diff --git a/capture.py b/capture.py
new file mode 100644
index 0000000..f1a15a9
--- /dev/null
+++ b/capture.py
@@ -0,0 +1,27 @@
+#!/usr/bin/env python3
+"""Save game state snapshots to ./capture/<label>.json while building facts.py.
+
+usage: python3 capture.py <label> [--wait SECONDS]
+"""
+import json, sys, time, pathlib
+sys.path.insert(0, str(pathlib.Path(__file__).parent))
+import sts2
+
+def snap(label, wait=1.0):
+    if wait: time.sleep(wait)
+    obs = sts2.state()
+    out = pathlib.Path(__file__).parent / "capture"
+    out.mkdir(exist_ok=True)
+    path = out / f"{label}.json"
+    path.write_text(json.dumps(obs, indent=2))
+    print(f"[{label}] state_type={obs.get('state_type')!r} "
+          f"msg={str(obs.get('message'))[:60]!r} -> {path.name} ({path.stat().st_size}B)")
+    return obs
+
+if __name__ == "__main__":
+    args = [a for a in sys.argv[1:] if not a.startswith("--")]
+    wait = 1.0
+    if "--wait" in sys.argv:
+        wait = float(sys.argv[sys.argv.index("--wait") + 1])
+    for lbl in args:
+        snap(lbl, wait)
diff --git a/sts2.py b/sts2.py
new file mode 100644
index 0000000..c290eff
--- /dev/null
+++ b/sts2.py
@@ -0,0 +1,227 @@
+#!/usr/bin/env python3
+"""
+sts2.py -- thin client for the STS2MCP localhost interface.
+
+The mod exposes the game on http://localhost:15526 with no auth.
+
+  GET  /api/v1/singleplayer?format=json|markdown   read state
+  POST /api/v1/singleplayer {"action": ..., ...}   perform one action
+
+Learning from the state model: every response carries `state_type`, and
+each state_type has a small fixed action set. That set IS our action
+space, so this module also maps state_type -> legal actions.
+
+The loop is strictly closed. Playing a card removes it from hand and
+shifts every later index, so we observe -> act once -> observe again.
+Never precompute an action list.
+"""
+
+from __future__ import annotations
+
+import json
+import urllib.error
+import urllib.request
+from dataclasses import dataclass
+from typing import Any
+
+BASE = "http://localhost:15526"
+TIMEOUT = 20.0
+
+
+class Sts2Error(RuntimeError):
+    pass
+
+
+# state_type -> legal actions. Source: STS2MCP docs/raw-simplified.md
+LEGAL_ACTIONS: dict[str, tuple[str, ...]] = {
+    "menu": ("menu_select",),
+    "game_over": ("menu_select",),
+    "monster": ("play_card", "use_potion", "end_turn"),
+    "elite": ("play_card", "use_potion", "end_turn"),
+    "boss": ("play_card", "use_potion", "end_turn"),
+    "hand_select": ("combat_select_card", "combat_confirm_selection"),
+    "rewards": ("claim_reward", "proceed"),
+    "card_reward": ("select_card_reward", "skip_card_reward"),
+    "map": ("choose_map_node",),
+    "event": ("choose_event_option", "advance_dialogue"),
+    "rest_site": ("choose_rest_option", "proceed"),
+    "shop": ("shop_purchase", "proceed"),
+    "fake_merchant": ("shop_purchase", "proceed"),
+    "treasure": ("claim_treasure_relic", "proceed"),
+    "card_select": ("select_card", "confirm_selection", "cancel_selection"),
+    "bundle_select": (
+        "select_bundle",
+        "confirm_bundle_selection",
+        "cancel_bundle_selection",
+    ),
+    "relic_select": ("select_relic", "skip_relic_selection"),
+    "crystal_sphere": (
+        "crystal_sphere_set_tool",
+        "crystal_sphere_click_cell",
+        "crystal_sphere_proceed",
+    ),
+    "overlay": (),
+    "unknown": (),
+}
+
+COMBAT_STATES = ("monster", "elite", "boss")
+
+
+@dataclass
+class ActionResult:
+    status: str
+    message: str
+    raw: dict
+
+    @property
+    def ok(self) -> bool:
+        return self.status == "ok"
+
+
+def _request(path: str, payload: dict | None = None, timeout: float = TIMEOUT) -> dict:
+    url = BASE + path
+    if payload is None:
+        request = urllib.request.Request(url, method="GET")
+    else:
+        request = urllib.request.Request(
+            url,
+            data=json.dumps(payload).encode("utf-8"),
+            method="POST",
+            headers={"Content-Type": "application/json"},
+        )
+    try:
+        with urllib.request.urlopen(request, timeout=timeout) as response:
+            body = response.read()
+    except urllib.error.HTTPError as exc:
+        detail = exc.read().decode("utf-8", "replace")
+        raise Sts2Error(f"HTTP {exc.code} on {path}: {detail[:300]}") from exc
+    except urllib.error.URLError as exc:
+        raise Sts2Error(
+            f"cannot reach the game on {BASE}. Is STS2 running with the mod loaded? ({exc})"
+        ) from exc
+
+    try:
+        return json.loads(body)
+    except json.JSONDecodeError as exc:
+        raise Sts2Error(f"non-JSON response from {path}: {body[:200]!r}") from exc
+
+
+# --------------------------------------------------------------------------
+# Reads
+# --------------------------------------------------------------------------
+
+def is_up() -> bool:
+    try:
+        data = _request("/", timeout=5.0)
+        return data.get("status") == "ok"
+    except Sts2Error:
+        return False
+
+
+def state(fmt: str = "json") -> dict:
+    """Current game state. fmt: 'json' or 'markdown' (markdown returns text)."""
+    if fmt == "markdown":
+        req = urllib.request.Request(f"{BASE}/api/v1/singleplayer?format=markdown")
+        with urllib.request.urlopen(req, timeout=TIMEOUT) as response:
+            return {"markdown": response.read().decode("utf-8")}
+    return _request("/api/v1/singleplayer?format=json")
+
+
+def state_type(observation: dict) -> str:
+    return observation.get("state_type", "unknown")
+
+
+def legal_actions(observation: dict) -> tuple[str, ...]:
+    return LEGAL_ACTIONS.get(state_type(observation), ())
+
+
+def profile() -> dict:
+    return _request("/api/v1/profile")
+
+
+def compendium() -> dict:
+    return _request("/api/v1/compendium")
+
+
+def wiki(query: str, item_type: str = "all", limit: int = 10) -> dict:
+    from urllib.parse import quote
+
+    return _request(
+        f"/api/v1/wiki?query={quote(query)}&item_type={item_type}&limit={limit}"
+    )
+
+
+def profiles() -> dict:
+    return _request("/api/v1/profiles")
+
+
+# --------------------------------------------------------------------------
+# Writes -- one action at a time
+# --------------------------------------------------------------------------
+
+def act(action: str, **params: Any) -> ActionResult:
+    payload = {"action": action}
+    payload.update(params)
+    data = _request("/api/v1/singleplayer", payload)
+    return ActionResult(
+        status=data.get("status", "?"),
+        # Errors arrive under `error`, not `message`. Reading only `message`
+        # makes every rejection look empty.
+        message=data.get("message") or data.get("error") or "",
+        raw=data,
+    )
+
+
+def play_card(card_index: int, target: str | None = None) -> ActionResult:
+    params: dict[str, Any] = {"card_index": card_index}
+    if target is not None:
+        params["target"] = target
+    return act("play_card", **params)
+
+
+def end_turn() -> ActionResult:
+    return act("end_turn")
+
+
+def use_potion(slot: int, target: str | None = None) -> ActionResult:
+    params: dict[str, Any] = {"slot": slot}
+    if target is not None:
+        params["target"] = target
+    return act("use_potion", **params)
+
+
+def menu_select(option: str, seed: str | None = None) -> ActionResult:
+    params: dict[str, Any] = {"option": option}
+    if seed is not None:
+        params["seed"] = seed
+    return act("menu_select", **params)
+
+
+# --------------------------------------------------------------------------
+# CLI
+# --------------------------------------------------------------------------
+
+def _main() -> int:
+    import sys
+
+    if not is_up():
+        print(f"game not reachable at {BASE}")
+        return 1
+
+    if len(sys.argv) > 1 and sys.argv[1] == "--dump":
+        obs = state()
+        print(json.dumps(obs, indent=2))
+        return 0
+
+    obs = state()
+    st = state_type(obs)
+    print(f"state_type : {st}")
+    print(f"run        : {obs.get('run')}")
+    print(f"legal      : {', '.join(legal_actions(obs)) or '(none - manual)'}")
+    if "options" in obs:
+        print(f"options    : {obs['options']}")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(_main())