diff --git a/src/inMemory.test.ts b/src/inMemory.test.ts
index f7e9e979..baf43446 100644
--- a/src/inMemory.test.ts
+++ b/src/inMemory.test.ts
@@ -1,5 +1,6 @@
 import { InMemoryTransport } from "./inMemory.js";
 import { JSONRPCMessage } from "./types.js";
+import { AuthInfo } from "./server/auth/types.js";
 
 describe("InMemoryTransport", () => {
   let clientTransport: InMemoryTransport;
@@ -35,6 +36,32 @@ describe("InMemoryTransport", () => {
     expect(receivedMessage).toEqual(message);
   });
 
+  test("should send message with auth info from client to server", async () => {
+    const message: JSONRPCMessage = {
+      jsonrpc: "2.0",
+      method: "test",
+      id: 1,
+    };
+
+    const authInfo: AuthInfo = {
+      token: "test-token",
+      clientId: "test-client",
+      scopes: ["read", "write"],
+      expiresAt: Date.now() / 1000 + 3600,
+    };
+
+    let receivedMessage: JSONRPCMessage | undefined;
+    let receivedAuthInfo: AuthInfo | undefined;
+    serverTransport.onmessage = (msg, extra) => {
+      receivedMessage = msg;
+      receivedAuthInfo = extra?.authInfo;
+    };
+
+    await clientTransport.send(message, { authInfo });
+    expect(receivedMessage).toEqual(message);
+    expect(receivedAuthInfo).toEqual(authInfo);
+  });
+
   test("should send message from server to client", async () => {
     const message: JSONRPCMessage = {
       jsonrpc: "2.0",
diff --git a/src/inMemory.ts b/src/inMemory.ts
index 106a9e7e..5dd6e81e 100644
--- a/src/inMemory.ts
+++ b/src/inMemory.ts
@@ -1,16 +1,22 @@
 import { Transport } from "./shared/transport.js";
-import { JSONRPCMessage } from "./types.js";
+import { JSONRPCMessage, RequestId } from "./types.js";
+import { AuthInfo } from "./server/auth/types.js";
+
+interface QueuedMessage {
+  message: JSONRPCMessage;
+  extra?: { authInfo?: AuthInfo };
+}
 
 /**
  * In-memory transport for creating clients and servers that talk to each other within the same process.
  */
 export class InMemoryTransport implements Transport {
   private _otherTransport?: InMemoryTransport;
-  private _messageQueue: JSONRPCMessage[] = [];
+  private _messageQueue: QueuedMessage[] = [];
 
   onclose?: () => void;
   onerror?: (error: Error) => void;
-  onmessage?: (message: JSONRPCMessage) => void;
+  onmessage?: (message: JSONRPCMessage, extra?: { authInfo?: AuthInfo }) => void;
   sessionId?: string;
 
   /**
@@ -27,10 +33,8 @@ export class InMemoryTransport implements Transport {
   async start(): Promise<void> {
     // Process any messages that were queued before start was called
     while (this._messageQueue.length > 0) {
-      const message = this._messageQueue.shift();
-      if (message) {
-        this.onmessage?.(message);
-      }
+      const queuedMessage = this._messageQueue.shift()!;
+      this.onmessage?.(queuedMessage.message, queuedMessage.extra);
     }
   }
 
@@ -41,15 +45,19 @@ export class InMemoryTransport implements Transport {
     this.onclose?.();
   }
 
-  async send(message: JSONRPCMessage): Promise<void> {
+  /**
+   * Sends a message with optional auth info.
+   * This is useful for testing authentication scenarios.
+   */
+  async send(message: JSONRPCMessage, options?: { relatedRequestId?: RequestId, authInfo?: AuthInfo }): Promise<void> {
     if (!this._otherTransport) {
       throw new Error("Not connected");
     }
 
     if (this._otherTransport.onmessage) {
-      this._otherTransport.onmessage(message);
+      this._otherTransport.onmessage(message, { authInfo: options?.authInfo });
     } else {
-      this._otherTransport._messageQueue.push(message);
+      this._otherTransport._messageQueue.push({ message, extra: { authInfo: options?.authInfo } });
     }
   }
 }
diff --git a/src/server/auth/types.ts b/src/server/auth/types.ts
index 93c5a493..c25c2b60 100644
--- a/src/server/auth/types.ts
+++ b/src/server/auth/types.ts
@@ -21,4 +21,10 @@ export interface AuthInfo {
    * When the token expires (in seconds since epoch).
    */
   expiresAt?: number;
+
+  /**
+   * Additional data associated with the token.
+   * This field should be used for any additional data that needs to be attached to the auth info.
+  */
+  extra?: Record<string, unknown>;
 }
\ No newline at end of file
diff --git a/src/server/sse.ts b/src/server/sse.ts
index 46948b47..03f6fefc 100644
--- a/src/server/sse.ts
+++ b/src/server/sse.ts
@@ -4,6 +4,7 @@ import { Transport } from "../shared/transport.js";
 import { JSONRPCMessage, JSONRPCMessageSchema } from "../types.js";
 import getRawBody from "raw-body";
 import contentType from "content-type";
+import { AuthInfo } from "./auth/types.js";
 import { URL } from 'url';
 
 const MAXIMUM_MESSAGE_SIZE = "4mb";
@@ -19,7 +20,7 @@ export class SSEServerTransport implements Transport {
 
   onclose?: () => void;
   onerror?: (error: Error) => void;
-  onmessage?: (message: JSONRPCMessage) => void;
+  onmessage?: (message: JSONRPCMessage, extra?: { authInfo?: AuthInfo }) => void;
 
   /**
    * Creates a new SSE server transport, which will direct the client to POST messages to the relative or absolute URL identified by `_endpoint`.
@@ -76,7 +77,7 @@ export class SSEServerTransport implements Transport {
    * This should be called when a POST request is made to send a message to the server.
    */
   async handlePostMessage(
-    req: IncomingMessage,
+    req: IncomingMessage & { auth?: AuthInfo },
     res: ServerResponse,
     parsedBody?: unknown,
   ): Promise<void> {
@@ -85,6 +86,7 @@ export class SSEServerTransport implements Transport {
       res.writeHead(500).end(message);
       throw new Error(message);
     }
+    const authInfo: AuthInfo | undefined = req.auth;
 
     let body: string | unknown;
     try {
@@ -104,7 +106,7 @@ export class SSEServerTransport implements Transport {
     }
 
     try {
-      await this.handleMessage(typeof body === 'string' ? JSON.parse(body) : body);
+      await this.handleMessage(typeof body === 'string' ? JSON.parse(body) : body, { authInfo });
     } catch {
       res.writeHead(400).end(`Invalid message: ${body}`);
       return;
@@ -116,7 +118,7 @@ export class SSEServerTransport implements Transport {
   /**
    * Handle a client message, regardless of how it arrived. This can be used to inform the server of messages that arrive via a means different than HTTP POST.
    */
-  async handleMessage(message: unknown): Promise<void> {
+  async handleMessage(message: unknown, extra?: { authInfo?: AuthInfo }): Promise<void> {
     let parsedMessage: JSONRPCMessage;
     try {
       parsedMessage = JSONRPCMessageSchema.parse(message);
@@ -125,7 +127,7 @@ export class SSEServerTransport implements Transport {
       throw error;
     }
 
-    this.onmessage?.(parsedMessage);
+    this.onmessage?.(parsedMessage, extra);
   }
 
   async close(): Promise<void> {
diff --git a/src/shared/protocol.ts b/src/shared/protocol.ts
index 07bfc02c..91fa8366 100644
--- a/src/shared/protocol.ts
+++ b/src/shared/protocol.ts
@@ -23,6 +23,7 @@ import {
   ServerCapabilities,
 } from "../types.js";
 import { Transport } from "./transport.js";
+import { AuthInfo } from "../server/auth/types.js";
 
 /**
  * Callback for progress notifications.
@@ -109,6 +110,11 @@ export type RequestHandlerExtra<SendRequestT extends Request,
      */
     signal: AbortSignal;
 
+    /**
+     * Information about a validated access token, provided to request handlers.
+     */
+    authInfo?: AuthInfo;
+
     /**
      * The session ID from the transport, if available.
      */
@@ -274,11 +280,11 @@ export abstract class Protocol<
       this._onerror(error);
     };
 
-    this._transport.onmessage = (message) => {
+    this._transport.onmessage = (message, extra) => {
       if (isJSONRPCResponse(message) || isJSONRPCError(message)) {
         this._onresponse(message);
       } else if (isJSONRPCRequest(message)) {
-        this._onrequest(message);
+        this._onrequest(message, extra);
       } else if (isJSONRPCNotification(message)) {
         this._onnotification(message);
       } else {
@@ -326,7 +332,7 @@ export abstract class Protocol<
       );
   }
 
-  private _onrequest(request: JSONRPCRequest): void {
+  private _onrequest(request: JSONRPCRequest, extra?: { authInfo?: AuthInfo }): void {
     const handler =
       this._requestHandlers.get(request.method) ?? this.fallbackRequestHandler;
 
@@ -351,20 +357,20 @@ export abstract class Protocol<
     const abortController = new AbortController();
     this._requestHandlerAbortControllers.set(request.id, abortController);
 
-    // Create extra object with both abort signal and sessionId from transport
-    const extra: RequestHandlerExtra<SendRequestT, SendNotificationT> = {
+    const fullExtra: RequestHandlerExtra<SendRequestT, SendNotificationT> = {
       signal: abortController.signal,
       sessionId: this._transport?.sessionId,
       sendNotification:
         (notification) =>
           this.notification(notification, { relatedRequestId: request.id }),
       sendRequest: (r, resultSchema, options?) =>
-        this.request(r, resultSchema, { ...options, relatedRequestId: request.id })
+        this.request(r, resultSchema, { ...options, relatedRequestId: request.id }),
+      authInfo: extra?.authInfo,
     };
 
     // Starting with Promise.resolve() puts any synchronous errors into the monad as well.
     Promise.resolve()
-      .then(() => handler(request, extra))
+      .then(() => handler(request, fullExtra))
       .then(
         (result) => {
           if (abortController.signal.aborted) {
diff --git a/src/shared/transport.ts b/src/shared/transport.ts
index e464653b..c2732391 100644
--- a/src/shared/transport.ts
+++ b/src/shared/transport.ts
@@ -1,3 +1,4 @@
+import { AuthInfo } from "../server/auth/types.js";
 import { JSONRPCMessage, RequestId } from "../types.js";
 
 /**
@@ -41,8 +42,11 @@ export interface Transport {
 
   /**
    * Callback for when a message (request or response) is received over the connection.
+   * 
+   * Includes the authInfo if the transport is authenticated.
+   * 
    */
-  onmessage?: (message: JSONRPCMessage) => void;
+  onmessage?: (message: JSONRPCMessage, extra?: { authInfo?: AuthInfo }) => void;
 
   /**
    * The session ID generated for this connection.