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

Fixes server-initiated requests (roots/list, sampling, elicitation)
hanging over HTTP transport. The client now retries opening the GET SSE
stream after receiving a session ID during initialization.

Fixes: #1167

Author: LucaButBoring

packages/client/src/client/streamableHttp.ts

@@ -141,6 +141,7 @@ export class StreamableHTTPClientTransport implements Transport {
     private _lastUpscopingHeader?: string; // Track last upscoping header to prevent infinite upscoping.
     private _serverRetryMs?: number; // Server-provided retry delay from SSE retry field
     private _reconnectionTimeout?: ReturnType<typeof setTimeout>;
+    private _sseStreamOpened = false; // Track if SSE stream was successfully opened
 
     onclose?: () => void;
     onerror?: (error: Error) => void;
@@ -247,6 +248,7 @@ export class StreamableHTTPClientTransport implements Transport {
                 });
             }
 
+            this._sseStreamOpened = true;
             this._handleSseStream(response.body, options, true);
         } catch (error) {
             this.onerror?.(error as Error);
@@ -486,10 +488,19 @@ 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) {
                 const text = await response.text?.().catch(() => null);
 

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,
@@ -140,7 +151,19 @@ 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
@@ -180,8 +203,19 @@ 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,

test/integration/test/stateManagementStreamableHttp.test.ts

@@ -352,6 +352,52 @@ describe('Zod v4', () => {
                 // Clean up
                 await transport.close();
             });
+
+            it('should support server-initiated roots/list request', async () => {
+                // This test reproduces GitHub issue #1167
+                // https://github.com/modelcontextprotocol/typescript-sdk/issues/1167
+                //
+                // The bug: server.listRoots() hangs when using HTTP transport because:
+                // 1. Client tries to open GET SSE stream before initialization
+                // 2. Server rejects with 400 "Server not initialized"
+                // 3. Client never retries opening SSE stream after initialization
+                // 4. Server's send() silently returns when no SSE stream exists
+                // 5. listRoots() promise never resolves
+
+                // Create client with roots capability
+                const client = new Client({ name: 'test-client', version: '1.0.0' }, { capabilities: { roots: { listChanged: true } } });
+
+                // Register handler for roots/list requests from server
+                client.setRequestHandler('roots/list', async () => {
+                    return {
+                        roots: [{ uri: 'file:///home/user/project', name: 'Test Project' }]
+                    };
+                });
+
+                const transport = new StreamableHTTPClientTransport(baseUrl);
+                await client.connect(transport);
+
+                // Verify client has session ID (stateful mode)
+                expect(transport.sessionId).toBeDefined();
+
+                // Now try to call listRoots from the server
+                const rootsPromise = mcpServer.server.listRoots();
+
+                // Use a short timeout to detect the hang
+                const timeoutPromise = new Promise<never>((_, reject) => {
+                    setTimeout(() => reject(new Error('listRoots() timed out - SSE stream not working')), 2000);
+                });
+
+                const result = await Promise.race([rootsPromise, timeoutPromise]);
+
+                expect(result.roots).toHaveLength(1);
+                expect(result.roots[0]).toEqual({
+                    uri: 'file:///home/user/project',
+                    name: 'Test Project'
+                });
+
+                await transport.close();
+            });
         });
     });
 });