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fix: restore globalThis.Response after lazy-loading MCP SDK #148

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fix: restore globalThis.Response after lazy-loading MCP SDK #148

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5 changes: 5 additions & 0 deletions .changeset/fix-global-response-pollution.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
"mcp-handler": patch
---

fix: restore globalThis.Response after lazy-loading MCP SDK to prevent Next.js route handler failures
109 changes: 71 additions & 38 deletions src/handler/mcp-api-handler.ts
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { SSEServerTransport } from "@modelcontextprotocol/sdk/server/sse.js";
import type { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import type { SSEServerTransport } from "@modelcontextprotocol/sdk/server/sse.js";
import {
type IncomingHttpHeaders,
IncomingMessage,
Expand All @@ -8,7 +8,7 @@ import {
import { createClient } from "redis";
import { Socket } from "node:net";
import { Readable } from "node:stream";
import { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/streamableHttp.js";
import type { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/streamableHttp.js";
import type { BodyType } from "./server-response-adapter";
import assert from "node:assert";
import type {
Expand All @@ -23,6 +23,36 @@ import { AuthInfo } from "@modelcontextprotocol/sdk/server/auth/types";
import { getAuthContext } from "../auth/auth-context";
import { ServerOptions } from ".";

// Lazy-loaded SDK modules. The @modelcontextprotocol/sdk has transitive
// dependencies (via undici) that replace globalThis.Response at import
// time. Eagerly importing the SDK at the module level causes Next.js
// route handlers to fail the `instanceof Response` check with
// "No response is returned from route handler" errors.
// See: https://github.com/vercel/mcp-handler/issues/140
let McpServerClass: typeof McpServer;
let SSEServerTransportClass: typeof SSEServerTransport;
let StreamableHTTPServerTransportClass: typeof StreamableHTTPServerTransport;

async function loadSdk() {
if (McpServerClass) return;

const OriginalResponse = globalThis.Response;

const [mcpMod, sseMod, httpMod] = await Promise.all([
import("@modelcontextprotocol/sdk/server/mcp.js"),
import("@modelcontextprotocol/sdk/server/sse.js"),
import("@modelcontextprotocol/sdk/server/streamableHttp.js"),
]);

// Restore the original Response so that other route handlers in the
// same process continue to pass Next.js's instanceof check.
globalThis.Response = OriginalResponse;

McpServerClass = mcpMod.McpServer;
SSEServerTransportClass = sseMod.SSEServerTransport;
StreamableHTTPServerTransportClass = httpMod.StreamableHTTPServerTransport;
}

interface SerializedRequest {
requestId: string;
url: string;
Expand Down Expand Up @@ -278,16 +308,14 @@ export function initializeMcpApiHandler(
let servers: McpServer[] = [];

let statelessServer: McpServer;
const statelessTransport = new StreamableHTTPServerTransport({
sessionIdGenerator: sessionIdGenerator,
});

let statelessTransport: StreamableHTTPServerTransport;

// Start periodic cleanup if not already running
if (!cleanupInterval) {
cleanupInterval = setInterval(() => {
const now = Date.now();
const staleThreshold = 5 * 60 * 1000; // 5 minutes

servers = servers.filter(server => {
const metadata = serverMetadata.get(server);
if (!metadata) {
Expand All @@ -302,7 +330,7 @@ export function initializeMcpApiHandler(
}
return false;
}

const age = now - metadata.createdAt.getTime();
if (age > staleThreshold) {
logger.log(`Removing stale server (session ${metadata.sessionId}, age: ${age}ms)`);
Expand All @@ -319,13 +347,15 @@ export function initializeMcpApiHandler(
serverMetadata.delete(server);
return false;
}

return true;
});
}, 30 * 1000); // Run every 30 seconds
}

return async function mcpApiHandler(req: Request, res: ServerResponse) {
await loadSdk();

const url = new URL(req.url || "", "https://example.com");
if (url.pathname === streamableHttpEndpoint) {
if (req.method === "GET") {
Expand Down Expand Up @@ -364,7 +394,10 @@ export function initializeMcpApiHandler(
);

if (!statelessServer) {
statelessServer = new McpServer(serverInfo, mcpServerOptions);
statelessTransport = new StreamableHTTPServerTransportClass({
sessionIdGenerator: sessionIdGenerator,
});
statelessServer = new McpServerClass(serverInfo, mcpServerOptions);
await initializeServer(statelessServer);
await statelessServer.connect(statelessTransport);
}
Expand Down Expand Up @@ -460,7 +493,7 @@ export function initializeMcpApiHandler(
});
logger.log("Got new SSE connection");
assert(sseMessageEndpoint, "sseMessageEndpoint is required");
const transport = new SSEServerTransport(sseMessageEndpoint, res);
const transport = new SSEServerTransportClass(sseMessageEndpoint, res);
const sessionId = transport.sessionId;

const eventRes = new EventEmittingResponse(
Expand All @@ -476,23 +509,23 @@ export function initializeMcpApiHandler(
undefined,
});

const server = new McpServer(serverInfo, serverOptions);
const server = new McpServerClass(serverInfo, serverOptions);

// Track cleanup state to prevent double cleanup
let isCleanedUp = false;
let interval: NodeJS.Timeout | null = null;
let timeout: NodeJS.Timeout | null = null;
let abortHandler: (() => void) | null = null;
let handleMessage: ((message: string) => Promise<void>) | null = null;
let logs: { type: LogLevel; messages: string[]; }[] = [];

// Comprehensive cleanup function
const cleanup = async (reason: string) => {
if (isCleanedUp) return;
isCleanedUp = true;

logger.log(`Cleaning up SSE connection: ${reason}`);

// Clear timers
if (timeout) {
clearTimeout(timeout);
Expand All @@ -502,13 +535,13 @@ export function initializeMcpApiHandler(
clearInterval(interval);
interval = null;
}

// Remove abort event listener
if (abortHandler) {
req.signal.removeEventListener("abort", abortHandler);
abortHandler = null;
}

// Unsubscribe from Redis
if (handleMessage) {
try {
Expand All @@ -518,7 +551,7 @@ export function initializeMcpApiHandler(
logger.error("Error unsubscribing from Redis:", error);
}
}

// Close server and transport
try {
if (server?.server) {
Expand All @@ -528,30 +561,30 @@ export function initializeMcpApiHandler(
await transport.close();
}
} catch (error) {
logger.error("Error closing server/transport:", error);
logger.error("Error closing stale server:", error);
}

// Remove server from array and WeakMap
servers = servers.filter((s) => s !== server);
serverMetadata.delete(server);

// End session event
eventRes.endSession("SSE");

// Clear logs array to free memory
logs = [];

// End response if not already ended
if (!res.headersSent) {
res.statusCode = 200;
res.end();
}
};

try {
await initializeServer(server);
servers.push(server);

// Store metadata in WeakMap
serverMetadata.set(server, {
sessionId,
Expand Down Expand Up @@ -674,12 +707,12 @@ export function initializeMcpApiHandler(

abortHandler = () => resolveTimeout("client hang up");
req.signal.addEventListener("abort", abortHandler);

// Handle response close event
res.on("close", () => {
cleanup("response closed");
});

// Handle response error event
res.on("error", (error) => {
logger.error("Response error:", error);
Expand Down Expand Up @@ -736,24 +769,24 @@ export function initializeMcpApiHandler(
let timeout: NodeJS.Timeout | null = null;
let hasResponded = false;
let isCleanedUp = false;

// Cleanup function to ensure all resources are freed
const cleanup = async () => {
if (isCleanedUp) return;
isCleanedUp = true;

if (timeout) {
clearTimeout(timeout);
timeout = null;
}

try {
await redis.unsubscribe(`responses:${sessionId}:${requestId}`);
} catch (error) {
logger.error("Error unsubscribing from Redis response channel:", error);
}
};

// Safe response handler to prevent double res.end()
const sendResponse = async (status: number, body: string) => {
if (!hasResponded) {
Expand All @@ -763,7 +796,7 @@ export function initializeMcpApiHandler(
await cleanup();
}
};

// Response handler
const handleResponse = async (message: string) => {
try {
Expand Down Expand Up @@ -805,7 +838,7 @@ export function initializeMcpApiHandler(
await cleanup();
}
});

// Handle response error event
res.on("error", async (error) => {
logger.error("Response error in message handler:", error);
Expand Down Expand Up @@ -880,9 +913,9 @@ function createFakeIncomingMessage(
req.method = method;
req.url = url;
req.headers = headers;
req.rawHeaders = Object.entries(headers).flatMap(([key, value]) =>
Array.isArray(value)
? value.flatMap(v => [key, v])
req.rawHeaders = Object.entries(headers).flatMap(([key, value]) =>
Array.isArray(value)
? value.flatMap(v => [key, v])
: [key, value ?? ""]
);

Expand Down