diff --git a/onchain/src/OptimizerV3Push3.sol b/onchain/src/OptimizerV3Push3.sol
index 78a6dd1..a24578f 100644
--- a/onchain/src/OptimizerV3Push3.sol
+++ b/onchain/src/OptimizerV3Push3.sol
@@ -56,8 +56,10 @@ contract OptimizerV3Push3 is IOptimizer {
         require(inputs[0].mantissa <= 1e18, "mantissa overflow");
 
         // Validate that shift is 0 (future-only field, not yet supported)
+        // and that no mantissa is negative (uint256 cast silently wraps negatives).
         for (uint256 k = 0; k < 8; k++) {
             require(inputs[k].shift == 0, "shift not yet supported");
+            require(inputs[k].mantissa >= 0, "negative mantissa");
         }
 
         uint256 percentagestaked = uint256(uint256(inputs[0].mantissa));
diff --git a/onchain/test/OptimizerV3Push3.t.sol b/onchain/test/OptimizerV3Push3.t.sol
index aba1183..bae7169 100644
--- a/onchain/test/OptimizerV3Push3.t.sol
+++ b/onchain/test/OptimizerV3Push3.t.sol
@@ -200,6 +200,30 @@ contract OptimizerV3Push3Test is Test {
         _assertBull(ci, as_, aw, dd);
     }
 
+    // ---- Negative mantissa guards ----
+
+    function testNegativeMantissaSlot0Reverts() public {
+        OptimizerInput[8] memory inp;
+        inp[0] = OptimizerInput({mantissa: -1, shift: 0});
+        vm.expectRevert("negative mantissa");
+        push3.calculateParams(inp);
+    }
+
+    function testNegativeMantissaSlot1Reverts() public {
+        OptimizerInput[8] memory inp;
+        inp[0] = OptimizerInput({mantissa: int256(_pct(95)), shift: 0});
+        inp[1] = OptimizerInput({mantissa: -1, shift: 0});
+        vm.expectRevert("negative mantissa");
+        push3.calculateParams(inp);
+    }
+
+    function testNegativeMantissaSlot5Reverts() public {
+        OptimizerInput[8] memory inp;
+        inp[5] = OptimizerInput({mantissa: -1, shift: 0});
+        vm.expectRevert("negative mantissa");
+        push3.calculateParams(inp);
+    }
+
     // ---- Fuzz ----
 
     function testFuzzNeverReverts(uint256 percentageStaked, uint256 averageTaxRate) public view {