diff --git a/test/src/concrete/Flow.construction.t.sol b/test/src/concrete/Flow.construction.t.sol
index 03206941..85fdaefa 100644
--- a/test/src/concrete/Flow.construction.t.sol
+++ b/test/src/concrete/Flow.construction.t.sol
@@ -6,7 +6,7 @@ import {Vm} from "forge-std/Test.sol";
 
 import {EvaluableConfigV3} from "rain.interpreter.interface/interface/IInterpreterCallerV2.sol";
 import {FlowTest} from "test/abstract/FlowTest.sol";
-import {EmptyFlowConfig} from "src/error/ErrFlow.sol";
+import {EmptyFlowConfig, UnsupportedFlowInputs} from "src/error/ErrFlow.sol";
 
 contract FlowConstructionTest is FlowTest {
     function testFlowConstructionEmptyConfigReverts() external {
@@ -16,6 +16,30 @@ contract FlowConstructionTest is FlowTest {
         I_CLONE_FACTORY.clone(impl, abi.encode(emptyConfig));
     }
 
+    /// Reverts with `UnsupportedFlowInputs` when the deployer reports any
+    /// non-zero `flowInputs` byte in the IO string. Pinning this protects
+    /// against a future deployer behaviour change leaking non-zero inputs
+    /// through the guard at `Flow.flowInit`.
+    /// forge-config: default.fuzz.runs = 100
+    function testFlowConstructionRevertsOnNonZeroFlowInputs(
+        address expression,
+        bytes memory bytecode,
+        uint256[] memory constants,
+        uint8 flowInputs
+    ) external {
+        vm.assume(flowInputs != 0);
+        // io: byte0 = flowInputs (non-zero), byte1 = 7 (>= MIN_FLOW_SENTINELS).
+        bytes memory io = abi.encodePacked(flowInputs, uint8(7));
+        expressionDeployerDeployExpression2MockCall(expression, io);
+
+        EvaluableConfigV3[] memory flowConfig = new EvaluableConfigV3[](1);
+        flowConfig[0] = EvaluableConfigV3(DEPLOYER, bytecode, constants);
+
+        address impl = deployFlowImplementation();
+        vm.expectRevert(UnsupportedFlowInputs.selector);
+        I_CLONE_FACTORY.clone(impl, abi.encode(flowConfig));
+    }
+
     function testFlowConstructionInitialize(address expression, bytes memory bytecode, uint256[] memory constants)
         external
     {