diff --git a/libs/decrease_position_utils/src/lib.rs b/libs/decrease_position_utils/src/lib.rs
index 2a91530..5a025e5 100644
--- a/libs/decrease_position_utils/src/lib.rs
+++ b/libs/decrease_position_utils/src/lib.rs
@@ -110,7 +110,7 @@ pub fn decrease_position(env: &Env, p: &DecreasePositionParams) -> DecreasePosit
     let size_delta_usd = p.size_delta_usd.min(position.size_in_usd);
 
     // 2. Update market funding + borrowing state
-    let index_price_mid = p.index_token_price.mid_price();
+    let index_price_mid = p.index_token_price.pick_price_for_pnl(p.is_long, false);
     update_funding_state(
         env,
         p.data_store,
diff --git a/libs/increase_position_utils/src/lib.rs b/libs/increase_position_utils/src/lib.rs
index 38c4cad..3844971 100644
--- a/libs/increase_position_utils/src/lib.rs
+++ b/libs/increase_position_utils/src/lib.rs
@@ -105,7 +105,7 @@ pub fn increase_position(env: &Env, p: &IncreasePositionParams) -> PositionProps
     // and price-impact pool writes are omitted to stay within Soroban's 40 ledger-entry
     // read budget. Funding/borrowing rates are zero when OI is zero (empty market), and
     // position open/close operations will refresh them once OI exists.
-    let index_price = p.index_token_price.mid_price();
+    let index_price = p.index_token_price.pick_price_for_pnl(p.is_long, true);
     let impact_usd: i128 = 0; // price impact skipped to save ledger entries
 
     // Execution price (no impact)
diff --git a/tests/static_execution_price_spread.py b/tests/static_execution_price_spread.py
new file mode 100644
index 0000000..bd1e9f7
--- /dev/null
+++ b/tests/static_execution_price_spread.py
@@ -0,0 +1,15 @@
+from pathlib import Path
+
+
+INCREASE = Path("libs/increase_position_utils/src/lib.rs").read_text()
+DECREASE = Path("libs/decrease_position_utils/src/lib.rs").read_text()
+
+
+def test_increase_uses_worst_case_spread_side():
+    assert "pick_price_for_pnl(p.is_long, true)" in INCREASE
+    assert "let index_price = p.index_token_price.mid_price();" not in INCREASE
+
+
+def test_decrease_uses_inverse_spread_side():
+    assert "pick_price_for_pnl(p.is_long, false)" in DECREASE
+    assert "let index_price_mid = p.index_token_price.mid_price();" not in DECREASE
\ No newline at end of file