diff --git a/src/mir/builder/calls/unified_emitter.rs b/src/mir/builder/calls/unified_emitter.rs
index c4605429..cdef6b5a 100644
--- a/src/mir/builder/calls/unified_emitter.rs
+++ b/src/mir/builder/calls/unified_emitter.rs
@@ -68,6 +68,11 @@ impl UnifiedCallEmitterBox {
         target: CallTarget,
         args: Vec<ValueId>,
     ) -> Result<(), String> {
+        // Phase 287 P4: Debug trace to see what CallTarget is passed
+        if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
+            eprintln!("[P287-TRACE] emit_unified_call_impl: target={:?}, dst={:?}, args={:?}", target, dst, args);
+        }
+
         // Emit resolve.try for method targets (dev-only; default OFF)
         let arity_for_try = args.len();
         if let CallTarget::Method {
diff --git a/src/mir/builder/rewrite/known.rs b/src/mir/builder/rewrite/known.rs
index c5a97e67..0c5d3eed 100644
--- a/src/mir/builder/rewrite/known.rs
+++ b/src/mir/builder/rewrite/known.rs
@@ -52,6 +52,22 @@ pub(crate) fn try_known_rewrite(
     if !builder.comp_ctx.user_defined_boxes.contains_key(cls) { // Phase 285LLVM-1.1: HashMap
         return None;
     }
+
+    // Phase 287 P4: Don't rewrite for primitive types
+    // (should use universal slot toString[#0], not user-defined static methods)
+    if let Some(recv_type) = builder.type_ctx.value_types.get(&object_value) {
+        use crate::mir::MirType;
+        match recv_type {
+            MirType::Integer | MirType::Float | MirType::Bool | MirType::String => {
+                if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
+                    eprintln!("[P287-GUARD] try_known_rewrite: BLOCKED primitive type {:?}", recv_type);
+                }
+                return None;
+            }
+            _ => {}
+        }
+    }
+
     // Policy gates（従来互換）
     let allow_userbox_rewrite =
         std::env::var("NYASH_DEV_REWRITE_USERBOX").ok().as_deref() == Some("1");
@@ -127,6 +143,22 @@ pub(crate) fn try_known_rewrite_to_dst(
     if !builder.comp_ctx.user_defined_boxes.contains_key(cls) { // Phase 285LLVM-1.1: HashMap
         return None;
     }
+
+    // Phase 287 P4: Don't rewrite for primitive types
+    // (should use universal slot toString[#0], not user-defined static methods)
+    if let Some(recv_type) = builder.type_ctx.value_types.get(&object_value) {
+        use crate::mir::MirType;
+        match recv_type {
+            MirType::Integer | MirType::Float | MirType::Bool | MirType::String => {
+                if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
+                    eprintln!("[P287-GUARD] try_known_rewrite_to_dst: BLOCKED primitive type {:?}", recv_type);
+                }
+                return None;
+            }
+            _ => {}
+        }
+    }
+
     let allow_userbox_rewrite =
         std::env::var("NYASH_DEV_REWRITE_USERBOX").ok().as_deref() == Some("1");
     let allow_new_origin = std::env::var("NYASH_DEV_REWRITE_NEW_ORIGIN")
@@ -207,6 +239,22 @@ pub(crate) fn try_unique_suffix_rewrite(
     if !builder.comp_ctx.user_defined_boxes.contains_key(&id.box_name) { // Phase 285LLVM-1.1: HashMap
         return None;
     }
+
+    // Phase 287 P4: Don't rewrite for primitive types
+    // (should use universal slot toString[#0], not user-defined static methods)
+    if let Some(recv_type) = builder.type_ctx.value_types.get(&object_value) {
+        use crate::mir::MirType;
+        match recv_type {
+            MirType::Integer | MirType::Float | MirType::Bool | MirType::String => {
+                if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
+                    eprintln!("[P287-GUARD] try_unique_suffix_rewrite: BLOCKED primitive type {:?}", recv_type);
+                }
+                return None;
+            }
+            _ => {}
+        }
+    }
+
     // unified
     let mut call_args = Vec::with_capacity(arg_values.len() + 1);
     call_args.push(object_value); // 'me'
@@ -263,6 +311,22 @@ pub(crate) fn try_unique_suffix_rewrite_to_dst(
     if !builder.comp_ctx.user_defined_boxes.contains_key(&id.box_name) { // Phase 285LLVM-1.1: HashMap
         return None;
     }
+
+    // Phase 287 P4: Don't rewrite for primitive types
+    // (should use universal slot toString[#0], not user-defined static methods)
+    if let Some(recv_type) = builder.type_ctx.value_types.get(&object_value) {
+        use crate::mir::MirType;
+        match recv_type {
+            MirType::Integer | MirType::Float | MirType::Bool | MirType::String => {
+                if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
+                    eprintln!("[P287-GUARD] try_unique_suffix_rewrite_to_dst: BLOCKED primitive type {:?}", recv_type);
+                }
+                return None;
+            }
+            _ => {}
+        }
+    }
+
     let _name_const = match crate::mir::builder::name_const::make_name_const_result(builder, &fname)
     {
         Ok(v) => v,
diff --git a/src/mir/builder/rewrite/special.rs b/src/mir/builder/rewrite/special.rs
index c8d474b8..331e6040 100644
--- a/src/mir/builder/rewrite/special.rs
+++ b/src/mir/builder/rewrite/special.rs
@@ -161,6 +161,40 @@ pub(crate) fn try_early_str_like_to_dst(
         None => return None,
     };
     if let Some(cls) = class_name_opt.clone() {
+        // Phase 287 P4: Conservative early rewrite guards
+
+        if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
+            eprintln!("[P287-GUARD] try_early_str_like_to_dst: cls={}, object_value={:?}", cls, object_value);
+            eprintln!("[P287-GUARD]   current_static_box={:?}", builder.comp_ctx.current_static_box());
+            eprintln!("[P287-GUARD]   value_type={:?}", builder.type_ctx.value_types.get(&object_value));
+        }
+
+        // Guard 1: Don't rewrite if class is current static box context
+        // (likely context contamination, not actual receiver type)
+        if let Some(current_box) = builder.comp_ctx.current_static_box() {
+            if cls == current_box {
+                if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
+                    eprintln!("[P287-GUARD] -> Guard 1 BLOCKED: cls == current_static_box ({})", current_box);
+                }
+                return None;
+            }
+        }
+
+        // Guard 2: Don't rewrite for primitive types
+        // (should use universal slot toString[#0])
+        if let Some(recv_type) = builder.type_ctx.value_types.get(&object_value) {
+            use crate::mir::MirType;
+            match recv_type {
+                MirType::Integer | MirType::Float | MirType::Bool | MirType::String => {
+                    if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
+                        eprintln!("[P287-GUARD] -> Guard 2 BLOCKED: primitive type {:?}", recv_type);
+                    }
+                    return None;
+                }
+                _ => {}
+            }
+        }
+
         let str_name = crate::mir::builder::calls::function_lowering::generate_method_function_name(
             &cls, "str", 0,
         );