fix: convert remaining capability throws to SdkError (#1761)

Co-authored-by: Matteo Gobbo <info@matteogobbo.it>
Co-authored-by: Konstantin Konstantinov <KKonstantinov@users.noreply.github.com>

Author: 3 people

.changeset/finish-sdkerror-capability.md

@@ -0,0 +1,7 @@
+---
+'@modelcontextprotocol/core': patch
+'@modelcontextprotocol/client': patch
+'@modelcontextprotocol/server': patch
+---
+
+Convert remaining capability-assertion throws to `SdkError(SdkErrorCode.CapabilityNotSupported, ...)`. Follow-up to #1454 which missed `Client.assertCapability()`, the task capability helpers in `experimental/tasks/helpers.ts`, and the sampling/elicitation capability checks in `experimental/tasks/server.ts`.

packages/client/src/client/client.ts

@@ -456,7 +456,7 @@ export class Client extends Protocol<ClientContext> {
 
     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 SdkError(SdkErrorCode.CapabilityNotSupported, `Server does not support ${capability} (required for ${method})`);
         }
     }
 

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

@@ -5,6 +5,8 @@
  * @experimental
  */
 
+import { SdkError, SdkErrorCode } from '../../errors/sdkErrors.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 {@linkcode SdkError} with {@linkcode SdkErrorCode.CapabilityNotSupported} if the capability is not supported
  *
  * @experimental
  */
@@ -32,13 +34,16 @@ export function assertToolsCallTaskCapability(
     entityName: 'Server' | 'Client'
 ): void {
     if (!requests) {
-        throw new Error(`${entityName} does not support task creation (required for ${method})`);
+        throw new SdkError(SdkErrorCode.CapabilityNotSupported, `${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 ${method})`);
+                throw new SdkError(
+                    SdkErrorCode.CapabilityNotSupported,
+                    `${entityName} does not support task creation for tools/call (required for ${method})`
+                );
             }
             break;
         }
@@ -57,7 +62,7 @@ export function assertToolsCallTaskCapability(
  * @param requests - The task requests capability object
  * @param method - The method being checked
  * @param entityName - `'Server'` or `'Client'` for error messages
- * @throws Error if the capability is not supported
+ * @throws {@linkcode SdkError} with {@linkcode SdkErrorCode.CapabilityNotSupported} if the capability is not supported
  *
  * @experimental
  */
@@ -67,20 +72,26 @@ export function assertClientRequestTaskCapability(
     entityName: 'Server' | 'Client'
 ): void {
     if (!requests) {
-        throw new Error(`${entityName} does not support task creation (required for ${method})`);
+        throw new SdkError(SdkErrorCode.CapabilityNotSupported, `${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 ${method})`);
+                throw new SdkError(
+                    SdkErrorCode.CapabilityNotSupported,
+                    `${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 ${method})`);
+                throw new SdkError(
+                    SdkErrorCode.CapabilityNotSupported,
+                    `${entityName} does not support task creation for elicitation/create (required for ${method})`
+                );
             }
             break;
         }

packages/server/src/experimental/tasks/server.ts

@@ -22,7 +22,7 @@ import type {
     ResponseMessage,
     ResultTypeMap
 } from '@modelcontextprotocol/core';
-import { getResultSchema, GetTaskPayloadResultSchema } from '@modelcontextprotocol/core';
+import { getResultSchema, GetTaskPayloadResultSchema, SdkError, SdkErrorCode } from '@modelcontextprotocol/core';
 
 import type { Server } from '../../server/server.js';
 
@@ -122,7 +122,7 @@ export class ExperimentalServerTasks {
 
         // Capability check - only required when tools/toolChoice are provided
         if ((params.tools || params.toolChoice) && !clientCapabilities?.sampling?.tools) {
-            throw new Error('Client does not support sampling tools capability.');
+            throw new SdkError(SdkErrorCode.CapabilityNotSupported, 'Client does not support sampling tools capability.');
         }
 
         // Message structure validation - always validate tool_use/tool_result pairs.
@@ -221,13 +221,13 @@ export class ExperimentalServerTasks {
         switch (mode) {
             case 'url': {
                 if (!clientCapabilities?.elicitation?.url) {
-                    throw new Error('Client does not support url elicitation.');
+                    throw new SdkError(SdkErrorCode.CapabilityNotSupported, 'Client does not support url elicitation.');
                 }
                 break;
             }
             case 'form': {
                 if (!clientCapabilities?.elicitation?.form) {
-                    throw new Error('Client does not support form elicitation.');
+                    throw new SdkError(SdkErrorCode.CapabilityNotSupported, 'Client does not support form elicitation.');
                 }
                 break;
             }