Missing key => Optional::missing(), distinct from explicit null. Always omitted from toArray()/definedOnly(); #[InferRules] infers 'sometimes' presence; works on hot path, fromResult(), nested/collection fields, and #[WhenLoaded]. First 2.0.0 change — touches TypeResolver's internal return signature.

toArray(context:), toJson(), definedOnly(), toResponse() pick which compiled serializer to render; propagates into nested/collection/flatten fields. Only the default context is persisted to .meta.php; other contexts compile lazily, cached separately so classes that never use context pay nothing. only()/except()/jsonSerialize() stay context-free (variadic-signature and JsonSerializable-interface constraints).

OrderData::jsonSchema() walks existing ClassMeta/ParameterMeta into JSON Schema draft 2020-12 — nested refs/$defs, enums, DataCollection, Flatten, Hidden, Optional, Discriminator oneOf, best-effort Rules mapping, and a ProvidesJsonSchema extension point on casts (implemented on all built-ins). TypeScriptGenerator builds on top for .d.ts generation (bin/sdo-typescript + artisan sdo:typescript)