diff --git a/src/mir/loop_builder.rs b/src/mir/loop_builder.rs
index 96e705ab..0e3b446a 100644
--- a/src/mir/loop_builder.rs
+++ b/src/mir/loop_builder.rs
@@ -340,6 +340,12 @@ impl<'a> LoopBuilder<'a> {
         use std::collections::HashSet;
         let mut writes = HashSet::new();
         for (name, &body_value) in &body_end_vars {
+            // Skip __pin$ temporary variables - they are always BodyLocalInternal
+            // (Task先生の発見: これらをcarrier扱いすると未定義ValueIdエラーの原因になる)
+            if name.starts_with("__pin$") && name.contains("$@") {
+                continue;
+            }
+
             if let Some(&base_value) = current_vars.get(name) {
                 if body_value != base_value {
                     writes.insert(name.clone());
diff --git a/src/mir/phi_core/loop_var_classifier.rs b/src/mir/phi_core/loop_var_classifier.rs
index 5829cb2e..8ad1f0db 100644
--- a/src/mir/phi_core/loop_var_classifier.rs
+++ b/src/mir/phi_core/loop_var_classifier.rs
@@ -174,6 +174,14 @@ impl LoopVarClassBox {
         inspector: &LocalScopeInspectorBox,
         exit_preds: &[BasicBlockId],
     ) -> LoopVarClass {
+        // Priority 0: __pin$ temporary variables are ALWAYS BodyLocalInternal
+        // Reason: These are compiler-generated temporaries for pinning expression values.
+        // They are defined inside loop bodies and should NEVER get exit PHIs.
+        // Task先生の発見: ValueId(289)等の未定義値エラーの根本原因！
+        if var_name.starts_with("__pin$") && var_name.contains("$@") {
+            return LoopVarClass::BodyLocalInternal;
+        }
+
         // Priority 1: Check if it's a pinned variable
         if pinned_vars.iter().any(|p| p == var_name) {
             return LoopVarClass::Pinned;
@@ -464,6 +472,39 @@ mod tests {
         assert!(!candidates.contains(&"ch".to_string()));  // ← Option C: ch is filtered out!
     }
 
+    /// Test Task先生の発見: __pin$ temporary variables should be BodyLocalInternal
+    #[test]
+    fn test_classify_pin_temporary_variables() {
+        let inspector = LocalScopeInspectorBox::new();
+        let classifier = LoopVarClassBox::new();
+
+        // __pin$ temporary variables should ALWAYS be BodyLocalInternal
+        // regardless of their definition locations
+        let pin_vars = vec![
+            "__pin$285$@binop_lhs",
+            "__pin$286$@binop_rhs",
+            "__pin$437$@binop_lhs",
+            "__pin$297$@assign",
+        ];
+
+        for pin_var in pin_vars {
+            let class = classifier.classify(
+                pin_var,
+                &[],  // Not pinned
+                &[],  // Not carrier
+                &inspector,
+                &[],  // No exit preds
+            );
+
+            assert_eq!(class, LoopVarClass::BodyLocalInternal,
+                "Variable '{}' should be BodyLocalInternal", pin_var);
+            assert!(!class.needs_exit_phi(),
+                "Variable '{}' should NOT need exit PHI", pin_var);
+            assert!(!class.needs_header_phi(),
+                "Variable '{}' should NOT need header PHI", pin_var);
+        }
+    }
+
     #[test]
     fn test_description() {
         assert_eq!(LoopVarClass::Pinned.description(), "Loop-crossing parameter");
diff --git a/src/runner/json_v0_bridge/lowering/loop_.rs b/src/runner/json_v0_bridge/lowering/loop_.rs
index a94b67ba..03316b47 100644
--- a/src/runner/json_v0_bridge/lowering/loop_.rs
+++ b/src/runner/json_v0_bridge/lowering/loop_.rs
@@ -276,6 +276,12 @@ pub(super) fn lower_loop_stmt(
     use std::collections::HashSet;
     let mut writes = HashSet::new();
     for (name, &body_value) in &body_vars {
+        // Skip __pin$ temporary variables - they are always BodyLocalInternal
+        // (Task先生の発見: これらをcarrier扱いすると未定義ValueIdエラーの原因になる)
+        if name.starts_with("__pin$") && name.contains("$@") {
+            continue;
+        }
+
         if let Some(&base_value) = base_vars.get(name) {
             if body_value != base_value {
                 writes.insert(name.clone());