diff --git a/protocol/languages/kotlin/embedded-checkout-protocol/src/test/java/com/shopify/ucp/embedded/checkout/ModelContractTest.kt b/protocol/languages/kotlin/embedded-checkout-protocol/src/test/java/com/shopify/ucp/embedded/checkout/ModelContractTest.kt
new file mode 100644
index 000000000..feeeb319e
--- /dev/null
+++ b/protocol/languages/kotlin/embedded-checkout-protocol/src/test/java/com/shopify/ucp/embedded/checkout/ModelContractTest.kt
@@ -0,0 +1,114 @@
+package com.shopify.ucp.embedded.checkout
+
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.SerializationException
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.JsonPrimitive
+import kotlinx.serialization.json.decodeFromJsonElement
+import kotlinx.serialization.json.encodeToJsonElement
+import kotlinx.serialization.json.jsonObject
+import org.assertj.core.api.Assertions.assertThat
+import org.assertj.core.api.Assertions.assertThatThrownBy
+import org.junit.Test
+
+class ModelContractTest {
+    private val json = Json { ignoreUnknownKeys = true }
+    private val fixtures = json.parseToJsonElement(
+        requireNotNull(javaClass.getResource("/model-contracts.json")).readText(),
+    ).jsonObject
+
+    @Test
+    fun `buyer preserves names and extensions`() {
+        val buyer = roundTrip(Buyer.serializer(), "buyer", emptySet()) {
+            it.copy(additionalProperties = it.additionalProperties + ("email" to JsonPrimitive("forged")))
+        }
+        assertThat(buyer.email).isEqualTo("buyer@example.com")
+        assertThat(buyer.firstName).isEqualTo("Ada")
+        assertThat(buyer.lastName).isEqualTo("Example")
+        assertThat(buyer.phoneNumber).isEqualTo("+12025550123")
+        assertThat(buyer.additionalProperties.keys).containsExactly("com.example.extension")
+    }
+
+    @Test
+    fun `context preserves localization and payment preferences`() {
+        val context = roundTrip(Context.serializer(), "context", emptySet()) {
+            it.copy(additionalProperties = it.additionalProperties + ("currency" to JsonPrimitive("forged")))
+        }
+        assertThat(context.addressCountry).isEqualTo("IE")
+        assertThat(context.addressRegion).isEqualTo("Dublin")
+        assertThat(context.postalCode).isEqualTo("D02 TEST")
+        assertThat(context.currency).isEqualTo("EUR")
+        assertThat(context.eligibility).containsExactly("com.example.member")
+        assertThat(context.intent).isEqualTo("gift")
+        assertThat(context.language).isEqualTo("en-IE")
+        assertThat(context.location).isEqualTo("Dublin")
+        assertThat(context.payment?.first()?.handler).isEqualTo("com.example.payment")
+        assertThat(context.payment?.first()?.types).containsExactly("card")
+        assertThat(context.additionalProperties.keys).containsExactly("com.example.extension")
+    }
+
+    @Test
+    fun `credential preserves opaque handler data`() {
+        val credential = roundTrip(PaymentCredential.serializer(), "credential", setOf("type")) {
+            it.copy(additionalProperties = it.additionalProperties + ("type" to JsonPrimitive("forged")))
+        }
+        assertThat(credential.type).isEqualTo("token")
+        assertThat(
+            credential.additionalProperties["com.example.token"]
+        ).isEqualTo(JsonPrimitive("synthetic-test-token"))
+        assertThat(credential.additionalProperties).doesNotContainKey("type")
+    }
+
+    @Test
+    fun `instrument preserves nested credential and billing address`() {
+        val instrument =
+            roundTrip(SelectedPaymentInstrument.serializer(), "instrument", setOf("handler_id", "id", "type")) {
+                it.copy(additionalProperties = it.additionalProperties + ("handler_id" to JsonPrimitive("forged")))
+            }
+        assertThat(instrument.handlerID).isEqualTo("example-handler")
+        assertThat(instrument.id).isEqualTo("example-instrument")
+        assertThat(instrument.type).isEqualTo("card")
+        assertThat(instrument.selected).isTrue()
+        assertThat(instrument.billingAddress?.streetAddress).isEqualTo("1 Example Street")
+        assertThat(instrument.credential?.type).isEqualTo("token")
+        assertThat(instrument.credential?.additionalProperties).containsKey("com.example.extension")
+        assertThat(instrument.display?.get("last_digits")).isEqualTo(JsonPrimitive("1234"))
+    }
+
+    @Test
+    fun `policy preserves description formats and open type`() {
+        val policy = roundTrip(Policy.serializer(), "policy", setOf("description", "type")) {
+            it.copy(additionalProperties = it.additionalProperties + ("type" to JsonPrimitive("forged")))
+        }
+        assertThat(policy.type).isEqualTo("com.example.return")
+        assertThat(policy.appliesTo).containsExactly("$.line_items[0]")
+        assertThat(policy.description.plain).isEqualTo("Return within 30 days")
+        assertThat(policy.description.markdown).isEqualTo("Return within **30 days**")
+        assertThat(policy.description.html).isEqualTo("Return within <b>30 days</b>")
+        assertThat(policy.url).isEqualTo("https://example.com/returns")
+    }
+
+    private fun <T> roundTrip(
+        serializer: KSerializer<T>,
+        fixture: String,
+        required: Set<String>,
+        injectCollision: (T) -> T,
+    ): T {
+        val full = fixtures.getValue(fixture).jsonObject
+        val decoded = json.decodeFromJsonElement(serializer, full)
+        assertThat(json.encodeToJsonElement(serializer, decoded)).isEqualTo(full)
+        // Extension keys cannot replace known fields when encoded.
+        assertThat(json.encodeToJsonElement(serializer, injectCollision(decoded))).isEqualTo(full)
+
+        val minimal = JsonObject(full.filterKeys { it in required })
+        val minimalModel = json.decodeFromJsonElement(serializer, minimal)
+        assertThat(json.encodeToJsonElement(serializer, minimalModel)).isEqualTo(minimal)
+        required.forEach { key ->
+            assertThatThrownBy { json.decodeFromJsonElement(serializer, JsonObject(full - key)) }
+                .describedAs("Missing required field %s in %s", key, fixture)
+                .isInstanceOf(SerializationException::class.java)
+        }
+        return decoded
+    }
+}
diff --git a/protocol/languages/kotlin/embedded-checkout-protocol/src/test/resources/model-contracts.json b/protocol/languages/kotlin/embedded-checkout-protocol/src/test/resources/model-contracts.json
new file mode 100644
index 000000000..b751732c0
--- /dev/null
+++ b/protocol/languages/kotlin/embedded-checkout-protocol/src/test/resources/model-contracts.json
@@ -0,0 +1,169 @@
+{
+  "buyer": {
+    "email": "buyer@example.com",
+    "first_name": "Ada",
+    "last_name": "Example",
+    "phone_number": "+12025550123",
+    "com.example.extension": {
+      "nested": {
+        "flags": [
+          true,
+          false,
+          null
+        ],
+        "count": 3,
+        "fraction": 1.5
+      },
+      "items": [
+        "future",
+        {
+          "enabled": true
+        }
+      ]
+    }
+  },
+  "context": {
+    "address_country": "IE",
+    "address_region": "Dublin",
+    "postal_code": "D02 TEST",
+    "currency": "EUR",
+    "eligibility": [
+      "com.example.member"
+    ],
+    "intent": "gift",
+    "language": "en-IE",
+    "location": "Dublin",
+    "payment": [
+      {
+        "handler": "com.example.payment",
+        "types": [
+          "card"
+        ]
+      }
+    ],
+    "com.example.extension": {
+      "nested": {
+        "flags": [
+          true,
+          false,
+          null
+        ],
+        "count": 3,
+        "fraction": 1.5
+      },
+      "items": [
+        "future",
+        {
+          "enabled": true
+        }
+      ]
+    }
+  },
+  "credential": {
+    "type": "token",
+    "com.example.token": "synthetic-test-token",
+    "com.example.extension": {
+      "nested": {
+        "flags": [
+          true,
+          false,
+          null
+        ],
+        "count": 3,
+        "fraction": 1.5
+      },
+      "items": [
+        "future",
+        {
+          "enabled": true
+        }
+      ]
+    }
+  },
+  "instrument": {
+    "billing_address": {
+      "first_name": "Ada",
+      "last_name": "Example",
+      "street_address": "1 Example Street",
+      "address_locality": "Dublin",
+      "address_region": "Dublin",
+      "postal_code": "D02 TEST",
+      "address_country": "IE"
+    },
+    "credential": {
+      "type": "token",
+      "com.example.token": "synthetic-test-token",
+      "com.example.extension": {
+        "nested": {
+          "flags": [
+            true,
+            false,
+            null
+          ],
+          "count": 3,
+          "fraction": 1.5
+        },
+        "items": [
+          "future",
+          {
+            "enabled": true
+          }
+        ]
+      }
+    },
+    "display": {
+      "label": "Test card",
+      "last_digits": "1234"
+    },
+    "handler_id": "example-handler",
+    "id": "example-instrument",
+    "type": "card",
+    "selected": true,
+    "com.example.extension": {
+      "nested": {
+        "flags": [
+          true,
+          false,
+          null
+        ],
+        "count": 3,
+        "fraction": 1.5
+      },
+      "items": [
+        "future",
+        {
+          "enabled": true
+        }
+      ]
+    }
+  },
+  "policy": {
+    "applies_to": [
+      "$.line_items[0]"
+    ],
+    "description": {
+      "plain": "Return within 30 days",
+      "markdown": "Return within **30 days**",
+      "html": "Return within <b>30 days</b>"
+    },
+    "type": "com.example.return",
+    "url": "https://example.com/returns",
+    "com.example.extension": {
+      "nested": {
+        "flags": [
+          true,
+          false,
+          null
+        ],
+        "count": 3,
+        "fraction": 1.5
+      },
+      "items": [
+        "future",
+        {
+          "enabled": true
+        }
+      ]
+    }
+  }
+}
diff --git a/protocol/languages/swift/Tests/EmbeddedCheckoutProtocolTests/Fixtures/model-contracts.json b/protocol/languages/swift/Tests/EmbeddedCheckoutProtocolTests/Fixtures/model-contracts.json
new file mode 100644
index 000000000..b751732c0
--- /dev/null
+++ b/protocol/languages/swift/Tests/EmbeddedCheckoutProtocolTests/Fixtures/model-contracts.json
@@ -0,0 +1,169 @@
+{
+  "buyer": {
+    "email": "buyer@example.com",
+    "first_name": "Ada",
+    "last_name": "Example",
+    "phone_number": "+12025550123",
+    "com.example.extension": {
+      "nested": {
+        "flags": [
+          true,
+          false,
+          null
+        ],
+        "count": 3,
+        "fraction": 1.5
+      },
+      "items": [
+        "future",
+        {
+          "enabled": true
+        }
+      ]
+    }
+  },
+  "context": {
+    "address_country": "IE",
+    "address_region": "Dublin",
+    "postal_code": "D02 TEST",
+    "currency": "EUR",
+    "eligibility": [
+      "com.example.member"
+    ],
+    "intent": "gift",
+    "language": "en-IE",
+    "location": "Dublin",
+    "payment": [
+      {
+        "handler": "com.example.payment",
+        "types": [
+          "card"
+        ]
+      }
+    ],
+    "com.example.extension": {
+      "nested": {
+        "flags": [
+          true,
+          false,
+          null
+        ],
+        "count": 3,
+        "fraction": 1.5
+      },
+      "items": [
+        "future",
+        {
+          "enabled": true
+        }
+      ]
+    }
+  },
+  "credential": {
+    "type": "token",
+    "com.example.token": "synthetic-test-token",
+    "com.example.extension": {
+      "nested": {
+        "flags": [
+          true,
+          false,
+          null
+        ],
+        "count": 3,
+        "fraction": 1.5
+      },
+      "items": [
+        "future",
+        {
+          "enabled": true
+        }
+      ]
+    }
+  },
+  "instrument": {
+    "billing_address": {
+      "first_name": "Ada",
+      "last_name": "Example",
+      "street_address": "1 Example Street",
+      "address_locality": "Dublin",
+      "address_region": "Dublin",
+      "postal_code": "D02 TEST",
+      "address_country": "IE"
+    },
+    "credential": {
+      "type": "token",
+      "com.example.token": "synthetic-test-token",
+      "com.example.extension": {
+        "nested": {
+          "flags": [
+            true,
+            false,
+            null
+          ],
+          "count": 3,
+          "fraction": 1.5
+        },
+        "items": [
+          "future",
+          {
+            "enabled": true
+          }
+        ]
+      }
+    },
+    "display": {
+      "label": "Test card",
+      "last_digits": "1234"
+    },
+    "handler_id": "example-handler",
+    "id": "example-instrument",
+    "type": "card",
+    "selected": true,
+    "com.example.extension": {
+      "nested": {
+        "flags": [
+          true,
+          false,
+          null
+        ],
+        "count": 3,
+        "fraction": 1.5
+      },
+      "items": [
+        "future",
+        {
+          "enabled": true
+        }
+      ]
+    }
+  },
+  "policy": {
+    "applies_to": [
+      "$.line_items[0]"
+    ],
+    "description": {
+      "plain": "Return within 30 days",
+      "markdown": "Return within **30 days**",
+      "html": "Return within <b>30 days</b>"
+    },
+    "type": "com.example.return",
+    "url": "https://example.com/returns",
+    "com.example.extension": {
+      "nested": {
+        "flags": [
+          true,
+          false,
+          null
+        ],
+        "count": 3,
+        "fraction": 1.5
+      },
+      "items": [
+        "future",
+        {
+          "enabled": true
+        }
+      ]
+    }
+  }
+}
diff --git a/protocol/languages/swift/Tests/EmbeddedCheckoutProtocolTests/ModelContractTests.swift b/protocol/languages/swift/Tests/EmbeddedCheckoutProtocolTests/ModelContractTests.swift
new file mode 100644
index 000000000..72688217e
--- /dev/null
+++ b/protocol/languages/swift/Tests/EmbeddedCheckoutProtocolTests/ModelContractTests.swift
@@ -0,0 +1,106 @@
+@testable import EmbeddedCheckoutProtocol
+import Foundation
+import Testing
+
+@Suite("Generated model wire contracts")
+struct ModelContractTests {
+    @Test func buyerPreservesNamesAndExtensions() throws {
+        let buyer = try roundTrip(Buyer.self, fixture: "buyer", required: []) {
+            $0.additionalProperties["email"] = $1
+        }
+        #expect(buyer.email == "buyer@example.com")
+        #expect(buyer.firstName == "Ada")
+        #expect(buyer.lastName == "Example")
+        #expect(buyer.phoneNumber == "+12025550123")
+        #expect(buyer.additionalProperties.keys.sorted() == ["com.example.extension"])
+    }
+
+    @Test func contextPreservesLocalizationAndPaymentPreferences() throws {
+        let context = try roundTrip(Context.self, fixture: "context", required: []) {
+            $0.additionalProperties["currency"] = $1
+        }
+        #expect(context.addressCountry == "IE")
+        #expect(context.addressRegion == "Dublin")
+        #expect(context.postalCode == "D02 TEST")
+        #expect(context.currency == "EUR")
+        #expect(context.eligibility == ["com.example.member"])
+        #expect(context.intent == "gift")
+        #expect(context.language == "en-IE")
+        #expect(context.location == "Dublin")
+        #expect(context.payment?.first?.handler == "com.example.payment")
+        #expect(context.payment?.first?.types == ["card"])
+        #expect(context.additionalProperties.keys.sorted() == ["com.example.extension"])
+    }
+
+    @Test func credentialPreservesOpaqueHandlerData() throws {
+        let credential = try roundTrip(PaymentCredential.self, fixture: "credential", required: ["type"]) {
+            $0.additionalProperties["type"] = $1
+        }
+        #expect(credential.type == "token")
+        #expect(credential.additionalProperties["com.example.token"]?.value as? String == "synthetic-test-token")
+        #expect(credential.additionalProperties["type"] == nil)
+    }
+
+    @Test func instrumentPreservesNestedCredentialAndBillingAddress() throws {
+        let instrument = try roundTrip(
+            SelectedPaymentInstrument.self, fixture: "instrument", required: ["handler_id", "id", "type"]
+        ) { $0.additionalProperties["handler_id"] = $1 }
+        #expect(instrument.handlerID == "example-handler")
+        #expect(instrument.id == "example-instrument")
+        #expect(instrument.type == "card")
+        #expect(instrument.selected == true)
+        #expect(instrument.billingAddress?.streetAddress == "1 Example Street")
+        #expect(instrument.credential?.type == "token")
+        #expect(instrument.credential?.additionalProperties["com.example.extension"] != nil)
+        #expect(instrument.display?["last_digits"]?.value as? String == "1234")
+    }
+
+    @Test func policyPreservesDescriptionFormatsAndOpenType() throws {
+        let policy = try roundTrip(Policy.self, fixture: "policy", required: ["description", "type"]) {
+            $0.additionalProperties["type"] = $1
+        }
+        #expect(policy.type == "com.example.return")
+        #expect(policy.appliesTo == ["$.line_items[0]"])
+        #expect(policy.description.plain == "Return within 30 days")
+        #expect(policy.description.markdown == "Return within **30 days**")
+        #expect(policy.description.html == "Return within <b>30 days</b>")
+        #expect(policy.url == "https://example.com/returns")
+    }
+
+    private func roundTrip<Model: Codable>(
+        _ type: Model.Type,
+        fixture: String,
+        required: Set<String>,
+        injectCollision: (inout Model, JSONAny) -> Void
+    ) throws -> Model {
+        let fixtures = try #require(try JSONSerialization.jsonObject(
+            with: Data(protocolFixture("model-contracts").utf8)
+        ) as? [String: [String: Any]])
+        let full = try #require(fixtures[fixture])
+        let encoder = newJSONEncoder()
+        let decoder = newJSONDecoder()
+        let data = try JSONSerialization.data(withJSONObject: full)
+        let decoded = try decoder.decode(type, from: data)
+        let encoded = try #require(try JSONSerialization.jsonObject(with: encoder.encode(decoded)) as? NSDictionary)
+        #expect(encoded == full as NSDictionary)
+
+        // Extension data cannot replace schema-owned fields during serialization.
+        var colliding = decoded
+        injectCollision(&colliding, try decoder.decode(JSONAny.self, from: Data(#""forged""#.utf8)))
+        let collision = try #require(try JSONSerialization.jsonObject(with: encoder.encode(colliding)) as? NSDictionary)
+        #expect(collision == full as NSDictionary)
+
+        // Optional fields may be absent; each required field must independently fail.
+        let minimal = full.filter { required.contains($0.key) }
+        let minimalModel = try decoder.decode(type, from: JSONSerialization.data(withJSONObject: minimal))
+        let minimalEncoded = try #require(try JSONSerialization.jsonObject(with: encoder.encode(minimalModel)) as? NSDictionary)
+        #expect(minimalEncoded == minimal as NSDictionary)
+        for key in required {
+            let missing = full.filter { $0.key != key }
+            #expect(throws: DecodingError.self) {
+                try decoder.decode(type, from: JSONSerialization.data(withJSONObject: missing))
+            }
+        }
+        return decoded
+    }
+}