diff --git a/.changeset/fix-node-set-cookie.md b/.changeset/fix-node-set-cookie.md
new file mode 100644
index 0000000000..a090aec10f
--- /dev/null
+++ b/.changeset/fix-node-set-cookie.md
@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/node": patch
+---
+
+Preserve every Set-Cookie field when adapting a web response to Node.
diff --git a/packages/middleware/node/src/toNodeHandler.ts b/packages/middleware/node/src/toNodeHandler.ts
index 5f26b1ad26..965c94cbad 100644
--- a/packages/middleware/node/src/toNodeHandler.ts
+++ b/packages/middleware/node/src/toNodeHandler.ts
@@ -66,7 +66,7 @@ export interface NodeIncomingMessageLike extends AsyncIterable<unknown> {
 
 /** Minimal duck-typed shape of a Node.js `ServerResponse` accepted by {@linkcode toNodeHandler}. */
 export interface NodeServerResponseLike {
-    writeHead(statusCode: number, headers?: Record<string, string>): unknown;
+    writeHead(statusCode: number, headers?: Record<string, string | string[]>): unknown;
     write(chunk: string | Uint8Array): unknown;
     end(chunk?: string | Uint8Array): unknown;
     on(event: string, listener: (...args: unknown[]) => void): unknown;
@@ -172,10 +172,12 @@ export function toNodeHandler(handler: FetchLikeMcpHandler, opts?: ToNodeHandler
             }
         }
 
-        const headers: Record<string, string> = {};
+        const headers: Record<string, string | string[]> = {};
         for (const [name, value] of response.headers) {
             headers[name] = value;
         }
+        const cookies = response.headers.getSetCookie();
+        if (cookies.length > 0) headers['set-cookie'] = cookies;
         res.writeHead(response.status, headers);
         if (response.body === null) {
             finished = true;
diff --git a/packages/middleware/node/test/toNodeHandler.test.ts b/packages/middleware/node/test/toNodeHandler.test.ts
index b879365f4a..b0bfb48ab0 100644
--- a/packages/middleware/node/test/toNodeHandler.test.ts
+++ b/packages/middleware/node/test/toNodeHandler.test.ts
@@ -421,7 +421,7 @@ describe('toNodeHandler', () => {
 
 interface FakeNodeResponse extends NodeServerResponseLike {
     statusCode: number;
-    headers: Record<string, string> | undefined;
+    headers: Record<string, string | string[]> | undefined;
 }
 
 function nodeRequestResponse(body: unknown): {
@@ -454,7 +454,7 @@ function nodeRequestResponse(body: unknown): {
     const res: FakeNodeResponse = {
         statusCode: 0,
         headers: undefined,
-        writeHead(statusCode: number, headers?: Record<string, string>) {
+        writeHead(statusCode: number, headers?: Record<string, string | string[]>) {
             this.statusCode = statusCode;
             this.headers = headers;
             return this;
@@ -484,3 +484,15 @@ function nodeRequestResponse(body: unknown): {
         }
     };
 }
+
+describe('response cookies', () => {
+    it('passes each Set-Cookie field separately to Node', async () => {
+        const cookies = ['first=one; Path=/; HttpOnly', 'second=two; Path=/; HttpOnly'];
+        const response = new Response(null, { headers: cookies.map(cookie => ['set-cookie', cookie] as [string, string]) });
+        const writeHead = vi.fn();
+        const res: NodeServerResponseLike = { writeHead, on: vi.fn(), write: vi.fn(), end: vi.fn() };
+        const req = Object.assign(Readable.from([]), { method: 'GET', url: '/', headers: { host: 'localhost' } });
+        await toNodeHandler({ fetch: async () => response })(req, res);
+        expect(writeHead).toHaveBeenCalledWith(200, { 'set-cookie': cookies });
+    });
+});