diff --git a/packages/zoe/src/contractSupport/index.js b/packages/zoe/src/contractSupport/index.js
index 7fb1b2a6df5..a096590eccb 100644
--- a/packages/zoe/src/contractSupport/index.js
+++ b/packages/zoe/src/contractSupport/index.js
@@ -36,6 +36,7 @@ export {
   withdrawFromSeat,
   saveAllIssuers,
   offerTo,
+  checkZCF,
 } from './zoeHelpers';
 
 export {
diff --git a/packages/zoe/src/contractSupport/zoeHelpers.js b/packages/zoe/src/contractSupport/zoeHelpers.js
index 6daa328c9d0..905f03109ad 100644
--- a/packages/zoe/src/contractSupport/zoeHelpers.js
+++ b/packages/zoe/src/contractSupport/zoeHelpers.js
@@ -476,3 +476,24 @@ export const offerTo = async (
 
   return harden({ userSeatPromise, deposited: depositedPromiseKit.promise });
 };
+
+/**
+ * Create a wrapped version of zcf that asserts an invariant
+ * before performing a reallocation.
+ *
+ * @param {ContractFacet} zcf
+ * @param {(stagings: SeatStaging[]) => void} assertFn - an assertion
+ * that must be true for the reallocate to occur
+ * @returns {ContractFacet}
+ */
+export const checkZCF = (zcf, assertFn) => {
+  const checkedZCF = harden({
+    ...zcf,
+    reallocate: (...stagings) => {
+      assertFn(stagings);
+      // @ts-ignore The types aren't right for spreading
+      zcf.reallocate(...stagings);
+    },
+  });
+  return checkedZCF;
+};
diff --git a/packages/zoe/src/contracts/multipoolAutoswap/constantProduct.js b/packages/zoe/src/contracts/multipoolAutoswap/constantProduct.js
new file mode 100644
index 00000000000..147105f22ce
--- /dev/null
+++ b/packages/zoe/src/contracts/multipoolAutoswap/constantProduct.js
@@ -0,0 +1,38 @@
+// @ts-check
+
+import { assert, details as X } from '@agoric/assert';
+import { natSafeMath } from '../../contractSupport';
+
+// A pool seat has Central and Secondary keywords, and a swap seat has
+// In and Out keywords
+const isPoolSeat = allocation => {
+  return allocation.Central !== undefined || allocation.Secondary !== undefined;
+};
+
+const calcK = allocation => {
+  return natSafeMath.multiply(
+    allocation.Secondary.value,
+    allocation.Central.value,
+  );
+};
+
+/**
+ *
+ * @param {SeatStaging[]} stagings
+ */
+export const assertConstantProduct = stagings => {
+  stagings.forEach(seatStaging => {
+    const seat = seatStaging.getSeat();
+    const priorAllocation = seat.getCurrentAllocation();
+    const stagedAllocation = seatStaging.getStagedAllocation();
+    if (isPoolSeat(stagedAllocation)) {
+      const oldK = calcK(priorAllocation);
+      const newK = calcK(stagedAllocation);
+      console.log('oldK', oldK, 'newK', newK);
+      assert(
+        newK >= oldK,
+        X`the product of the pool tokens must not decrease as the result of a trade. ${oldK} decreased to ${newK}`,
+      );
+    }
+  });
+};
diff --git a/packages/zoe/src/contracts/multipoolAutoswap/multipoolAutoswap.js b/packages/zoe/src/contracts/multipoolAutoswap/multipoolAutoswap.js
index fd9226ab7dc..909e19061e3 100644
--- a/packages/zoe/src/contracts/multipoolAutoswap/multipoolAutoswap.js
+++ b/packages/zoe/src/contracts/multipoolAutoswap/multipoolAutoswap.js
@@ -5,12 +5,13 @@ import { makeWeakStore } from '@agoric/store';
 import { Far } from '@agoric/marshal';
 
 import { AssetKind, makeIssuerKit, AmountMath } from '@agoric/ertp';
-import { assertIssuerKeywords } from '../../contractSupport';
+import { assertIssuerKeywords, checkZCF } from '../../contractSupport';
 import { makeAddPool } from './pool';
 import { makeGetCurrentPrice } from './getCurrentPrice';
 import { makeMakeSwapInvitation } from './swap';
 import { makeMakeAddLiquidityInvitation } from './addLiquidity';
 import { makeMakeRemoveLiquidityInvitation } from './removeLiquidity';
+import { assertConstantProduct } from './constantProduct';
 
 import '../../../exported';
 
@@ -126,7 +127,7 @@ const start = zcf => {
     makeSwapInInvitation,
     makeSwapOutInvitation,
   } = makeMakeSwapInvitation(
-    zcf,
+    checkZCF(zcf, assertConstantProduct),
     isSecondary,
     isCentral,
     getPool,
diff --git a/packages/zoe/test/unitTests/contracts/multipoolAutoswap/test-constantProduct.js b/packages/zoe/test/unitTests/contracts/multipoolAutoswap/test-constantProduct.js
new file mode 100644
index 00000000000..b4d33575799
--- /dev/null
+++ b/packages/zoe/test/unitTests/contracts/multipoolAutoswap/test-constantProduct.js
@@ -0,0 +1,64 @@
+// @ts-check
+
+// eslint-disable-next-line import/no-extraneous-dependencies
+import { test } from '@agoric/zoe/tools/prepare-test-env-ava';
+
+import { AmountMath } from '@agoric/ertp';
+
+import { checkZCF } from '../../../../src/contractSupport';
+import { assertConstantProduct } from '../../../../src/contracts/multipoolAutoswap/constantProduct';
+import { setupZCFTest } from '../../zcf/setupZcfTest';
+
+test('constantProduct invariant', async t => {
+  const { zcf } = await setupZCFTest();
+
+  const checkedZCF = checkZCF(zcf, assertConstantProduct);
+
+  const { zcfSeat: poolSeat } = checkedZCF.makeEmptySeatKit();
+  const { zcfSeat: swapSeat } = checkedZCF.makeEmptySeatKit();
+
+  // allocate some secondary and central to the poolSeat
+  const centralMint = await checkedZCF.makeZCFMint('Central');
+  const { brand: centralBrand } = centralMint.getIssuerRecord();
+  const secondaryMint = await checkedZCF.makeZCFMint('Secondary');
+  const { brand: secondaryBrand } = secondaryMint.getIssuerRecord();
+  centralMint.mintGains(
+    { Central: AmountMath.make(centralBrand, 10n ** 6n) },
+    poolSeat,
+  );
+  secondaryMint.mintGains(
+    { Secondary: AmountMath.make(secondaryBrand, 10n ** 6n) },
+    poolSeat,
+  );
+
+  const poolSeatAllocation = poolSeat.getCurrentAllocation();
+  t.deepEqual(poolSeatAllocation, {
+    Central: AmountMath.make(centralBrand, 10n ** 6n),
+    Secondary: AmountMath.make(secondaryBrand, 10n ** 6n),
+  });
+
+  // const oldK =
+  //   poolSeatAllocation.Secondary.value * poolSeatAllocation.Central.value;
+
+  // const newK = 0;
+
+  // Let's give the swap user all the tokens and take
+  // nothing, a clear violation of the constant product
+  t.throws(
+    () =>
+      checkedZCF.reallocate(
+        poolSeat.stage({
+          Central: AmountMath.make(centralBrand, 0n),
+          Secondary: AmountMath.make(secondaryBrand, 0n),
+        }),
+        swapSeat.stage({
+          In: poolSeatAllocation.Central,
+          Out: poolSeatAllocation.Secondary,
+        }),
+      ),
+    {
+      message:
+        'the product of the pool tokens must not decrease as the result of a trade. "[1000000000000n]" decreased to "[0n]"',
+    },
+  );
+});
diff --git a/packages/zoe/test/unitTests/contracts/test-multipoolAutoswap.js b/packages/zoe/test/unitTests/contracts/multipoolAutoswap/test-multipoolAutoswap.js
similarity index 99%
rename from packages/zoe/test/unitTests/contracts/test-multipoolAutoswap.js
rename to packages/zoe/test/unitTests/contracts/multipoolAutoswap/test-multipoolAutoswap.js
index f2c9e70e0ff..e7aa7153df1 100644
--- a/packages/zoe/test/unitTests/contracts/test-multipoolAutoswap.js
+++ b/packages/zoe/test/unitTests/contracts/multipoolAutoswap/test-multipoolAutoswap.js
@@ -6,23 +6,26 @@ import { test } from '@agoric/zoe/tools/prepare-test-env-ava';
 import bundleSource from '@agoric/bundle-source';
 import { makeIssuerKit, amountMath } from '@agoric/ertp';
 import { E } from '@agoric/eventual-send';
-import fakeVatAdmin from '../../../tools/fakeVatAdmin';
+import fakeVatAdmin from '../../../../tools/fakeVatAdmin';
 
 // noinspection ES6PreferShortImport
-import { makeZoe } from '../../../src/zoeService/zoe';
-import { setup } from '../setupBasicMints';
+import { makeZoe } from '../../../../src/zoeService/zoe';
+import { setup } from '../../setupBasicMints';
 import {
   makeTrader,
   updatePoolState,
   scaleForAddLiquidity,
   scaleForRemoveLiquidity,
   priceFromTargetOutput,
-} from '../../autoswapJig';
-import { assertPayoutDeposit, assertAmountsEqual } from '../../zoeTestHelpers';
-import buildManualTimer from '../../../tools/manualTimer';
-import { getAmountOut } from '../../../src/contractSupport';
+} from '../../../autoswapJig';
+import {
+  assertPayoutDeposit,
+  assertAmountsEqual,
+} from '../../../zoeTestHelpers';
+import buildManualTimer from '../../../../tools/manualTimer';
+import { getAmountOut } from '../../../../src/contractSupport';
 
-const multipoolAutoswapRoot = `${__dirname}/../../../src/contracts/multipoolAutoswap/multipoolAutoswap`;
+const multipoolAutoswapRoot = `${__dirname}/../../../../src/contracts/multipoolAutoswap/multipoolAutoswap`;
 
 test('multipoolAutoSwap with valid offers', async t => {
   const { moolaR, simoleanR, moola, simoleans } = setup();
diff --git a/packages/zoe/test/unitTests/contracts/test-multipoolPriceAuthority.js b/packages/zoe/test/unitTests/contracts/multipoolAutoswap/test-multipoolPriceAuthority.js
similarity index 88%
rename from packages/zoe/test/unitTests/contracts/test-multipoolPriceAuthority.js
rename to packages/zoe/test/unitTests/contracts/multipoolAutoswap/test-multipoolPriceAuthority.js
index 625640dd37e..f5e9dde0180 100644
--- a/packages/zoe/test/unitTests/contracts/test-multipoolPriceAuthority.js
+++ b/packages/zoe/test/unitTests/contracts/multipoolAutoswap/test-multipoolPriceAuthority.js
@@ -4,9 +4,9 @@
 import { test } from '@agoric/zoe/tools/prepare-test-env-ava';
 import { makeNotifierKit } from '@agoric/notifier';
 import { makeIssuerKit, amountMath, AssetKind } from '@agoric/ertp';
-import { makePriceAuthority } from '../../../src/contracts/multipoolAutoswap/priceAuthority';
-import { setup } from '../setupBasicMints';
-import buildManualTimer from '../../../tools/manualTimer';
+import { makePriceAuthority } from '../../../../src/contracts/multipoolAutoswap/priceAuthority';
+import { setup } from '../../setupBasicMints';
+import buildManualTimer from '../../../../tools/manualTimer';
 
 test('multipoolAutoSwap PriceAuthority exception path', async t => {
   const { moolaR, simoleanR } = setup();