diff --git a/packages/inter-protocol/src/auction/auctionBook.js b/packages/inter-protocol/src/auction/auctionBook.js
index 508f15840d6..6af7805c275 100644
--- a/packages/inter-protocol/src/auction/auctionBook.js
+++ b/packages/inter-protocol/src/auction/auctionBook.js
@@ -59,6 +59,29 @@ const trace = makeTracer('AucBook', false);
  * offerBidScaling: Ratio,
  * })} BidSpec
  */
+/**
+ *
+ * @param {Brand<'nat'>} currencyBrand
+ * @param {Brand<'nat'>} collateralBrand
+ */
+export const makeBidSpecShape = (currencyBrand, collateralBrand) => {
+  const currencyAmountShape = { brand: currencyBrand, value: M.nat() };
+  const collateralAmountShape = { brand: collateralBrand, value: M.nat() };
+  return M.splitRecord(
+    { want: collateralAmountShape },
+    {
+      // xxx should have exactly one of these properties
+      offerPrice: makeBrandedRatioPattern(
+        currencyAmountShape,
+        collateralAmountShape,
+      ),
+      offerBidScaling: makeBrandedRatioPattern(
+        currencyAmountShape,
+        currencyAmountShape,
+      ),
+    },
+  );
+};
 
 /** @typedef {import('@agoric/vat-data').Baggage} Baggage */
 
@@ -145,6 +168,7 @@ export const makeAuctionBook = async (
 
   let curAuctionPrice = zeroRatio;
 
+  // FIXME the maker callbacks return a non-durable object
   const scaledBidBook = provide(baggage, 'scaledBidBook', () => {
     const ratioPattern = makeBrandedRatioPattern(
       currencyAmountShape,
diff --git a/packages/inter-protocol/src/auction/auctioneer.js b/packages/inter-protocol/src/auction/auctioneer.js
index 06332a6daaf..955f7d9c69a 100644
--- a/packages/inter-protocol/src/auction/auctioneer.js
+++ b/packages/inter-protocol/src/auction/auctioneer.js
@@ -23,7 +23,7 @@ import { makeTracer, BASIS_POINTS } from '@agoric/internal';
 import { FullProposalShape } from '@agoric/zoe/src/typeGuards.js';
 import { appendToStoredArray } from '@agoric/store/src/stores/store-utils.js';
 import { mustMatch } from '@agoric/store';
-import { makeAuctionBook } from './auctionBook.js';
+import { makeAuctionBook, makeBidSpecShape } from './auctionBook.js';
 import { AuctionState } from './util.js';
 import { makeScheduler } from './scheduler.js';
 import { auctioneerParamTypes } from './params.js';
@@ -268,6 +268,16 @@ export const start = async (zcf, privateArgs, baggage) => {
       M.splitRecord({ give: { Collateral: AmountShape } }),
     );
 
+  const bidProposalShape = M.splitRecord(
+    {
+      give: { Currency: { brand: brands.Currency, value: M.nat() } },
+    },
+    {
+      want: M.or({ Collateral: AmountShape }, {}),
+      exit: FullProposalShape.exit,
+    },
+  );
+
   const publicFacet = augmentPublicFacet(
     harden({
       /** @param {Brand<'nat'>} collateralBrand */
@@ -275,24 +285,18 @@ export const start = async (zcf, privateArgs, baggage) => {
         mustMatch(collateralBrand, BrandShape);
         books.has(collateralBrand) ||
           Fail`No book for brand ${collateralBrand}`;
+        const bidSpecShape = makeBidSpecShape(brands.Currency, collateralBrand);
         /**
          * @param {ZCFSeat} zcfSeat
          * @param {import('./auctionBook.js').BidSpec} bidSpec
          */
         const newBidHandler = (zcfSeat, bidSpec) => {
+          // xxx consider having Zoe guard the offerArgs with a provided shape
+          mustMatch(bidSpec, bidSpecShape);
           const auctionBook = books.get(collateralBrand);
           auctionBook.addOffer(bidSpec, zcfSeat, isActive());
           return 'Your offer has been received';
         };
-        const bidProposalShape = M.splitRecord(
-          {
-            give: { Currency: { brand: brands.Currency, value: M.nat() } },
-          },
-          {
-            want: M.or({ Collateral: AmountShape }, {}),
-            exit: FullProposalShape.exit,
-          },
-        );
 
         return zcf.makeInvitation(
           newBidHandler,