diff --git a/onchain/src/Optimizer.sol b/onchain/src/Optimizer.sol
index fe7687b..54db899 100644
--- a/onchain/src/Optimizer.sol
+++ b/onchain/src/Optimizer.sol
@@ -248,6 +248,9 @@ contract Optimizer is Initializable, UUPSUpgradeable {
         virtual
         returns (uint256 capitalInefficiency, uint256 anchorShare, uint24 anchorWidth, uint256 discoveryDepth)
     {
+        // Guard against negative mantissa — uint256() cast silently wraps negatives.
+        require(inputs[0].mantissa >= 0, "negative mantissa");
+        require(inputs[1].mantissa >= 0, "negative mantissa");
         // Extract slots 0 and 1 (shift=0 assumed — mantissa IS the value)
         uint256 percentageStaked = uint256(inputs[0].mantissa);
         uint256 averageTaxRate = uint256(inputs[1].mantissa);
diff --git a/onchain/test/Optimizer.t.sol b/onchain/test/Optimizer.t.sol
index d22eef6..2b5fff5 100644
--- a/onchain/test/Optimizer.t.sol
+++ b/onchain/test/Optimizer.t.sol
@@ -324,6 +324,26 @@ contract OptimizerTest is Test {
         assertEq(w, 10, "totalWidth < 10 should be clamped to minimum of 10");
     }
 
+    /**
+     * @notice calculateParams reverts when inputs[0].mantissa is negative
+     */
+    function testCalculateParamsRevertsOnNegativeMantissa0() public {
+        OptimizerInput[8] memory inputs;
+        inputs[0] = OptimizerInput({mantissa: -1, shift: 0});
+        vm.expectRevert("negative mantissa");
+        optimizer.calculateParams(inputs);
+    }
+
+    /**
+     * @notice calculateParams reverts when inputs[1].mantissa is negative
+     */
+    function testCalculateParamsRevertsOnNegativeMantissa1() public {
+        OptimizerInput[8] memory inputs;
+        inputs[1] = OptimizerInput({mantissa: -1, shift: 0});
+        vm.expectRevert("negative mantissa");
+        optimizer.calculateParams(inputs);
+    }
+
     /**
      * @notice Non-admin calling upgradeTo should revert with UnauthorizedAccount
      */