/**
* JSON-RPC line parser for MCP protocol
* Messages are newline-delimited JSON over stdio
*/
import type { JsonRpcMessage, LineBuffer } from './types.js';
export type ParseResult =
| { type: 'single'; message: JsonRpcMessage }
| { type: 'batch'; messages: JsonRpcMessage[] }
| null;
/**
* Parse a single line as JSON-RPC message or batch
* Returns null if the line is empty or not valid JSON-RPC
*/
export function parseJsonRpcLine(line: string): JsonRpcMessage | null {
const result = parseJsonRpcLineEx(line);
if (result?.type === 'single') return result.message;
return null;
}
/**
* Extended parser that also detects JSON-RPC batch arrays.
* The MCP spec (2025-03-26) allows batch messages: arrays of JSON-RPC objects.
*/
export function parseJsonRpcLineEx(line: string): ParseResult {
const trimmed = line.trim();
if (!trimmed) {
return null;
}
try {
const parsed = JSON.parse(trimmed);
// Batch message: JSON array of JSON-RPC objects
if (Array.isArray(parsed)) {
const messages: JsonRpcMessage[] = [];
for (const item of parsed) {
if (item && typeof item === 'object' && item.jsonrpc === '2.0') {
messages.push(item as JsonRpcMessage);
}
}
if (messages.length > 0) {
return { type: 'batch', messages };
}
return null;
}
if (!parsed || typeof parsed !== 'object' || parsed.jsonrpc !== '2.0') {
return null;
}
return { type: 'single', message: parsed as JsonRpcMessage };
} catch {
return null;
}
}
/**
* Create a line buffer that accumulates chunks and emits complete lines
* Handles partial lines that arrive across multiple chunks
*/
const MAX_LINE_LENGTH = 10 * 1024 * 1024; // 10MB — reject lines larger than this
export function createLineBuffer(onLine: (line: string) => void): LineBuffer {
let buffer = '';
return {
push(chunk: string): void {
buffer += chunk;
// Prevent OOM from missing newlines
if (buffer.length > MAX_LINE_LENGTH && !buffer.includes('\n')) {
process.stderr.write(`[mcpwall] Warning: discarding oversized message (${buffer.length} bytes)\n`);
buffer = '';
return;
}
const lines = buffer.split('\n');
buffer = lines.pop() || '';
for (const line of lines) {
if (line) {
onLine(line);
}
}
},
flush(): void {
if (buffer.trim()) {
onLine(buffer);
buffer = '';
}
}
};
}
