diff --git a/src/lib/LibICloneableFactoryV4.sol b/src/lib/LibICloneableFactoryV4.sol
index e8e58c0..f86a641 100644
--- a/src/lib/LibICloneableFactoryV4.sol
+++ b/src/lib/LibICloneableFactoryV4.sol
@@ -14,6 +14,10 @@ import {
 /// `initialize` included — to nothing.
 error ZeroImplementationCodeSize();
 
+/// Thrown when an implementation's code begins with `0xef`: an EIP-7702
+/// delegated account.
+error DelegatedImplementation();
+
 /// Thrown when the clone address already has code. On the open-salt path the
 /// occupant is the exact clone asked for; on the namespaced path it is the
 /// clone the same deployer deployed at that salt, with whatever `data` that
@@ -125,12 +129,20 @@ library LibICloneableFactoryV4 {
     }
 
     /// Reverts with `ZeroImplementationCodeSize` if `implementation` has no
-    /// code. Always a mistake: the clone would delegate every call to nothing.
+    /// code, or `DelegatedImplementation` if its code begins with `0xef`.
     /// @param implementation The contract to clone.
     function checkImplementationCode(address implementation) internal view {
         if (implementation.code.length == 0) {
             revert ZeroImplementationCodeSize();
         }
+        uint256 firstByte;
+        assembly ("memory-safe") {
+            extcodecopy(implementation, 0, 0, 1)
+            firstByte := byte(0, mload(0))
+        }
+        if (firstByte == 0xef) {
+            revert DelegatedImplementation();
+        }
     }
 
     /// The shared tail of both clone entry points: guard the implementation,
diff --git a/test/src/lib/LibICloneableFactoryV4.checkImplementationCode.t.sol b/test/src/lib/LibICloneableFactoryV4.checkImplementationCode.t.sol
index 52b3d00..c469e58 100644
--- a/test/src/lib/LibICloneableFactoryV4.checkImplementationCode.t.sol
+++ b/test/src/lib/LibICloneableFactoryV4.checkImplementationCode.t.sol
@@ -4,14 +4,15 @@ pragma solidity =0.8.25;
 
 import {Test} from "forge-std-1.16.1/src/Test.sol";
 
-import {LibICloneableFactoryV4, ZeroImplementationCodeSize} from "src/lib/LibICloneableFactoryV4.sol";
+import {
+    LibICloneableFactoryV4,
+    DelegatedImplementation,
+    ZeroImplementationCodeSize
+} from "src/lib/LibICloneableFactoryV4.sol";
 import {TestCloneable} from "test/concrete/TestCloneable.sol";
 
 /// @title LibICloneableFactoryV4CheckImplementationCodeTest
-/// @notice Tests `LibICloneableFactoryV4.checkImplementationCode`: a codeless
-/// implementation is always a mistake — its clone would delegate every call,
-/// `initialize` included, to nothing — so it reverts with a typed error
-/// before any deploy happens.
+/// @notice Tests `LibICloneableFactoryV4.checkImplementationCode`.
 contract LibICloneableFactoryV4CheckImplementationCodeTest is Test {
     /// External wrapper so `vm.expectRevert` sees the internal library call at
     /// its own call depth.
@@ -32,61 +33,22 @@ contract LibICloneableFactoryV4CheckImplementationCodeTest is Test {
         LibICloneableFactoryV4.checkImplementationCode(address(implementation));
     }
 
-    /// Any nonempty code is enough to pass: the guard is a code-size check,
-    /// not a validation of what the code is.
+    /// Any nonempty code not beginning with `0xef` passes.
     function testCheckImplementationCodeEtched(address implementation, bytes memory code) external {
         vm.assume(implementation.code.length == 0);
         vm.assume(uint160(implementation) > 0x0a);
         vm.assume(code.length > 0);
-        // EIP-3541 forbids DEPLOYING any code whose first byte is `0xef`, so
-        // no CREATE or CREATE2 can put such code at an address. That leaves
-        // exactly one way an account can hold it — an EIP-7702 delegation
-        // designator, which is `0xef0100` followed by an address and is
-        // therefore EXACTLY 23 bytes. That case is real, so it is not excluded
-        // here, it is pinned by its own test below.
-        //
-        // What this exclusion drops is the rest: `0xef`-leading blobs of any
-        // other length, which no chain can produce. It cannot weaken the
-        // property under test, because the guard only ever looks at code
-        // LENGTH.
-        //
-        // It is also what keeps this test from failing for a harness reason:
-        // `vm.etch` parses a `0xef01` prefix as an EIP-7702 delegation
-        // designator and rejects it unless the blob is exactly 23 bytes
-        // ("Eip7702 is not 23 bytes long"), so a fuzz run that drew one died
-        // in the cheatcode rather than in the code under test.
         vm.assume(code[0] != 0xef);
         vm.etch(implementation, code);
         LibICloneableFactoryV4.checkImplementationCode(implementation);
     }
 
-    /// The one `0xef`-leading code a real account can hold: an EIP-7702
-    /// delegation designator, `0xef0100 || address`, exactly 23 bytes. The
-    /// fuzz test above cannot reach it, so it is pinned here as a fixed case.
-    ///
-    /// It PASSES the guard, and that is the point worth having on the record.
-    /// `EXTCODESIZE` on a delegated EOA returns 23, not zero, so the size
-    /// check cannot tell an ordinary implementation contract from an EOA that
-    /// has delegated — and unlike a deployed contract, a delegation is
-    /// REVOCABLE by the account holder at any time. A caller who wants an
-    /// immutable implementation does not get that from this guard; the guard
-    /// promises only that something is there.
-    ///
-    /// Scoped honestly: `foundry.toml` pins `evm_version = "cancun"`, which
-    /// predates EIP-7702, so what is asserted here is that the 23-byte
-    /// designator is storable at an address and passes the SIZE check. The
-    /// execution semantics of delegation are not exercised and this test does
-    /// not claim them.
+    /// An EIP-7702 delegation designator reverts `DelegatedImplementation`.
     function testCheckImplementationCodeEip7702Designator(address delegated, address delegate) external {
         vm.assume(delegated.code.length == 0);
         vm.assume(uint160(delegated) > 0x0a);
-
-        bytes memory designator = abi.encodePacked(hex"ef0100", delegate);
-        assertEq(designator.length, 23, "an EIP-7702 designator is 23 bytes");
-
-        vm.etch(delegated, designator);
-
-        assertEq(delegated.code.length, 23, "EXTCODESIZE sees the designator, not zero");
-        LibICloneableFactoryV4.checkImplementationCode(delegated);
+        vm.etch(delegated, abi.encodePacked(hex"ef0100", delegate));
+        vm.expectRevert(abi.encodeWithSelector(DelegatedImplementation.selector));
+        this.checkImplementationCodeExternal(delegated);
     }
 }