diff --git a/tests/charon_llbc/src/lib.rs b/tests/charon_llbc/src/lib.rs
index eb40e9f..b5193b4 100644
--- a/tests/charon_llbc/src/lib.rs
+++ b/tests/charon_llbc/src/lib.rs
@@ -1,9 +1,13 @@
 //! Minimal Rust crate used to smoke-test the charon_llbc Bazel rule.
 //! Kept deliberately small: `charon rustc --preset=aeneas` should translate
 //! this without hitting any unsupported features.
+//!
+//! MUTANT #2 (mutation-testing spike, rules_lean#23): `add` drops the `y`
+//! term entirely. Expect: KILLED — add.spec requires z.val = x.val + y.val.
 
 pub fn add(x: u32, y: u32) -> u32 {
-    x + y
+    let _ = y;
+    x
 }
 
 pub fn identity(x: u64) -> u64 {