diff --git a/src/shared/protocol.ts b/src/shared/protocol.ts
index b072e578..07bfc02c 100644
--- a/src/shared/protocol.ts
+++ b/src/shared/protocol.ts
@@ -3,6 +3,10 @@ import {
   CancelledNotificationSchema,
   ClientCapabilities,
   ErrorCode,
+  isJSONRPCError,
+  isJSONRPCRequest,
+  isJSONRPCResponse,
+  isJSONRPCNotification,
   JSONRPCError,
   JSONRPCNotification,
   JSONRPCRequest,
@@ -271,12 +275,14 @@ export abstract class Protocol<
     };
 
     this._transport.onmessage = (message) => {
-      if (!("method" in message)) {
+      if (isJSONRPCResponse(message) || isJSONRPCError(message)) {
         this._onresponse(message);
-      } else if ("id" in message) {
+      } else if (isJSONRPCRequest(message)) {
         this._onrequest(message);
-      } else {
+      } else if (isJSONRPCNotification(message)) {
         this._onnotification(message);
+      } else {
+        this._onerror(new Error(`Unknown message type: ${JSON.stringify(message)}`));
       }
     };
 
@@ -437,7 +443,7 @@ export abstract class Protocol<
     this._progressHandlers.delete(messageId);
     this._cleanupTimeout(messageId);
 
-    if ("result" in response) {
+    if (isJSONRPCResponse(response)) {
       handler(response);
     } else {
       const error = new McpError(
diff --git a/src/types.ts b/src/types.ts
index da4afd2a..db6bf125 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -78,6 +78,9 @@ export const JSONRPCRequestSchema = z
   .merge(RequestSchema)
   .strict();
 
+export const isJSONRPCRequest = (value: unknown): value is JSONRPCRequest =>
+  JSONRPCRequestSchema.safeParse(value).success;
+
 /**
  * A notification which does not expect a response.
  */
@@ -88,6 +91,11 @@ export const JSONRPCNotificationSchema = z
   .merge(NotificationSchema)
   .strict();
 
+export const isJSONRPCNotification = (
+  value: unknown
+): value is JSONRPCNotification =>
+  JSONRPCNotificationSchema.safeParse(value).success;
+
 /**
  * A successful (non-error) response to a request.
  */
@@ -99,6 +107,9 @@ export const JSONRPCResponseSchema = z
   })
   .strict();
 
+export const isJSONRPCResponse = (value: unknown): value is JSONRPCResponse =>
+  JSONRPCResponseSchema.safeParse(value).success;
+
 /**
  * Error codes defined by the JSON-RPC specification.
  */
@@ -139,6 +150,9 @@ export const JSONRPCErrorSchema = z
   })
   .strict();
 
+export const isJSONRPCError = (value: unknown): value is JSONRPCError =>
+  JSONRPCErrorSchema.safeParse(value).success;
+
 export const JSONRPCMessageSchema = z.union([
   JSONRPCRequestSchema,
   JSONRPCNotificationSchema,