From 4ee8087eec3d73befa82d4f95083cd192b09db01 Mon Sep 17 00:00:00 2001
From: Matan Tsach <matan.tsach@gmail.com>
Date: Mon, 9 Mar 2026 12:24:48 +0200
Subject: [PATCH 1/2] fix: allow transport restart after close()

close() aborts _abortController but never resets it to undefined, so
start() sees a truthy guard and throws "already started." Same issue in
SSEClientTransport with _eventSource. Also reset _endpoint in SSE to
avoid stale endpoint URLs after restart.

Fixes #1641

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 packages/client/src/client/sse.ts             |  3 ++
 packages/client/src/client/streamableHttp.ts  |  1 +
 packages/client/test/client/sse.test.ts       | 28 ++++++++++++++
 .../client/test/client/streamableHttp.test.ts | 37 +++++++++++++++++++
 4 files changed, 69 insertions(+)

diff --git a/packages/client/src/client/sse.ts b/packages/client/src/client/sse.ts
index 133aa0004..c01dd3afa 100644
--- a/packages/client/src/client/sse.ts
+++ b/packages/client/src/client/sse.ts
@@ -239,7 +239,10 @@ 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?.();
     }
 
diff --git a/packages/client/src/client/streamableHttp.ts b/packages/client/src/client/streamableHttp.ts
index 79a20adfc..06e71be1c 100644
--- a/packages/client/src/client/streamableHttp.ts
+++ b/packages/client/src/client/streamableHttp.ts
@@ -452,6 +452,7 @@ export class StreamableHTTPClientTransport implements Transport {
             this._reconnectionTimeout = undefined;
         }
         this._abortController?.abort();
+        this._abortController = undefined;
         this.onclose?.();
     }
 
diff --git a/packages/client/test/client/sse.test.ts b/packages/client/test/client/sse.test.ts
index 0b0aff67b..18f253d13 100644
--- a/packages/client/test/client/sse.test.ts
+++ b/packages/client/test/client/sse.test.ts
@@ -1528,4 +1528,32 @@ describe('SSEClientTransport', () => {
             expect(globalFetchSpy).not.toHaveBeenCalled();
         });
     });
+
+    describe('Transport lifecycle', () => {
+        it('should allow start() after close() for transport reuse', async () => {
+            transport = new SSEClientTransport(resourceBaseUrl);
+            await transport.start();
+
+            // close() the transport
+            await transport.close();
+
+            // Second start() should succeed — not throw "already started"
+            await transport.start();
+
+            // Verify transport is functional by sending a message
+            const message: JSONRPCMessage = {
+                jsonrpc: '2.0',
+                method: 'test',
+                params: {},
+                id: 'test-1'
+            };
+
+            await transport.send(message);
+
+            // Wait for request processing
+            await new Promise(resolve => setTimeout(resolve, 50));
+
+            expect(lastServerRequest.method).toBe('POST');
+        });
+    });
 });
diff --git a/packages/client/test/client/streamableHttp.test.ts b/packages/client/test/client/streamableHttp.test.ts
index 8a550feae..fe63fc709 100644
--- a/packages/client/test/client/streamableHttp.test.ts
+++ b/packages/client/test/client/streamableHttp.test.ts
@@ -1659,4 +1659,41 @@ describe('StreamableHTTPClientTransport', () => {
             });
         });
     });
+
+    describe('Transport lifecycle', () => {
+        it('should allow start() after close() for transport reuse', async () => {
+            const fetchMock = globalThis.fetch as Mock;
+
+            // First start()
+            await transport.start();
+
+            // close() the transport
+            await transport.close();
+
+            // Second start() should succeed — not throw "already started"
+            await transport.start();
+
+            // Verify transport is functional by sending a message
+            fetchMock.mockResolvedValueOnce({
+                ok: true,
+                status: 200,
+                headers: new Headers({ 'content-type': 'application/json' }),
+                json: async () => ({
+                    jsonrpc: '2.0',
+                    result: {},
+                    id: 'test-1'
+                })
+            });
+
+            const message: JSONRPCMessage = {
+                jsonrpc: '2.0',
+                method: 'test',
+                params: {},
+                id: 'test-1'
+            };
+
+            await transport.send(message);
+            expect(fetchMock).toHaveBeenCalledTimes(1);
+        });
+    });
 });

From c26226730448677a71b6798bde808cf9a04d46a0 Mon Sep 17 00:00:00 2001
From: Matan Tsach <matan.tsach@gmail.com>
Date: Fri, 13 Mar 2026 12:42:37 +0200
Subject: [PATCH 2/2] chore: add changeset for transport restart fix

---
 .changeset/popular-moles-begin.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/popular-moles-begin.md

diff --git a/.changeset/popular-moles-begin.md b/.changeset/popular-moles-begin.md
new file mode 100644
index 000000000..35c685a28
--- /dev/null
+++ b/.changeset/popular-moles-begin.md
@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/client": patch
+---
+
+fix: allow transport restart after close()