feat: add mcp client instruction support

src/client/index.test.ts

@@ -36,6 +36,7 @@ test("should initialize with matching protocol version", async () => {
               name: "test",
               version: "1.0",
             },
+            instructions: "test instructions",
           },
         });
       }
@@ -66,6 +67,9 @@ test("should initialize with matching protocol version", async () => {
       }),
     }),
   );
+
+  // Should have the instructions returned
+  expect(client.getInstructions()).toEqual("test instructions");
 });
 
 test("should initialize with supported older protocol version", async () => {
@@ -111,6 +115,9 @@ test("should initialize with supported older protocol version", async () => {
     name: "test",
     version: "1.0",
   });
+
+  // Expect no instructions
+  expect(client.getInstructions()).toBeUndefined();
 });
 
 test("should reject unsupported protocol version", async () => {

src/client/index.ts

@@ -85,6 +85,7 @@ export class Client<
   private _serverCapabilities?: ServerCapabilities;
   private _serverVersion?: Implementation;
   private _capabilities: ClientCapabilities;
+  private _instructions?: string;
 
   /**
    * Initializes this client with the given name and version information.
@@ -152,6 +153,8 @@ export class Client<
       this._serverCapabilities = result.capabilities;
       this._serverVersion = result.serverInfo;
 
+      this._instructions = result.instructions;
+
       await this.notification({
         method: "notifications/initialized",
       });
@@ -176,6 +179,13 @@ export class Client<
     return this._serverVersion;
   }
 
+  /**
+   * After initialization has completed, this may be populated with information about the server's instructions.
+   */
+  getInstructions(): string | undefined {
+    return this._instructions;
+  }
+
   protected assertCapabilityForMethod(method: RequestT["method"]): void {
     switch (method as ClientRequest["method"]) {
       case "logging/setLevel":

src/types.ts

@@ -304,6 +304,12 @@ export const InitializeResultSchema = ResultSchema.extend({
   protocolVersion: z.string(),
   capabilities: ServerCapabilitiesSchema,
   serverInfo: ImplementationSchema,
+  /**
+   * Instructions describing how to use the server and its features.
+   *
+   * This can be used by clients to improve the LLM's understanding of available tools, resources, etc. It can be thought of like a "hint" to the model. For example, this information MAY be added to the system prompt.
+   */
+  instructions: z.optional(z.string()),
 });
 
 /**