Allow streamable HTTP transport restart after close

.changeset/streamable-http-restart-close.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/client': patch
+---
+
+Allow StreamableHTTPClientTransport to be restarted after close by clearing its aborted controller.

packages/client/src/client/streamableHttp.ts

@@ -345,7 +345,7 @@ export class StreamableHTTPClientTransport implements Transport {
 
         const reconnect = (): void => {
             this._cancelReconnection = undefined;
-            if (this._abortController?.signal.aborted) return;
+            if (!this._abortController || this._abortController.signal.aborted) return;
             this._startOrAuthSse(options).catch(error => {
                 this.onerror?.(new Error(`Failed to reconnect SSE stream: ${error instanceof Error ? error.message : String(error)}`));
                 try {
@@ -513,6 +513,7 @@ export class StreamableHTTPClientTransport implements Transport {
         } finally {
             this._cancelReconnection = undefined;
             this._abortController?.abort();
+            this._abortController = undefined;
             this.onclose?.();
         }
     }

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

@@ -149,6 +149,18 @@ describe('StreamableHTTPClientTransport', () => {
         await reconnectTransport.close().catch(() => {});
     });
 
+    it('can be started again after close()', async () => {
+        await transport.start();
+        const firstAbortController = transport['_abortController'];
+
+        await transport.close();
+
+        expect(firstAbortController?.signal.aborted).toBe(true);
+        expect(transport['_abortController']).toBeUndefined();
+        await expect(transport.start()).resolves.toBeUndefined();
+        expect(transport['_abortController']).toBeDefined();
+    });
+
     it('should terminate session with DELETE request', async () => {
         // First, simulate getting a session ID
         const message: JSONRPCMessage = {