Assert tool request handlers do not already exist

src/server/index.ts

@@ -343,7 +343,9 @@ export class Server<
   }
 
   private setToolRequestHandlers() {
-    // TODO: Check that these handlers do not already exist
+    this.assertCanSetRequestHandler(ListToolsRequestSchema.shape.method.value);
+    this.assertCanSetRequestHandler(CallToolRequestSchema.shape.method.value);
+
     // TODO: Register tool capability
 
     this.setRequestHandler(ListToolsRequestSchema, (): ListToolsResult => ({

src/shared/protocol.ts

@@ -511,6 +511,15 @@ export abstract class Protocol<
     this._requestHandlers.delete(method);
   }
 
+  /**
+   * Asserts that a request handler has not already been set for the given method, in preparation for a new one being automatically installed.
+   */
+  protected assertCanSetRequestHandler(method: string): void {
+    if (this._requestHandlers.has(method)) {
+      throw new Error(`A request handler for ${method} already exists, which would be overridden`);
+    }
+  }
+
   /**
    * Registers a handler to invoke when this protocol object receives a notification with the given method.
    *