylist. Adding a new skip_schema: true field here is sufficient — no TS-side edit needed.

group: &'static str

UI group for the add-property dropdown. Fields with the same group are displayed together. Empty string for skip_schema fields. One of: “This Page”, “Child Pages”, “Child Styles”, “Whole Site”. The “Other” group is handled entirely on the TS side for unknown fields.

file_kinds: Option<&'static [ExtKind]>

For Widget::FilePicker fields, the extension kinds the picker should restrict search results to (e.g. cover → image or video; logo → image only). None means unrestricted. This is the schema-side SSOT the chip bar reads instead of hardcoding a key -> ExtKind[] switch.