diff --git a/crates/fidc-core/src/numeric_expr_vm.rs b/crates/fidc-core/src/numeric_expr_vm.rs
index bfd4756..a854143 100644
--- a/crates/fidc-core/src/numeric_expr_vm.rs
+++ b/crates/fidc-core/src/numeric_expr_vm.rs
@@ -168,36 +168,40 @@ impl Program {
                 Instruction::Push(value) => scratch.stack.push(value),
                 Instruction::LoadVariable(index) => {
                     let index = usize::from(index);
-                    let cached = scratch.variables[index];
-                    let value = match cached {
-                        Some(value) => value,
-                        None => {
-                            let expected_type = self.variable_types[index];
-                            let value = resolve(index, &self.variables[index], expected_type)?;
-                            if value.value_type() != expected_type {
-                                return Err(EvalError::new(format!(
-                                    "variable {} expected {:?}, got {:?}",
-                                    self.variables[index],
-                                    expected_type,
-                                    value.value_type()
-                                )));
-                            }
-                            scratch.variables[index] = Some(value);
-                            value
+                    let value = if scratch.variable_generations[index] == scratch.generation {
+                        scratch.variables[index]
+                    } else {
+                        let expected_type = self.variable_types[index];
+                        let value = resolve(index, &self.variables[index], expected_type)?;
+                        if value.value_type() != expected_type {
+                            return Err(EvalError::new(format!(
+                                "variable {} expected {:?}, got {:?}",
+                                self.variables[index],
+                                expected_type,
+                                value.value_type()
+                            )));
                         }
+                        scratch.variables[index] = value;
+                        scratch.variable_generations[index] = scratch.generation;
+                        value
                     };
                     scratch.stack.push(value);
                 }
                 Instruction::LoadLocal(index) => {
                     let index = usize::from(index);
-                    let value = scratch.locals[index].ok_or_else(|| {
-                        EvalError::new(format!("local slot {index} was not initialized"))
-                    })?;
+                    if scratch.local_generations[index] != scratch.generation {
+                        return Err(EvalError::new(format!(
+                            "local slot {index} was not initialized"
+                        )));
+                    }
+                    let value = scratch.locals[index];
                     scratch.stack.push(value);
                 }
                 Instruction::StoreLocal(index) => {
+                    let index = usize::from(index);
                     let value = pop(&mut scratch.stack)?;
-                    scratch.locals[usize::from(index)] = Some(value);
+                    scratch.locals[index] = value;
+                    scratch.local_generations[index] = scratch.generation;
                 }
                 Instruction::Unary(operator) => {
                     let value = pop(&mut scratch.stack)?;
@@ -260,8 +264,11 @@ impl Program {
 #[derive(Debug, Default)]
 pub(crate) struct Scratch {
     stack: Vec<Value>,
-    variables: Vec<Option<Value>>,
-    locals: Vec<Option<Value>>,
+    variables: Vec<Value>,
+    variable_generations: Vec<u32>,
+    locals: Vec<Value>,
+    local_generations: Vec<u32>,
+    generation: u32,
 }
 
 impl Scratch {
@@ -273,10 +280,21 @@ impl Scratch {
                 .len()
                 .saturating_sub(self.stack.capacity()),
         );
-        self.variables.clear();
-        self.variables.resize(program.variables.len(), None);
-        self.locals.clear();
-        self.locals.resize(program.local_count, None);
+        self.generation = self.generation.wrapping_add(1);
+        if self.generation == 0 {
+            self.variable_generations.fill(0);
+            self.local_generations.fill(0);
+            self.generation = 1;
+        }
+        if self.variables.len() < program.variables.len() {
+            self.variables
+                .resize(program.variables.len(), Value::Number(0.0));
+            self.variable_generations.resize(program.variables.len(), 0);
+        }
+        if self.locals.len() < program.local_count {
+            self.locals.resize(program.local_count, Value::Number(0.0));
+            self.local_generations.resize(program.local_count, 0);
+        }
     }
 }
 
@@ -1362,6 +1380,27 @@ mod tests {
         }
     }
 
+    #[test]
+    fn scratch_generation_rollover_invalidates_cached_slots() {
+        let program = compile("left + right", |_| Some(ValueType::Number)).expect("compile");
+        let mut scratch = Scratch::default();
+        let first = program
+            .evaluate(&mut scratch, |_index, name, _expected| {
+                Ok(Value::Number(if name == "left" { 1.0 } else { 2.0 }))
+            })
+            .expect("first evaluation");
+        assert_eq!(first, Value::Number(3.0));
+
+        scratch.generation = u32::MAX;
+        let second = program
+            .evaluate(&mut scratch, |_index, name, _expected| {
+                Ok(Value::Number(if name == "left" { 10.0 } else { 20.0 }))
+            })
+            .expect("evaluation after generation rollover");
+        assert_eq!(second, Value::Number(30.0));
+        assert_eq!(scratch.generation, 1);
+    }
+
     #[test]
     fn matches_rhai_for_representative_numeric_boolean_corpus() {
         let source = r#"