diff --git a/package-lock.json b/package-lock.json
index fe1f9783c..f4222ba98 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,13 @@
 {
   "name": "@modelcontextprotocol/sdk",
-  "version": "0.3.3",
+  "version": "0.4.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "@modelcontextprotocol/sdk",
-      "version": "0.3.3",
+      "version": "0.4.0",
+      "license": "MIT",
       "dependencies": {
         "content-type": "^1.0.5",
         "raw-body": "^3.0.0",
diff --git a/src/types.ts b/src/types.ts
index a0d2d8040..0d55a75bb 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -723,7 +723,7 @@ export const CallToolResultSchema = ResultSchema.extend({
   content: z.array(
     z.union([TextContentSchema, ImageContentSchema, EmbeddedResourceSchema]),
   ),
-  isError: z.boolean().default(false),
+  isError: z.boolean().default(false).optional(),
 });
 
 /**