diff --git a/.changeset/streamable-http-onclose-types.md b/.changeset/streamable-http-onclose-types.md
new file mode 100644
index 0000000000..f10c204ab7
--- /dev/null
+++ b/.changeset/streamable-http-onclose-types.md
@@ -0,0 +1,6 @@
+---
+'@modelcontextprotocol/node': patch
+'@modelcontextprotocol/server': patch
+---
+
+Align Streamable HTTP onclose types with the shared Transport contract.
diff --git a/packages/middleware/node/src/streamableHttp.ts b/packages/middleware/node/src/streamableHttp.ts
index 68a0c224f0..a70588d4d7 100644
--- a/packages/middleware/node/src/streamableHttp.ts
+++ b/packages/middleware/node/src/streamableHttp.ts
@@ -100,11 +100,11 @@ export class NodeStreamableHTTPServerTransport implements Transport {
     /**
      * Sets callback for when the transport is closed.
      */
-    set onclose(handler: (() => void) | undefined) {
+    set onclose(handler: Transport['onclose']) {
         this._webStandardTransport.onclose = handler;
     }
 
-    get onclose(): (() => void) | undefined {
+    get onclose(): Transport['onclose'] {
         return this._webStandardTransport.onclose;
     }
 
diff --git a/packages/middleware/node/test/streamableHttp.typecheck.ts b/packages/middleware/node/test/streamableHttp.typecheck.ts
new file mode 100644
index 0000000000..701d636ad6
--- /dev/null
+++ b/packages/middleware/node/test/streamableHttp.typecheck.ts
@@ -0,0 +1,5 @@
+import type { Transport } from '@modelcontextprotocol/core';
+import { NodeStreamableHTTPServerTransport } from '../src/streamableHttp.js';
+
+const transport: Transport = new NodeStreamableHTTPServerTransport({});
+transport.onclose = undefined;
diff --git a/packages/server/src/server/streamableHttp.ts b/packages/server/src/server/streamableHttp.ts
index fd3563a077..2f563c3d94 100644
--- a/packages/server/src/server/streamableHttp.ts
+++ b/packages/server/src/server/streamableHttp.ts
@@ -242,7 +242,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
     private _supportedProtocolVersions: string[];
 
     sessionId?: string;
-    onclose?: () => void;
+    onclose?: Transport['onclose'];
     onerror?: (error: Error) => void;
     onmessage?: (message: JSONRPCMessage, extra?: MessageExtraInfo) => void;