diff --git a/onchain/src/IOptimizer.sol b/onchain/src/IOptimizer.sol
index 047bcdd..4531a8a 100644
--- a/onchain/src/IOptimizer.sol
+++ b/onchain/src/IOptimizer.sol
@@ -3,7 +3,9 @@ pragma solidity ^0.8.19;
 
 /**
  * @notice Dyadic rational input: mantissa × 2^(-shift).
- *         For shift == 0 (current usage via _toDyadic), value == mantissa.
+ *         shift is reserved for future use and MUST be 0.  All production
+ *         Optimizer implementations require shift == 0 and revert otherwise.
+ *         When shift == 0 (as produced by _toDyadic), value == mantissa.
  */
 struct OptimizerInput {
     int256 mantissa;
diff --git a/onchain/src/Optimizer.sol b/onchain/src/Optimizer.sol
index 7c596e7..e08d835 100644
--- a/onchain/src/Optimizer.sol
+++ b/onchain/src/Optimizer.sol
@@ -356,11 +356,13 @@ contract Optimizer is Initializable, UUPSUpgradeable, IOptimizer {
         virtual
         returns (uint256 capitalInefficiency, uint256 anchorShare, uint24 anchorWidth, uint256 discoveryDepth)
     {
-        // Guard against negative mantissa — uint256() cast silently wraps negatives.
+        // Guard against non-zero shift and negative mantissa.
+        // shift is reserved for future use; uint256() cast silently wraps negatives.
         for (uint256 k; k < 8; k++) {
+            require(inputs[k].shift == 0, "shift not yet supported");
             require(inputs[k].mantissa >= 0, "negative mantissa");
         }
-        // Extract slots 0 and 1 (shift=0 assumed — mantissa IS the value)
+        // Extract slots 0 and 1 (shift=0 enforced above — mantissa IS the value)
         uint256 percentageStaked = uint256(inputs[0].mantissa);
         uint256 averageTaxRate = uint256(inputs[1].mantissa);
 
diff --git a/onchain/src/OptimizerV3.sol b/onchain/src/OptimizerV3.sol
index 8fa995e..e7375f9 100644
--- a/onchain/src/OptimizerV3.sol
+++ b/onchain/src/OptimizerV3.sol
@@ -22,6 +22,13 @@ contract OptimizerV3 is Optimizer {
         override
         returns (uint256 ci, uint256 anchorShare, uint24 anchorWidth, uint256 discoveryDepth)
     {
+        // Guard against non-zero shift and negative mantissa.
+        // shift is reserved for future use; uint256() cast silently wraps negatives.
+        for (uint256 k; k < 8; k++) {
+            require(inputs[k].shift == 0, "shift not yet supported");
+            require(inputs[k].mantissa >= 0, "negative mantissa");
+        }
+
         // ── BEGIN TRANSPILER OUTPUT (optimizer_v3.push3) ──
         // Do NOT edit by hand — regenerate via: npx tsx tools/push3-transpiler/transpile-cli.ts
 
diff --git a/onchain/test/Optimizer.t.sol b/onchain/test/Optimizer.t.sol
index 18e0728..57daa62 100644
--- a/onchain/test/Optimizer.t.sol
+++ b/onchain/test/Optimizer.t.sol
@@ -359,6 +359,18 @@ contract OptimizerTest is Test {
         }
     }
 
+    /**
+     * @notice calculateParams reverts when any slot has shift != 0
+     */
+    function testCalculateParamsRevertsOnNonZeroShift() public {
+        for (uint256 k = 0; k < 8; k++) {
+            OptimizerInput[8] memory inputs;
+            inputs[k] = OptimizerInput({ mantissa: 0, shift: 1 });
+            vm.expectRevert("shift not yet supported");
+            optimizer.calculateParams(inputs);
+        }
+    }
+
     /**
      * @notice Non-admin calling upgradeTo should revert with UnauthorizedAccount
      */
diff --git a/onchain/test/OptimizerV3.t.sol b/onchain/test/OptimizerV3.t.sol
new file mode 100644
index 0000000..fc3b662
--- /dev/null
+++ b/onchain/test/OptimizerV3.t.sol
@@ -0,0 +1,85 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+pragma solidity ^0.8.19;
+
+import {OptimizerV3} from "../src/OptimizerV3.sol";
+import {OptimizerInput} from "../src/IOptimizer.sol";
+import "forge-std/Test.sol";
+
+/**
+ * @title OptimizerV3Test
+ * @notice Unit tests for OptimizerV3.calculateParams input validation guards
+ *         (shift and negative-mantissa) and basic bear/bull output correctness.
+ */
+contract OptimizerV3Test is Test {
+    OptimizerV3 v3;
+
+    function setUp() public {
+        v3 = new OptimizerV3();
+    }
+
+    // ---- Helpers ----
+
+    function _inputs(uint256 percentageStaked, uint256 averageTaxRate)
+        internal
+        pure
+        returns (OptimizerInput[8] memory inp)
+    {
+        inp[0] = OptimizerInput({mantissa: int256(percentageStaked), shift: 0});
+        inp[1] = OptimizerInput({mantissa: int256(averageTaxRate), shift: 0});
+    }
+
+    // ---- Shift guard ----
+
+    function testNonZeroShiftReverts() public {
+        for (uint256 k = 0; k < 8; k++) {
+            OptimizerInput[8] memory inp;
+            inp[k] = OptimizerInput({mantissa: 0, shift: 1});
+            vm.expectRevert("shift not yet supported");
+            v3.calculateParams(inp);
+        }
+    }
+
+    // ---- Negative mantissa guard ----
+
+    function testNegativeMantissaSlot0Reverts() public {
+        OptimizerInput[8] memory inp;
+        inp[0] = OptimizerInput({mantissa: -1, shift: 0});
+        vm.expectRevert("negative mantissa");
+        v3.calculateParams(inp);
+    }
+
+    function testNegativeMantissaSlot1Reverts() public {
+        OptimizerInput[8] memory inp;
+        inp[0] = OptimizerInput({mantissa: int256(95e16), shift: 0});
+        inp[1] = OptimizerInput({mantissa: -1, shift: 0});
+        vm.expectRevert("negative mantissa");
+        v3.calculateParams(inp);
+    }
+
+    function testNegativeMantissaHighSlotReverts() public {
+        OptimizerInput[8] memory inp;
+        inp[5] = OptimizerInput({mantissa: -1, shift: 0});
+        vm.expectRevert("negative mantissa");
+        v3.calculateParams(inp);
+    }
+
+    // ---- Basic output correctness ----
+
+    function testBearAtLowStaking() public view {
+        (uint256 ci, uint256 as_, uint24 aw, uint256 dd) =
+            v3.calculateParams(_inputs(50e16, 0));
+        assertEq(ci, 0);
+        assertEq(as_, 3e17);
+        assertEq(aw, 100);
+        assertEq(dd, 3e17);
+    }
+
+    function testBullAtHighStaking() public view {
+        (uint256 ci, uint256 as_, uint24 aw, uint256 dd) =
+            v3.calculateParams(_inputs(96e16, 0));
+        assertEq(ci, 0);
+        assertEq(as_, 1e18);
+        assertEq(aw, 20);
+        assertEq(dd, 1e18);
+    }
+}
diff --git a/onchain/test/OptimizerV3Push3.t.sol b/onchain/test/OptimizerV3Push3.t.sol
index bae7169..f606c41 100644
--- a/onchain/test/OptimizerV3Push3.t.sol
+++ b/onchain/test/OptimizerV3Push3.t.sol
@@ -224,6 +224,17 @@ contract OptimizerV3Push3Test is Test {
         push3.calculateParams(inp);
     }
 
+    // ---- Shift guard ----
+
+    function testNonZeroShiftReverts() public {
+        for (uint256 k = 0; k < 8; k++) {
+            OptimizerInput[8] memory inp;
+            inp[k] = OptimizerInput({mantissa: 0, shift: 1});
+            vm.expectRevert("shift not yet supported");
+            push3.calculateParams(inp);
+        }
+    }
+
     // ---- Fuzz ----
 
     function testFuzzNeverReverts(uint256 percentageStaked, uint256 averageTaxRate) public view {