diff --git a/.changeset/dev-166-builder-fee-404.md b/.changeset/dev-166-builder-fee-404.md
new file mode 100644
index 0000000..9e64f89
--- /dev/null
+++ b/.changeset/dev-166-builder-fee-404.md
@@ -0,0 +1,5 @@
+---
+"@polymarket/client": patch
+---
+
+Map unknown builder fee responses to `UserInputError`.
diff --git a/packages/client/src/actions/clob.ts b/packages/client/src/actions/clob.ts
index f20d012..9b04fdf 100644
--- a/packages/client/src/actions/clob.ts
+++ b/packages/client/src/actions/clob.ts
@@ -410,7 +410,17 @@ export async function fetchBuilderFeeRates(
   return unwrap(
     client.clob
       .get(`/fees/builder-fees/${params.builderCode}`)
-      .andThen(validateWith(FetchBuilderFeeRatesResponseSchema)),
+      .andThen(validateWith(FetchBuilderFeeRatesResponseSchema))
+      .mapErr((error) => {
+        if (error instanceof RequestRejectedError && error.status === 404) {
+          return new UserInputError(
+            `Unknown builder code: ${params.builderCode}`,
+            { cause: error },
+          );
+        }
+
+        return error;
+      }),
   );
 }
 
diff --git a/packages/client/tests/integration/orders.test.ts b/packages/client/tests/integration/orders.test.ts
index e12d581..4d5ba09 100644
--- a/packages/client/tests/integration/orders.test.ts
+++ b/packages/client/tests/integration/orders.test.ts
@@ -1,10 +1,11 @@
-import { OrderSide, OrderType } from '@polymarket/bindings';
+import { BuilderCodeSchema, OrderSide, OrderType } from '@polymarket/bindings';
 import { OrderPostStatus } from '@polymarket/bindings/clob';
 import {
   createSecureClient,
   InsufficientLiquidityError,
   type Market,
   type SecureClient,
+  UserInputError,
 } from '@polymarket/client';
 import { fetchNegRisk } from '@polymarket/client/actions';
 import { expectPresent } from '@polymarket/types';
@@ -20,6 +21,9 @@ import { expectAcceptedOrderResponse } from './helpers';
 import { findHighVolumeLowPriceMarket } from './markets';
 
 const market = await findHighVolumeLowPriceMarket(publicClient);
+const UNKNOWN_BUILDER_CODE = BuilderCodeSchema.parse(
+  '0xabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcdefabcd',
+);
 let marketOrderCleanup:
   | {
       market: Market;
@@ -168,6 +172,26 @@ describe('Orders', { timeout: 60_000 }, () => {
 
       expect(order.builder).toBe(builderCode);
     });
+
+    it('reports unknown builder codes as user input errors when resolving buy amounts against max spend', async ({
+      annotate,
+      secureClientWithDepositWallet,
+    }) => {
+      const yesTokenId = expectPresent(market.outcomes.yes.tokenId);
+      const minimumOrderSize = expectPresent(market.trading.minimumOrderSize);
+      annotate(`Market ID: ${market.id}`);
+      annotate(`Token ID: ${yesTokenId}`);
+
+      await expect(
+        secureClientWithDepositWallet.createMarketOrder({
+          amount: minimumOrderSize,
+          builderCode: UNKNOWN_BUILDER_CODE,
+          maxSpend: minimumOrderSize,
+          side: OrderSide.BUY,
+          tokenId: yesTokenId,
+        }),
+      ).rejects.toThrow(UserInputError);
+    });
   });
 
   describe('placeLimitOrder', () => {