diff --git a/backend/tests/test_api.py b/backend/tests/test_api.py
new file mode 100644
index 0000000..e7582a7
--- /dev/null
+++ b/backend/tests/test_api.py
@@ -0,0 +1,61 @@
+import pytest
+from fastapi.testclient import TestClient
+
+from tft.api import app as app_mod
+from tft.constants.loader import load_constants
+from tft.model import artifact as artifact_mod
+from tests.test_sim import make_static
+
+
+@pytest.fixture
+def client(monkeypatch):
+    static = make_static()
+    static["meta"]["patch"] = "test"
+    art = artifact_mod.build(static)
+    cfg = load_constants(17)
+    monkeypatch.setattr(app_mod, "_load_game_deps", lambda: (art, cfg))
+    return TestClient(app_mod.app)
+
+
+def test_full_game_via_api(client):
+    state = client.post("/api/game", params={"seed": 3}).json()
+    game_id = state["game_id"]
+    assert state["round"]["label"] == "1-1"
+    assert state["player"]["hp"] == 100
+    assert len(state["opponents"]) == 7
+
+    steps = 0
+    while not state["over"]:
+        if state["augment_offer"]:
+            client.post(f"/api/game/{game_id}/action",
+                        json={"action": "pick_augment", "choice": 0})
+        affordable = [i for i, s in enumerate(state["shop"])
+                      if s and s["cost"] <= state["player"]["gold"]]
+        if affordable:
+            client.post(f"/api/game/{game_id}/action",
+                        json={"action": "buy", "slot": affordable[0]})
+        resp = client.post(f"/api/game/{game_id}/action", json={"action": "next"})
+        assert resp.status_code == 200
+        state = resp.json()
+        steps += 1
+        assert steps < 100
+    assert 1 <= state["placement"] <= 8
+
+
+def test_invalid_action_is_400(client):
+    game_id = client.post("/api/game", params={"seed": 1}).json()["game_id"]
+    resp = client.post(f"/api/game/{game_id}/action",
+                       json={"action": "sell", "where": "bench", "idx": 99})
+    assert resp.status_code == 400
+    resp = client.post(f"/api/game/{game_id}/action", json={"action": "dance"})
+    assert resp.status_code == 400
+    assert client.post("/api/game/nope/action", json={"action": "next"}).status_code == 404
+
+
+def test_state_shape(client):
+    state = client.post("/api/game", params={"seed": 5}).json()
+    for key in ("round", "player", "board", "bench", "shop", "traits",
+                "opponents", "items", "over", "log"):
+        assert key in state
+    assert state["player"]["score"] == 0.0
+    assert all(o["board"] is not None for o in state["opponents"])
diff --git a/backend/tft/api/app.py b/backend/tft/api/app.py
index 154c69d..363bbf3 100644
--- a/backend/tft/api/app.py
+++ b/backend/tft/api/app.py
@@ -1,8 +1,182 @@
-from fastapi import FastAPI
+import json
+import uuid
+
+from fastapi import FastAPI, HTTPException
+from pydantic import BaseModel
+
+from tft import paths
+from tft.model.score import active_trait_tiers
+from tft.sim.game import Game, InvalidAction
 
 app = FastAPI(title="tft-sim")
 
+GAMES: dict[str, Game] = {}
+
+
+class ActionRequest(BaseModel):
+    action: str
+    slot: int | None = None
+    where: str | None = None
+    idx: int | None = None
+    item_idx: int | None = None
+    board_idx: int | None = None
+    choice: int | None = None
+
 
 @app.get("/health")
 def health() -> dict:
     return {"status": "ok"}
+
+
+def _load_game_deps():
+    from tft.constants.loader import load_constants
+    from tft.model import artifact as artifact_mod
+
+    pointer = paths.latest_static_pointer()
+    if not pointer.exists():
+        raise HTTPException(503, "no static data: run `tft.cli fetch-static` first")
+    set_number = json.loads(pointer.read_text())["set"]
+    return artifact_mod.load(set_number), load_constants(set_number)
+
+
+@app.post("/api/game")
+def new_game(seed: int | None = None) -> dict:
+    artifact, cfg = _load_game_deps()
+    game_id = uuid.uuid4().hex[:8]
+    GAMES[game_id] = Game(artifact, cfg, seed=seed)
+    return {"game_id": game_id, **state(game_id)}
+
+
+@app.get("/api/game/{game_id}")
+def state(game_id: str) -> dict:
+    game = GAMES.get(game_id)
+    if game is None:
+        raise HTTPException(404, "unknown game")
+    return serialize(game)
+
+
+@app.post("/api/game/{game_id}/action")
+def action(game_id: str, req: ActionRequest) -> dict:
+    game = GAMES.get(game_id)
+    if game is None:
+        raise HTTPException(404, "unknown game")
+    try:
+        if req.action == "buy":
+            game.buy(req.slot)
+        elif req.action == "sell":
+            game.sell(req.where, req.idx)
+        elif req.action == "reroll":
+            game.reroll()
+        elif req.action == "buy_xp":
+            game.buy_xp()
+        elif req.action == "move":
+            game.move(req.where, req.idx)
+        elif req.action == "equip":
+            game.equip(req.item_idx, req.board_idx)
+        elif req.action == "pick_augment":
+            game.pick_augment(req.choice)
+        elif req.action == "next":
+            game.step()
+        else:
+            raise HTTPException(400, f"unknown action {req.action}")
+    except InvalidAction as e:
+        raise HTTPException(400, str(e))
+    except TypeError:
+        raise HTTPException(400, f"missing parameters for action {req.action}")
+    return serialize(game)
+
+
+def unit_view(game: Game, u: dict) -> dict:
+    info = game.artifact["static"]["units"][u["api_name"]]
+    return {
+        "api_name": u["api_name"],
+        "name": info.get("name", u["api_name"]),
+        "cost": info["cost"],
+        "stars": u["stars"],
+        "traits": u.get("shown_traits", info["traits"]),
+        "icon": info.get("icon"),
+        "items": [
+            {"api_name": i, "name": game.artifact["static"]["items"].get(i, {}).get("name", i),
+             "icon": game.artifact["static"]["items"].get(i, {}).get("icon")}
+            for i in u["items"]
+        ],
+    }
+
+
+def serialize(game: Game) -> dict:
+    static = game.artifact["static"]
+    tiers = active_trait_tiers(game.board, static["traits"], static["units"])
+    counts: dict[str, int] = {}
+    for u in game.board:
+        for t in set(static["units"][u["api_name"]]["traits"]):
+            counts[t] = counts.get(t, 0) + 1
+    traits_panel = sorted(
+        (
+            {
+                "api_name": t,
+                "name": static["traits"][t].get("name", t),
+                "count": n,
+                "tier": tiers.get(t, 0),
+                "breakpoints": [bp["min_units"] for bp in static["traits"][t]["breakpoints"]],
+                "icon": static["traits"][t].get("icon"),
+            }
+            for t, n in counts.items()
+        ),
+        key=lambda x: (-x["tier"], -x["count"]),
+    )
+
+    stage = game.round["stage"]
+    return {
+        "round": {"label": game.round["label"], "stage": stage,
+                  "kind": game.round["kind"]},
+        "player": {
+            "hp": game.hp, "gold": game.gold, "level": game.level, "xp": game.xp,
+            "xp_needed": game.cfg["xp"]["to_next"][game.level - 1]
+            if game.level < game.cfg["xp"]["max_level"] else None,
+            "streak": game.streak,
+            "score": round(game.player_score(), 1),
+        },
+        "board": [unit_view(game, u) for u in game.board],
+        "bench": [unit_view(game, u) for u in game.bench],
+        "items": [
+            {"api_name": i, "name": static["items"].get(i, {}).get("name", i),
+             "icon": static["items"].get(i, {}).get("icon")}
+            for i in game.items
+        ],
+        "shop": [
+            None if api is None else {
+                "api_name": api,
+                "name": static["units"][api].get("name", api),
+                "cost": static["units"][api]["cost"],
+                "traits": [static["traits"][t].get("name", t) for t in static["units"][api]["traits"]
+                           if t in static["traits"]],
+                "icon": static["units"][api].get("icon"),
+            }
+            for api in game.shop
+        ],
+        "traits": traits_panel,
+        "augment_offer": [
+            {"api_name": a, "name": static["augments"].get(a, {}).get("name", a),
+             "icon": static["augments"].get(a, {}).get("icon")}
+            for a in game.augment_offer
+        ],
+        "augments": [static["augments"].get(a, {}).get("name", a) for a in game.augments],
+        "opponents": sorted(
+            (
+                {
+                    "name": b.name, "archetype": b.archetype, "hp": max(b.hp, 0),
+                    "alive": b.alive, "placement": b.placement,
+                    "level": b.level(stage) if b.alive else None,
+                    "score": round(b.score(stage, game.artifact), 1) if b.alive else None,
+                    "board": [unit_view(game, u) for u in b.board],
+                }
+                for b in game.bots
+            ),
+            key=lambda x: -x["hp"],
+        ),
+        "reroll_cost": game.cfg["shop"]["reroll_cost"],
+        "xp_cost": game.cfg["xp"]["buy_cost"],
+        "over": game.over,
+        "placement": game.placement,
+        "log": game.log[-8:],
+    }