fix(mcp): 405 the standalone GET SSE stream to stop /mcp OOM (#325)

The stateless MCP server returns JSON on POST (enableJsonResponse) and
pushes no server-initiated messages, so the optional standalone GET SSE
stream serves no purpose. Left enabled, each GET holds an SSE stream open
indefinitely (25s keepalive, no eventStore) and pins a fresh per-request
McpServer (~5MB of tools + Zod schemas); a few dozen concurrent connected
clients exceed the 128MB isolate limit. This was 100% of the /mcp
exceededMemory OOMs (GET only; POST never OOMed).

Return 405 (spec-compliant 'no standalone stream') before building the
server, so GET allocates nothing. Also removes the bulk of the elevated
GET canceled / responseStreamDisconnected outcomes.

src/server/mcp/transport.test.ts

@@ -160,6 +160,28 @@ describe("handleSelfHostedOpenSeoMcpRequest", () => {
     });
   });
 
+  // The OOM came from the GET SSE stream pinning a per-request McpServer, so
+  // GET must 405 without ever building one.
+  it("returns 405 for the standalone GET SSE stream without building a server", async () => {
+    const { handleSelfHostedOpenSeoMcpRequest } =
+      await import("@/server/mcp/transport");
+
+    const response = await handleSelfHostedOpenSeoMcpRequest(
+      new Request("https://open-seo.test/mcp", {
+        method: "GET",
+        headers: { Accept: "text/event-stream" },
+      }),
+      "local_noauth",
+      {},
+      ctx,
+    );
+
+    expect(response.status).toBe(405);
+    expect(response.headers.get("Allow")).toContain("POST");
+    // nextServerId only advances when a server is built — a GET must not.
+    expect(serverMocks.nextServerId).toBe(0);
+  });
+
   it("lets the MCP transport handle OPTIONS without auth context", async () => {
     const { handleSelfHostedOpenSeoMcpRequest } =
       await import("@/server/mcp/transport");

src/server/mcp/transport.ts

@@ -97,6 +97,24 @@ function handleOpenSeoMcpRequest(
   env: unknown,
   ctx: ExecutionContext,
 ): Promise<Response> {
+  // Decline the optional standalone GET SSE stream: this server is stateless
+  // (POST returns JSON) and pushes no server-initiated messages, so the stream
+  // does nothing but leak memory — each GET is held open by a keepalive and
+  // pins a per-request McpServer (~5MB), so a few dozen concurrent clients OOM
+  // the 128MB isolate. 405 is the spec's "no stream" response; returning it
+  // before building the server means a GET allocates nothing.
+  if (request.method === "GET") {
+    return Promise.resolve(
+      new Response("Method Not Allowed", {
+        status: 405,
+        headers: {
+          Allow: "POST, DELETE, OPTIONS",
+          "Access-Control-Allow-Origin": "*",
+        },
+      }),
+    );
+  }
+
   const server = createOpenSeoMcpServer();
   const handler = createMcpHandler(server, {
     route: MCP_ROUTE,