diff --git a/src/mir/builder/control_flow.rs b/src/mir/builder/control_flow.rs
index d7d3811d..67b4995e 100644
--- a/src/mir/builder/control_flow.rs
+++ b/src/mir/builder/control_flow.rs
@@ -56,7 +56,18 @@ impl super::MirBuilder {
             eprintln!("[cf_loop] CALLED from somewhere");
             eprintln!("[cf_loop] Current stack (simulated): check build_statement vs build_expression_impl");
         }
-        // Delegate to LoopBuilder for consistent handling
+
+        // Phase 186: LoopBuilder Hard Freeze - Require explicit legacy mode opt-in
+        if !crate::config::env::joinir_dev::legacy_loopbuilder_enabled() {
+            return Err(format!(
+                "[joinir/freeze] Loop lowering failed: JoinIR does not support this pattern, and LoopBuilder is disabled.\n\
+                 Function: {}\n\
+                 Hint: Set NYASH_LEGACY_LOOPBUILDER=1 to enable legacy path (dev-only)",
+                self.current_function.as_ref().map(|f| f.signature.name.as_str()).unwrap_or("<unknown>")
+            ));
+        }
+
+        // Delegate to LoopBuilder for consistent handling (legacy path only)
         let mut loop_builder = crate::mir::loop_builder::LoopBuilder::new(self);
         loop_builder.build_loop(condition, body)
     }