diff --git a/protocol/languages/typescript/src/index.d.ts b/protocol/languages/typescript/src/index.d.ts
index 303978c3c..00a97175f 100644
--- a/protocol/languages/typescript/src/index.d.ts
+++ b/protocol/languages/typescript/src/index.d.ts
@@ -7,3 +7,4 @@ export { Client, type DecodeErrorContext } from './client';
 export { windowOpenSuccess, windowOpenRejected } from './window_open';
 export { EmbeddedCheckoutProtocol } from './embedded_checkout_protocol';
 export { ProtocolValidationError, type ProtocolValidationReason, } from './protocol_codec_runtime';
+export { decodeCheckoutSnapshot } from './protocol_codec_runtime';
diff --git a/protocol/languages/typescript/src/index.ts b/protocol/languages/typescript/src/index.ts
index 9b8d3f592..4e8a75ffb 100644
--- a/protocol/languages/typescript/src/index.ts
+++ b/protocol/languages/typescript/src/index.ts
@@ -40,3 +40,4 @@ export {
   ProtocolValidationError,
   type ProtocolValidationReason,
 } from './protocol_codec_runtime';
+export {decodeCheckoutSnapshot} from './protocol_codec_runtime';
diff --git a/protocol/languages/typescript/src/protocol_codec_runtime.d.ts b/protocol/languages/typescript/src/protocol_codec_runtime.d.ts
index b974ac8ac..f32305e9a 100644
--- a/protocol/languages/typescript/src/protocol_codec_runtime.d.ts
+++ b/protocol/languages/typescript/src/protocol_codec_runtime.d.ts
@@ -7,5 +7,7 @@ export declare class ProtocolValidationError extends TypeError {
     constructor(modelPath: string, reason: ProtocolValidationReason);
 }
 export declare function decodeProtocolObject(value: unknown, modelName: string): JSONRecord;
+/** Decode Kit checkout fields using the generated schema, without protocol metadata. */
+export declare function decodeCheckoutSnapshot(value: unknown): JSONRecord;
 export declare function encodeProtocolObject(value: unknown, modelName: string): unknown;
 export {};
diff --git a/protocol/languages/typescript/src/protocol_codec_runtime.ts b/protocol/languages/typescript/src/protocol_codec_runtime.ts
index 940336c98..929cde7ca 100644
--- a/protocol/languages/typescript/src/protocol_codec_runtime.ts
+++ b/protocol/languages/typescript/src/protocol_codec_runtime.ts
@@ -54,6 +54,19 @@ export function decodeProtocolObject(
   return walkObject(input, renameMap[modelName], 'decode', modelName) as JSONRecord;
 }
 
+/** Decode Kit checkout fields using the generated schema, without protocol metadata. */
+export function decodeCheckoutSnapshot(value: unknown): JSONRecord {
+  const input = {...requireObject(value, 'Checkout')};
+  delete input.ucp;
+  requireFields(input, (REQUIRED_FIELDS.Checkout ?? []).filter(field => field !== 'ucp'), 'Checkout');
+  requireStringFields(input, ['currency', 'id', 'status'], 'Checkout');
+  for (const field of ['line_items', 'links', 'totals']) {
+    if (!Array.isArray(input[field])) throw new TypeError('Invalid Checkout');
+  }
+  requireNestedFields(input, 'Checkout');
+  return walkObject(input, renameMap.Checkout, 'decode', 'Checkout') as JSONRecord;
+}
+
 export function encodeProtocolObject(
   value: unknown,
   modelName: string,
@@ -79,8 +92,10 @@ function walkObject(
   const targetIndex = direction === 'decode' ? 1 : 0;
 
   const entryBySource = new Map<string, RenameEntry>();
+  const sourceByTarget = new Map<string, string>();
   for (const entry of entries ?? []) {
     entryBySource.set(entry[sourceIndex], entry);
+    sourceByTarget.set(entry[targetIndex], entry[sourceIndex]);
   }
 
   const output: JSONRecord = {};
@@ -93,9 +108,15 @@ function walkObject(
     }
     const entry = entryBySource.get(key);
     if (entry) {
-      output[entry[targetIndex]] = walkChild(item, entry[2], direction);
+      setOwnProperty(output, entry[targetIndex], walkChild(item, entry[2], direction));
     } else {
-      output[key] = item;
+      // Camel-case aliases are reserved for schema fields, even when the wire
+      // field is absent. Extensions must not overwrite typed checkout values.
+      const source = sourceByTarget.get(key);
+      if (direction === 'decode' && source !== undefined) {
+        throw new ProtocolValidationError(`${modelName}.${source}`, 'invalid_type');
+      }
+      setOwnProperty(output, key, item);
     }
   }
   return output;
@@ -133,11 +154,17 @@ function mapValues(
 ): JSONRecord {
   const output: JSONRecord = {};
   for (const [key, item] of Object.entries(value)) {
-    output[key] = walkChild(item, child, direction);
+    setOwnProperty(output, key, walkChild(item, child, direction));
   }
   return output;
 }
 
+// Assignment invokes Object.prototype.__proto__; define an own data property
+// instead so arbitrary extension and dictionary keys cannot change the prototype.
+function setOwnProperty(output: JSONRecord, key: string, value: unknown): void {
+  Object.defineProperty(output, key, {value, enumerable: true, writable: true, configurable: true});
+}
+
 function walkUnion(
   value: unknown,
   members: RenameChild[],
diff --git a/protocol/languages/typescript/test/checkout-snapshot.test.ts b/protocol/languages/typescript/test/checkout-snapshot.test.ts
new file mode 100644
index 000000000..8077ac628
--- /dev/null
+++ b/protocol/languages/typescript/test/checkout-snapshot.test.ts
@@ -0,0 +1,79 @@
+import {expect, test} from 'vitest';
+import {decodeCheckoutSnapshot} from '../src';
+
+const snapshot = {
+  id: 'checkout-1',
+  currency: 'USD',
+  status: 'incomplete',
+  line_items: [],
+  links: [],
+  totals: [],
+};
+
+test('decodes Kit snapshots without protocol metadata', () => {
+  expect(decodeCheckoutSnapshot(snapshot)).toEqual({
+    id: 'checkout-1', currency: 'USD', status: 'incomplete',
+    lineItems: [], links: [], totals: [],
+  });
+  const withMetadata = {...snapshot, ucp: {version: 'ignored'}};
+  expect(decodeCheckoutSnapshot(withMetadata)).not.toHaveProperty('ucp');
+  expect(withMetadata).toHaveProperty('ucp.version', 'ignored');
+});
+
+test('converts schema fields while retaining extension and dictionary keys', () => {
+  const decoded = decodeCheckoutSnapshot({
+    ...snapshot,
+    buyer: {first_name: 'Test', merchant_field: {nested_key: true}},
+    actions: {'com.example.verify': [{id: 'a1', config: {custom_key: true}}]},
+    attribution: {source_name: 'sample'},
+    signals: {buyer_signal: {custom_key: 1}},
+    custom_extension: {line_items: ['unchanged']},
+    policies: [{id: 'p1', type: 'return', applies_to: ['$.line_items[0]']}],
+    order: {id: 'order-1', permalink_url: 'https://example.test/orders/1'},
+  });
+  expect(decoded).toMatchObject({
+    buyer: {firstName: 'Test', merchant_field: {nested_key: true}},
+    actions: {'com.example.verify': [{id: 'a1', config: {custom_key: true}}]},
+    attribution: {source_name: 'sample'},
+    signals: {buyer_signal: {custom_key: 1}},
+    custom_extension: {line_items: ['unchanged']},
+    policies: [{appliesTo: ['$.line_items[0]']}],
+    order: {permalinkUrl: 'https://example.test/orders/1'},
+  });
+});
+
+test('treats undefined optional fields as absent', () => {
+  const decoded = decodeCheckoutSnapshot({...snapshot, order: undefined, fulfillment: undefined});
+  expect(decoded).not.toHaveProperty('order');
+  expect(decoded).not.toHaveProperty('fulfillment');
+});
+
+test.each([
+  null, [], {}, {...snapshot, id: 1}, {...snapshot, status: 2},
+  {...snapshot, line_items: null}, {...snapshot, totals: undefined},
+  {...snapshot, order: {id: 'order-1'}},
+])('rejects malformed snapshots', value => {
+  expect(() => decodeCheckoutSnapshot(value)).toThrow(TypeError);
+});
+
+
+test('preserves __proto__ as an own extension without inheriting checkout fields', () => {
+  const extension = JSON.parse('{"__proto__":{"order":{"id":1}}}');
+  const decoded = decodeCheckoutSnapshot({...snapshot, ...extension, buyer: extension});
+  expect(Object.getPrototypeOf(decoded)).toBe(Object.prototype);
+  expect(Object.prototype.hasOwnProperty.call(decoded, '__proto__')).toBe(true);
+  expect(decoded.__proto__).toEqual({order: {id: 1}});
+  expect(decoded).not.toHaveProperty('order');
+  expect(Object.getPrototypeOf(decoded.buyer)).toBe(Object.prototype);
+  expect(Object.prototype.hasOwnProperty.call(decoded.buyer, '__proto__')).toBe(true);
+});
+
+test.each([
+  {...snapshot, lineItems: 'extension'},
+  {lineItems: 'extension', ...snapshot},
+  {...snapshot, continueUrl: 123},
+  {...snapshot, buyer: {first_name: 'Test', firstName: 123}},
+  {...snapshot, order: {id: 'order-1', permalink_url: 'https://example.test', permalinkUrl: 123}},
+])('rejects extensions that occupy schema aliases regardless of key order', value => {
+  expect(() => decodeCheckoutSnapshot(value)).toThrow(TypeError);
+});
diff --git a/protocol/languages/typescript/test/codec-runtime.test.ts b/protocol/languages/typescript/test/codec-runtime.test.ts
index c008d0fbc..31e982065 100644
--- a/protocol/languages/typescript/test/codec-runtime.test.ts
+++ b/protocol/languages/typescript/test/codec-runtime.test.ts
@@ -252,3 +252,18 @@ function expectValidationError(
     });
   }
 }
+
+
+test('preserves __proto__ dictionary keys through decoding and encoding', () => {
+  const input = {
+    ...wire,
+    ucp: {version: '2026-01-11', payment_handlers: JSON.parse('{"__proto__":[{"available_instruments":[]}]}')},
+  };
+  const decoded = decodeProtocolObject(input, 'Checkout') as typeof input & {
+    ucp: {paymentHandlers: Record<string, unknown>};
+  };
+  expect(Object.getPrototypeOf(decoded.ucp.paymentHandlers)).toBe(Object.prototype);
+  expect(Object.prototype.hasOwnProperty.call(decoded.ucp.paymentHandlers, '__proto__')).toBe(true);
+  expect(decoded.ucp.paymentHandlers.__proto__).toEqual([{availableInstruments: []}]);
+  expect(encodeProtocolObject(decoded, 'Checkout')).toEqual(input);
+});