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* @module mcp-client/transport/jsonrpc
* @description JSON-RPC 2.0 transport base class for the MCP client stack.
*
* `MCPTransportClient` owns the wire-level orchestration:
* - HTTP POST via `performPost` (fetch + Node.js fallback)
* - JSON-RPC 2.0 `tools/call` envelope (built in `./request-builder.ts`)
* - Response parsing + gateway payload dereferencing (`./response-parser.ts`)
* - JSON-RPC error envelope classification (`./error-envelope.ts`)
* - Exponential-backoff retry on transient transport errors (`./retry.ts`)
* - Lazy session re-init on session-init / rate-limit errors (`./session.ts`)
* - Statistics tracking (`requests`, `errors`, `successRate`)
*
* The orchestrating `MCPClient` in `../client.ts` extends this base and wires
* per-domain method wrappers on top.
*
* @author Hack23 AB
* @license Apache-2.0
*/
import type {
MCPClientConfig,
MCPStats,
JsonRpcResponse,
} from '../../types/mcp.js';
import { performPost } from '../transport.js';
import { DEFAULT_MAX_RETRIES, DEFAULT_MCP_SERVER_URL, getDefaultTimeout } from '../config/defaults.js';
import { DEFAULT_MCP_AUTH_TOKEN } from '../config/auth.js';
import { calculateRetryDelay, isRetryableNetworkError } from './retry.js';
import { initializeSession, parseSSEResponse, type SessionInitContext } from './session.js';
import {
assertValidToolName,
buildJsonRpcRequest,
buildRequestHeaders,
nextJsonRpcId,
} from './request-builder.js';
import { parseJsonRpcEnvelope, resolveResultContent } from './response-parser.js';
import { classifyJsonRpcError, formatRequestFailure } from './error-envelope.js';
/**
* Wire-level MCP transport. Domain-agnostic JSON-RPC 2.0 client with
* retry, SSE parsing, and lazy session bootstrap.
*/
export class MCPTransportClient implements SessionInitContext {
readonly baseURL: string;
readonly timeout: number;
readonly maxRetries: number;
readonly customHeaders: Readonly<Record<string, string>>;
readonly authToken: string;
requestCount: number;
errorCount: number;
sessionId: string | null;
constructor(config: MCPClientConfig | string = {}) {
if (typeof config === 'string') {
this.baseURL = config;
this.timeout = getDefaultTimeout();
this.maxRetries = DEFAULT_MAX_RETRIES;
this.customHeaders = {};
this.authToken = DEFAULT_MCP_AUTH_TOKEN;
} else {
this.baseURL = config.baseURL ?? config.serverUrl ?? DEFAULT_MCP_SERVER_URL;
this.timeout = config.timeout ?? getDefaultTimeout();
this.maxRetries = config.maxRetries ?? DEFAULT_MAX_RETRIES;
this.customHeaders = config.headers ?? {};
this.authToken = config.authToken ?? DEFAULT_MCP_AUTH_TOKEN;
}
this.requestCount = 0;
this.errorCount = 0;
this.sessionId = null;
}
/** SessionInitContext glue — returns the next monotonic JSON-RPC id. */
nextJsonRpcId(): number {
return nextJsonRpcId();
}
// -----------------------------------------------------------------------
// Core request
// -----------------------------------------------------------------------
async request(
tool: string,
params: Record<string, unknown> = {},
retryCount = 0,
_skipPrefix = false,
): Promise<Record<string, unknown>> {
assertValidToolName(tool);
if (retryCount === 0) {
this.requestCount++;
}
const controller = new AbortController();
const timeoutId = setTimeout(() => controller.abort(), this.timeout);
try {
const jsonRpcRequest = buildJsonRpcRequest(tool, params);
if (this.authToken && !this.sessionId) {
try {
await this.initializeSession();
} catch {
// Session init is optional
}
}
const headers = buildRequestHeaders(this.customHeaders, this.authToken, this.sessionId);
const response = await performPost(
this.baseURL,
headers,
JSON.stringify(jsonRpcRequest),
controller.signal,
);
if (!response.ok) {
let errorBody = '';
try { errorBody = await response.text(); } catch { /* ignore */ }
throw new Error(
`MCP server error: ${response.status} ${response.statusText}${errorBody ? ' - ' + errorBody : ''}`,
);
}
const jsonRpcResponse: JsonRpcResponse = await parseJsonRpcEnvelope(response);
const errorOutcome = classifyJsonRpcError(jsonRpcResponse);
Iif (errorOutcome.kind === 'session_init' && retryCount < 2) {
this.sessionId = null;
const delay = (retryCount + 1) * 2000;
console.warn(`⚠️ Session error, re-initializing after ${delay}ms...`);
await this.sleep(delay);
await this.initializeSession();
return this.request(tool, params, retryCount + 1);
}
if (errorOutcome.kind !== 'none') {
throw new Error(errorOutcome.message);
}
const result = (jsonRpcResponse.result ?? {}) as Record<string, unknown>;
return resolveResultContent(result);
} catch (error: unknown) {
const err = error as Error;
if (retryCount < this.maxRetries - 1 && isRetryableNetworkError(err)) {
const delay = calculateRetryDelay(retryCount);
console.warn(
`⚠️ Request failed (${err.message.substring(0, 60)}), retrying after ${delay}ms (${retryCount + 1}/${this.maxRetries - 1})...`,
);
this.sessionId = null;
await this.sleep(delay);
return this.request(tool, params, retryCount + 1);
}
this.errorCount++;
throw new Error(formatRequestFailure(err, this.baseURL), { cause: error });
} finally {
clearTimeout(timeoutId);
}
}
// -----------------------------------------------------------------------
// Session / utility delegators
// -----------------------------------------------------------------------
async sleep(ms: number): Promise<void> {
return new Promise((resolve) => setTimeout(resolve, ms));
}
parseSSEResponse(text: string): JsonRpcResponse {
return parseSSEResponse(text);
}
async initializeSession(): Promise<void> {
return initializeSession(this);
}
// -----------------------------------------------------------------------
// Statistics
// -----------------------------------------------------------------------
getStats(): MCPStats {
return {
requests: this.requestCount,
errors: this.errorCount,
successRate:
this.requestCount > 0
? Math.round(((this.requestCount - this.errorCount) / this.requestCount) * 100) + '%'
: '0%',
};
}
resetStats(): void {
this.requestCount = 0;
this.errorCount = 0;
}
}
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