fix: add UnsupportedCapabilityError to handle capabilities custom exception

packages/client/src/client/client.ts

@@ -66,7 +66,8 @@ import {
     ResourceListChangedNotificationSchema,
     safeParse,
     SUPPORTED_PROTOCOL_VERSIONS,
-    ToolListChangedNotificationSchema
+    ToolListChangedNotificationSchema,
+    UnsupportedCapabilityError
 } from '@modelcontextprotocol/core';
 
 import { ExperimentalClientTasks } from '../experimental/tasks/client.js';
@@ -479,7 +480,7 @@ export class Client<
 
     protected assertCapability(capability: keyof ServerCapabilities, method: string): void {
         if (!this._serverCapabilities?.[capability]) {
-            throw new Error(`Server does not support ${capability} (required for ${method})`);
+            throw new UnsupportedCapabilityError(`Server does not support ${capability} (required for ${method})`);
         }
     }
 
@@ -562,14 +563,14 @@ export class Client<
         switch (method as ClientRequest['method']) {
             case 'logging/setLevel':
                 if (!this._serverCapabilities?.logging) {
-                    throw new Error(`Server does not support logging (required for ${method})`);
+                    throw new UnsupportedCapabilityError(`Server does not support logging (required for ${method})`);
                 }
                 break;
 
             case 'prompts/get':
             case 'prompts/list':
                 if (!this._serverCapabilities?.prompts) {
-                    throw new Error(`Server does not support prompts (required for ${method})`);
+                    throw new UnsupportedCapabilityError(`Server does not support prompts (required for ${method})`);
                 }
                 break;
 
@@ -579,25 +580,25 @@ export class Client<
             case 'resources/subscribe':
             case 'resources/unsubscribe':
                 if (!this._serverCapabilities?.resources) {
-                    throw new Error(`Server does not support resources (required for ${method})`);
+                    throw new UnsupportedCapabilityError(`Server does not support resources (required for ${method})`);
                 }
 
                 if (method === 'resources/subscribe' && !this._serverCapabilities.resources.subscribe) {
-                    throw new Error(`Server does not support resource subscriptions (required for ${method})`);
+                    throw new UnsupportedCapabilityError(`Server does not support resource subscriptions (required for ${method})`);
                 }
 
                 break;
 
             case 'tools/call':
             case 'tools/list':
                 if (!this._serverCapabilities?.tools) {
-                    throw new Error(`Server does not support tools (required for ${method})`);
+                    throw new UnsupportedCapabilityError(`Server does not support tools (required for ${method})`);
                 }
                 break;
 
             case 'completion/complete':
                 if (!this._serverCapabilities?.completions) {
-                    throw new Error(`Server does not support completions (required for ${method})`);
+                    throw new UnsupportedCapabilityError(`Server does not support completions (required for ${method})`);
                 }
                 break;
 
@@ -615,7 +616,9 @@ export class Client<
         switch (method as ClientNotification['method']) {
             case 'notifications/roots/list_changed':
                 if (!this._capabilities.roots?.listChanged) {
-                    throw new Error(`Client does not support roots list changed notifications (required for ${method})`);
+                    throw new UnsupportedCapabilityError(
+                        `Client does not support roots list changed notifications (required for ${method})`
+                    );
                 }
                 break;
 
@@ -643,19 +646,19 @@ export class Client<
         switch (method) {
             case 'sampling/createMessage':
                 if (!this._capabilities.sampling) {
-                    throw new Error(`Client does not support sampling capability (required for ${method})`);
+                    throw new UnsupportedCapabilityError(`Client does not support sampling capability (required for ${method})`);
                 }
                 break;
 
             case 'elicitation/create':
                 if (!this._capabilities.elicitation) {
-                    throw new Error(`Client does not support elicitation capability (required for ${method})`);
+                    throw new UnsupportedCapabilityError(`Client does not support elicitation capability (required for ${method})`);
                 }
                 break;
 
             case 'roots/list':
                 if (!this._capabilities.roots) {
-                    throw new Error(`Client does not support roots capability (required for ${method})`);
+                    throw new UnsupportedCapabilityError(`Client does not support roots capability (required for ${method})`);
                 }
                 break;
 
@@ -664,7 +667,7 @@ export class Client<
             case 'tasks/result':
             case 'tasks/cancel':
                 if (!this._capabilities.tasks) {
-                    throw new Error(`Client does not support tasks capability (required for ${method})`);
+                    throw new UnsupportedCapabilityError(`Client does not support tasks capability (required for ${method})`);
                 }
                 break;
 

packages/core/src/experimental/tasks/helpers.ts

@@ -5,6 +5,8 @@
  * @experimental
  */
 
+import { UnsupportedCapabilityError } from '../../types/types.js';
+
 /**
  * Type representing the task requests capability structure.
  * This is derived from ClientTasksCapability.requests and ServerTasksCapability.requests.
@@ -22,7 +24,7 @@ interface TaskRequestsCapability {
  * @param requests - The task requests capability object
  * @param method - The method being checked
  * @param entityName - 'Server' or 'Client' for error messages
- * @throws Error if the capability is not supported
+ * @throws UnsupportedCapabilityError if the capability is not supported
  *
  * @experimental
  */
@@ -32,13 +34,15 @@ export function assertToolsCallTaskCapability(
     entityName: 'Server' | 'Client'
 ): void {
     if (!requests) {
-        throw new Error(`${entityName} does not support task creation (required for ${method})`);
+        throw new UnsupportedCapabilityError(`${entityName} does not support task creation (required for ${method})`);
     }
 
     switch (method) {
         case 'tools/call':
             if (!requests.tools?.call) {
-                throw new Error(`${entityName} does not support task creation for tools/call (required for ${method})`);
+                throw new UnsupportedCapabilityError(
+                    `${entityName} does not support task creation for tools/call (required for ${method})`
+                );
             }
             break;
 
@@ -55,7 +59,7 @@ export function assertToolsCallTaskCapability(
  * @param requests - The task requests capability object
  * @param method - The method being checked
  * @param entityName - 'Server' or 'Client' for error messages
- * @throws Error if the capability is not supported
+ * @throws UnsupportedCapabilityError if the capability is not supported
  *
  * @experimental
  */
@@ -65,19 +69,23 @@ export function assertClientRequestTaskCapability(
     entityName: 'Server' | 'Client'
 ): void {
     if (!requests) {
-        throw new Error(`${entityName} does not support task creation (required for ${method})`);
+        throw new UnsupportedCapabilityError(`${entityName} does not support task creation (required for ${method})`);
     }
 
     switch (method) {
         case 'sampling/createMessage':
             if (!requests.sampling?.createMessage) {
-                throw new Error(`${entityName} does not support task creation for sampling/createMessage (required for ${method})`);
+                throw new UnsupportedCapabilityError(
+                    `${entityName} does not support task creation for sampling/createMessage (required for ${method})`
+                );
             }
             break;
 
         case 'elicitation/create':
             if (!requests.elicitation?.create) {
-                throw new Error(`${entityName} does not support task creation for elicitation/create (required for ${method})`);
+                throw new UnsupportedCapabilityError(
+                    `${entityName} does not support task creation for elicitation/create (required for ${method})`
+                );
             }
             break;
 

packages/core/src/types/types.ts

@@ -238,7 +238,8 @@ export enum ErrorCode {
     InternalError = -32603,
 
     // MCP-specific error codes
-    UrlElicitationRequired = -32042
+    UrlElicitationRequired = -32042,
+    UnsupportedCapability = -32043
 }
 
 /**
@@ -2369,6 +2370,12 @@ export class UrlElicitationRequiredError extends McpError {
     }
 }
 
+export class UnsupportedCapabilityError extends McpError {
+    constructor(message: string) {
+        super(ErrorCode.UnsupportedCapability, message);
+    }
+}
+
 type Primitive = string | number | boolean | bigint | null | undefined;
 type Flatten<T> = T extends Primitive
     ? T

test/integration/test/client/client.test.ts

@@ -645,7 +645,7 @@ test('should respect client notification capabilities', async () => {
     await clientWithoutCapability.connect(clientTransport);
 
     // This should throw because the client doesn't have the roots.listChanged capability
-    await expect(clientWithoutCapability.sendRootsListChanged()).rejects.toThrow(/^Client does not support/);
+    await expect(clientWithoutCapability.sendRootsListChanged()).rejects.toThrow(/^MCP error -32043: Client does not support/);
 });
 
 /***