[Protocol] Add support for additional properties

[markmur]

What changes are you making?

This preserves untyped protocol extension properties across generated TypeScript, Swift, and Kotlin models.

What is wrong with the current state?

UCP schemas intentionally allow extension fields through open objects and map-like schemas. The current generated clients do not handle that consistently: Swift and Kotlin decode typed model fields but drop unknown object members, while TypeScript had map-like extension data represented as generated named interfaces such as Signals. That makes current spec fields look more permanent than they are and means future extension keys can disappear or require generator changes when the spec shape changes.

There is also a collision risk when unknown fields are re-encoded. If an extension map contains a key that matches a typed field, the typed field should remain authoritative.

What this fixes

The model generator now derives extension handling from the schema instead of from spec-specific names:

• open object models get an additionalProperties bucket in Swift and Kotlin
• map-like schemas using propertyNames plus additionalProperties become native untyped maps in all languages
• Swift and Kotlin custom encoders re-emit unknown fields, but filter keys that collide with typed fields
• tests cover preservation of top-level extension keys, nested extension maps, and typed-field collision behavior

Language implications

TypeScript

Open generated models expose unknown fields through additionalProperties, matching Swift and Kotlin. Map-like fields are still emitted inline as { [key: string]: any }. For example, checkout.signals is no longer a generated Signals interface.

import {notificationDescriptors} from "./generated/ProtocolNotifications";

const checkout = notificationDescriptors.complete.decode(params);

const topLevelValue = checkout.additionalProperties?.["com.example.foo"];
const signalValue = checkout.signals?.["com.example.device_id"];

Swift

Open generated models expose unknown fields as additionalProperties: [String: JSONAny]. Map-like fields such as signals are [String: JSONAny]?.

client = client.on(EmbeddedCheckoutProtocol.Event.complete) { checkout in
    let topLevelValue = checkout.additionalProperties["com.example.foo"]?.value as? String
    let signalValue = checkout.signals?["com.example.device_id"]?.value as? String
}

Kotlin

Open generated models expose unknown fields as additionalProperties: Map<String, JsonElement>. Map-like fields such as signals are JsonObject?.

import kotlinx.serialization.json.JsonPrimitive
import kotlinx.serialization.json.contentOrNull

val checkout = EmbeddedCheckoutProtocol.complete.decode(params) ?: return

val topLevelValue = checkout.additionalProperties["com.example.foo"]
val topLevelString = (topLevelValue as? JsonPrimitive)?.contentOrNull

val signalValue = checkout.signals?.get("com.example.device_id")
val signalString = (signalValue as? JsonPrimitive)?.contentOrNull

[github-actions]

React Native — Coverage Report

Lines	Statements	Branches	Functions
	91.85% (327/356)	87.98% (183/208)	100% (86/86)

[github-actions]

Package Size

Platform	Artifact	Base	Head	Delta
React Native	npm tarball	99.1 KiB	98.9 KiB	-203 B
Android	release AAR	164.5 KiB	164.5 KiB	0 B

React Native file breakdown

File	Base	Head	Delta
node_modules/@shopify/checkout-kit-protocol/src/generated/Models.ts	85.8 KiB	85.4 KiB	-436 B
node_modules/@shopify/checkout-kit-protocol/src/generated/Models.d.ts	53.2 KiB	53.0 KiB	-217 B
ios/ShopifyCheckoutKit.swift	15.0 KiB	15.0 KiB	0 B
ios/AcceleratedCheckoutButtons.swift	14.0 KiB	14.0 KiB	0 B
lib/commonjs/index.js	13.1 KiB	13.1 KiB	0 B
src/components/AcceleratedCheckoutButtons.tsx	12.9 KiB	12.9 KiB	0 B
src/index.ts	12.5 KiB	12.5 KiB	0 B
lib/commonjs/components/AcceleratedCheckoutButtons.js	11.4 KiB	11.4 KiB	0 B
android/src/main/java/com/shopify/reactnative/checkoutkit/ShopifyCheckoutKitModule.java	11.1 KiB	11.1 KiB	0 B
lib/module/index.js	10.4 KiB	10.4 KiB	0 B
lib/commonjs/components/AcceleratedCheckoutButtons.js.map	10.3 KiB	10.3 KiB	0 B
lib/module/components/AcceleratedCheckoutButtons.js	10.2 KiB	10.2 KiB	0 B
lib/module/components/AcceleratedCheckoutButtons.js.map	9.1 KiB	9.1 KiB	0 B
src/index.d.ts	8.9 KiB	8.9 KiB	0 B
node_modules/@shopify/checkout-kit-protocol/src/generated/ProtocolNotifications.ts	8.5 KiB	8.5 KiB	0 B
lib/module/index.js.map	8.2 KiB	8.2 KiB	0 B
src/present-dispatcher.ts	8.0 KiB	8.0 KiB	0 B
lib/commonjs/index.js.map	7.7 KiB	7.7 KiB	0 B
lib/commonjs/present-dispatcher.js	6.8 KiB	6.8 KiB	0 B
lib/module/present-dispatcher.js	6.5 KiB	6.5 KiB	0 B
…and 103 smaller files

Android file breakdown

File	Base	Head	Delta
classes.jar	174.1 KiB	174.1 KiB	0 B
res/layout/dialog_checkout.xml	1.3 KiB	1.3 KiB	0 B
proguard.txt	798 B	798 B	0 B
AndroidManifest.xml	578 B	578 B	0 B
res/values/values.xml	564 B	564 B	0 B
R.txt	522 B	522 B	0 B
res/drawable/close.xml	431 B	431 B	0 B
res/menu/checkout_menu.xml	354 B	354 B	0 B
META-INF/com/android/build/gradle/aar-metadata.properties	157 B	157 B	0 B

Measured from the PR base SHA and PR head SHA. The file breakdown shows uncompressed sizes within each package artifact, so individual files do not sum to the compressed artifact total. This comment reports package artifact sizes only; it is not a final app binary-size report.

[kieran-osgood-shopify]

Everything looks good to me

This has me thinking about whether we're approaching the point we might want to consider a fork of quicktype for our purposes, or even some patch-package style changes so we dont have a total fork, to add some flag driven behaviour for us

It might wind up more maintainable than our script which relies on so much string.replace if we can insert some of the behaviour we are inserting here

[markmur]

This has me thinking about whether we're approaching the point we might want to consider a fork of quicktype for our purposes

Yeah @kieran-osgood-shopify, this is what @westeezy was considering too. I think it's a nice idea to keep the generation logic encapsulated for our purposes and written in a way we know and like.