fix(client): retry SSE stream after receiving session ID

packages/client/src/client/streamableHttp.ts

@@ -184,6 +184,7 @@ export class StreamableHTTPClientTransport implements Transport {
     private _serverRetryMs?: number; // Server-provided retry delay from SSE retry field
     private readonly _reconnectionScheduler?: ReconnectionScheduler;
     private _cancelReconnection?: () => void;
+    private _sseStreamOpened = false; // Track if SSE stream was successfully opened
 
     onclose?: () => void;
     onerror?: (error: Error) => void;
@@ -294,6 +295,7 @@ export class StreamableHTTPClientTransport implements Transport {
                 });
             }
 
+            this._sseStreamOpened = true;
             this._handleSseStream(response.body, options, true);
         } catch (error) {
             this.onerror?.(error as Error);
@@ -556,10 +558,19 @@ export class StreamableHTTPClientTransport implements Transport {
 
             // Handle session ID received during initialization
             const sessionId = response.headers.get('mcp-session-id');
+            const hadSessionId = this._sessionId !== undefined;
             if (sessionId) {
                 this._sessionId = sessionId;
             }
 
+            // If we just received a session ID for the first time and SSE stream is not open,
+            // try to open it now. This handles the case where the initial SSE connection
+            // during start() was rejected because the server wasn't initialized yet.
+            // See: https://github.com/modelcontextprotocol/typescript-sdk/issues/1167
+            if (sessionId && !hadSessionId && !this._sseStreamOpened) {
+                this._startOrAuthSse({ resumptionToken: undefined }).catch(error => this.onerror?.(error));
+            }
+
             if (!response.ok) {
                 if (response.status === 401 && this._authProvider) {
                     // Store WWW-Authenticate params for interactive finishAuth() path

packages/client/test/client/streamableHttp.test.ts

@@ -104,8 +104,19 @@ describe('StreamableHTTPClientTransport', () => {
             headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'test-session-id' })
         });
 
+        // Mock the SSE stream GET request that happens after receiving session ID
+        (globalThis.fetch as Mock).mockResolvedValueOnce({
+            ok: false,
+            status: 405,
+            headers: new Headers(),
+            body: { cancel: vi.fn() }
+        });
+
         await transport.send(message);
 
+        // Allow the async SSE connection attempt to complete
+        await new Promise(resolve => setTimeout(resolve, 10));
+
         // Send a second message that should include the session ID
         (globalThis.fetch as Mock).mockResolvedValueOnce({
             ok: true,
@@ -167,7 +178,19 @@ describe('StreamableHTTPClientTransport', () => {
             headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'test-session-id' })
         });
 
+        // Mock the SSE stream GET request that happens after receiving session ID
+        (globalThis.fetch as Mock).mockResolvedValueOnce({
+            ok: false,
+            status: 405,
+            headers: new Headers(),
+            body: { cancel: vi.fn() }
+        });
+
         await transport.send(message);
+
+        // Allow the async SSE connection attempt to complete
+        await new Promise(resolve => setTimeout(resolve, 10));
+
         expect(transport.sessionId).toBe('test-session-id');
 
         // Now terminate the session
@@ -207,8 +230,19 @@ describe('StreamableHTTPClientTransport', () => {
             headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'test-session-id' })
         });
 
+        // Mock the SSE stream GET request that happens after receiving session ID
+        (globalThis.fetch as Mock).mockResolvedValueOnce({
+            ok: false,
+            status: 405,
+            headers: new Headers(),
+            body: { cancel: vi.fn() }
+        });
+
         await transport.send(message);
 
+        // Allow the async SSE connection attempt to complete
+        await new Promise(resolve => setTimeout(resolve, 10));
+
         // Now terminate the session, but server responds with 405
         (globalThis.fetch as Mock).mockResolvedValueOnce({
             ok: false,