move get sse stream from a separate method to automatic connection when notifications/initialized received

src/client/streamableHttp.test.ts

@@ -164,8 +164,7 @@ describe("StreamableHTTPClientTransport", () => {
     // We expect the 405 error to be caught and handled gracefully
     // This should not throw an error that breaks the transport
     await transport.start();
-    await expect(transport.openSseStream()).rejects.toThrow("Failed to open SSE stream: Method Not Allowed");
-
+    await expect(transport["_startOrAuthStandaloneSSE"]()).resolves.not.toThrow("Failed to open SSE stream: Method Not Allowed");
     // Check that GET was attempted
     expect(global.fetch).toHaveBeenCalledWith(
       expect.anything(),
@@ -209,7 +208,7 @@ describe("StreamableHTTPClientTransport", () => {
     transport.onmessage = messageSpy;
 
     await transport.start();
-    await transport.openSseStream();
+    await transport["_startOrAuthStandaloneSSE"]();
 
     // Give time for the SSE event to be processed
     await new Promise(resolve => setTimeout(resolve, 50));
@@ -295,7 +294,7 @@ describe("StreamableHTTPClientTransport", () => {
     });
 
     await transport.start();
-    await transport.openSseStream();
+    await transport["_startOrAuthStandaloneSSE"]();
     await new Promise(resolve => setTimeout(resolve, 50));
 
     // Now simulate attempting to reconnect
@@ -306,7 +305,7 @@ describe("StreamableHTTPClientTransport", () => {
       body: null
     });
 
-    await transport.openSseStream();
+    await transport["_startOrAuthStandaloneSSE"]();
 
     // Check that Last-Event-ID was included
     const calls = (global.fetch as jest.Mock).mock.calls;
@@ -366,7 +365,7 @@ describe("StreamableHTTPClientTransport", () => {
 
     await transport.start();
 
-    await transport.openSseStream();
+    await transport["_startOrAuthStandaloneSSE"]();
     expect((actualReqInit.headers as Headers).get("x-custom-header")).toBe("CustomValue");
 
     requestInit.headers["X-Custom-Header"] = "SecondCustomValue";

src/client/streamableHttp.ts

@@ -1,5 +1,5 @@
 import { Transport } from "../shared/transport.js";
-import { JSONRPCMessage, JSONRPCMessageSchema } from "../types.js";
+import { isJSONRPCNotification, JSONRPCMessage, JSONRPCMessageSchema } from "../types.js";
 import { auth, AuthResult, OAuthClientProvider, UnauthorizedError } from "./auth.js";
 import { EventSourceParserStream } from "eventsource-parser/stream";
 
@@ -126,6 +126,12 @@ export class StreamableHTTPClientTransport implements Transport {
           return await this._authThenStart();
         }
 
+        // 405 indicates that the server does not offer an SSE stream at GET endpoint
+        // This is an expected case that should not trigger an error
+        if (response.status === 405) {
+          return;
+        }
+
         throw new StreamableHTTPError(
           response.status,
           `Failed to open SSE stream: ${response.statusText}`,
@@ -243,6 +249,12 @@ export class StreamableHTTPClientTransport implements Transport {
 
       // If the response is 202 Accepted, there's no body to process
       if (response.status === 202) {
+        // if the accepted notification is initialized, we start the SSE stream
+        // if it's supported by the server
+        if (isJSONRPCNotification(message) && message.method === "notifications/initialized") {
+          // We don't need to handle 405 here anymore as it's handled in _startOrAuthStandaloneSSE
+          this._startOrAuthStandaloneSSE().catch(err => this.onerror?.(err));
+        }
         return;
       }
 
@@ -279,20 +291,4 @@ export class StreamableHTTPClientTransport implements Transport {
       throw error;
     }
   }
-
-  /**
-   * Opens SSE stream to receive messages from the server.
-   *
-   * This allows the server to push messages to the client without requiring the client
-   * to first send a request via HTTP POST. Some servers may not support this feature.
-   * If authentication is required but fails, this method will throw an UnauthorizedError.
-   */
-  async openSseStream(): Promise<void> {
-    if (!this._abortController) {
-      throw new Error(
-        "StreamableHTTPClientTransport not started! Call connect() before openSseStream().",
-      );
-    }
-    await this._startOrAuthStandaloneSSE();
-  }
 }

src/examples/client/simpleStreamableHttp.ts

@@ -25,20 +25,10 @@ async function main(): Promise<void> {
   const transport = new StreamableHTTPClientTransport(
     new URL('http://localhost:3000/mcp')
   );
-  let supportsStandaloneSse = false;
-
-  // Connect the client using the transport and initialize the server
+  
+  // Connect the client using the transport and initialize the server  
   await client.connect(transport);
   console.log('Connected to MCP server');
-  console.log('Opening SSE stream to receive server notifications...');
-  try {
-    await transport.openSseStream();
-    supportsStandaloneSse = true;
-    console.log('SSE stream established successfully. Waiting for notifications...');
-  }
-  catch (error) {
-    console.error('Failed to open SSE stream:', error);
-  }
 
   // Set up notification handlers for server-initiated messages
   client.setNotificationHandler(LoggingMessageNotificationSchema, (notification) => {
@@ -136,11 +126,8 @@ async function main(): Promise<void> {
   } catch (error) {
     console.log(`Resources not supported by this server (${error})`);
   }
-  if (supportsStandaloneSse) {
-    // Instead of closing immediately, keep the connection open to receive notifications
-    console.log('\nKeeping connection open to receive notifications. Press Ctrl+C to exit.');
-  }
-
+  // Keep the connection open to receive notifications
+  console.log('\nKeeping connection open to receive notifications. Press Ctrl+C to exit.');
 }
 
 main().catch((error: unknown) => {

src/server/streamableHttp.test.ts

@@ -578,7 +578,7 @@ describe("StreamableHTTPServerTransport", () => {
 
       expect(mockResponse.writeHead).toHaveBeenCalledWith(200, expect.objectContaining({
         "Content-Type": "text/event-stream",
-        "Cache-Control": "no-cache",
+        "Cache-Control": "no-cache, no-transform",
         "Connection": "keep-alive",
         "mcp-session-id": transport.sessionId,
       }));