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