diff --git a/src/mir/loop_builder.rs b/src/mir/loop_builder.rs
index bd03ea80..96e705ab 100644
--- a/src/mir/loop_builder.rs
+++ b/src/mir/loop_builder.rs
@@ -384,88 +384,21 @@ impl<'a> LoopBuilder<'a> {
         // BreakFinderBox / FuncScannerBox 等で、loop body 内で新規宣言された local 変数が
         // loop header に戻った時に undefined になる問題を修正
         //
-        // Step 5-5-A: PHI pred mismatch 根本修正
-        // - Preheader入力（poison）とLatch入力の両方を即座に追加
-        // - 空PHI生成→後で埋める方式は不完全だった（seal_phisが処理しない）
+        // Step 5-5-B: EXPERIMENTAL - Body-local Header PHI generation DISABLED
+        // Reason: Option C design states body-local variables should NOT have header PHIs
+        // - BodyLocalExit: needs EXIT PHI only, NOT header PHI
+        // - BodyLocalInternal: needs NO PHI at all
+        //
+        // TODO Step 5-3: Integrate Option C classification (LoopVarClassBox) here
+        //
+        // TEMPORARY DISABLE to test hypothesis that header PHIs are the root cause
         let trace_loop_phi = std::env::var("HAKO_LOOP_PHI_TRACE").ok().as_deref() == Some("1");
-        if trace_loop_phi {
-            eprintln!("[loop-phi/body-local] Checking for body-local variables");
-            eprintln!("  current_vars.len()={}, body_end_vars.len()={}", current_vars.len(), body_end_vars.len());
-        }
 
-        // Detect body-local variables (variables declared in body, not in preheader)
-        let body_local_vars: Vec<(String, ValueId)> = body_end_vars
-            .iter()
-            .filter(|(name, _value)| !current_vars.contains_key(name.as_str()))
-            .map(|(name, value)| (name.clone(), *value))
-            .collect();
-
-        if !body_local_vars.is_empty() {
-            if trace_loop_phi {
-                eprintln!("[loop-phi/body-local] Found {} body-local variables", body_local_vars.len());
-            }
-
-            // Step 5-5-A: Add PHI nodes with BOTH inputs (preheader + latch)
-            for (var_name, _body_value) in &body_local_vars {
-                if trace_loop_phi {
-                    eprintln!("[loop-phi/body-local] Adding PHI for body-local var: {}", var_name);
-                }
-
-                let phi_id = self.new_value();
-
-                // Step 5-5-A: Preheader input is poison (variable doesn't exist yet)
-                let preheader_poison = self.new_value();
-
-                // Emit Const::Poison instruction in preheader block
-                if let Some(ref mut func) = self.parent_builder.current_function {
-                    if let Some(preheader_block) = func.blocks.get_mut(&preheader_id) {
-                        preheader_block.add_instruction(MirInstruction::Const {
-                            dst: preheader_poison,
-                            value: crate::mir::ConstValue::Null,  // Poison semantics
-                        });
-                    }
-                }
-
-                // Step 5-5-A FIX: Get latch value from current variable map
-                // The variable map was updated at line 374-377 with body_end_vars,
-                // so looking it up now gives us the correct ValueId for the latch block.
-                let latch_value = self.get_current_variable_map()
-                    .get(var_name)
-                    .copied()
-                    .unwrap_or(phi_id);  // Fallback to phi_id if not found
-
-                // Step 5-5-A: Insert PHI with BOTH inputs (preheader poison + latch value)
-                if let Some(ref mut func) = self.parent_builder.current_function {
-                    if let Some(header_block) = func.blocks.get_mut(&header_id) {
-                        // Find position after existing PHIs
-                        let phi_count = header_block.phi_instructions().count();
-                        header_block.instructions.insert(
-                            phi_count,
-                            MirInstruction::Phi {
-                                dst: phi_id,
-                                inputs: vec![
-                                    (preheader_id, preheader_poison),  // ← Preheader input (poison)
-                                    (latch_id, latch_value),           // ← Latch input (from variable map)
-                                ],
-                            },
-                        );
-                    }
-                }
-
-                // Rebind variable to PHI
-                self.update_variable(var_name.clone(), phi_id);
-
-                if trace_loop_phi {
-                    eprintln!(
-                        "[loop-phi/body-local] Created PHI {}: preheader={} (poison), latch={} ({})",
-                        phi_id.0, preheader_poison.0, latch_id.0, latch_value.0
-                    );
-                }
-            }
-
-            if trace_loop_phi {
-                eprintln!("[loop-phi/body-local] Added {} body-local PHIs with complete inputs", body_local_vars.len());
-            }
+        // DISABLED: Body-local header PHI generation
+        // This code was causing undefined value errors because it created header PHIs
+        // for variables that should only have exit PHIs (or no PHIs at all)
+        if false {  // Disabled for Step 5-5-B experiment
+            // [Original code removed - see git history if needed]
         }
 
         // Pass 4: Generate continue_merge PHIs first, then seal header PHIs