diff --git a/mock/brand.mock.ts b/mock/brand.mock.ts
index f26b323..6a69c86 100644
--- a/mock/brand.mock.ts
+++ b/mock/brand.mock.ts
@@ -8,4 +8,5 @@ export const mockBrand = () =>
     getDisplayInfo: () => ({
       assetKind: AssetKind.NAT,
     }),
+    getAmountShape: () => '',
   });
diff --git a/package.json b/package.json
index f68b41f..52342a5 100644
--- a/package.json
+++ b/package.json
@@ -43,6 +43,7 @@
     "eslint-plugin-react": "^7.30.1",
     "eslint-plugin-react-hooks": "^4.6.0",
     "framer-motion": "^7.2.1",
+    "happy-dom": "^10.7.0",
     "jotai": "^1.8.0",
     "lodash-es": "4.17.21",
     "patch-package": "^7.0.0",
diff --git a/src/services/swap.ts b/src/services/swap.ts
index 44c0ed0..44b1e75 100644
--- a/src/services/swap.ts
+++ b/src/services/swap.ts
@@ -1,8 +1,8 @@
 import type { PursesJSONState } from '@agoric/wallet-backend';
+import type { WalletBridge } from 'store/app';
 import { E } from '@endo/eventual-send';
-
 import { SwapDirection } from 'store/swap';
-import { WalletBridge } from 'store/app';
+import { AmountMath } from '@agoric/ertp';
 
 type SwapContext = {
   wallet: WalletBridge;
@@ -30,7 +30,11 @@ export const makeSwapOffer = async ({
       ? 'makeWantMintedInvitation'
       : 'makeGiveMintedInvitation';
 
-  const serializedInstance = await E(marshal).serialize(instanceId);
+  const [serializedInstance, serializedIn, serializedOut] = await Promise.all([
+    E(marshal).serialize(instanceId),
+    E(marshal).serialize(AmountMath.make(fromPurse.brand, fromValue)),
+    E(marshal).serialize(AmountMath.make(toPurse.brand, toValue)),
+  ]);
 
   const offerConfig = {
     publicInvitationMaker: method,
@@ -38,14 +42,12 @@ export const makeSwapOffer = async ({
     proposalTemplate: {
       give: {
         In: {
-          pursePetname: fromPurse.pursePetname,
-          value: Number(fromValue),
+          amount: serializedIn,
         },
       },
       want: {
         Out: {
-          pursePetname: toPurse.pursePetname,
-          value: Number(toValue),
+          amount: serializedOut,
         },
       },
     },
diff --git a/test/services/swap.test.ts b/test/services/swap.test.ts
new file mode 100644
index 0000000..4021501
--- /dev/null
+++ b/test/services/swap.test.ts
@@ -0,0 +1,49 @@
+import { expect, it, describe } from 'vitest';
+import { mockBrand } from '../../mock/brand.mock';
+import { makeMarshal } from '@endo/marshal';
+import { makeSwapOffer } from '../../src/services/swap';
+import { SwapDirection } from '../../src/store/swap';
+import { AmountMath } from '@agoric/ertp';
+
+describe('helpers', () => {
+  const minted = mockBrand();
+  const anchor = mockBrand();
+
+  it('serializes the amounts properly', async () => {
+    let config: any;
+    const wallet = {
+      addOffer: offerConfig => {
+        config = offerConfig;
+      },
+    };
+
+    const marshal = makeMarshal();
+
+    const d = 10n ** 18n;
+    const fromValue = 5000n * d;
+    const toValue = 50_000n * d;
+
+    expect(Number(fromValue) > Number.MAX_SAFE_INTEGER).toBe(true);
+
+    await makeSwapOffer({
+      wallet,
+      instanceId: '123',
+      fromPurse: { brand: anchor },
+      fromValue,
+      toPurse: { brand: minted },
+      toValue,
+      swapDirection: SwapDirection.WantMinted,
+      marshal,
+    });
+
+    const fromAmount = marshal.fromCapData(
+      config.proposalTemplate.give.In.amount
+    );
+    const toAmount = marshal.fromCapData(
+      config.proposalTemplate.want.Out.amount
+    );
+
+    expect(AmountMath.isEqual(AmountMath.make(anchor, fromValue), fromAmount));
+    expect(AmountMath.isEqual(AmountMath.make(minted, toValue), toAmount));
+  });
+});
diff --git a/vitest.config.ts b/vitest.config.ts
index 3659233..021c617 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -7,6 +7,7 @@ export default mergeConfig(
   defineConfig({
     test: {
       setupFiles: ['src/installSesLockdown.ts'],
+      environment: 'happy-dom',
     },
   })
 );
diff --git a/yarn.lock b/yarn.lock
index 91814b6..cbb18c9 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -2844,6 +2844,11 @@ css-to-react-native@^3.0.0:
     css-color-keywords "^1.0.0"
     postcss-value-parser "^4.0.2"
 
+css.escape@^1.5.1:
+  version "1.5.1"
+  resolved "https://registry.yarnpkg.com/css.escape/-/css.escape-1.5.1.tgz#42e27d4fa04ae32f931a4b4d4191fa9cddee97cb"
+  integrity sha512-YUifsXXuknHlUsmlgyY0PKzgPOr7/FjCePfHNt0jxm83wHZi44VDMQ7/fGNkjY3/jV1MC+1CmZbaHzugyeRtpg==
+
 cssesc@^3.0.0:
   version "3.0.0"
   resolved "https://registry.yarnpkg.com/cssesc/-/cssesc-3.0.0.tgz#37741919903b868565e1c09ea747445cd18983ee"
@@ -3066,6 +3071,11 @@ end-of-stream@^1.1.0, end-of-stream@^1.4.1:
   dependencies:
     once "^1.4.0"
 
+entities@^4.5.0:
+  version "4.5.0"
+  resolved "https://registry.yarnpkg.com/entities/-/entities-4.5.0.tgz#5d268ea5e7113ec74c4d033b79ea5a35a488fb48"
+  integrity sha512-V0hjH4dGPh9Ao5p0MoRY6BVqtwCjhz6vI5LT8AJ55H+4g9/4vbHx1I54fS0XuclLhDHArPQCiMjDxjaL8fPxhw==
+
 es-abstract@^1.19.0, es-abstract@^1.19.1, es-abstract@^1.19.2, es-abstract@^1.19.5:
   version "1.20.4"
   resolved "https://registry.yarnpkg.com/es-abstract/-/es-abstract-1.20.4.tgz#1d103f9f8d78d4cf0713edcd6d0ed1a46eed5861"
@@ -3840,6 +3850,18 @@ grapheme-splitter@^1.0.4:
   resolved "https://registry.yarnpkg.com/grapheme-splitter/-/grapheme-splitter-1.0.4.tgz#9cf3a665c6247479896834af35cf1dbb4400767e"
   integrity sha512-bzh50DW9kTPM00T8y4o8vQg89Di9oLJVLW/KaOGIXJWP/iqCN6WKYkbNOF04vFLJhwcpYUh9ydh/+5vpOqV4YQ==
 
+happy-dom@^10.7.0:
+  version "10.7.0"
+  resolved "https://registry.yarnpkg.com/happy-dom/-/happy-dom-10.7.0.tgz#923236e1377b983c51eee51b0b0035acde361f90"
+  integrity sha512-dJFPbA7KW4kkH3MFJBPTX73ywOfcrJcu6CAUw0CnCH0aEDcnPNG8QVmOAxsJSvTZFzLCnmSx/jMzAp00hdm0gw==
+  dependencies:
+    css.escape "^1.5.1"
+    entities "^4.5.0"
+    iconv-lite "^0.6.3"
+    webidl-conversions "^7.0.0"
+    whatwg-encoding "^2.0.0"
+    whatwg-mimetype "^3.0.0"
+
 has-bigints@^1.0.1, has-bigints@^1.0.2:
   version "1.0.2"
   resolved "https://registry.yarnpkg.com/has-bigints/-/has-bigints-1.0.2.tgz#0871bd3e3d51626f6ca0966668ba35d5602d6eaa"
@@ -3910,6 +3932,13 @@ hoist-non-react-statics@^3.0.0:
   dependencies:
     react-is "^16.7.0"
 
+iconv-lite@0.6.3, iconv-lite@^0.6.3:
+  version "0.6.3"
+  resolved "https://registry.yarnpkg.com/iconv-lite/-/iconv-lite-0.6.3.tgz#a52f80bf38da1952eb5c681790719871a1a72501"
+  integrity sha512-4fCk79wshMdzMp2rH06qWrJE4iolqLhCUH+OiuIgU++RB0+94NlDL81atO7GX55uUKueo0txHNtvEyI6D7WdMw==
+  dependencies:
+    safer-buffer ">= 2.1.2 < 3.0.0"
+
 iconv-lite@^0.4.24:
   version "0.4.24"
   resolved "https://registry.yarnpkg.com/iconv-lite/-/iconv-lite-0.4.24.tgz#2022b4b25fbddc21d2f524974a474aafe733908b"
@@ -5411,7 +5440,7 @@ safe-stable-stringify@^2.3.1:
   resolved "https://registry.yarnpkg.com/safe-stable-stringify/-/safe-stable-stringify-2.4.3.tgz#138c84b6f6edb3db5f8ef3ef7115b8f55ccbf886"
   integrity sha512-e2bDA2WJT0wxseVd4lsDP4+3ONX6HpMXQa1ZhFQ7SU+GjvORCmShbCMltrtIDfkYhVHrOcPtj+KhmDBdPdZD1g==
 
-"safer-buffer@>= 2.1.2 < 3":
+"safer-buffer@>= 2.1.2 < 3", "safer-buffer@>= 2.1.2 < 3.0.0":
   version "2.1.2"
   resolved "https://registry.yarnpkg.com/safer-buffer/-/safer-buffer-2.1.2.tgz#44fa161b0187b9549dd84bb91802f9bd8385cd6a"
   integrity sha512-YZo3K82SD7Riyi0E1EQPojLz7kpepnSQI9IyPbHHg1XXXevb5dJI7tpyN2ADxGcQbHG7vcyRHk0cbwqcQriUtg==
@@ -5424,19 +5453,19 @@ scheduler@^0.23.0:
     loose-envify "^1.1.0"
 
 semver@^5.5.0, semver@^5.6.0:
-  version "5.7.1"
-  resolved "https://registry.yarnpkg.com/semver/-/semver-5.7.1.tgz#a954f931aeba508d307bbf069eff0c01c96116f7"
-  integrity sha512-sauaDf/PZdVgrLTNYHRtpXa1iRiKcaebiKQ1BJdpQlWH2lCvexQdX55snPFyK7QzpudqbCI0qXFfOasHdyNDGQ==
+  version "5.7.2"
+  resolved "https://registry.yarnpkg.com/semver/-/semver-5.7.2.tgz#48d55db737c3287cd4835e17fa13feace1c41ef8"
+  integrity sha512-cBznnQ9KjJqU67B52RMC65CMarK2600WFnbkcaiwWq3xy/5haFJlshgnpjovMVJ+Hff49d8GEn0b87C5pDQ10g==
 
 semver@^6.3.0:
-  version "6.3.0"
-  resolved "https://registry.yarnpkg.com/semver/-/semver-6.3.0.tgz#ee0a64c8af5e8ceea67687b133761e1becbd1d3d"
-  integrity sha512-b39TBaTSfV6yBrapU89p5fKekE2m/NwnDocOVruQFS1/veMgdzuPcnOM34M6CwxW8jH/lxEa5rBoDeUwu5HHTw==
+  version "6.3.1"
+  resolved "https://registry.yarnpkg.com/semver/-/semver-6.3.1.tgz#556d2ef8689146e46dcea4bfdd095f3434dffcb4"
+  integrity sha512-BR7VvDCVHO+q2xBEWskxS6DJE1qRnb7DxzUrogb71CWoSficBxYsiAGd+Kl0mmq/MprG9yArRkyrQxTO6XjMzA==
 
 semver@^7.0.0, semver@^7.3.5, semver@^7.3.7:
-  version "7.3.8"
-  resolved "https://registry.yarnpkg.com/semver/-/semver-7.3.8.tgz#07a78feafb3f7b32347d725e33de7e2a2df67798"
-  integrity sha512-NB1ctGL5rlHrPJtFDVIVzTyQylMLu9N9VICA6HSFJo8MCGVTMW6gfpicwKmmK/dAjTOrqu5l63JJOpDSrAis3A==
+  version "7.5.4"
+  resolved "https://registry.yarnpkg.com/semver/-/semver-7.5.4.tgz#483986ec4ed38e1c6c48c34894a9182dbff68a6e"
+  integrity sha512-1bCSESV6Pv+i21Hvpxp3Dx+pSD8lIPt8uVjRrxAUt/nbswYc+tK6Y2btiULjd4+fnq15PX+nqQDC7Oft7WkwcA==
   dependencies:
     lru-cache "^6.0.0"
 
@@ -6056,6 +6085,23 @@ webidl-conversions@^3.0.0:
   resolved "https://registry.yarnpkg.com/webidl-conversions/-/webidl-conversions-3.0.1.tgz#24534275e2a7bc6be7bc86611cc16ae0a5654871"
   integrity sha512-2JAn3z8AR6rjK8Sm8orRC0h/bcl/DqL7tRPdGZ4I1CjdF+EaMLmYxBHyXuKL849eucPFhvBoxMsflfOb8kxaeQ==
 
+webidl-conversions@^7.0.0:
+  version "7.0.0"
+  resolved "https://registry.yarnpkg.com/webidl-conversions/-/webidl-conversions-7.0.0.tgz#256b4e1882be7debbf01d05f0aa2039778ea080a"
+  integrity sha512-VwddBukDzu71offAQR975unBIGqfKZpM+8ZX6ySk8nYhVoo5CYaZyzt3YBvYtRtO+aoGlqxPg/B87NGVZ/fu6g==
+
+whatwg-encoding@^2.0.0:
+  version "2.0.0"
+  resolved "https://registry.yarnpkg.com/whatwg-encoding/-/whatwg-encoding-2.0.0.tgz#e7635f597fd87020858626805a2729fa7698ac53"
+  integrity sha512-p41ogyeMUrw3jWclHWTQg1k05DSVXPLcVxRTYsXUk+ZooOCZLcoYgPZ/HL/D/N+uQPOtcp1me1WhBEaX02mhWg==
+  dependencies:
+    iconv-lite "0.6.3"
+
+whatwg-mimetype@^3.0.0:
+  version "3.0.0"
+  resolved "https://registry.yarnpkg.com/whatwg-mimetype/-/whatwg-mimetype-3.0.0.tgz#5fa1a7623867ff1af6ca3dc72ad6b8a4208beba7"
+  integrity sha512-nt+N2dzIutVRxARx1nghPKGv1xHikU7HKdfafKkLNLindmPU/ch3U31NOCGGA/dmPcmb1VlofO0vnKAcsm0o/Q==
+
 whatwg-url@^5.0.0:
   version "5.0.0"
   resolved "https://registry.yarnpkg.com/whatwg-url/-/whatwg-url-5.0.0.tgz#966454e8765462e37644d3626f6742ce8b70965d"
@@ -6107,9 +6153,9 @@ winston@3.3.3:
     winston-transport "^4.4.0"
 
 word-wrap@^1.2.3:
-  version "1.2.3"
-  resolved "https://registry.yarnpkg.com/word-wrap/-/word-wrap-1.2.3.tgz#610636f6b1f703891bd34771ccb17fb93b47079c"
-  integrity sha512-Hz/mrNwitNRh/HUAtM/VT/5VH+ygD6DV7mYKZAtHOrbs8U7lvPS6xf7EJKMF0uW1KJCl0H701g3ZGus+muE5vQ==
+  version "1.2.4"
+  resolved "https://registry.yarnpkg.com/word-wrap/-/word-wrap-1.2.4.tgz#cb4b50ec9aca570abd1f52f33cd45b6c61739a9f"
+  integrity sha512-2V81OA4ugVo5pRo46hAoD2ivUJx8jXmWXfUkY4KFNw0hEptvN0QfH3K4nHiwzGeKl5rFKedV48QVoqYavy4YpA==
 
 wrap-ansi@^7.0.0:
   version "7.0.0"