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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;
/** Maximum reconnect back-off delay in milliseconds */
const RECONNECT_MAX_DELAY_MS = 30000;
/** HTTP header for API rate-limit retry delay */
const RETRY_AFTER_HEADER = 'X-Retry-After';
/** Log prefix for rate-limit warnings */
const RATE_LIMIT_MSG = 'Rate limited. Retry after';
/**
* Typed error thrown when the MCP gateway returns 401 (session expired).
* Callers can detect this with `instanceof MCPSessionExpiredError` to trigger re-authentication.
*/
export class MCPSessionExpiredError extends Error {
constructor(statusText: string) {
super(`MCP session expired (401): ${statusText}`);
this.name = 'MCPSessionExpiredError';
}
}
/**
* Typed error thrown when the MCP gateway returns 429 (rate limited).
* Carries the parsed `retryAfterMs` delay so callers can honour the back-off period.
* `retryAfterMs` is 0 when no Retry-After / X-Retry-After header was present.
*/
export class MCPRateLimitError extends Error {
readonly retryAfterMs: number;
constructor(retryAfterMs: number, message: string) {
super(message);
this.name = 'MCPRateLimitError';
this.retryAfterMs = retryAfterMs;
}
}
/**
* Parse a `Retry-After` or `X-Retry-After` header value into milliseconds.
* Accepts delta-seconds ("30"), numeric-with-suffix ("30s"), or an HTTP-date string.
* Returns 0 when the value is empty or unparseable.
*
* @param retryAfter - Raw Retry-After / X-Retry-After header value
* @returns Delay in milliseconds, or 0 if the value cannot be parsed
*/
function parseRetryAfterMs(retryAfter: string): number {
const normalized = retryAfter.trim().replace(/s$/i, '');
Iif (!normalized) return 0;
const numericDelay = Number(normalized);
if (!Number.isNaN(numericDelay)) return numericDelay * 1000;
const retryDate = new Date(retryAfter);
Eif (!Number.isNaN(retryDate.getTime())) {
return Math.max(0, retryDate.getTime() - Date.now());
}
return 0;
}
/**
* Returns true only for transient, retriable failures: request timeouts,
* network-level connection-closed/reset errors, and "not connected" states.
*
* Uses an allow-list of known transient error patterns so that unknown or
* server-level errors (e.g., tool runtime failures) are NOT retried:
* - timeout — AbortSignal timeout or custom timeout message
* - connection closed / reset / refused — network-level transport failures
* - not connected — local "not yet connected" guard error
* - socket hang up — Node.js HTTP socket-level disconnection
*
* Everything else (MCPSessionExpiredError, TypeError, rate-limit errors,
* unknown errors) returns false so `callToolWithRetry` surfaces them immediately.
*
* @param error - The caught error to classify
* @returns `true` if the error is safe to retry
*/
export function isRetriableError(error: Error): boolean {
// Never retry session expiry or programmer errors — these require intervention
if (error instanceof MCPSessionExpiredError || error instanceof TypeError) {
return false;
}
const msg = error.message?.toLowerCase() ?? '';
// Never retry rate-limit errors — callers must honour the Retry-After delay.
// `instanceof MCPRateLimitError` is the primary guard for typed errors;
// the string prefix fallback handles any legacy plain Error with a rate-limit message.
if (error instanceof MCPRateLimitError || msg.startsWith(RATE_LIMIT_MSG.toLowerCase())) {
return false;
}
return (
msg.includes('timeout') ||
msg.includes('connection closed') ||
msg.includes('connection reset') ||
msg.includes('not connected') ||
msg.includes('econnreset') ||
msg.includes('econnrefused') ||
msg.includes('socket hang up')
);
}
/**
* Parse a `Retry-After` or `X-Retry-After` header value (which may be either a
* delay-in-seconds number, a numeric string with an optional trailing "s" suffix
* (e.g. "30s"), or an HTTP-date string) into a human-readable message.
*
* @param retryAfter - Raw header value
* @returns Formatted string describing the delay (e.g. "30s" or "45s (until Thu, 01 Jan 2026 …)")
*/
export function formatRetryAfter(retryAfter: string): string {
// Accept both bare numbers ("30") and numeric-with-suffix ("30s")
const normalized = retryAfter.trim().replace(/s$/i, '');
if (!normalized) {
return retryAfter;
}
const numericDelay = Number(normalized);
if (!Number.isNaN(numericDelay)) {
return `${numericDelay}s`;
}
const retryDate = new Date(retryAfter);
if (Number.isNaN(retryDate.getTime())) {
return retryAfter;
}
const delayMs = retryDate.getTime() - Date.now();
if (delayMs > 0) {
const delaySeconds = Math.ceil(delayMs / 1000);
return `${delaySeconds}s (until ${retryDate.toUTCString()})`;
}
return retryDate.toUTCString();
}
/**
* 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;
private maxRetries: number;
private reconnectingPromise: Promise<void> | null;
private timeoutCount: number;
private reconnectCount: 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 = Math.max(1, options.maxConnectionAttempts ?? 3);
this.connectionRetryDelay = options.connectionRetryDelay ?? 1000;
this.maxRetries = options.maxRetries ?? 2;
this.reconnectingPromise = null;
this.timeoutCount = 0;
this.reconnectCount = 0;
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;
}
/**
* Get connection health metrics for telemetry
*
* @returns Object with timeout count, reconnection count, and current connection status
*/
getConnectionHealth(): { timeoutCount: number; reconnectCount: number; connected: boolean } {
return {
timeoutCount: this.timeoutCount,
reconnectCount: this.reconnectCount,
connected: this.connected,
};
}
/**
* Compute the delay before the next connection attempt.
* Respects `Retry-After` carried by {@link MCPRateLimitError}; otherwise uses
* exponential back-off (`connectionRetryDelay * 2^(attempt - 1)`).
*
* @param error - The error from the failed attempt
* @param attempt - Number of attempts made so far (1-indexed)
* @returns Delay in milliseconds
*/
private _computeConnectionDelay(error: unknown, attempt: number): number {
Eif (error instanceof MCPRateLimitError && error.retryAfterMs > 0) {
return error.retryAfterMs;
}
return this.connectionRetryDelay * Math.pow(2, attempt - 1);
}
/**
* Handle a single connection attempt error: re-throw immediately for non-retriable errors
* (e.g. session expiry), increment the attempt counter, and return the delay to wait
* before the next attempt. Throws when the maximum attempts have been exhausted.
*
* @param error - The error from the failed attempt
* @returns Delay in milliseconds to wait before the next attempt
*/
private _handleConnectionAttemptError(error: unknown): number {
if (error instanceof MCPSessionExpiredError) {
throw error;
}
this.connectionAttempts++;
if (this.connectionAttempts >= this.maxConnectionAttempts) {
console.error(
'❌ Failed to connect to MCP server after',
this.maxConnectionAttempts,
'attempts'
);
throw error;
}
return this._computeConnectionDelay(error, this.connectionAttempts);
}
/**
* 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) {
const delay = this._handleConnectionAttemptError(error);
console.warn(
`⚠️ Connection attempt ${this.connectionAttempts} failed. Retrying in ${delay}ms...`
);
await new Promise((resolve) => setTimeout(resolve, delay));
}
}
}
/**
* 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;
}
}
}
/**
* Build the Authorization header value for gateway requests.
* Keys that already contain a valid RFC 7235 scheme token followed by
* whitespace (e.g. "Bearer …", "Token …", "AWS4-HMAC-SHA256 …") are passed
* through unchanged. Otherwise the raw key is sent directly unless
* EP_MCP_GATEWAY_AUTH_SCHEME is set to a valid token, in which case that
* scheme prefix is prepended. The EP MCP gateway expects raw-token auth by
* default (no "Bearer " prefix).
*
* @param apiKey - Raw or pre-prefixed gateway API key
* @returns The full Authorization header value, or empty string for empty keys
* @throws {Error} When the API key contains CR or LF characters (header injection risk)
*/
private _buildAuthorizationHeader(apiKey: string): string {
const trimmedKey = apiKey.trim();
if (!trimmedKey) {
return '';
}
// Reject CR/LF in the key to prevent HTTP header injection.
if (/[\r\n]/.test(trimmedKey)) {
throw new Error(
'Invalid gateway API key: control characters (CR/LF) are not allowed in Authorization header values.'
);
}
// RFC 7235 tchar token pattern for scheme validation.
// This regex exclusively allows valid tchar characters, which by definition
// excludes control characters — no separate control-char check is needed
// for the scheme token itself.
const tokenRegex = /^[!#$%&'*+\-.^_`|~0-9A-Za-z]+$/;
// If the key already starts with a valid RFC 7235 scheme token followed
// by whitespace, treat it as a fully formed Authorization value and pass
// it through unchanged.
const firstSpaceIndex = trimmedKey.indexOf(' ');
if (firstSpaceIndex > 0) {
const possibleScheme = trimmedKey.slice(0, firstSpaceIndex);
Eif (tokenRegex.test(possibleScheme)) {
return trimmedKey;
}
}
const rawScheme =
typeof process !== 'undefined' && process.env && process.env['EP_MCP_GATEWAY_AUTH_SCHEME'];
const scheme = typeof rawScheme === 'string' ? rawScheme.trim() : '';
if (scheme && tokenRegex.test(scheme)) {
return `${scheme} ${trimmedKey}`;
}
return trimmedKey;
}
/**
* 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'] = this._buildAuthorizationHeader(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),
});
if (!response.ok) {
this._throwGatewayResponseError(response);
}
const sessionId = response.headers.get('mcp-session-id');
if (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);
}
}
/**
* Throw an appropriate error for a non-OK gateway response.
* Extracted to keep `_sendGatewayRequest` within cognitive-complexity limits.
*
* @param response - The non-OK fetch Response
*/
private _throwGatewayResponseError(response: Response): never {
if (response.status === 401) {
this.mcpSessionId = null;
this.connected = false;
throw new MCPSessionExpiredError(response.statusText);
}
if (response.status === 429) {
const rawRetryAfter =
response.headers.get(RETRY_AFTER_HEADER) ?? response.headers.get('Retry-After');
const retryAfter = (rawRetryAfter ?? '').trim();
if (retryAfter !== '') {
const retryMessage = formatRetryAfter(retryAfter);
const retryAfterMs = parseRetryAfterMs(retryAfter);
console.warn(`⏳ ${RATE_LIMIT_MSG} ${retryMessage}`);
throw new MCPRateLimitError(retryAfterMs, `${RATE_LIMIT_MSG} ${retryMessage}`);
}
const statusText = response.statusText || 'Too Many Requests';
throw new MCPRateLimitError(
0,
`${RATE_LIMIT_MSG} (status ${response.status} ${statusText}; ${RETRY_AFTER_HEADER}/Retry-After header missing)`
);
}
throw new Error(`Gateway error ${response.status}: ${response.statusText}`);
}
/**
* 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',
};
Iif (this.gatewayApiKey) {
headers['Authorization'] = this._buildAuthorizationHeader(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),
});
Eif (!response.ok) {
this._throwGatewayResponseError(response);
}
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;
}
/**
* Attempt to reconnect to the MCP server with exponential back-off.
* Concurrent callers await the same in-flight reconnect instead of no-oping,
* ensuring the connection is re-established before all waiting callers continue.
*
* @returns Promise that resolves when reconnection succeeds or all attempts are exhausted
*/
private async reconnect(): Promise<void> {
if (this.reconnectingPromise !== null) {
return this.reconnectingPromise;
}
this.reconnectCount++;
console.log(`🔄 Reconnecting to ${this.serverLabel} (attempt ${this.reconnectCount})...`);
this.reconnectingPromise = this._doReconnect();
try {
await this.reconnectingPromise;
} finally {
this.reconnectingPromise = null;
}
}
/**
* Internal reconnect helper.
*
* Waits for an exponential back-off delay derived from the current
* `reconnectCount`, then delegates to `connect()` which handles its own
* retry loop. This avoids composing N×N attempts.
*
* @returns Promise that resolves when reconnection succeeds or logs on failure
*/
private async _doReconnect(): Promise<void> {
// Derive a single outer back-off delay from reconnectCount so successive
// reconnect bursts are spaced further apart, capped at RECONNECT_MAX_DELAY_MS.
// Normalize maxConnectionAttempts to ≥1 to avoid a negative upper bound when
// the user configures 0 attempts (which would give 2^-1 = 0.5 s backoff).
const normalizedMaxAttempts = Math.max(1, this.maxConnectionAttempts);
// Clamp to [0, normalizedMaxAttempts - 1]: first floor to ≥0, then ceil to ≤max.
const attemptIndex = Math.min(Math.max(0, this.reconnectCount - 1), normalizedMaxAttempts - 1);
const delay = Math.min(
this.connectionRetryDelay * Math.pow(2, attemptIndex),
RECONNECT_MAX_DELAY_MS
);
await new Promise((r) => setTimeout(r, delay));
try {
this.connected = false;
await this.connect();
} catch (error) {
console.error(
`❌ Reconnection to ${this.serverLabel} failed: ${
error instanceof Error ? error.message : String(error)
}`
);
}
}
/**
* Log a retry warning and, if disconnected, attempt to reconnect before waiting.
*
* @param lastError - The error from the failed attempt
* @param attempt - Zero-based current attempt index
* @param retries - Total retry count
* @returns Promise that resolves after logging, optional reconnect, and inter-retry delay
*/
private async _handleRetryAttempt(
lastError: Error,
attempt: number,
retries: number
): Promise<void> {
if (lastError.message.toLowerCase().includes('timeout')) {
this.timeoutCount++;
console.warn(
`⏱️ Request timeout (total: ${this.timeoutCount}), retrying ${attempt + 1}/${retries}...`
);
} else {
console.warn(`⚠️ Request failed, retrying ${attempt + 1}/${retries}: ${lastError.message}`);
}
if (!this.connected) {
await this.reconnect();
}
await new Promise((r) => setTimeout(r, this.connectionRetryDelay * (attempt + 1)));
}
/**
* Call an MCP tool with automatic retry on timeout or connection loss.
* Reconnects automatically if the connection was lost between attempts.
*
* Only transient failures are retried (see `isRetriableError`). Non-retriable
* errors — rate-limit (429), session-expired (401), and programmer errors such
* as `TypeError` — are re-thrown immediately without consuming any retry budget.
*
* @param name - Tool name
* @param args - Tool arguments (plain object, non-null, not an array)
* @param maxRetries - Override the default retry count from options
* @returns Tool execution result
*/
async callToolWithRetry(
name: string,
args: object = {},
maxRetries?: number
): Promise<MCPToolResult> {
const retries = maxRetries ?? this.maxRetries;
if (retries < 0) {
throw new RangeError(`maxRetries must be >= 0, received ${retries}`);
}
let lastError: Error = new Error(`Failed to call tool '${name}' after ${retries} retries`);
for (let attempt = 0; attempt <= retries; attempt++) {
try {
return await this.callTool(name, args);
} catch (error) {
lastError = error instanceof Error ? error : new Error(String(error));
if (!isRetriableError(lastError)) throw lastError;
if (attempt === retries) break;
await this._handleRetryAttempt(lastError, attempt, retries);
}
}
throw lastError;
}
}
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