fix(client): allow transport restart after close()

.changeset/fix-transport-restart-after-close.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/client': patch
+---
+
+Allow `StreamableHTTPClientTransport` and `SSEClientTransport` to restart after `close()`. `close()` now clears `_abortController` (previously aborted but not unset, blocking the start guard) and `_sessionId` (previously leaked into post-restart requests, causing 404s).

packages/client/src/client/sse.ts

@@ -241,7 +241,10 @@ export class SSEClientTransport implements Transport {
 
     async close(): Promise<void> {
         this._abortController?.abort();
+        this._abortController = undefined;
         this._eventSource?.close();
+        this._eventSource = undefined;
+        this._endpoint = undefined;
         this.onclose?.();
     }
 

packages/client/src/client/streamableHttp.ts

@@ -474,7 +474,7 @@ export class StreamableHTTPClientTransport implements Transport {
     }
 
     async start() {
-        if (this._abortController) {
+        if (this._abortController && !this._abortController.signal.aborted) {
             throw new Error(
                 'StreamableHTTPClientTransport already started! If using Client class, note that connect() calls start() automatically.'
             );
@@ -509,6 +509,7 @@ export class StreamableHTTPClientTransport implements Transport {
         } finally {
             this._cancelReconnection = undefined;
             this._abortController?.abort();
+            this._sessionId = undefined;
             this.onclose?.();
         }
     }

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

@@ -1924,4 +1924,47 @@ describe('StreamableHTTPClientTransport', () => {
             expect(onclose).toHaveBeenCalledTimes(1);
         });
     });
+
+    describe('Transport restart after close()', () => {
+        it('should allow start() after close() and not send stale session ID', async () => {
+            transport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'));
+
+            const fetchMock = globalThis.fetch as Mock;
+
+            // First lifecycle: start, receive a session ID, close
+            await transport.start();
+
+            fetchMock.mockResolvedValueOnce({
+                ok: true,
+                status: 200,
+                headers: new Headers({
+                    'content-type': 'application/json',
+                    'mcp-session-id': 'stale-session-abc'
+                }),
+                json: async () => ({ jsonrpc: '2.0', result: {}, id: 'init-1' })
+            });
+
+            await transport.send({ jsonrpc: '2.0', method: 'initialize', params: {}, id: 'init-1' });
+            expect(transport.sessionId).toBe('stale-session-abc');
+
+            await transport.close();
+            expect(transport.sessionId).toBeUndefined();
+
+            // Second lifecycle: start() should not throw
+            await transport.start();
+
+            fetchMock.mockResolvedValueOnce({
+                ok: true,
+                status: 202,
+                headers: new Headers(),
+                text: async () => ''
+            });
+
+            await transport.send({ jsonrpc: '2.0', method: 'notifications/initialized' });
+
+            // The post-restart request must NOT include the stale session ID
+            const postRestartHeaders = fetchMock.mock.calls[1]![1]?.headers as Headers;
+            expect(postRestartHeaders.get('mcp-session-id')).toBeNull();
+        });
+    });
 });