Handle capabilities checks at the request() level, make non-strict by default

src/shared/protocol.ts

@@ -21,11 +21,23 @@ import { Transport } from "./transport.js";
  */
 export type ProgressCallback = (progress: Progress) => void;
 
+/**
+ * Additional initialization options.
+ */
+export type ProtocolOptions = {
+  /**
+   * Whether to restrict emitted requests to only those that the remote side has indicated that they can handle, through their advertised capabilities.
+   *
+   * Currently this defaults to false, for backwards compatibility with SDK versions that did not advertise capabilities correctly. In future, this will default to true.
+   */
+  enforceStrictCapabilities?: boolean;
+};
+
 /**
  * Implements MCP protocol framing on top of a pluggable transport, including
  * features like request/response linking, notifications, and progress.
  */
-export class Protocol<
+export abstract class Protocol<
   SendRequestT extends Request,
   SendNotificationT extends Notification,
   SendResultT extends Result,
@@ -70,7 +82,7 @@ export class Protocol<
    */
   fallbackNotificationHandler?: (notification: Notification) => Promise<void>;
 
-  constructor() {
+  constructor(private _options?: ProtocolOptions) {
     this.setNotificationHandler(ProgressNotificationSchema, (notification) => {
       this._onprogress(notification as unknown as ProgressNotification);
     });
@@ -245,6 +257,15 @@ export class Protocol<
     await this._transport?.close();
   }
 
+  /**
+   * A method to check if a capability is supported by the remote side, for the given method to be called.
+   *
+   * This should be implemented by subclasses.
+   */
+  protected abstract assertCapabilityForMethod(
+    method: SendRequestT["method"],
+  ): void;
+
   /**
    * Sends a request and wait for a response, with optional progress notifications in the meantime (if supported by the server).
    *
@@ -261,6 +282,10 @@ export class Protocol<
         return;
       }
 
+      if (this._options?.enforceStrictCapabilities === true) {
+        this.assertCapabilityForMethod(request.method);
+      }
+
       const messageId = this._requestMessageId++;
       const jsonrpcRequest: JSONRPCRequest = {
         ...request,