validate_stdout.py

#!/usr/bin/env python3
from __future__ import annotations

import argparse
import json
import sys
from pathlib import Path
from typing import TextIO


def validate_stream(handle: TextIO) -> int:
    checked = 0
    for line_number, raw in enumerate(handle, 1):
        text = raw.strip()
        if not text:
            continue
        checked += 1
        try:
            message = json.loads(text)
        except json.JSONDecodeError as exc:
            print(
                f"FAIL line {line_number}: not valid JSON: {exc.msg}; sample={text[:160]!r}",
                file=sys.stderr,
            )
            return 2
        if not isinstance(message, dict):
            print(f"FAIL line {line_number}: expected a JSON object, got {type(message).__name__}", file=sys.stderr)
            return 3
        if message.get("jsonrpc") != "2.0":
            print(
                f"FAIL line {line_number}: JSON object is not JSON-RPC 2.0; sample={text[:160]!r}",
                file=sys.stderr,
            )
            return 4
    print(f"PASS: checked {checked} non-empty stdout line(s); all are JSON-RPC 2.0 objects")
    return 0


def main() -> int:
    parser = argparse.ArgumentParser(
        description="Validate that captured MCP stdio stdout contains only JSON-RPC 2.0 objects."
    )
    parser.add_argument("path", nargs="?", help="Captured stdout file. Reads stdin when omitted.")
    args = parser.parse_args()

    if args.path:
        with Path(args.path).open("r", encoding="utf-8") as handle:
            return validate_stream(handle)
    return validate_stream(sys.stdin)


if __name__ == "__main__":
    raise SystemExit(main())