diff --git a/onchain/src/IOptimizer.sol b/onchain/src/IOptimizer.sol
index 047bcdd..0a4d537 100644
--- a/onchain/src/IOptimizer.sol
+++ b/onchain/src/IOptimizer.sol
@@ -3,7 +3,10 @@ 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 current
+ *         Optimizer implementations (Optimizer, OptimizerV3, OptimizerV3Push3)
+ *         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..e520456 100644
--- a/onchain/src/Optimizer.sol
+++ b/onchain/src/Optimizer.sol
@@ -356,8 +356,10 @@ 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)
diff --git a/onchain/src/OptimizerV3.sol b/onchain/src/OptimizerV3.sol
index 8fa995e..ecfc30e 100644
--- a/onchain/src/OptimizerV3.sol
+++ b/onchain/src/OptimizerV3.sol
@@ -22,6 +22,11 @@ contract OptimizerV3 is Optimizer {
         override
         returns (uint256 ci, uint256 anchorShare, uint24 anchorWidth, uint256 discoveryDepth)
     {
+        // Guard against non-zero shift (reserved for future use).
+        for (uint256 k; k < 8; k++) {
+            require(inputs[k].shift == 0, "shift not yet supported");
+        }
+
         // ── 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/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 {