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#!/usr/bin/env node
import cors from "cors";
import { parseArgs } from "node:util";
import { parse as shellParseArgs } from "shell-quote";
import nodeFetch, { Headers as NodeHeaders } from "node-fetch";
// Type-compatible wrappers for node-fetch to work with browser-style types
const fetch = nodeFetch;
const Headers = NodeHeaders;
import {
SSEClientTransport,
SseError,
} from "@modelcontextprotocol/sdk/client/sse.js";
import {
StdioClientTransport,
getDefaultEnvironment,
} from "@modelcontextprotocol/sdk/client/stdio.js";
import {
StreamableHTTPClientTransport,
StreamableHTTPError,
} from "@modelcontextprotocol/sdk/client/streamableHttp.js";
import { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/streamableHttp.js";
import { SSEServerTransport } from "@modelcontextprotocol/sdk/server/sse.js";
import { Transport } from "@modelcontextprotocol/sdk/shared/transport.js";
import express from "express";
import rateLimit from "express-rate-limit";
import { findActualExecutable } from "spawn-rx";
import mcpProxy from "./mcpProxy.js";
import { randomUUID, randomBytes, timingSafeEqual } from "node:crypto";
import { fileURLToPath } from "url";
import { dirname, join } from "path";
import { readFileSync } from "fs";
const DEFAULT_MCP_PROXY_LISTEN_PORT = "6277";
const sandboxRateLimiter = rateLimit({
windowMs: 15 * 60 * 1000, // 15 minutes
max: 100, // limit each IP to 100 /sandbox requests per windowMs
});
const defaultEnvironment = {
...getDefaultEnvironment(),
...(process.env.MCP_ENV_VARS ? JSON.parse(process.env.MCP_ENV_VARS) : {}),
};
const { values } = parseArgs({
args: process.argv.slice(2),
options: {
env: { type: "string", default: "" },
args: { type: "string", default: "" },
command: { type: "string", default: "" },
transport: { type: "string", default: "" },
"server-url": { type: "string", default: "" },
},
});
/**
* Helper function to detect 401 Unauthorized errors from various transport types.
* StreamableHTTPClientTransport throws a generic Error with "HTTP 401" in the message
* when there's no authProvider configured, while SSEClientTransport throws SseError.
*/
const is401Error = (error: unknown): boolean => {
if (error instanceof SseError && error.code === 401) return true;
if (error instanceof StreamableHTTPError && error.code === 401) return true;
if (
error instanceof Error &&
(error.message.includes("HTTP 401") || error.message.includes("(401)"))
)
return true;
return false;
};
// Function to get HTTP headers.
const getHttpHeaders = (req: express.Request): Record<string, string> => {
const headers: Record<string, string> = {};
// Iterate over all headers in the request
for (const key in req.headers) {
const lowerKey = key.toLowerCase();
// Check if the header is one we want to forward
if (
lowerKey.startsWith("mcp-") ||
lowerKey === "authorization" ||
lowerKey === "last-event-id"
) {
// Exclude the proxy's own authentication header and the Client <-> Proxy session ID header
if (lowerKey !== "x-mcp-proxy-auth" && lowerKey !== "mcp-session-id") {
const value = req.headers[key];
if (typeof value === "string") {
// If the value is a string, use it directly
headers[key] = value;
} else if (Array.isArray(value)) {
// If the value is an array, use the last element
const lastValue = value.at(-1);
if (lastValue !== undefined) {
headers[key] = lastValue;
}
}
// If value is undefined, it's skipped, which is correct.
}
}
}
// Handle the custom auth header separately. We expect `x-custom-auth-header`
// to be a string containing the name of the actual authentication header.
const customAuthHeaderName = req.headers["x-custom-auth-header"];
if (typeof customAuthHeaderName === "string") {
const lowerCaseHeaderName = customAuthHeaderName.toLowerCase();
const value = req.headers[lowerCaseHeaderName];
if (typeof value === "string") {
headers[customAuthHeaderName] = value;
} else if (Array.isArray(value)) {
// If the actual auth header was sent multiple times, use the last value.
const lastValue = value.at(-1);
if (lastValue !== undefined) {
headers[customAuthHeaderName] = lastValue;
}
}
}
// Handle multiple custom headers (new approach)
if (req.headers["x-custom-auth-headers"] !== undefined) {
try {
const customHeaderNames = JSON.parse(
req.headers["x-custom-auth-headers"] as string,
) as string[];
if (Array.isArray(customHeaderNames)) {
customHeaderNames.forEach((headerName) => {
const lowerCaseHeaderName = headerName.toLowerCase();
if (req.headers[lowerCaseHeaderName] !== undefined) {
const value = req.headers[lowerCaseHeaderName];
headers[headerName] = Array.isArray(value)
? value[value.length - 1]
: value;
}
});
}
} catch (error) {
console.warn("Failed to parse x-custom-auth-headers:", error);
}
}
return headers;
};
/**
* Updates a headers object in-place, preserving the original Accept header.
* This is necessary to ensure that transports holding a reference to the headers
* object see the updates.
* @param currentHeaders The headers object to update.
* @param newHeaders The new headers to apply.
*/
const updateHeadersInPlace = (
currentHeaders: Record<string, string>,
newHeaders: Record<string, string>,
) => {
// Preserve the Accept header, which is set at transport creation and
// is not present in subsequent client requests.
const accept = currentHeaders["Accept"];
// Clear the old headers and apply the new ones.
Object.keys(currentHeaders).forEach((key) => delete currentHeaders[key]);
Object.assign(currentHeaders, newHeaders);
// Restore the Accept header.
if (accept) {
currentHeaders["Accept"] = accept;
}
};
const app = express();
app.use(cors());
app.use((req, res, next) => {
res.header("Access-Control-Expose-Headers", "mcp-session-id");
next();
});
const webAppTransports: Map<string, Transport> = new Map<string, Transport>(); // Web app transports by web app sessionId
const serverTransports: Map<string, Transport> = new Map<string, Transport>(); // Server Transports by web app sessionId
const sessionHeaderHolders: Map<string, { headers: HeadersInit }> = new Map(); // For dynamic header updates
// Use provided token from environment or generate a new one
const sessionToken =
process.env.MCP_PROXY_AUTH_TOKEN || randomBytes(32).toString("hex");
const authDisabled = !!process.env.DANGEROUSLY_OMIT_AUTH;
// Origin validation middleware to prevent DNS rebinding attacks
const originValidationMiddleware = (
req: express.Request,
res: express.Response,
next: express.NextFunction,
) => {
const origin = req.headers.origin;
// Default origins based on CLIENT_PORT or use environment variable
const clientPort = process.env.CLIENT_PORT || "6274";
const defaultOrigin = `http://localhost:${clientPort}`;
const allowedOrigins = process.env.ALLOWED_ORIGINS?.split(",") || [
defaultOrigin,
];
if (!origin || !allowedOrigins.includes(origin)) {
console.error(`Invalid origin: ${origin}`);
res.status(403).json({
error: "Forbidden - invalid origin",
message:
"Request blocked to prevent DNS rebinding attacks. Configure allowed origins via environment variable.",
});
return;
}
next();
};
const authMiddleware = (
req: express.Request,
res: express.Response,
next: express.NextFunction,
) => {
if (authDisabled) {
return next();
}
const sendUnauthorized = () => {
res.status(401).json({
error: "Unauthorized",
message:
"Authentication required. Use the session token shown in the console when starting the server.",
});
};
const authHeader = req.headers["x-mcp-proxy-auth"];
const authHeaderValue = Array.isArray(authHeader)
? authHeader[0]
: authHeader;
if (!authHeaderValue || !authHeaderValue.startsWith("Bearer ")) {
sendUnauthorized();
return;
}
const providedToken = authHeaderValue.substring(7); // Remove 'Bearer ' prefix
const expectedToken = sessionToken;
// Convert to buffers for timing-safe comparison
const providedBuffer = Buffer.from(providedToken);
const expectedBuffer = Buffer.from(expectedToken);
// Check length first to prevent timing attacks
if (providedBuffer.length !== expectedBuffer.length) {
sendUnauthorized();
return;
}
// Perform timing-safe comparison
if (!timingSafeEqual(providedBuffer, expectedBuffer)) {
sendUnauthorized();
return;
}
next();
};
/**
* Converts a Node.js ReadableStream to a web-compatible ReadableStream
* This is necessary for the EventSource polyfill which expects web streams
*/
const createWebReadableStream = (nodeStream: any): ReadableStream => {
let closed = false;
return new ReadableStream({
start(controller) {
nodeStream.on("data", (chunk: any) => {
if (!closed) {
controller.enqueue(chunk);
}
});
nodeStream.on("end", () => {
if (!closed) {
closed = true;
controller.close();
}
});
nodeStream.on("error", (err: any) => {
if (!closed) {
closed = true;
controller.error(err);
}
});
},
cancel() {
closed = true;
nodeStream.destroy();
},
});
};
/**
* Creates a `fetch` function that merges dynamic session headers with the
* headers from the actual request, ensuring that request-specific headers like
* `Content-Type` are preserved. For SSE requests, it also converts Node.js
* streams to web-compatible streams.
*/
const createCustomFetch = (headerHolder: { headers: HeadersInit }) => {
return async (
input: RequestInfo | URL,
init?: RequestInit,
): Promise<Response> => {
// Determine the headers from the original request/init.
// The SDK may pass a Request object or a URL and an init object.
const originalHeaders =
input instanceof Request ? input.headers : init?.headers;
// Start with our dynamic session headers.
const finalHeaders = new Headers(headerHolder.headers);
// Merge the SDK's request-specific headers, letting them overwrite.
// This is crucial for preserving Content-Type on POST requests.
new Headers(originalHeaders).forEach((value, key) => {
finalHeaders.set(key, value);
});
// Convert Headers to a plain object for node-fetch compatibility
const headersObject: Record<string, string> = {};
finalHeaders.forEach((value, key) => {
headersObject[key] = value;
});
// Get the response from node-fetch (cast input and init to handle type differences)
const response = await fetch(
input as any,
{ ...init, headers: headersObject } as any,
);
// Check if this is an SSE request by looking at the Accept header
const acceptHeader = finalHeaders.get("Accept");
const isSSE = acceptHeader?.includes("text/event-stream");
if (isSSE && response.body) {
// For SSE requests, we need to convert the Node.js stream to a web ReadableStream
// because the EventSource polyfill expects web-compatible streams
const webStream = createWebReadableStream(response.body);
// Create a new response with the web-compatible stream
// Convert node-fetch headers to plain object for web Response compatibility
const responseHeaders: Record<string, string> = {};
response.headers.forEach((value: string, key: string) => {
responseHeaders[key] = value;
});
return new Response(webStream, {
status: response.status,
statusText: response.statusText,
headers: responseHeaders,
}) as Response;
}
// For non-SSE requests, return the response as-is (cast to handle type differences)
return response as unknown as Response;
};
};
const createTransport = async (
req: express.Request,
): Promise<{
transport: Transport;
headerHolder?: { headers: HeadersInit };
}> => {
const query = req.query;
console.log("Query parameters:", JSON.stringify(query));
const transportType = query.transportType as string;
if (transportType === "stdio") {
const command = (query.command as string).trim();
const origArgs = shellParseArgs(query.args as string) as string[];
const queryEnv = query.env ? JSON.parse(query.env as string) : {};
const env = { ...defaultEnvironment, ...process.env, ...queryEnv };
const { cmd, args } = findActualExecutable(command, origArgs);
console.log(`STDIO transport: command=${cmd}, args=${args}`);
const transport = new StdioClientTransport({
command: cmd,
args,
env,
stderr: "pipe",
});
await transport.start();
return { transport };
} else if (transportType === "sse") {
const url = query.url as string;
const headers = getHttpHeaders(req);
headers["Accept"] = "text/event-stream";
const headerHolder = { headers };
console.log(
`SSE transport: url=${url}, headers=${JSON.stringify(headers)}`,
);
const transport = new SSEClientTransport(new URL(url), {
eventSourceInit: {
fetch: createCustomFetch(headerHolder),
},
requestInit: {
headers: headerHolder.headers,
},
});
await transport.start();
return { transport, headerHolder };
} else if (transportType === "streamable-http") {
const headers = getHttpHeaders(req);
headers["Accept"] = "text/event-stream, application/json";
const headerHolder = { headers };
const transport = new StreamableHTTPClientTransport(
new URL(query.url as string),
{
// Pass a custom fetch to inject the latest headers on each request
fetch: createCustomFetch(headerHolder),
},
);
await transport.start();
return { transport, headerHolder };
} else {
console.error(`Invalid transport type: ${transportType}`);
throw new Error("Invalid transport type specified");
}
};
app.get(
"/mcp",
originValidationMiddleware,
authMiddleware,
async (req, res) => {
const sessionId = req.headers["mcp-session-id"] as string;
console.log(`Received GET message for sessionId ${sessionId}`);
const headerHolder = sessionHeaderHolders.get(sessionId);
if (headerHolder) {
updateHeadersInPlace(
headerHolder.headers as Record<string, string>,
getHttpHeaders(req),
);
}
try {
const transport = webAppTransports.get(
sessionId,
) as StreamableHTTPServerTransport;
if (!transport) {
res.status(404).end("Session not found");
return;
} else {
await transport.handleRequest(req, res);
}
} catch (error) {
console.error("Error in /mcp route:", error);
res.status(500).json(error);
}
},
);
app.post(
"/mcp",
originValidationMiddleware,
authMiddleware,
async (req, res) => {
const sessionId = req.headers["mcp-session-id"] as string | undefined;
if (sessionId) {
console.log(`Received POST message for sessionId ${sessionId}`);
const headerHolder = sessionHeaderHolders.get(sessionId);
if (headerHolder) {
updateHeadersInPlace(
headerHolder.headers as Record<string, string>,
getHttpHeaders(req),
);
}
try {
const transport = webAppTransports.get(
sessionId,
) as StreamableHTTPServerTransport;
if (!transport) {
res.status(404).end("Transport not found for sessionId " + sessionId);
} else {
await (transport as StreamableHTTPServerTransport).handleRequest(
req,
res,
);
}
} catch (error) {
console.error("Error in /mcp route:", error);
res.status(500).json(error);
}
} else {
console.log("New StreamableHttp connection request");
try {
const { transport: serverTransport, headerHolder } =
await createTransport(req);
const webAppTransport = new StreamableHTTPServerTransport({
sessionIdGenerator: randomUUID,
onsessioninitialized: (sessionId) => {
webAppTransports.set(sessionId, webAppTransport);
serverTransports.set(sessionId, serverTransport!); // eslint-disable-line @typescript-eslint/no-non-null-assertion
if (headerHolder) {
sessionHeaderHolders.set(sessionId, headerHolder);
}
console.log("Client <-> Proxy sessionId: " + sessionId);
},
onsessionclosed: (sessionId) => {
webAppTransports.delete(sessionId);
serverTransports.delete(sessionId);
sessionHeaderHolders.delete(sessionId);
},
});
console.log("Created StreamableHttp client transport");
await webAppTransport.start();
mcpProxy({
transportToClient: webAppTransport,
transportToServer: serverTransport,
});
await (webAppTransport as StreamableHTTPServerTransport).handleRequest(
req,
res,
req.body,
);
} catch (error) {
if (is401Error(error)) {
console.error(
"Received 401 Unauthorized from MCP server:",
error instanceof Error ? error.message : error,
);
res.status(401).json(error);
return;
}
console.error("Error in /mcp POST route:", error);
res.status(500).json(error);
}
}
},
);
app.delete(
"/mcp",
originValidationMiddleware,
authMiddleware,
async (req, res) => {
const sessionId = req.headers["mcp-session-id"] as string | undefined;
console.log(`Received DELETE message for sessionId ${sessionId}`);
if (sessionId) {
try {
const serverTransport = serverTransports.get(
sessionId,
) as StreamableHTTPClientTransport;
if (!serverTransport) {
res.status(404).end("Transport not found for sessionId " + sessionId);
} else {
await serverTransport.terminateSession();
await serverTransport.close();
webAppTransports.delete(sessionId);
serverTransports.delete(sessionId);
sessionHeaderHolders.delete(sessionId);
console.log(`Transports removed for sessionId ${sessionId}`);
}
res.status(200).end();
} catch (error) {
console.error("Error in /mcp route:", error);
res.status(500).json(error);
}
}
},
);
app.get(
"/stdio",
originValidationMiddleware,
authMiddleware,
async (req, res) => {
try {
console.log("New STDIO connection request");
const { transport: serverTransport } = await createTransport(req);
const proxyFullAddress = (req.query.proxyFullAddress as string) || "";
const prefix = proxyFullAddress || "";
const endpoint = `${prefix}/message`;
const webAppTransport = new SSEServerTransport(endpoint, res);
webAppTransports.set(webAppTransport.sessionId, webAppTransport);
console.log("Created client transport");
serverTransports.set(webAppTransport.sessionId, serverTransport);
console.log("Created server transport");
await webAppTransport.start();
(serverTransport as StdioClientTransport).stderr!.on("data", (chunk) => {
if (chunk.toString().includes("MODULE_NOT_FOUND")) {
// Server command not found, remove transports
const message = "Command not found, transports removed";
webAppTransport.send({
jsonrpc: "2.0",
method: "notifications/message",
params: {
level: "emergency",
logger: "proxy",
data: {
message,
},
},
});
webAppTransport.close();
serverTransport.close();
webAppTransports.delete(webAppTransport.sessionId);
serverTransports.delete(webAppTransport.sessionId);
sessionHeaderHolders.delete(webAppTransport.sessionId);
console.error(message);
} else {
// Inspect message and attempt to assign a RFC 5424 Syslog Protocol level
let level;
let message = chunk.toString().trim();
let ucMsg = chunk.toString().toUpperCase();
if (ucMsg.includes("DEBUG")) {
level = "debug";
} else if (ucMsg.includes("INFO")) {
level = "info";
} else if (ucMsg.includes("NOTICE")) {
level = "notice";
} else if (ucMsg.includes("WARN")) {
level = "warning";
} else if (ucMsg.includes("ERROR")) {
level = "error";
} else if (ucMsg.includes("CRITICAL")) {
level = "critical";
} else if (ucMsg.includes("ALERT")) {
level = "alert";
} else if (ucMsg.includes("EMERGENCY")) {
level = "emergency";
} else if (ucMsg.includes("SIGINT")) {
message = "SIGINT received. Server shutdown.";
level = "emergency";
} else if (ucMsg.includes("SIGHUP")) {
message = "SIGHUP received. Server shutdown.";
level = "emergency";
} else if (ucMsg.includes("SIGTERM")) {
message = "SIGTERM received. Server shutdown.";
level = "emergency";
} else {
level = "info";
}
webAppTransport.send({
jsonrpc: "2.0",
method: "notifications/message",
params: {
level,
logger: "stdio",
data: {
message,
},
},
});
}
});
mcpProxy({
transportToClient: webAppTransport,
transportToServer: serverTransport,
});
} catch (error) {
if (is401Error(error)) {
console.error(
"Received 401 Unauthorized from MCP server. Authentication failure.",
);
res.status(401).json(error);
return;
}
console.error("Error in /stdio route:", error);
res.status(500).json(error);
}
},
);
app.get(
"/sse",
originValidationMiddleware,
authMiddleware,
async (req, res) => {
try {
console.log(
"New SSE connection request. NOTE: The SSE transport is deprecated and has been replaced by StreamableHttp",
);
const { transport: serverTransport, headerHolder } =
await createTransport(req);
const proxyFullAddress = (req.query.proxyFullAddress as string) || "";
const prefix = proxyFullAddress || "";
const endpoint = `${prefix}/message`;
const webAppTransport = new SSEServerTransport(endpoint, res);
webAppTransports.set(webAppTransport.sessionId, webAppTransport);
console.log("Created client transport");
serverTransports.set(webAppTransport.sessionId, serverTransport!); // eslint-disable-line @typescript-eslint/no-non-null-assertion
if (headerHolder) {
sessionHeaderHolders.set(webAppTransport.sessionId, headerHolder);
}
console.log("Created server transport");
await webAppTransport.start();
mcpProxy({
transportToClient: webAppTransport,
transportToServer: serverTransport,
});
} catch (error) {
if (is401Error(error)) {
console.error(
"Received 401 Unauthorized from MCP server. Authentication failure.",
);
res.status(401).json(error);
return;
} else if (error instanceof SseError && error.code === 404) {
console.error(
"Received 404 not found from MCP server. Does the MCP server support SSE?",
);
res.status(404).json(error);
return;
} else if (JSON.stringify(error).includes("ECONNREFUSED")) {
console.error("Connection refused. Is the MCP server running?");
res.status(500).json(error);
}
console.error("Error in /sse route:", error);
res.status(500).json(error);
}
},
);
app.post(
"/message",
originValidationMiddleware,
authMiddleware,
async (req, res) => {
try {
const sessionId = req.query.sessionId as string;
console.log(`Received POST message for sessionId ${sessionId}`);
const headerHolder = sessionHeaderHolders.get(sessionId);
if (headerHolder) {
updateHeadersInPlace(
headerHolder.headers as Record<string, string>,
getHttpHeaders(req),
);
}
const transport = webAppTransports.get(sessionId) as SSEServerTransport;
if (!transport) {
res.status(404).end("Session not found");
return;
}
await transport.handlePostMessage(req, res);
} catch (error) {
console.error("Error in /message route:", error);
res.status(500).json(error);
}
},
);
app.get("/health", (req, res) => {
res.json({
status: "ok",
});
});
app.get("/config", originValidationMiddleware, authMiddleware, (req, res) => {
try {
res.json({
defaultEnvironment,
defaultCommand: values.command,
defaultArgs: values.args,
defaultTransport: values.transport,
defaultServerUrl: values["server-url"],
});
} catch (error) {
console.error("Error in /config route:", error);
res.status(500).json(error);
}
});
app.get(
"/sandbox",
sandboxRateLimiter as express.RequestHandler,
(req, res) => {
const __filename = fileURLToPath(import.meta.url);
const __dirname = dirname(__filename);
const filePath = join(__dirname, "..", "static", "sandbox_proxy.html");
let sandboxHtml;
try {
sandboxHtml = readFileSync(filePath, "utf-8");
} catch (e) {
sandboxHtml = "MCP Apps sandbox not loaded: " + e;
}
res.set("Cache-Control", "no-cache, no-store, max-age=0");
res.send(sandboxHtml);
},
);
const PORT = parseInt(
process.env.SERVER_PORT || DEFAULT_MCP_PROXY_LISTEN_PORT,
10,
);
const HOST = process.env.HOST || "localhost";
const server = app.listen(PORT, HOST);
server.on("listening", () => {
console.log(`⚙️ Proxy server listening on ${HOST}:${PORT}`);
if (!authDisabled) {
console.log(
`🔑 Session token: ${sessionToken}\n ` +
`Use this token to authenticate requests or set DANGEROUSLY_OMIT_AUTH=true to disable auth`,
);
} else {
console.log(
`⚠️ WARNING: Authentication is disabled. This is not recommended.`,
);
}
});
server.on("error", (err) => {
if (err.message.includes(`EADDRINUSE`)) {
console.error(`❌ Proxy Server PORT IS IN USE at port ${PORT} ❌ `);
} else {
console.error(err.message);
}
process.exit(1);
});