change sessionIdGenerator to be either undefined or generate a string

Author: ihrpr

README.md

@@ -325,10 +325,9 @@ app.use(express.json());
 
 // Handle all MCP requests (GET, POST, DELETE) at a single endpoint
 app.all('/mcp', async (req, res) => {
-  // Create a transport with sessionIdGenerator set to return undefined
-  // This disables session tracking completely
+  // Disable session tracking by setting sessionIdGenerator to undefined  
   const transport = new StreamableHTTPServerTransport({
-    sessionIdGenerator: () => undefined,
+    sessionIdGenerator: undefined,
     req,
     res
   });

src/examples/README.md

@@ -134,7 +134,7 @@ The Streamable HTTP transport can be configured to operate without tracking sess
 
 To enable stateless mode, configure the `StreamableHTTPServerTransport` with:
 ```typescript
-sessionIdGenerator: () => undefined
+sessionIdGenerator: undefined
 ```
 
 This disables session management entirely, and the server won't generate or expect session IDs.

src/integration-tests/stateManagementStreamableHttp.test.ts

@@ -68,7 +68,7 @@ describe('Streamable HTTP Transport Session Management', () => {
     const serverTransport = new StreamableHTTPServerTransport({
       sessionIdGenerator: withSessionManagement
         ? () => randomUUID()   // With session management, generate UUID
-        : () => undefined      // Without session management, return undefined
+        : undefined     // Without session management, return undefined
     });
 
     await mcpServer.connect(serverTransport);

src/server/streamableHttp.test.ts

@@ -10,7 +10,7 @@ import { z } from "zod";
  * Test server configuration for StreamableHTTPServerTransport tests
  */
 interface TestServerConfig {
-  sessionIdGenerator?: () => string | undefined;
+  sessionIdGenerator: (() => string) | undefined;
   enableJsonResponse?: boolean;
   customRequestHandler?: (req: IncomingMessage, res: ServerResponse, parsedBody?: unknown) => Promise<void>;
   eventStore?: EventStore;
@@ -19,7 +19,7 @@ interface TestServerConfig {
 /**
  * Helper to create and start test HTTP server with MCP setup
  */
-async function createTestServer(config: TestServerConfig = {}): Promise<{
+async function createTestServer(config: TestServerConfig = { sessionIdGenerator: (() => randomUUID()) }): Promise<{
   server: Server;
   transport: StreamableHTTPServerTransport;
   mcpServer: McpServer;
@@ -40,7 +40,7 @@ async function createTestServer(config: TestServerConfig = {}): Promise<{
   );
 
   const transport = new StreamableHTTPServerTransport({
-    sessionIdGenerator: config.sessionIdGenerator ?? (() => randomUUID()),
+    sessionIdGenerator: config.sessionIdGenerator,
     enableJsonResponse: config.enableJsonResponse ?? false,
     eventStore: config.eventStore
   });
@@ -681,7 +681,7 @@ describe("StreamableHTTPServerTransport with JSON Response Mode", () => {
   let sessionId: string;
 
   beforeEach(async () => {
-    const result = await createTestServer({ enableJsonResponse: true });
+    const result = await createTestServer({ sessionIdGenerator: (() => randomUUID()), enableJsonResponse: true });
     server = result.server;
     transport = result.transport;
     baseUrl = result.baseUrl;
@@ -784,7 +784,8 @@ describe("StreamableHTTPServerTransport with pre-parsed body", () => {
           console.error("Error handling request:", error);
           if (!res.headersSent) res.writeHead(500).end();
         }
-      }
+      },
+      sessionIdGenerator: (() => randomUUID())
     });
 
     server = result.server;
@@ -1063,7 +1064,7 @@ describe("StreamableHTTPServerTransport in stateless mode", () => {
   let baseUrl: URL;
 
   beforeEach(async () => {
-    const result = await createTestServer({ sessionIdGenerator: () => undefined });
+    const result = await createTestServer({ sessionIdGenerator: undefined });
     server = result.server;
     transport = result.transport;
     baseUrl = result.baseUrl;

src/server/streamableHttp.ts

@@ -37,7 +37,7 @@ export interface StreamableHTTPServerTransportOptions {
    * 
    * Return undefined to disable session management.
    */
-  sessionIdGenerator: () => string | undefined;
+  sessionIdGenerator: (() => string) | undefined;
 
   /**
    * A callback for session initialization events
@@ -98,7 +98,7 @@ export interface StreamableHTTPServerTransportOptions {
  */
 export class StreamableHTTPServerTransport implements Transport {
   // when sessionId is not set (undefined), it means the transport is in stateless mode
-  private sessionIdGenerator: () => string | undefined;
+  private sessionIdGenerator: (() => string) | undefined;
   private _started: boolean = false;
   private _streamMapping: Map<string, ServerResponse> = new Map();
   private _requestToStreamMapping: Map<RequestId, string> = new Map();
@@ -365,7 +365,7 @@ export class StreamableHTTPServerTransport implements Transport {
           }));
           return;
         }
-        this.sessionId = this.sessionIdGenerator();
+        this.sessionId = this.sessionIdGenerator?.();
         this._initialized = true;
 
         // If we have a session ID and an onsessioninitialized handler, call it immediately