From 0fde67952ef32c701a3591be85b5a122735ce4a9 Mon Sep 17 00:00:00 2001
From: Genmin <joey@joeyroth.com>
Date: Wed, 29 Apr 2026 16:54:08 -0700
Subject: [PATCH 1/3] fix: avoid duplicate SSE close callbacks

---
 src/server/sse.ts       |  6 ++++--
 test/server/sse.test.ts | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/src/server/sse.ts b/src/server/sse.ts
index 4931beae60..1976c2a7d1 100644
--- a/src/server/sse.ts
+++ b/src/server/sse.ts
@@ -124,8 +124,10 @@ export class SSEServerTransport implements Transport {
 
         this._sseResponse = this.res;
         this.res.on('close', () => {
-            this._sseResponse = undefined;
-            this.onclose?.();
+            if (this._sseResponse !== undefined) {
+                this._sseResponse = undefined;
+                this.onclose?.();
+            }
         });
     }
 
diff --git a/test/server/sse.test.ts b/test/server/sse.test.ts
index 0e996d1d64..09b3116188 100644
--- a/test/server/sse.test.ts
+++ b/test/server/sse.test.ts
@@ -442,6 +442,39 @@ describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
                 await transport.close();
                 expect(transport.onclose).toHaveBeenCalled();
             });
+
+            it('should only call onclose once when close ends the SSE response', async () => {
+                const onclose = vi.fn();
+                const server = createServer(async (_req, res) => {
+                    const transport = new SSEServerTransport('/messages', res);
+                    transport.onclose = onclose;
+
+                    await transport.start();
+                    await transport.close();
+                });
+
+                const baseUrl = await listenOnRandomPort(server);
+
+                try {
+                    await new Promise<void>((resolve, reject) => {
+                        const req = http.request(baseUrl, { headers: { Accept: 'text/event-stream' } }, response => {
+                            response.resume();
+                            response.on('end', resolve);
+                            response.on('error', reject);
+                        });
+
+                        req.on('error', reject);
+                        req.end();
+                    });
+
+                    await new Promise(resolve => setTimeout(resolve, 50));
+                    expect(onclose).toHaveBeenCalledTimes(1);
+                } finally {
+                    await new Promise<void>((resolve, reject) => {
+                        server.close(error => (error ? reject(error) : resolve()));
+                    });
+                }
+            });
         });
 
         describe('send method', () => {

From 3e3b2908d75a49ca49ab1a7d0f30d5cecba7a1c7 Mon Sep 17 00:00:00 2001
From: Genmin <joey@joeyroth.com>
Date: Thu, 30 Apr 2026 07:35:04 -0700
Subject: [PATCH 2/3] chore: add sse close changeset

---
 .changeset/fix-v1x-sse-close-once.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/fix-v1x-sse-close-once.md

diff --git a/.changeset/fix-v1x-sse-close-once.md b/.changeset/fix-v1x-sse-close-once.md
new file mode 100644
index 0000000000..6f17c231ae
--- /dev/null
+++ b/.changeset/fix-v1x-sse-close-once.md
@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/sdk": patch
+---
+
+fix: avoid duplicate SSE close callbacks

From 1c9fe2c5dd10d3559187a46f8004d50281127968 Mon Sep 17 00:00:00 2001
From: Genmin <joey@joeyroth.com>
Date: Thu, 30 Apr 2026 07:53:46 -0700
Subject: [PATCH 3/3] chore: format sse close changeset

---
 .changeset/fix-v1x-sse-close-once.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.changeset/fix-v1x-sse-close-once.md b/.changeset/fix-v1x-sse-close-once.md
index 6f17c231ae..fb7c886b47 100644
--- a/.changeset/fix-v1x-sse-close-once.md
+++ b/.changeset/fix-v1x-sse-close-once.md
@@ -1,5 +1,5 @@
 ---
-"@modelcontextprotocol/sdk": patch
+'@modelcontextprotocol/sdk': patch
 ---
 
 fix: avoid duplicate SSE close callbacks