[Protocol] Add support for additional properties

protocol/languages/kotlin/embedded-checkout-protocol/src/test/java/com/shopify/ucp/embedded/checkout/ExtensionPreservationTest.kt

@@ -0,0 +1,45 @@
+package com.shopify.ucp.embedded.checkout
+
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonPrimitive
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Test
+
+class ExtensionPreservationTest {
+
+    private val json = Json { ignoreUnknownKeys = true }
+
+    @Test
+    fun `preserves unknown extension keys on Checkout`() {
+        val wire = """
+            {
+              "id": "checkout-123",
+              "currency": "USD",
+              "line_items": [],
+              "links": [],
+              "status": "incomplete",
+              "totals": [],
+              "ucp": {"payment_handlers": {}, "version": "2026-04-08"},
+              "signals": {"dev.ucp.buyer_ip": "203.0.113.7", "com.example.device_id": "abc-123"},
+              "com.example.foo": "bar"
+            }
+        """.trimIndent()
+
+        val checkout = json.decodeFromString(Checkout.serializer(), wire)
+        val reEncoded = json.parseToJsonElement(json.encodeToString(Checkout.serializer(), checkout)).jsonObject
+
+        assertThat(reEncoded["com.example.foo"]?.jsonPrimitive?.content).isEqualTo("bar")
+
+        val signals = reEncoded["signals"]?.jsonObject
+        assertThat(signals?.get("dev.ucp.buyer_ip")?.jsonPrimitive?.content).isEqualTo("203.0.113.7")
+        assertThat(signals?.get("com.example.device_id")?.jsonPrimitive?.content).isEqualTo("abc-123")
+
+        val colliding = checkout.copy(
+            additionalProperties = checkout.additionalProperties + ("id" to JsonPrimitive("extension-id")),
+        )
+        val collisionEncoded = json.parseToJsonElement(json.encodeToString(Checkout.serializer(), colliding)).jsonObject
+        assertThat(collisionEncoded["id"]?.jsonPrimitive?.content).isEqualTo("checkout-123")
+    }
+}

protocol/languages/swift/Tests/EmbeddedCheckoutProtocolTests/ModelDecodingTests.swift

@@ -153,6 +153,36 @@ struct ModelDecodingTests {
         #expect(colorScheme == [.light, .dark])
         #expect(config.delegate == ["window.open"])
     }
+
+    @Test func preservesUnknownExtensionKeysOnCheckout() throws {
+        let json = """
+        {
+          "id": "checkout-123",
+          "currency": "USD",
+          "line_items": [],
+          "links": [],
+          "status": "incomplete",
+          "totals": [],
+          "ucp": {"payment_handlers": {}, "version": "2026-04-08"},
+          "signals": {"dev.ucp.buyer_ip": "203.0.113.7", "com.example.device_id": "abc-123"},
+          "com.example.foo": "bar"
+        }
+        """
+        var checkout = try JSONDecoder().decode(Checkout.self, from: Data(json.utf8))
+        let reEncoded = try JSONEncoder().encode(checkout)
+        let object = try #require(try JSONSerialization.jsonObject(with: reEncoded) as? [String: Any])
+
+        #expect(object["com.example.foo"] as? String == "bar")
+
+        let signals = try #require(object["signals"] as? [String: Any])
+        #expect(signals["dev.ucp.buyer_ip"] as? String == "203.0.113.7")
+        #expect(signals["com.example.device_id"] as? String == "abc-123")
+
+        checkout.additionalProperties["id"] = try JSONDecoder().decode(JSONAny.self, from: Data("\"extension-id\"".utf8))
+        let collisionEncoded = try JSONEncoder().encode(checkout)
+        let collisionObject = try #require(try JSONSerialization.jsonObject(with: collisionEncoded) as? [String: Any])
+        #expect(collisionObject["id"] as? String == "checkout-123")
+    }
 }
 
 private func fixtureString(_ name: String) throws -> String {

protocol/languages/typescript/src/generated/Models.d.ts

@@ -51,7 +51,9 @@ export interface Checkout {
      */
     order?: OrderConfirmation;
     payment?: Payment;
-    signals?: Signals;
+    signals?: {
+        [key: string]: any;
+    };
     /**
      * Checkout state indicating the current phase and required action. See Checkout Status
      * lifecycle documentation for state transition details.
@@ -715,17 +717,6 @@ export interface PaymentCredential {
  * use reverse-domain naming to ensure provenance and prevent collisions when multiple
  * extensions contribute to the shared namespace.
  */
-export interface Signals {
-    /**
-     * Client's IP address (IPv4 or IPv6).
-     */
-    devUcpBuyerIp?: string;
-    /**
-     * Client's HTTP User-Agent header or equivalent.
-     */
-    devUcpUserAgent?: string;
-    [property: string]: any;
-}
 /**
  * Checkout state indicating the current phase and required action. See Checkout Status
  * lifecycle documentation for state transition details.

protocol/languages/typescript/src/generated/Models.ts

@@ -69,7 +69,7 @@ export interface Checkout {
      */
     order?:   OrderConfirmation;
     payment?: Payment;
-    signals?: Signals;
+    signals?: { [key: string]: any };
     /**
      * Checkout state indicating the current phase and required action. See Checkout Status
      * lifecycle documentation for state transition details.
@@ -758,17 +758,6 @@ export interface PaymentCredential {
  * use reverse-domain naming to ensure provenance and prevent collisions when multiple
  * extensions contribute to the shared namespace.
  */
-export interface Signals {
-    /**
-     * Client's IP address (IPv4 or IPv6).
-     */
-    devUcpBuyerIp?: string;
-    /**
-     * Client's HTTP User-Agent header or equivalent.
-     */
-    devUcpUserAgent?: string;
-    [property: string]: any;
-}
 
 /**
  * Checkout state indicating the current phase and required action. See Checkout Status
@@ -2132,7 +2121,7 @@ const typeMap: any = {
         { json: "messages", js: "messages", typ: u(undefined, a(r("Message"))) },
         { json: "order", js: "order", typ: u(undefined, r("OrderConfirmation")) },
         { json: "payment", js: "payment", typ: u(undefined, r("Payment")) },
-        { json: "signals", js: "signals", typ: u(undefined, r("Signals")) },
+        { json: "signals", js: "signals", typ: u(undefined, m("any")) },
         { json: "status", js: "status", typ: r("CheckoutStatus") },
         { json: "totals", js: "totals", typ: a(r("CheckoutTotal")) },
         { json: "ucp", js: "ucp", typ: r("UcpCheckoutResponseSchema") },
@@ -2283,10 +2272,6 @@ const typeMap: any = {
     "PaymentCredential": o([
         { json: "type", js: "type", typ: "" },
     ], "any"),
-    "Signals": o([
-        { json: "dev.ucp.buyer_ip", js: "devUcpBuyerIp", typ: u(undefined, "") },
-        { json: "dev.ucp.user_agent", js: "devUcpUserAgent", typ: u(undefined, "") },
-    ], "any"),
     "CheckoutTotal": o([
         { json: "amount", js: "amount", typ: 0 },
         { json: "display_text", js: "displayText", typ: u(undefined, "") },

protocol/languages/typescript/test/extension-preservation.test.ts

@@ -0,0 +1,27 @@
+import {expect, test} from 'vitest';
+
+import {Convert} from '../src/generated/Models';
+
+test('preserves unknown top-level and nested extension keys through Convert round-trip', () => {
+  const wire = {
+    id: 'checkout-123',
+    currency: 'USD',
+    line_items: [],
+    links: [],
+    status: 'incomplete',
+    totals: [],
+    ucp: {payment_handlers: {}, version: '2026-04-08'},
+    signals: {
+      'dev.ucp.buyer_ip': '203.0.113.7',
+      'com.example.device_id': 'abc-123',
+    },
+    'com.example.foo': 'bar',
+  };
+
+  const checkout = Convert.toCheckout(JSON.stringify(wire));
+  const roundTripped = JSON.parse(Convert.checkoutToJson(checkout));
+
+  expect(roundTripped['com.example.foo']).toBe('bar');
+  expect(roundTripped.signals['dev.ucp.buyer_ip']).toBe('203.0.113.7');
+  expect(roundTripped.signals['com.example.device_id']).toBe('abc-123');
+});