diff --git a/changelog.d/fixes/13859-mcp-sdk-externalize.md b/changelog.d/fixes/13859-mcp-sdk-externalize.md
new file mode 100644
index 0000000000..8a21b7a3e1
--- /dev/null
+++ b/changelog.d/fixes/13859-mcp-sdk-externalize.md
@@ -0,0 +1 @@
+- fix(build): externalize @modelcontextprotocol/sdk in standalone server webpack config to prevent TDZ ReferenceError on MCP initialize (#13859)
diff --git a/next.config.mjs b/next.config.mjs
index 1508523280..36c0282e0f 100644
--- a/next.config.mjs
+++ b/next.config.mjs
@@ -366,6 +366,13 @@ const nextConfig = {
     "ws",
     "bufferutil",
     "utf-8-validate",
+    // The SDK's client graph has a module-level `class extends Client` cycle
+    // against the TLA Client module. Bundled into route chunks it throws
+    // "Cannot access 'l' before initialization" during evaluation and every
+    // /api/mcp/stream initialize answers HTTP 500. Node's native ESM loader
+    // resolves the same circular graph via live bindings, so keep the SDK
+    // out of the webpack server bundle.
+    "@modelcontextprotocol/sdk",
     "child_process",
     "fs",
     "path",
diff --git a/tests/unit/mcp-sdk-external.test.ts b/tests/unit/mcp-sdk-external.test.ts
new file mode 100644
index 0000000000..2b70bad82d
--- /dev/null
+++ b/tests/unit/mcp-sdk-external.test.ts
@@ -0,0 +1,25 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+
+// The bundled MCP SDK client graph contains a module-level `class extends
+// Client` cycle against the async Client module. Under Next's standalone
+// webpack TLA runtime that cycle throws `ReferenceError: Cannot access 'l'
+// before initialization` during module evaluation, which takes down
+// /api/mcp/stream with HTTP 500 on every initialize. Node's native ESM
+// loader resolves the same circular graph through live bindings, so the SDK
+// must stay external to the server bundle.
+test("next.config keeps @modelcontextprotocol/sdk out of the server bundle", () => {
+  const src = readFileSync(new URL("../../next.config.mjs", import.meta.url), "utf8");
+  // Strip // line comments first: a commented-out entry must not satisfy the check.
+  const uncommented = src
+    .split("\n")
+    .map((line) => line.replace(/[/][/].*$/, ""))
+    .join("\n");
+  const externalBlock = uncommented.match(/serverExternalPackages:\s*\[([\s\S]*?)\]/);
+  assert.ok(externalBlock, "serverExternalPackages block must exist");
+  assert.ok(
+    /["']@modelcontextprotocol[/]sdk["']/.test(externalBlock[1]),
+    "serverExternalPackages must externalize @modelcontextprotocol/sdk",
+  );
+});