diff --git a/src/server/streamableHttp.test.ts b/src/server/streamableHttp.test.ts
index e54bea01..3a0a5c06 100644
--- a/src/server/streamableHttp.test.ts
+++ b/src/server/streamableHttp.test.ts
@@ -29,7 +29,8 @@ interface TestServerConfig {
   enableJsonResponse?: boolean;
   customRequestHandler?: (req: IncomingMessage, res: ServerResponse, parsedBody?: unknown) => Promise<void>;
   eventStore?: EventStore;
-  onsessionclosed?: (sessionId: string) => void;
+  onsessioninitialized?: (sessionId: string) => void | Promise<void>;
+  onsessionclosed?: (sessionId: string) => void | Promise<void>;
 }
 
 /**
@@ -59,6 +60,7 @@ async function createTestServer(config: TestServerConfig = { sessionIdGenerator:
     sessionIdGenerator: config.sessionIdGenerator,
     enableJsonResponse: config.enableJsonResponse ?? false,
     eventStore: config.eventStore,
+    onsessioninitialized: config.onsessioninitialized,
     onsessionclosed: config.onsessionclosed
   });
 
@@ -114,6 +116,7 @@ async function createTestAuthServer(config: TestServerConfig = { sessionIdGenera
     sessionIdGenerator: config.sessionIdGenerator,
     enableJsonResponse: config.enableJsonResponse ?? false,
     eventStore: config.eventStore,
+    onsessioninitialized: config.onsessioninitialized,
     onsessionclosed: config.onsessionclosed
   });
 
@@ -1666,6 +1669,213 @@ describe("StreamableHTTPServerTransport onsessionclosed callback", () => {
   });
 });
 
+// Test async callbacks for onsessioninitialized and onsessionclosed
+describe("StreamableHTTPServerTransport async callbacks", () => {
+  it("should support async onsessioninitialized callback", async () => {
+    const initializationOrder: string[] = [];
+    
+    // Create server with async onsessioninitialized callback
+    const result = await createTestServer({
+      sessionIdGenerator: () => randomUUID(),
+      onsessioninitialized: async (sessionId: string) => {
+        initializationOrder.push('async-start');
+        // Simulate async operation
+        await new Promise(resolve => setTimeout(resolve, 10));
+        initializationOrder.push('async-end');
+        initializationOrder.push(sessionId);
+      },
+    });
+    
+    const tempServer = result.server;
+    const tempUrl = result.baseUrl;
+
+    // Initialize to trigger the callback
+    const initResponse = await sendPostRequest(tempUrl, TEST_MESSAGES.initialize);
+    const tempSessionId = initResponse.headers.get("mcp-session-id");
+    
+    // Give time for async callback to complete
+    await new Promise(resolve => setTimeout(resolve, 50));
+    
+    expect(initializationOrder).toEqual(['async-start', 'async-end', tempSessionId]);
+    
+    // Clean up
+    tempServer.close();
+  });
+
+  it("should support sync onsessioninitialized callback (backwards compatibility)", async () => {
+    const capturedSessionId: string[] = [];
+    
+    // Create server with sync onsessioninitialized callback
+    const result = await createTestServer({
+      sessionIdGenerator: () => randomUUID(),
+      onsessioninitialized: (sessionId: string) => {
+        capturedSessionId.push(sessionId);
+      },
+    });
+    
+    const tempServer = result.server;
+    const tempUrl = result.baseUrl;
+
+    // Initialize to trigger the callback
+    const initResponse = await sendPostRequest(tempUrl, TEST_MESSAGES.initialize);
+    const tempSessionId = initResponse.headers.get("mcp-session-id");
+    
+    expect(capturedSessionId).toEqual([tempSessionId]);
+    
+    // Clean up
+    tempServer.close();
+  });
+
+  it("should support async onsessionclosed callback", async () => {
+    const closureOrder: string[] = [];
+    
+    // Create server with async onsessionclosed callback
+    const result = await createTestServer({
+      sessionIdGenerator: () => randomUUID(),
+      onsessionclosed: async (sessionId: string) => {
+        closureOrder.push('async-close-start');
+        // Simulate async operation
+        await new Promise(resolve => setTimeout(resolve, 10));
+        closureOrder.push('async-close-end');
+        closureOrder.push(sessionId);
+      },
+    });
+    
+    const tempServer = result.server;
+    const tempUrl = result.baseUrl;
+
+    // Initialize to get a session ID
+    const initResponse = await sendPostRequest(tempUrl, TEST_MESSAGES.initialize);
+    const tempSessionId = initResponse.headers.get("mcp-session-id");
+    expect(tempSessionId).toBeDefined();
+
+    // DELETE the session
+    const deleteResponse = await fetch(tempUrl, {
+      method: "DELETE",
+      headers: {
+        "mcp-session-id": tempSessionId || "",
+        "mcp-protocol-version": "2025-03-26",
+      },
+    });
+
+    expect(deleteResponse.status).toBe(200);
+    
+    // Give time for async callback to complete
+    await new Promise(resolve => setTimeout(resolve, 50));
+    
+    expect(closureOrder).toEqual(['async-close-start', 'async-close-end', tempSessionId]);
+
+    // Clean up
+    tempServer.close();
+  });
+
+  it("should propagate errors from async onsessioninitialized callback", async () => {
+    const consoleErrorSpy = jest.spyOn(console, 'error').mockImplementation();
+    
+    // Create server with async onsessioninitialized callback that throws
+    const result = await createTestServer({
+      sessionIdGenerator: () => randomUUID(),
+      onsessioninitialized: async (_sessionId: string) => {
+        throw new Error('Async initialization error');
+      },
+    });
+    
+    const tempServer = result.server;
+    const tempUrl = result.baseUrl;
+
+    // Initialize should fail when callback throws
+    const initResponse = await sendPostRequest(tempUrl, TEST_MESSAGES.initialize);
+    expect(initResponse.status).toBe(400);
+    
+    // Clean up
+    consoleErrorSpy.mockRestore();
+    tempServer.close();
+  });
+
+  it("should propagate errors from async onsessionclosed callback", async () => {
+    const consoleErrorSpy = jest.spyOn(console, 'error').mockImplementation();
+    
+    // Create server with async onsessionclosed callback that throws
+    const result = await createTestServer({
+      sessionIdGenerator: () => randomUUID(),
+      onsessionclosed: async (_sessionId: string) => {
+        throw new Error('Async closure error');
+      },
+    });
+    
+    const tempServer = result.server;
+    const tempUrl = result.baseUrl;
+
+    // Initialize to get a session ID
+    const initResponse = await sendPostRequest(tempUrl, TEST_MESSAGES.initialize);
+    const tempSessionId = initResponse.headers.get("mcp-session-id");
+
+    // DELETE should fail when callback throws
+    const deleteResponse = await fetch(tempUrl, {
+      method: "DELETE",
+      headers: {
+        "mcp-session-id": tempSessionId || "",
+        "mcp-protocol-version": "2025-03-26",
+      },
+    });
+
+    expect(deleteResponse.status).toBe(500);
+    
+    // Clean up
+    consoleErrorSpy.mockRestore();
+    tempServer.close();
+  });
+
+  it("should handle both async callbacks together", async () => {
+    const events: string[] = [];
+    
+    // Create server with both async callbacks
+    const result = await createTestServer({
+      sessionIdGenerator: () => randomUUID(),
+      onsessioninitialized: async (sessionId: string) => {
+        await new Promise(resolve => setTimeout(resolve, 5));
+        events.push(`initialized:${sessionId}`);
+      },
+      onsessionclosed: async (sessionId: string) => {
+        await new Promise(resolve => setTimeout(resolve, 5));
+        events.push(`closed:${sessionId}`);
+      },
+    });
+    
+    const tempServer = result.server;
+    const tempUrl = result.baseUrl;
+
+    // Initialize to trigger first callback
+    const initResponse = await sendPostRequest(tempUrl, TEST_MESSAGES.initialize);
+    const tempSessionId = initResponse.headers.get("mcp-session-id");
+    
+    // Wait for async callback
+    await new Promise(resolve => setTimeout(resolve, 20));
+    
+    expect(events).toContain(`initialized:${tempSessionId}`);
+
+    // DELETE to trigger second callback
+    const deleteResponse = await fetch(tempUrl, {
+      method: "DELETE",
+      headers: {
+        "mcp-session-id": tempSessionId || "",
+        "mcp-protocol-version": "2025-03-26",
+      },
+    });
+
+    expect(deleteResponse.status).toBe(200);
+    
+    // Wait for async callback
+    await new Promise(resolve => setTimeout(resolve, 20));
+    
+    expect(events).toContain(`closed:${tempSessionId}`);
+    expect(events).toHaveLength(2);
+
+    // Clean up
+    tempServer.close();
+  });
+});
+
 // Test DNS rebinding protection
 describe("StreamableHTTPServerTransport DNS rebinding protection", () => {
   let server: Server;
diff --git a/src/server/streamableHttp.ts b/src/server/streamableHttp.ts
index 37164c86..3bf84e43 100644
--- a/src/server/streamableHttp.ts
+++ b/src/server/streamableHttp.ts
@@ -47,7 +47,7 @@ export interface StreamableHTTPServerTransportOptions {
    * and need to keep track of them.
    * @param sessionId The generated session ID
    */
-  onsessioninitialized?: (sessionId: string) => void;
+  onsessioninitialized?: (sessionId: string) => void | Promise<void>;
 
   /**
    * A callback for session close events
@@ -59,7 +59,7 @@ export interface StreamableHTTPServerTransportOptions {
    * session open/running.
    * @param sessionId The session ID that was closed
   */
-  onsessionclosed?: (sessionId: string) => void;
+  onsessionclosed?: (sessionId: string) => void | Promise<void>;
 
   /**
    * If true, the server will return JSON responses instead of starting an SSE stream.
@@ -138,8 +138,8 @@ export class StreamableHTTPServerTransport implements Transport {
   private _enableJsonResponse: boolean = false;
   private _standaloneSseStreamId: string = '_GET_stream';
   private _eventStore?: EventStore;
-  private _onsessioninitialized?: (sessionId: string) => void;
-  private _onsessionclosed?: (sessionId: string) => void;
+  private _onsessioninitialized?: (sessionId: string) => void | Promise<void>;
+  private _onsessionclosed?: (sessionId: string) => void | Promise<void>;
   private _allowedHosts?: string[];
   private _allowedOrigins?: string[];
   private _enableDnsRebindingProtection: boolean;
@@ -460,7 +460,7 @@ export class StreamableHTTPServerTransport implements Transport {
         // If we have a session ID and an onsessioninitialized handler, call it immediately
         // This is needed in cases where the server needs to keep track of multiple sessions
         if (this.sessionId && this._onsessioninitialized) {
-          this._onsessioninitialized(this.sessionId);
+          await Promise.resolve(this._onsessioninitialized(this.sessionId));
         }
 
       }
@@ -552,7 +552,7 @@ export class StreamableHTTPServerTransport implements Transport {
     if (!this.validateProtocolVersion(req, res)) {
       return;
     }
-    this._onsessionclosed?.(this.sessionId!);
+    await Promise.resolve(this._onsessionclosed?.(this.sessionId!));
     await this.close();
     res.writeHead(200).end();
   }