diff --git a/src/main.py b/src/main.py
index 4bb7b60..34f850f 100644
--- a/src/main.py
+++ b/src/main.py
@@ -110,6 +110,14 @@ DAILY_TRADE_SESSIONS = 4  # Number of trading sessions per day
 TRADE_SESSION_INTERVAL_HOURS = 6  # Hours between sessions
 
 
+def _resolve_sell_qty_for_pnl(*, sell_qty: int | None, buy_qty: int | None) -> int:
+    """Choose quantity basis for SELL outcome PnL with safe fallback."""
+    resolved_sell = int(sell_qty or 0)
+    if resolved_sell > 0:
+        return resolved_sell
+    return max(0, int(buy_qty or 0))
+
+
 async def _retry_connection(coro_factory: Any, *args: Any, label: str = "", **kwargs: Any) -> Any:
     """Call an async function retrying on ConnectionError with exponential backoff.
 
@@ -1667,8 +1675,9 @@ async def trading_cycle(
             )
             if buy_trade and buy_trade.get("price") is not None:
                 buy_price = float(buy_trade["price"])
-                buy_qty = int(buy_trade.get("quantity") or 1)
-                trade_pnl = (trade_price - buy_price) * buy_qty
+                buy_qty = int(buy_trade.get("quantity") or 0)
+                sell_qty = _resolve_sell_qty_for_pnl(sell_qty=quantity, buy_qty=buy_qty)
+                trade_pnl = (trade_price - buy_price) * sell_qty
                 decision_logger.update_outcome(
                     decision_id=buy_trade["decision_id"],
                     pnl=trade_pnl,
@@ -2772,8 +2781,12 @@ async def run_daily_session(
                     )
                     if buy_trade and buy_trade.get("price") is not None:
                         buy_price = float(buy_trade["price"])
-                        buy_qty = int(buy_trade.get("quantity") or 1)
-                        trade_pnl = (trade_price - buy_price) * buy_qty
+                        buy_qty = int(buy_trade.get("quantity") or 0)
+                        sell_qty = _resolve_sell_qty_for_pnl(
+                            sell_qty=quantity,
+                            buy_qty=buy_qty,
+                        )
+                        trade_pnl = (trade_price - buy_price) * sell_qty
                         decision_logger.update_outcome(
                             decision_id=buy_trade["decision_id"],
                             pnl=trade_pnl,
diff --git a/tests/test_main.py b/tests/test_main.py
index 63ee0da..d5ff5c3 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -27,6 +27,7 @@ from src.main import (
     _extract_held_qty_from_balance,
     _handle_market_close,
     _retry_connection,
+    _resolve_sell_qty_for_pnl,
     _run_context_scheduler,
     _run_evolution_loop,
     _start_dashboard_server,
@@ -119,6 +120,18 @@ class TestExtractAvgPriceFromBalance:
         result = _extract_avg_price_from_balance(balance, "005930", is_domestic=True)
         assert result == 0.0
 
+
+def test_resolve_sell_qty_for_pnl_prefers_sell_qty() -> None:
+    assert _resolve_sell_qty_for_pnl(sell_qty=30, buy_qty=100) == 30
+
+
+def test_resolve_sell_qty_for_pnl_uses_buy_qty_fallback_when_sell_qty_missing() -> None:
+    assert _resolve_sell_qty_for_pnl(sell_qty=None, buy_qty=12) == 12
+
+
+def test_resolve_sell_qty_for_pnl_returns_zero_when_both_missing() -> None:
+    assert _resolve_sell_qty_for_pnl(sell_qty=None, buy_qty=None) == 0
+
     def test_returns_zero_when_field_empty_string(self) -> None:
         """Returns 0.0 when pchs_avg_pric is an empty string."""
         balance = {"output1": [{"pdno": "005930", "pchs_avg_pric": ""}]}
@@ -2750,6 +2763,9 @@ async def test_sell_order_uses_broker_balance_qty_not_db() -> None:
     assert call_kwargs["order_type"] == "SELL"
     # Must use broker-confirmed qty (5), NOT DB-recorded ordered qty (10)
     assert call_kwargs["quantity"] == 5
+    updated_buy = decision_logger.get_decision_by_id(buy_decision_id)
+    assert updated_buy is not None
+    assert updated_buy.outcome_pnl == -25.0
 
 
 @pytest.mark.asyncio