fix(smithy-client): add prototype chain fallback for service exception

.changeset/rich-emus-cry.md

@@ -0,0 +1,5 @@
+---
+"@smithy/smithy-client": patch
+---
+
+prototype chain fallback for service exception

packages/smithy-client/src/exceptions.spec.ts

@@ -97,6 +97,17 @@ describe("Exception Hierarchy Tests", () => {
       expect(clientException instanceof ClientServiceException).toBe(true);
       expect(clientException instanceof ModeledClientServiceException).toBe(false);
     });
+
+    it("should handle exceptions where name indicates error type", () => {
+      const egError = new ClientServiceException();
+      egError.name = "EgTooLarge"; // specific error type name
+      // Should maintain instanceof chain
+      expect(egError instanceof Error).toBe(true);
+      expect(egError instanceof ServiceException).toBe(true);
+      expect(egError instanceof ClientServiceException).toBe(true);
+      // The name property can be used to identify specific error types
+      expect(egError.name).toBe("EgTooLarge");
+    });
   });
 
   describe("ModeledClientServiceException Instance Tests", () => {

packages/smithy-client/src/exceptions.ts

@@ -45,9 +45,10 @@ export class ServiceException extends Error implements SmithyException, Metadata
     if (!value) return false;
     const candidate = value as ServiceException;
     return (
-      Boolean(candidate.$fault) &&
-      Boolean(candidate.$metadata) &&
-      (candidate.$fault === "client" || candidate.$fault === "server")
+      ServiceException.prototype.isPrototypeOf(candidate) ||
+      (Boolean(candidate.$fault) &&
+        Boolean(candidate.$metadata) &&
+        (candidate.$fault === "client" || candidate.$fault === "server"))
     );
   }