fix: allow error deserializers to populate error response body

.changeset/lazy-olives-film.md

@@ -0,0 +1,5 @@
+---
+"@smithy/middleware-serde": patch
+---
+
+allow deserializers to populate error response body

packages/middleware-serde/src/deserializerMiddleware.spec.ts

@@ -78,7 +78,12 @@ describe("deserializerMiddleware", () => {
   });
 
   it("adds a hint about $response to the message of the thrown error", async () => {
-    const exception = Object.assign(new Error("MockException"), mockNextResponse.response);
+    const exception = Object.assign(new Error("MockException"), mockNextResponse.response, {
+      $response: {
+        body: "",
+      },
+      $responseBodyText: "oh no",
+    });
     mockDeserializer.mockReset();
     mockDeserializer.mockRejectedValueOnce(exception);
     try {
@@ -88,6 +93,7 @@ describe("deserializerMiddleware", () => {
       expect(e.message).toContain(
         "to see the raw response, inspect the hidden field {error}.$response on this object."
       );
+      expect(e.$response.body).toEqual("oh no");
     }
   });
 });

packages/middleware-serde/src/deserializerMiddleware.ts

@@ -33,6 +33,14 @@ export const deserializerMiddleware = <Input extends object, Output extends obje
       // only apply this to non-ServiceException.
       const hint = `Deserialization error: to see the raw response, inspect the hidden field {error}.$response on this object.`;
       error.message += "\n  " + hint;
+
+      if (typeof error.$responseBodyText !== "undefined") {
+        // if $responseBodyText was collected by the error parser, assign it to
+        // replace the response body, because it was consumed and is now empty.
+        if (error.$response) {
+          error.$response.body = error.$responseBodyText;
+        }
+      }
     }
 
     throw error;