diff --git a/src/mir/loop_builder.rs b/src/mir/loop_builder.rs
index ab223291..bba016a1 100644
--- a/src/mir/loop_builder.rs
+++ b/src/mir/loop_builder.rs
@@ -654,17 +654,42 @@ impl<'a> LoopBuilder<'a> {
         // Region 情報（entry/exit/slots）をログに出すよ。
         crate::mir::region::observer::observe_control_form(self.parent_builder, &form);
 
-        // [LoopForm] exit PHI for Case A/B (uses exit_snapshots + exit_preds)
-        // - Case A: header+break → exit PHI includes both paths
-        // - Case B: break-only → exit PHI excludes header (not a predecessor)
-        let exit_snaps = self.exit_snapshots.clone();
-        crate::mir::phi_core::loopform_builder::build_exit_phis_for_control(
-            &loopform,
-            self,
-            &form,
-            &exit_snaps,
-            branch_source_block,
-        )?;
+        // Phase 27.6-2: JoinIR Exit φ バイパスチェック
+        let fn_name = self
+            .parent_builder
+            .current_function
+            .as_ref()
+            .map(|f| f.signature.name.as_str())
+            .unwrap_or("");
+
+        let exit_bypass = crate::mir::phi_core::exit_phi_builder::joinir_exit_bypass_enabled()
+            && crate::mir::phi_core::exit_phi_builder::is_joinir_exit_bypass_target(fn_name);
+
+        if exit_bypass {
+            // Phase 27.6-2: JoinIR 実験経路では Exit φ を生成しない。
+            // ループ内で定義された値だけで exit 後を構成する（JoinIR の k_exit 引数として表現）。
+            //
+            // ⚠️ 重要: このモードでは MIR は不完全（Exit φ 抜け）であり、VM で実行できない。
+            // JoinIR runner 専用モードであることに注意。
+            if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
+                eprintln!(
+                    "[loopform/exit-bypass] func={} exit={:?} header={:?} (JoinIR experiment only)",
+                    fn_name, exit_id, header_id
+                );
+            }
+        } else {
+            // [LoopForm] exit PHI for Case A/B (uses exit_snapshots + exit_preds)
+            // - Case A: header+break → exit PHI includes both paths
+            // - Case B: break-only → exit PHI excludes header (not a predecessor)
+            let exit_snaps = self.exit_snapshots.clone();
+            crate::mir::phi_core::loopform_builder::build_exit_phis_for_control(
+                &loopform,
+                self,
+                &form,
+                &exit_snaps,
+                branch_source_block,
+            )?;
+        }
 
         // Pop loop context
         crate::mir::builder::loops::pop_loop_context(self.parent_builder);
diff --git a/src/mir/phi_core/exit_phi_builder.rs b/src/mir/phi_core/exit_phi_builder.rs
index 35d1e6c5..c2116c71 100644
--- a/src/mir/phi_core/exit_phi_builder.rs
+++ b/src/mir/phi_core/exit_phi_builder.rs
@@ -394,6 +394,30 @@ impl<T: crate::mir::phi_core::loopform_builder::LoopFormOps> LoopFormOps for T {
     }
 }
 
+// ============================================================================
+// Phase 27.6-2: JoinIR Exit φ バイパス用ヘルパー関数
+// ============================================================================
+
+/// JoinIR Exit φ 縮退実験トグル
+///
+/// - NYASH_JOINIR_EXPERIMENT=1
+/// - NYASH_JOINIR_EXIT_EXP=1
+///
+/// の両方が立っているときだけ true。
+pub(crate) fn joinir_exit_bypass_enabled() -> bool {
+    crate::mir::join_ir::env_flag_is_1("NYASH_JOINIR_EXPERIMENT")
+        && crate::mir::join_ir::env_flag_is_1("NYASH_JOINIR_EXIT_EXP")
+}
+
+/// Exit φ バイパス対象の関数かどうか
+///
+/// 当面は minimal/trim の 2 本だけ:
+/// - Main.skip/1
+/// - FuncScannerBox.trim/1
+pub(crate) fn is_joinir_exit_bypass_target(func_name: &str) -> bool {
+    matches!(func_name, "Main.skip/1" | "FuncScannerBox.trim/1")
+}
+
 // ============================================================================
 // Unit Tests
 // ============================================================================