diff --git a/src/mir/builder.rs b/src/mir/builder.rs
index e8785e8d..4e53097d 100644
--- a/src/mir/builder.rs
+++ b/src/mir/builder.rs
@@ -180,6 +180,14 @@ pub struct MirBuilder {
     pub(super) local_ssa_map: HashMap<(BasicBlockId, ValueId, u8), ValueId>,
     /// BlockSchedule cache: deduplicate materialize copies per (bb, src)
     pub(super) schedule_mat_map: HashMap<(BasicBlockId, ValueId), ValueId>,
+
+    /// Guard flag to prevent re-entering emit_unified_call from BoxCall fallback.
+    /// Used when RouterPolicyBox in emit_unified_call has already decided to
+    /// route a given Method call to BoxCall; emit_box_or_plugin_call must not
+    /// bounce back into the unified path for the same call, otherwise an
+    /// infinite recursion (emit_unified_call → emit_box_or_plugin_call →
+    /// emit_unified_call …) can occur when routing decisions disagree.
+    pub(super) in_unified_boxcall_fallback: bool,
 }
 
 impl MirBuilder {
@@ -232,6 +240,8 @@ impl MirBuilder {
 
             local_ssa_map: HashMap::new(),
             schedule_mat_map: HashMap::new(),
+
+            in_unified_boxcall_fallback: false,
         }
     }
 
diff --git a/src/mir/builder/calls/emit.rs b/src/mir/builder/calls/emit.rs
index b63b580c..f935a6fe 100644
--- a/src/mir/builder/calls/emit.rs
+++ b/src/mir/builder/calls/emit.rs
@@ -108,7 +108,16 @@ impl MirBuilder {
                     eprintln!("[router-guard] {}.{} → BoxCall fallback (recv=%{})", box_name, method, r.0);
                 }
                 let effects = EffectMask::READ.add(Effect::ReadHeap);
-                return self.emit_box_or_plugin_call(dst, *r, method.clone(), None, args, effects);
+                // Prevent BoxCall helper from bouncing back into emit_unified_call
+                // for the same call. RouterPolicyBox has already decided on
+                // Route::BoxCall for this callee, so emit_box_or_plugin_call
+                // must not re-enter the unified path even if its own heuristics
+                // would otherwise choose Unified.
+                let prev_flag = self.in_unified_boxcall_fallback;
+                self.in_unified_boxcall_fallback = true;
+                let res = self.emit_box_or_plugin_call(dst, *r, method.clone(), None, args, effects);
+                self.in_unified_boxcall_fallback = prev_flag;
+                return res;
             }
         }
 
diff --git a/src/mir/builder/utils.rs b/src/mir/builder/utils.rs
index d11e4978..99abb223 100644
--- a/src/mir/builder/utils.rs
+++ b/src/mir/builder/utils.rs
@@ -181,7 +181,14 @@ impl super::MirBuilder {
                 );
             }
         }
-        if use_unified_env && matches!(route, crate::mir::builder::router::policy::Route::Unified) {
+        // Unified path from BoxCall helper is only allowed when we are not
+        // already in a BoxCall fallback originating from emit_unified_call.
+        // in_unified_boxcall_fallback is set by emit_unified_call's RouterPolicy
+        // guard when it has already decided that this call must be a BoxCall.
+        if use_unified_env
+            && matches!(route, crate::mir::builder::router::policy::Route::Unified)
+            && !self.in_unified_boxcall_fallback
+        {
             let target = super::builder_calls::CallTarget::Method {
                 box_type,
                 method: method.clone(),