from __future__ import annotations

import sys
from pathlib import Path

_WARNED_PATHS: set[Path] = set()


def load_system_prompt(*, default: str, path: str = "SYSTEM.md") -> str:
    """Load the system prompt from disk, with a fallback default."""
    prompt_path = Path(path)
    try:
        content = prompt_path.read_text(encoding="utf-8").strip()
    except FileNotFoundError:
        _warn_once(prompt_path, "not found")
        return default
    except OSError as exc:  # pragma: no cover - environment specific
        _warn_once(prompt_path, f"unreadable ({exc})")
        return default

    if not content:
        _warn_once(prompt_path, "empty")
        return default
    return content


def _warn_once(path: Path, reason: str) -> None:
    if path in _WARNED_PATHS:
        return
    _WARNED_PATHS.add(path)
    print(
        f"[Prompt] {path} is {reason}; using built-in default prompt.",
        file=sys.stderr,
    )