fix: prevent double-prefixed McpError message when reconstructing from wire

src/shared/protocol.ts

@@ -892,7 +892,7 @@ export abstract class Protocol<SendRequestT extends Request, SendNotificationT e
             if (isJSONRPCResultResponse(response)) {
                 resolver(response);
             } else {
-                const error = new McpError(response.error.code, response.error.message, response.error.data);
+                const error = McpError.fromError(response.error.code, response.error.message, response.error.data);
                 resolver(error);
             }
             return;

src/types.ts

@@ -2314,20 +2314,33 @@ export class McpError extends Error {
         this.name = 'McpError';
     }
 
+    /**
+     * Strip the "MCP error NNNN: " prefix that McpError's constructor adds,
+     * so that fromError does not double-prefix when reconstructing an error
+     * whose message already carries the prefix from the wire.
+     */
+    private static _stripErrorPrefix(message: string): string {
+        return message.replace(/^MCP error -?\d+: /, '');
+    }
+
     /**
      * Factory method to create the appropriate error type based on the error code and data
      */
     static fromError(code: number, message: string, data?: unknown): McpError {
+        // Strip the "MCP error NNNN: " prefix if present, to avoid double-prefixing
+        // when the message comes from a serialized McpError on the wire.
+        const cleanMessage = McpError._stripErrorPrefix(message);
+
         // Check for specific error types
         if (code === ErrorCode.UrlElicitationRequired && data) {
             const errorData = data as { elicitations?: unknown[] };
             if (errorData.elicitations) {
-                return new UrlElicitationRequiredError(errorData.elicitations as ElicitRequestURLParams[], message);
+                return new UrlElicitationRequiredError(errorData.elicitations as ElicitRequestURLParams[], cleanMessage);
             }
         }
 
         // Default to generic McpError
-        return new McpError(code, message, data);
+        return new McpError(code, cleanMessage, data);
     }
 }
 

test/shared/protocol.test.ts

@@ -5403,6 +5403,32 @@ describe('Error handling for missing resolvers', () => {
         });
     });
 
+    describe('McpError.fromError should not double-prefix', () => {
+        it('should not double-prefix when message already has MCP error prefix', () => {
+            // Simulate the message as it arrives from the wire (already prefixed by server's McpError constructor)
+            const prefixedMessage = 'MCP error -32601: Unknown tool: nope';
+            const error = McpError.fromError(ErrorCode.MethodNotFound, prefixedMessage);
+
+            // Should have exactly one prefix, not two
+            expect(error.message).toBe('MCP error -32601: Unknown tool: nope');
+            expect(error.message).not.toContain('MCP error -32601: MCP error');
+        });
+
+        it('should add prefix when message does not have it', () => {
+            // Direct usage without prefix (e.g., internal code creating an error)
+            const error = McpError.fromError(ErrorCode.InternalError, 'Something went wrong');
+
+            // Should have the prefix added
+            expect(error.message).toBe('MCP error -32603: Something went wrong');
+        });
+
+        it('should handle negative error codes correctly', () => {
+            const prefixedMessage = 'MCP error -32600: Invalid request';
+            const error = McpError.fromError(ErrorCode.InvalidRequest, prefixedMessage);
+            expect(error.message).toBe('MCP error -32600: Invalid request');
+        });
+    });
+
     describe('Response and error message routing integration', () => {
         it('should handle mixed response and error messages in queue', async () => {
             await protocol.connect(transport);