From 82156515523402bd230d54ef6682762922711e87 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Fr=C3=A9d=C3=A9ric=20Barthelet?=
 <frederic.barthelet@gmail.com>
Date: Mon, 7 Apr 2025 18:24:50 +0200
Subject: [PATCH 1/2] Add helper functions to distinguish between jsonRPC
 message types

---
 src/shared/protocol.ts | 13 +++++++++----
 src/types.ts           | 11 +++++++++++
 2 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/src/shared/protocol.ts b/src/shared/protocol.ts
index b072e578..19ff97d3 100644
--- a/src/shared/protocol.ts
+++ b/src/shared/protocol.ts
@@ -3,6 +3,9 @@ import {
   CancelledNotificationSchema,
   ClientCapabilities,
   ErrorCode,
+  isJSONRPCRequest,
+  isJSONRPCResponse,
+  isJSONRPCNotification,
   JSONRPCError,
   JSONRPCNotification,
   JSONRPCRequest,
@@ -271,12 +274,14 @@ export abstract class Protocol<
     };
 
     this._transport.onmessage = (message) => {
-      if (!("method" in message)) {
+      if (isJSONRPCResponse(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 +442,7 @@ 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..a68c3ff1 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.
  */

From 4e749dba4de93020f860c713492eb8b6872da746 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Fr=C3=A9d=C3=A9ric=20Barthelet?=
 <frederic.barthelet@gmail.com>
Date: Thu, 10 Apr 2025 00:06:07 +0200
Subject: [PATCH 2/2] Add isJSONRPCError method

---
 src/shared/protocol.ts | 3 ++-
 src/types.ts           | 3 +++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/shared/protocol.ts b/src/shared/protocol.ts
index 19ff97d3..07bfc02c 100644
--- a/src/shared/protocol.ts
+++ b/src/shared/protocol.ts
@@ -3,6 +3,7 @@ import {
   CancelledNotificationSchema,
   ClientCapabilities,
   ErrorCode,
+  isJSONRPCError,
   isJSONRPCRequest,
   isJSONRPCResponse,
   isJSONRPCNotification,
@@ -274,7 +275,7 @@ export abstract class Protocol<
     };
 
     this._transport.onmessage = (message) => {
-      if (isJSONRPCResponse(message)) {
+      if (isJSONRPCResponse(message) || isJSONRPCError(message)) {
         this._onresponse(message);
       } else if (isJSONRPCRequest(message)) {
         this._onrequest(message);
diff --git a/src/types.ts b/src/types.ts
index a68c3ff1..db6bf125 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -150,6 +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,