turnstone/examples/door-game/understone/server.py

"""MCP server for Understone — the only module that imports ``mcp`` (or ``starlette``).

Nine ``door_*`` tools form the entire player interface. Every handler is a
synchronous ``def`` that takes and returns ``str``; no exception is allowed
to cross the MCP boundary (each handler catches, logs server-side, and
returns an in-fiction line). The handlers are thin wrappers over a single
module-level :class:`~understone.game.Game`; all rules live behind that
façade.

Three extra HTTP routes (``/watch`` and its two JSON feeds) serve the
read-only spectator page from :mod:`understone.watch`. They are registered via
FastMCP's ``custom_route`` and ride inside the streamable-http app; the
``starlette`` request/response types appear ONLY here, mirroring the MCP SDK's
own ``custom_route`` examples. The routes are unauthenticated by design and
strictly read-only — they never mutate or persist world state.

Usage::

    understone                 # via entry point (stdio transport)
    python -m understone       # via module
    understone validate PATH   # check a content pack loads cleanly
    understone newpack PATH    # scaffold a new pack + authoring manual
    understone worlds          # list the bundled worlds and their soundness

Environment variables
---------------------
UNDERSTONE_DB         SQLite path (default: ./understone.db)
UNDERSTONE_WORLD      Content-pack directory (default: packaged world/data)
UNDERSTONE_TRANSPORT  "stdio" (default) or "streamable-http"
UNDERSTONE_HOST       Bind host for http transport (default: 127.0.0.1)
UNDERSTONE_PORT       Bind port for http transport (default: 8077)
UNDERSTONE_PATH       HTTP path for the MCP endpoint (default: /mcp)
"""

from __future__ import annotations

import argparse
import logging
import os
from pathlib import Path
from typing import TYPE_CHECKING

from mcp.server.fastmcp import FastMCP
from mcp.server.transport_security import TransportSecuritySettings
from starlette.responses import HTMLResponse, JSONResponse, Response

from understone import cli, sim, watch
from understone.errors import WorldLoadError
from understone.game import Game
from understone.persistence import Store
from understone.world import PACKAGED_WORLD_DIR
from understone.world.loader import load_world

if TYPE_CHECKING:
    from starlette.applications import Starlette
    from starlette.requests import Request

log = logging.getLogger(__name__)

_PREMISE = (
    "Understone is a shared-world BBS door game played through these tools. "
    "Call door_help first to learn how to narrate it."
)

# The DM manual returned by door_help — a module constant so it is stable and
# greppable. It teaches an assistant how to run the game responsibly.
_DM_MANUAL = """\
UNDERSTONE — A GUIDE FOR THE GAME MASTER

WHAT THIS IS
  Understone is a multiplayer, BBS-style ANSI door game — a small text RPG in
  the lineage of the classic BBS door games. Many players share ONE
  persistent world hosted by this server. You are the storyteller at the
  terminal; the server is the rules engine and the single source of truth.

THE GOLDEN RULE
  The server is authoritative. Never invent dice rolls, loot, gold, hit
  points, map tiles, or outcomes. Every number and event comes back from a
  tool call. Narrate AROUND the facts the tools return — never ahead of them.
  If you want something to happen, call the tool and see what the world says.

THE TWO MODES OF PLAY
  1. The overworld (TILE mode). Tools return an ASCII "keyframe": a bordered
     map window centred on the player. '@' is the player, '☻' is another
     adventurer, glyphs are buildings (⌂ inn, $ shop, ✚ healer, ∩ dungeon).
     Movement here is FREE — it costs no daily turns.
  2. Location interiors (MENU mode). Stepping onto a building opens a menu of
     options like (R)est, (B)uy, (H)eal, (D)escend, (L)eave.

PRESENTING FRAMES
  When a tool returns a map or a menu, show it to the player VERBATIM inside a
  fenced code block so the box-drawing lines stay aligned. Then add your prose
  underneath. Do not redraw or paraphrase the frame.

NARRATION
  Be vivid and in-fiction. Turn the terse result lines ("You travel 3 steps.",
  "+8 XP, +3 gold.") into atmosphere. Keep your additions consistent with the
  returned facts and the high-fantasy tone of the Vale of Understone.

THE DAILY RHYTHM
  Each adventurer has a small budget of turns per real-world UTC day. Only
  fighting, descending, and challenging the Wyrm spend a turn; moving,
  resting, shopping and looking do not. When the budget is gone, the day is
  done — encourage the player to return tomorrow. This is a correspondence
  game: a little each day.

WANDERING THE FOREST (the texture of a walk)
  A step through wild country may turn up more than a monster. The server
  rolls a private encounter table as the player walks: most often a foe (which
  stops the walk for a fight or flight), but sometimes a purse of gold, a
  healing spring, a small trap (it can never kill — it floors at 1 HP), or a
  scrap of Vale lore. The non-combat finds are applied at once, narrated in
  the move result, and do NOT stop the walk; at most one such event happens
  per move. These finds are PRIVATE — they are not Herald news — so narrate
  them as the quiet texture of travelling, and watch the lore: it whispers of
  something coiled beneath the dungeon.

DELVING DEEP (the reasons to come back)
  Beneath the daily reset are four standing draws that reward a returning hero.
  * THE RUNG LADDER. The dungeon is a ladder of guardians fought one rung per
    'descend' (each costs a daily turn). A descent faces the NEXT rung past your
    deepest; a win advances your depth and you climb back out, a loss bounces
    you home but your depth PERSISTS — you re-enter where you left off. Reaching
    the last rung opens the Wyrm's door (the depth gate above). Narrate the deep
    as a slow, earned descent, a rung at a time.
  * THE SATCHEL AND THE DEATH-SAVE. A small satchel carries a few potions
    (buying one at the shop now STOWS it instead of drinking it). 'quaff'
    (anywhere, no turn) drinks the strongest. The heart of it: if a fight would
    KILL the active fighter and they carry a potion, the strongest is drunk
    AUTOMATICALLY — they survive standing at the potion's value, no bounce, no
    spawn reset. Play that beat big: the elixir burning down their throat at the
    edge of death. (A sleeping ambush victim never auto-quaffs — they are
    asleep.)
  * THE FORGE — GOLD AND ORE. The shop's forge adds a +1 edge to the equipped
    weapon or armour ('forge' target="weapon"/"armour"), up to a cap, each tier
    dearer than the last. A step costs gold AND forge ore — a material the hero
    EARNS in combat, never buys: every cleared dungeon rung drops some, and a won
    forest fight sometimes turns up a little. So the forge is fed by descending,
    not just by a fat purse; a hero short of ore is told so. Swapping or selling
    a forged piece loses the edge with it.
  * RARE BEASTS. A few named beasts prowl the forest, surfacing seldom. Felling
    one is loud — a public Herald flash — and it always guards a draught that
    drops into the satchel (if there is room). Treat a rare kill as a small
    legend in itself.

THE WYRM BELOW (the endgame, and how to win)
  Deep under the dungeon sleeps the Wyrm Below — a fixed, fearsome boss and
  the ONLY win condition. At the dungeon, a sufficiently seasoned hero may
  'challenge' it (door_action action="challenge"). The Wyrm gates on BOTH
  level AND depth: an under-level hero is turned away first, and even a high
  hero who has not plumbed the deep to its floor (see DELVING DEEP) is told the
  Wyrm will not stir. Once both are met, the challenge spends a daily turn and
  resolves in one call, like a fight.
    * On victory the hero FREES THE VALE. The triumph is heralded to everyone,
      the run is carved into the Hall of Legends (shown by door_rank), and the
      hero is reborn in a classic-door-game-style legacy reset: level, gold, gear and stats
      return to first-day values and they stand again at the town — but they
      keep a permanent ★ for the win, and may set out to do it all again. Their
      remaining turns for the day and their place in the world carry over.
    * On defeat the Wyrm devours them; they wake at the spawn, barely alive.
  Play this beat big: it is the climax of a whole run. Narrate the reset as the
  Vale renewing itself around an undying legend, not as a death.

BESTOWING FORTUNE (use sparingly)
  door_bestow lets you, the storyteller, grant a little gold or healing to
  mark a great story moment — a heroic rescue, a clever solution, a poignant
  death-defiance. It NEVER grants items (gear comes from the shop) and NEVER
  grants turns (the clock does not bend). It is capped by a small daily pool
  per player, and every bestowal is written to the public log for all to see.
  Treat it as seasoning, not a salt-shaker: reserve it for the rare, earned
  beat, and never promise a reward you cannot actually deliver within the cap.

THE SOCIAL LAYER (rivals, mail, and dice)
  Understone is a SHARED world, and three verbs let players touch one another.
  * AMBUSH (door_action action="ambush" target=<player>, on the overworld).
    A classic-door-game-style player-kill: you fall upon a RIVAL WHO HAS NOT YET ACTED
    TODAY and rob them. The SLEEP RULE is the heart of it — a player who has
    already taken their turn that day is awake and cannot be ambushed, so the
    surest defence is simply to play. The gatekeeper shields the young (both of
    you must clear a level floor) and only matches near-equals (a level band).
    On a win you take a slice of their gold and they wake at the spawn at 1 HP;
    a public Herald crows the deed and the victim gets a PRIVATE note. But the
    sleeper may WAKE: lose, and YOU are the one who flees bleeding, shamed on
    the feed and gaining nothing. You get one attempt per rival per day, win or
    lose. Narrate ambush as a real betrayal — and losing one as just deserts.
  * POST (door_action action="post" target=<player> text=<message>, anywhere).
    Leave a private note at the inn for another player; they read it on their
    next door_log under "While you were away". It costs no turn, is capped per
    day, and the note is PRIVATE — it never reaches the public Herald or the
    lobby TV. Good for taunts after an ambush, alliances, or a kind word.
  * GAMBLE (door_action action="gamble" amount=<gold>, at the inn).
    Wager gold on a single throw of 2d6 against the house: roll higher to
    double your stake, tie to push, roll lower to lose it. It costs no turn but
    is capped per day. A big win is heralded; a quiet one is just a story you
    tell. Remind players the house has no mercy and the odds are even at best.

THE VAULT (banking coin at the inn)
  The inn keeps a strongbox. DEPOSIT (door_action action="deposit" amount=<gold>)
  moves coin from the hero's hand into the vault; WITHDRAW (action="withdraw"
  amount=<gold>) draws it back. Neither costs a turn. Two things make the vault
  matter: banked gold is SAFE FROM AMBUSH (a sleeping-robber lifts only what the
  victim carries), so banking before logging off is the way to protect a purse;
  and banked gold SURVIVES THE WYRM-WIN RESET — it is the one wealth a reborn
  hero keeps, alongside their ★. Suggest a wary player bank their winnings.

TOOL CHEAT-SHEET
  door_help                      This manual.
  door_join(player)              Sign in (creates or resumes a character).
  door_status(player)            Read the character sheet.
  door_look(player)              Redraw the current view (map or menu).
  door_move(player, ...)         Walk the overworld (free). steps="NNEE" or
                                 heading="east" + distance=3 (max 8 per call).
  door_action(player, action)    Context verb: fight, flee, ambush (a rival),
                                 rest, deposit/withdraw (the inn vault), buy,
                                 sell, forge (a +1 edge, gold + ore), heal,
                                 gamble (dice at the inn), descend (one rung),
                                 challenge (the Wyrm), post (a note), quaff (a
                                 carried potion), leave.
  door_log(player)               Read the Understone Herald (the shared feed).
  door_rank(player)              The leaderboard + Hall of Legends (★ = wins).
  door_bestow(player, reason...) Grant a little gold/healing for a story beat.

GETTING STARTED
  Ask the player their adventurer's name, call door_join, present the opening
  keyframe, and set the scene: a small town at the western edge of a wooded
  vale, a road running east toward darker country and a dungeon mouth.
"""

_BLANK_NAME = 'The gatekeeper squints. "I didn\'t catch your name, traveller."'

# Module-level game singleton, built lazily so tests can inject their own.
_GAME: Game | None = None


def _build_game(watch_url: str | None = None) -> Game:
    """Construct the module Game from environment configuration."""
    db_path = os.environ.get("UNDERSTONE_DB", "understone.db")
    world_dir = os.environ.get("UNDERSTONE_WORLD") or str(PACKAGED_WORLD_DIR)
    world = load_world(world_dir)
    store = Store(db_path)
    return Game(world, store, watch_url=watch_url)


def _game() -> Game:
    """Return the module game, building it on first use."""
    global _GAME
    if _GAME is None:
        _GAME = _build_game()
    return _GAME


def _set_game(game: Game) -> None:
    """Install a prebuilt game (used by create_app / tests)."""
    global _GAME
    _GAME = game


def _guard_name(player: str) -> str | None:
    """Return the blank-name refusal when *player* is empty, else None."""
    return None if player.strip() else _BLANK_NAME


mcp: FastMCP = FastMCP(
    "understone",
    instructions=_PREMISE,
)


@mcp.tool()
def door_help() -> str:
    """Read the Understone game-master manual — start here.

    Returns a short guide for running this multiplayer, BBS-style ANSI door
    game (a classic text RPG / dungeon adventure): the two play modes, how
    to present the ASCII map frames, the daily-turn rhythm, and the full tool
    cheat-sheet. Call door_help before your first session to learn how to run
    the game, then call door_join to begin.
    """
    watch_line = _game().watch_line()
    if watch_line:
        return f"{_DM_MANUAL}\nTHE LOBBY TV\n  {watch_line}\n"
    return _DM_MANUAL


@mcp.tool()
def door_join(player: str) -> str:
    """Sign an adventurer into the shared world of Understone — call this first.

    Understone is a multiplayer, BBS-style ANSI door game: a text adventure /
    dungeon RPG in the spirit of the classic BBS door games, played entirely
    through these tools. This creates a new character at the town, or resumes
    an existing one by name, and returns the opening overworld map frame. New
    to running it? Call door_help before your first session.

    Args:
        player: The adventurer's name (their identity in the world).
    """
    blank = _guard_name(player)
    if blank is not None:
        return blank
    try:
        return _game().join(player)
    except Exception:
        log.exception("door_join failed for %r", player)
        return _unexpected()


@mcp.tool()
def door_status(player: str) -> str:
    """Show an adventurer's character sheet (level, HP, gear, gold, turns).

    Read-only. Use it to check progress before deciding what to do next.

    Args:
        player: The adventurer's name.
    """
    blank = _guard_name(player)
    if blank is not None:
        return blank
    try:
        return _game().status(player)
    except Exception:
        log.exception("door_status failed for %r", player)
        return _unexpected()


@mcp.tool()
def door_look(player: str) -> str:
    """Redraw what the adventurer currently sees (read-only).

    On the overworld this is an ASCII map keyframe centred on the player
    ('@' is you, '☻' are other players, glyphs are buildings). Inside a
    building it is that location's menu. Present the result verbatim in a
    fenced code block, then narrate.

    Args:
        player: The adventurer's name.
    """
    blank = _guard_name(player)
    if blank is not None:
        return blank
    try:
        return _game().look(player)
    except Exception:
        log.exception("door_look failed for %r", player)
        return _unexpected()


@mcp.tool()
def door_move(player: str, steps: str = "", heading: str = "", distance: int = 1) -> str:
    """Walk the overworld — movement is free and never costs a daily turn.

    Only valid on the overworld (in a building, use door_action 'leave'
    first). Give EITHER a compact ``steps`` string of cardinal letters such
    as "NNEE", OR a ``heading`` ("north"/"south"/"east"/"west") with a
    ``distance``. At most 8 cells move per call; the walk stops early at
    walls, water, a building door, or a wandering monster.

    Args:
        player: The adventurer's name.
        steps: Cardinal letters, e.g. "NNEE" (takes precedence if given).
        heading: A compass direction used with distance.
        distance: How many cells to travel along heading (1-8).
    """
    blank = _guard_name(player)
    if blank is not None:
        return blank
    try:
        return _game().move(player, steps, heading, distance)
    except Exception:
        log.exception("door_move failed for %r", player)
        return _unexpected()


@mcp.tool()
def door_action(
    player: str,
    action: str,
    target: str = "",
    item: str = "",
    text: str = "",
    amount: int = 0,
) -> str:
    """Take a context-sensitive action in the world.

    The legal verbs depend on where the adventurer is. On the overworld:
    'fight' or 'flee' a wandering monster (fighting spends one daily turn), or
    'ambush' a named rival who has not yet acted today — a sleeping-rival
    robbery in the spirit of the classic door-game player-kill (target=<name>, spends a turn).
    Inside a building: 'rest' (inn), 'buy'/'sell'/'forge' (shop), 'heal'
    (healer), or 'leave'. At the inn you may also 'gamble' a stake of gold at
    dice (amount=<gold>), or bank coin in the vault with 'deposit'/'withdraw'
    (amount=<gold>) — banked gold is safe from ambush and survives a Wyrm-win
    reset. Buying a potion now stows it in your satchel rather than drinking it;
    'forge' (target="weapon"/"armour", at the shop) spends gold AND forge ore
    (won in the deep) to add a +1 edge to your equipped gear, up to a cap. At
    the dungeon:
    'descend' ONE rung of the deep — the next guardian past your deepest, one
    per turn — or 'challenge' the Wyrm Below, the endgame boss and the only way
    to win. Descending a rung advances your depth; reaching the floor opens the
    Wyrm's door. The challenge is gated by BOTH level and depth (you must have
    plumbed the deep to its floor) and, once allowed, spends a daily turn and
    resolves in a single call like a fight: a victory frees the Vale and begins
    a new life (see door_help), a defeat bounces you home. Anywhere and at no
    turn cost: 'post' leaves a private note for another player (target=<name>,
    text=<message>) read on their next door_log, and 'quaff' drinks the
    strongest potion from your satchel. An illegal verb returns the verbs valid
    right here.

    Args:
        player: The adventurer's name.
        action: The verb to attempt (fight, flee, ambush, rest, deposit,
            withdraw, buy, sell, forge, heal, gamble, descend, challenge, post,
            quaff, leave).
        target: The other player's name for 'ambush'/'post', or the slot
            ("weapon"/"armour") for 'forge'.
        item: For shop 'buy', the item id to purchase.
        text: For 'post', the note left for the target (<= 120 characters).
        amount: For inn 'gamble', the gold wagered on the dice; for the vault
            'deposit'/'withdraw', the gold moved.
    """
    blank = _guard_name(player)
    if blank is not None:
        return blank
    try:
        return _game().action(player, action, target, item, text, amount)
    except Exception:
        log.exception("door_action failed for %r action=%r", player, action)
        return _unexpected()


@mcp.tool()
def door_log(player: str) -> str:
    """Catch up on what happened in the shared world while the player was away.

    Returns the events since this adventurer last checked (fights, deaths,
    blessings, descents by anyone) and advances their personal marker.

    Args:
        player: The adventurer's name.
    """
    blank = _guard_name(player)
    if blank is not None:
        return blank
    try:
        return _game().log(player)
    except Exception:
        log.exception("door_log failed for %r", player)
        return _unexpected()


@mcp.tool()
def door_rank(player: str = "") -> str:
    """Show the Roll of Heroes — the top-ten leaderboard.

    Ordered by level, then experience, then name. If the caller names
    themselves and they place in the top ten, their row is marked. Each ★
    beside a name is one slaying of the Wyrm Below. Below the table, the Hall
    of Legends lists the most recent completed runs (name, level at the kill,
    days the run took, date); it is omitted while no one has yet won.

    Args:
        player: The caller's name (optional; marks their row when present).
    """
    try:
        return _game().rank(player)
    except Exception:
        log.exception("door_rank failed for %r", player)
        return _unexpected()


@mcp.tool()
def door_bestow(player: str, reason: str, gold: int = 0, heal: int = 0) -> str:
    """Grant a small gift of gold or healing to mark a great story moment.

    This is the game master's discretionary channel, to be used SPARINGLY for
    earned, story-driven beats. It grants only gold and/or healing — never
    items (gear comes from the shop) and never turns (the daily clock does not
    bend). Each grant is capped by a small daily pool per adventurer and is
    written to the public log, so spend it on the rare moment that deserves
    it; do not promise more than the cap allows. When a pack sets healing to
    cost nothing, a bestowed heal is free and draws nothing from the pool.

    Args:
        player: The adventurer receiving the gift.
        reason: A short, in-fiction reason (<= 120 characters).
        gold: Gold to grant (>= 0).
        heal: HP to restore (>= 0); only the missing portion is applied and
            charged against the pool.
    """
    blank = _guard_name(player)
    if blank is not None:
        return blank
    try:
        return _game().bestow(player, reason, gold, heal)
    except Exception:
        log.exception("door_bestow failed for %r", player)
        return _unexpected()


# FastMCP.custom_route has no return annotation upstream (mcp 1.27.2), so mypy
# reads the decorator as untyped; the ignore is scoped to that single gap.
@mcp.custom_route("/watch", methods=["GET"])  # type: ignore[untyped-decorator]
async def watch_page(_request: Request) -> Response:
    """Serve the read-only CRT spectator page (static HTML, no world reads)."""
    return HTMLResponse(watch.WATCH_HTML)


@mcp.custom_route("/watch/world.json", methods=["GET"])  # type: ignore[untyped-decorator]
async def watch_world(_request: Request) -> Response:
    """Serve the STATIC map payload (dimensions, coloured rows, locations)."""
    return JSONResponse(watch.build_world_payload(_game().world))


@mcp.custom_route("/watch/state.json", methods=["GET"])  # type: ignore[untyped-decorator]
async def watch_state(_request: Request) -> Response:
    """Serve the DYNAMIC snapshot (players, Herald, Hall) — read-only.

    The builder reads the module Game with no ``await`` in between, so each
    response is a consistent point-in-time snapshot of the shared world.
    """
    return JSONResponse(watch.build_state_payload(_game()))


def _unexpected() -> str:
    """In-fiction line for an unexpected server-side error."""
    return (
        "A strange fog rolls through the Vale and the moment slips away. "
        "(Something went wrong; try again.)"
    )


def create_app(
    db_path: str, world_dir: str | None = None, watch_url: str | None = None
) -> Starlette:
    """Build the streamable-HTTP ASGI app backed by a fresh game.

    Used by both ``main`` (for the http transport) and the integration tests,
    so tests can point at a temp DB without environment juggling. ``watch_url``,
    when given, is the spectator page URL the join banner and help manual
    advertise; ``main`` derives it from the bind host/port.
    """
    world = load_world(world_dir or str(PACKAGED_WORLD_DIR))
    store = Store(db_path)
    _set_game(Game(world, store, watch_url=watch_url))
    return mcp.streamable_http_app()


def _serve() -> None:
    """Serve the Understone MCP world over the configured transport.

    Honours UNDERSTONE_TRANSPORT: "stdio" (default) or "streamable-http".
    For http, host/port/path are read from the environment and applied to the
    FastMCP settings before serving. This is the actual transport launch; it is
    kept separate from argument parsing so the parse step has no side effects.
    """
    logging.basicConfig(level=logging.INFO)
    transport = os.environ.get("UNDERSTONE_TRANSPORT", "stdio")

    if transport == "streamable-http":
        host = os.environ.get("UNDERSTONE_HOST", "127.0.0.1")
        port = int(os.environ.get("UNDERSTONE_PORT", "8077"))
        mcp.settings.host = host
        mcp.settings.port = port
        mcp.settings.streamable_http_path = os.environ.get("UNDERSTONE_PATH", "/mcp")
        mcp.settings.stateless_http = False
        # FastMCP freezes DNS-rebinding protection (a localhost-only Host
        # allowlist) at CONSTRUCTION, and this module builds `mcp` at import
        # with the default 127.0.0.1 host — so a 0.0.0.0/LAN bind would 421
        # "Invalid Host" on /mcp for every remote node (the multi-node case).
        # Binding off localhost means we intend to accept other hosts, so drop
        # the allowlist here (matching the SDK's own default for a non-localhost
        # bind). These routes are unauthenticated by design — serve only on a
        # trusted network. UNDERSTONE_HOST controls the bind.
        if host not in ("127.0.0.1", "localhost", "::1"):
            mcp.settings.transport_security = TransportSecuritySettings(
                enable_dns_rebinding_protection=False
            )
        # The spectator page is only reachable over http, so its URL is composed
        # here from the bind address. A 0.0.0.0 bind should advertise a host a
        # browser can actually reach (see the README Watch section).
        watch_url = f"http://{host}:{port}/watch"
        # Build the game eagerly (with the watch URL) so a config error surfaces
        # before serving and the join/help advertisements carry the page link.
        try:
            _set_game(_build_game(watch_url))
        except WorldLoadError as exc:
            raise SystemExit(f"failed to load world: {exc}") from exc
        mcp.run(transport="streamable-http")
        return

    try:
        _game()
    except WorldLoadError as exc:
        raise SystemExit(f"failed to load world: {exc}") from exc
    mcp.run(transport="stdio")


def _build_parser() -> argparse.ArgumentParser:
    """Build the ``understone`` argument parser: serve (default), validate, newpack.

    Parsing is deliberately free of side effects — no world load, no port bind —
    so the resolved subcommand can be inspected without serving anything.
    """
    parser = argparse.ArgumentParser(
        prog="understone",
        description=(
            "Understone — a BBS-style ANSI door game served over MCP, plus the "
            "tools to author its world packs."
        ),
    )
    sub = parser.add_subparsers(dest="cmd")
    sub.add_parser("serve", help="serve the MCP world (the default with no command)")
    validate = sub.add_parser("validate", help="validate a content pack and print a report")
    validate.add_argument("path", type=Path, help="the pack directory to validate")
    newpack = sub.add_parser("newpack", help="scaffold a new content pack from the bundled world")
    newpack.add_argument("path", type=Path, help="the directory to create the pack in")
    sub.add_parser("worlds", help="list the bundled worlds and whether each is sound")
    sim = sub.add_parser("simulate", help="run a greedy balance bot over a pack and report")
    sim.add_argument("path", type=Path, help="the pack directory to simulate")
    sim.add_argument("--days", type=int, default=30, help="sim-days to play (default 30)")
    sim.add_argument("--seed", type=int, default=1, help="RNG seed (default 1)")
    sim.add_argument(
        "--seeds",
        type=int,
        default=None,
        help="run a sweep of this many seeds from --seed and aggregate",
    )
    return parser


def main(argv: list[str] | None = None) -> None:
    """Run the Understone command line: serve, or author a world pack.

    With no arguments (the entry point and ``python -m understone``) this serves
    the MCP world exactly as before. ``validate PATH`` and ``newpack PATH`` drive
    the pack-authoring loop and exit with the verb's status code.
    """
    args = _build_parser().parse_args(argv)
    if args.cmd == "validate":
        raise SystemExit(cli.cli_validate(args.path))
    if args.cmd == "newpack":
        raise SystemExit(cli.cli_newpack(args.path))
    if args.cmd == "worlds":
        raise SystemExit(cli.cli_worlds())
    if args.cmd == "simulate":
        raise SystemExit(sim.cli_simulate(args.path, args.days, args.seed, seeds=args.seeds))
    _serve()