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// SPDX-License-Identifier: Apache-2.0
/**
* @module MCP/MCPConnection
* @description Base MCP client — JSON-RPC 2.0 transport over stdio or HTTP gateway.
* Supports two transport modes:
* - **stdio**: Spawns the EP MCP server binary as a child process (default)
* - **gateway**: Connects to an MCP Gateway via HTTP (for agentic workflow environments)
*
* Gateway mode is activated when `EP_MCP_GATEWAY_URL` env var is set or
* `gatewayUrl` is provided in options.
*/
import { spawn, type ChildProcess } from 'child_process';
import { resolve, dirname } from 'path';
import { fileURLToPath } from 'url';
import type {
MCPClientOptions,
MCPToolResult,
JSONRPCRequest,
JSONRPCResponse,
PendingRequest,
} from '../types/index.js';
/** npm binary name for the European Parliament MCP server */
const BINARY_NAME = 'european-parliament-mcp-server';
/** Platform-specific binary filename (Windows uses .cmd shim) */
const BINARY_FILE = process.platform === 'win32' ? `${BINARY_NAME}.cmd` : BINARY_NAME;
/** Default binary resolved from node_modules/.bin relative to this file's compiled location */
const DEFAULT_SERVER_BINARY = resolve(
dirname(fileURLToPath(import.meta.url)),
`../../node_modules/.bin/${BINARY_FILE}`
);
/** Request timeout in milliseconds — EU Parliament API responses commonly take 30+ seconds */
const REQUEST_TIMEOUT_MS = 60000;
/** Connection startup delay in milliseconds */
const CONNECTION_STARTUP_DELAY_MS = 500;
/**
* Parse an SSE (Server-Sent Events) response body to extract the first valid JSON-RPC message.
*
* The MCP Streamable HTTP protocol sends JSON-RPC responses as SSE `data:` lines.
* This function returns the **first** successfully parsed JSON-RPC message; any
* subsequent `data:` lines are ignored. This matches the MCP protocol expectation
* of one JSON-RPC response per HTTP request/response cycle.
*
* @param body - Raw SSE response text (may contain multiple lines including `event:` and `data:`)
* @returns The first valid JSON-RPC response found, or null if no valid message exists
*/
export function parseSSEResponse(body: string): JSONRPCResponse | null {
const lines = body.split('\n');
for (const line of lines) {
const trimmed = line.trim();
if (trimmed.startsWith('data:')) {
const jsonStr = trimmed.slice(5).trim();
if (jsonStr) {
try {
return JSON.parse(jsonStr) as JSONRPCResponse;
} catch {
// Continue to next data line
}
}
}
}
return null;
}
/**
* Base MCP connection managing JSON-RPC 2.0 transport over stdio or HTTP gateway.
* Extended by domain-specific clients to add tool wrapper methods.
*/
export class MCPConnection {
private serverPath: string;
private connected: boolean;
private process: ChildProcess | null;
private requestId: number;
private pendingRequests: Map<number, PendingRequest>;
private connectionAttempts: number;
private maxConnectionAttempts: number;
private connectionRetryDelay: number;
/** Gateway URL for HTTP transport mode */
private gatewayUrl: string | null;
/** API key for gateway authentication */
private gatewayApiKey: string | null;
/** MCP session ID returned by the gateway */
private mcpSessionId: string | null;
/** Human-readable server name for log messages */
protected serverLabel: string;
constructor(options: MCPClientOptions = {}) {
this.serverPath =
options.serverPath ?? process.env['EP_MCP_SERVER_PATH'] ?? DEFAULT_SERVER_BINARY;
this.connected = false;
this.process = null;
this.requestId = 0;
this.pendingRequests = new Map();
this.connectionAttempts = 0;
this.maxConnectionAttempts = options.maxConnectionAttempts ?? 3;
this.connectionRetryDelay = options.connectionRetryDelay ?? 1000;
this.serverLabel = options.serverLabel ?? 'European Parliament MCP Server';
const rawGatewayUrl = (options.gatewayUrl ?? process.env['EP_MCP_GATEWAY_URL'] ?? '').trim();
this.gatewayUrl = rawGatewayUrl || null;
this.gatewayApiKey = options.gatewayApiKey ?? process.env['EP_MCP_GATEWAY_API_KEY'] ?? null;
this.mcpSessionId = null;
}
/**
* Check if client is connected
*
* @returns Connection status
*/
isConnected(): boolean {
return this.connected;
}
/**
* Check if using gateway HTTP transport
*
* @returns True if gateway mode is active
*/
isGatewayMode(): boolean {
return Boolean(this.gatewayUrl);
}
/**
* Get the configured gateway URL
*
* @returns Gateway URL or null if using stdio transport
*/
getGatewayUrl(): string | null {
return this.gatewayUrl;
}
/**
* Get the configured gateway API key
*
* @returns Gateway API key or null if not set
*/
getGatewayApiKey(): string | null {
return this.gatewayApiKey;
}
/**
* Get the current MCP session ID
*
* @returns Session ID returned by the gateway, or null if not yet connected
*/
getMcpSessionId(): string | null {
return this.mcpSessionId;
}
/**
* Connect to the MCP server with retry logic
*/
async connect(): Promise<void> {
if (this.connected) {
return;
}
if (this.gatewayUrl) {
console.log(`🔌 Connecting to ${this.serverLabel} via gateway...`);
console.log(` Gateway URL: ${this.gatewayUrl}`);
} else {
console.log(`🔌 Connecting to ${this.serverLabel}...`);
}
this.connectionAttempts = 0;
while (this.connectionAttempts < this.maxConnectionAttempts) {
try {
if (this.gatewayUrl) {
await this._attemptGatewayConnection();
} else {
await this._attemptConnection();
}
this.connectionAttempts = 0; // Reset on success
return;
} catch (error) {
this.connectionAttempts++;
Iif (this.connectionAttempts < this.maxConnectionAttempts) {
const delay = this.connectionRetryDelay * Math.pow(2, this.connectionAttempts - 1);
console.warn(
`⚠️ Connection attempt ${this.connectionAttempts} failed. Retrying in ${delay}ms...`
);
await new Promise((resolve) => setTimeout(resolve, delay));
} else {
console.error(
'❌ Failed to connect to MCP server after',
this.maxConnectionAttempts,
'attempts'
);
throw error;
}
}
}
}
/**
* Validate a gateway response body, throwing on JSON-RPC errors.
*
* @param contentType - Response content-type header
* @param body - Raw response body text
*/
private _validateGatewayResponseBody(contentType: string, body: string): void {
Iif (contentType.includes('text/event-stream')) {
const parsed = parseSSEResponse(body);
if (parsed?.error) {
throw new Error(parsed.error.message ?? 'MCP gateway initialization error');
}
return;
}
Iif (!body) {
return;
}
try {
const jsonResponse = JSON.parse(body) as JSONRPCResponse;
Iif (jsonResponse.error) {
throw new Error(jsonResponse.error.message ?? 'MCP gateway initialization error');
}
} catch (e) {
// Non-JSON body is acceptable for init — some gateways return empty/plain text
if (e instanceof Error && e.message.includes('MCP gateway')) {
throw e;
}
}
}
/**
* Attempt a single connection via MCP Gateway (HTTP transport)
*/
private async _attemptGatewayConnection(): Promise<void> {
try {
const headers: Record<string, string> = {
'Content-Type': 'application/json',
Accept: 'application/json, text/event-stream',
};
Iif (this.gatewayApiKey) {
headers['Authorization'] = `Bearer ${this.gatewayApiKey}`;
}
const initRequest: JSONRPCRequest = {
jsonrpc: '2.0',
id: ++this.requestId,
method: 'initialize',
params: {
protocolVersion: '2024-11-05',
capabilities: {},
clientInfo: { name: 'ep-mcp-client', version: '1.0.0' },
},
};
const response = await fetch(this.gatewayUrl!, {
method: 'POST',
headers,
body: JSON.stringify(initRequest),
signal: AbortSignal.timeout(REQUEST_TIMEOUT_MS),
});
Iif (!response.ok) {
throw new Error(`Gateway returned ${response.status}: ${response.statusText}`);
}
const sessionId = response.headers.get('mcp-session-id');
Eif (sessionId) {
this.mcpSessionId = sessionId;
}
// Parse and validate the initialization response body
const contentType = response.headers.get('content-type') ?? '';
const body = await response.text();
this._validateGatewayResponseBody(contentType, body);
this.connected = true;
console.log(`✅ Connected to ${this.serverLabel} via gateway`);
} catch (error) {
const message = error instanceof Error ? error.message : String(error);
console.error('❌ Failed to connect to MCP gateway:', message);
throw error;
}
}
/**
* Attempt a single connection via stdio (spawns server binary)
*/
private async _attemptConnection(): Promise<void> {
try {
const isJavaScriptFile: boolean = this.serverPath.toLowerCase().endsWith('.js');
const command: string = isJavaScriptFile ? process.execPath : this.serverPath;
const args: string[] = isJavaScriptFile ? [this.serverPath] : [];
this.process = spawn(command, args, {
stdio: ['pipe', 'pipe', 'pipe'],
});
let buffer = '';
let startupError: Error | null = null;
this.process.stdout?.on('data', (data: Buffer) => {
buffer += data.toString();
const lines = buffer.split('\n');
buffer = lines.pop() ?? '';
for (const line of lines) {
if (line.trim()) {
this.handleMessage(line);
}
}
});
this.process.stderr?.on('data', (data: Buffer) => {
const message = data.toString().trim();
Eif (message) {
console.error(`MCP Server: ${message}`);
}
});
this.process.on('close', (code: number | null) => {
console.log(`MCP Server exited with code ${code}`);
this.connected = false;
for (const [id, { reject }] of this.pendingRequests.entries()) {
reject(new Error('MCP server connection closed'));
this.pendingRequests.delete(id);
}
});
this.process.on('error', (err: Error) => {
startupError = err;
this.connected = false;
});
await new Promise((resolve) => setTimeout(resolve, CONNECTION_STARTUP_DELAY_MS));
Iif (startupError) {
throw startupError;
}
this.connected = true;
console.log(`✅ Connected to ${this.serverLabel}`);
} catch (error) {
const message = error instanceof Error ? error.message : String(error);
console.error('❌ Failed to spawn MCP server:', message);
throw error;
}
}
/**
* Disconnect from the MCP server
*/
disconnect(): void {
if (this.process) {
this.process.kill();
this.process = null;
}
this.connected = false;
this.mcpSessionId = null;
}
/**
* Handle incoming messages from MCP server (stdio mode only)
*
* @param line - JSON message line from server
*/
handleMessage(line: string): void {
try {
const message = JSON.parse(line) as JSONRPCResponse;
if (message.id && this.pendingRequests.has(message.id)) {
const pending = this.pendingRequests.get(message.id)!;
this.pendingRequests.delete(message.id);
if (message.error) {
pending.reject(new Error(message.error.message ?? 'MCP server error'));
} else {
pending.resolve(message.result);
}
} else if (!message.id && message.method) {
console.log(`MCP Notification: ${message.method}`);
}
} catch (error) {
const errorMessage = error instanceof Error ? error.message : String(error);
console.error('Error parsing MCP message:', errorMessage);
console.error('Problematic line:', line);
}
}
/**
* Send a request via MCP Gateway (HTTP transport)
*
* @param method - RPC method name
* @param params - Method parameters
* @returns Server response
*/
private async _sendGatewayRequest(
method: string,
params: Record<string, unknown> = {}
): Promise<unknown> {
const id = ++this.requestId;
const request: JSONRPCRequest = {
jsonrpc: '2.0',
id,
method,
params,
};
const headers: Record<string, string> = {
'Content-Type': 'application/json',
Accept: 'application/json, text/event-stream',
};
if (this.gatewayApiKey) {
headers['Authorization'] = `Bearer ${this.gatewayApiKey}`;
}
if (this.mcpSessionId) {
headers['Mcp-Session-Id'] = this.mcpSessionId;
}
const response = await fetch(this.gatewayUrl!, {
method: 'POST',
headers,
body: JSON.stringify(request),
signal: AbortSignal.timeout(REQUEST_TIMEOUT_MS),
});
if (!response.ok) {
throw new Error(`Gateway error ${response.status}: ${response.statusText}`);
}
const sessionId = response.headers.get('mcp-session-id');
if (sessionId) {
this.mcpSessionId = sessionId;
}
const contentType = response.headers.get('content-type') ?? '';
const body = await response.text();
if (contentType.includes('text/event-stream')) {
const parsed = parseSSEResponse(body);
if (!parsed) {
throw new Error('Failed to parse SSE response from gateway');
}
if (parsed.error) {
throw new Error(parsed.error.message ?? 'MCP gateway error');
}
return parsed.result;
}
const jsonResponse = JSON.parse(body) as JSONRPCResponse;
if (jsonResponse.error) {
throw new Error(jsonResponse.error.message ?? 'MCP gateway error');
}
return jsonResponse.result;
}
/**
* Send a request to the MCP server
*
* @param method - RPC method name
* @param params - Method parameters
* @returns Server response
*/
async sendRequest(method: string, params: Record<string, unknown> = {}): Promise<unknown> {
if (!this.connected) {
throw new Error('Not connected to MCP server');
}
Iif (this.gatewayUrl) {
return await this._sendGatewayRequest(method, params);
}
const id = ++this.requestId;
const request: JSONRPCRequest = {
jsonrpc: '2.0',
id,
method,
params,
};
return await new Promise((resolve, reject) => {
this.pendingRequests.set(id, { resolve, reject });
const message = JSON.stringify(request) + '\n';
this.process?.stdin?.write(message);
setTimeout(() => {
if (this.pendingRequests.has(id)) {
this.pendingRequests.delete(id);
reject(new Error('Request timeout'));
}
}, REQUEST_TIMEOUT_MS);
});
}
/**
* List available MCP tools
*
* @returns List of available tools
*/
async listTools(): Promise<unknown> {
return await this.sendRequest('tools/list');
}
/**
* Call an MCP tool
*
* @param name - Tool name
* @param args - Tool arguments (must be a plain object, non-null, not an array)
* @returns Tool execution result
*/
async callTool(name: string, args: object = {}): Promise<MCPToolResult> {
if (args === null || Array.isArray(args) || typeof args !== 'object') {
throw new TypeError(
'MCP tool arguments must be a plain object (non-null object, not an array or function)'
);
}
return (await this.sendRequest('tools/call', { name, arguments: args })) as MCPToolResult;
}
}
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