diff --git a/backtest/capacity.py b/backtest/capacity.py
index 721d290..274395b 100644
--- a/backtest/capacity.py
+++ b/backtest/capacity.py
@@ -272,7 +272,10 @@ def _run_at_aum(
                 # Standard turnover cost
                 turnover = abs(trade_weights).sum() / 2
                 total_turnover += turnover
-                cost_rate = self.cost_model.compute_costs(turnover)
+                # Two-way traded value: compute_costs halves stamp tax on the
+                # assumption that half of its input is sells, so it needs
+                # buys + sells, not the one-sided turnover kept above.
+                cost_rate = self.cost_model.compute_costs(turnover * 2)
                 capital -= cost_rate * capital
 
                 # Extra impact cost
diff --git a/backtest/engine.py b/backtest/engine.py
index 64a9132..a4b0c00 100644
--- a/backtest/engine.py
+++ b/backtest/engine.py
@@ -224,7 +224,13 @@ def _simulate_pnl(self, returns: pd.DataFrame) -> None:
 
                 turnover = (target_weights - current_weights).abs().sum() / 2
                 if turnover > 0:
-                    rebalance_cost = self.cost_model.compute_costs(turnover)
+                    # `turnover` is one-sided (the convention documented in
+                    # portfolio/constraints.py), but CostModel.compute_costs
+                    # charges commission and slippage on the value it is given
+                    # and halves stamp tax on the assumption that half of it is
+                    # sells -- which only holds for the *two-way* traded value.
+                    # Passing one-sided turnover halved every component.
+                    rebalance_cost = self.cost_model.compute_costs(turnover * 2)
                 turnover_records.append((next_rdate, turnover))
 
                 current_weights = target_weights.copy()
diff --git a/tests/test_backtest/test_cost_model.py b/tests/test_backtest/test_cost_model.py
index c4049b9..ee21da6 100644
--- a/tests/test_backtest/test_cost_model.py
+++ b/tests/test_backtest/test_cost_model.py
@@ -1,9 +1,40 @@
 """Tests for transaction cost model."""
 
 import pandas as pd
+import pytest
 from quant_platform.backtest.cost_model import CostModel
 
 
+def test_compute_costs_scalar_input_is_the_two_way_traded_value():
+    """The contract the backtest engine depends on (regression for BUG-03).
+
+    `compute_costs` charges commission and slippage on the value it is given,
+    and halves stamp tax on the assumption that half of that value is sells.
+    That assumption only holds for the *two-way* traded value (buys + sells),
+    not for one-sided turnover.
+
+    The engine used to pass one-sided turnover -- the convention documented in
+    portfolio/constraints.py -- which halved commission, stamp tax and slippage
+    together. On a 30%-one-sided-turnover monthly strategy that understated the
+    cost by ~47 bp/year.
+    """
+    model = CostModel(commission=0.0003, stamp_tax=0.001, slippage=0.0005)
+    one_sided = 0.30
+    two_way = one_sided * 2
+
+    expected = (
+        two_way * 0.0003       # commission, both sides
+        + one_sided * 0.001    # stamp tax, sells only
+        + two_way * 0.0005     # slippage, both sides
+    )
+
+    assert model.compute_costs(two_way) == pytest.approx(expected)
+    # And feeding it one-sided is exactly the halving that was the defect.
+    assert model.compute_costs(two_way) == pytest.approx(
+        2 * model.compute_costs(one_sided)
+    )
+
+
 def test_cost_model_basic():
     model = CostModel()
     turnover = pd.Series([100000, 50000], index=["A", "B"])
diff --git a/tests/test_backtest/test_engine.py b/tests/test_backtest/test_engine.py
index d6dd2d7..0ef9d88 100644
--- a/tests/test_backtest/test_engine.py
+++ b/tests/test_backtest/test_engine.py
@@ -39,7 +39,13 @@ def test_initial_rebalance_charges_cost_and_records_turnover(self):
         engine.weights_history = {dates[1]: target}
         engine._simulate_pnl(returns)
 
-        assert engine.portfolio_values.iloc[-1] == pytest.approx(999_500.0)
+        # One-sided turnover is 0.5 (cash -> fully invested), so the two-way
+        # traded value is 1.0 and the cost is 1.0 * commission = 0.001.
+        # This used to assert 999_500 -- the half-charged figure produced by
+        # passing one-sided turnover to a cost model that expects two-way.
+        assert engine.portfolio_values.iloc[-1] == pytest.approx(999_000.0)
         assert engine.turnover_history is not None
         assert len(engine.turnover_history) == 1
+        # The recorded turnover stays one-sided: it is a metric, not the base
+        # the cost model is fed.
         assert engine.turnover_history.iloc[0] == pytest.approx(0.5)