diff --git a/src/llvm_py/instructions/boxcall.py b/src/llvm_py/instructions/boxcall.py
index ec985ea1..3a628a0c 100644
--- a/src/llvm_py/instructions/boxcall.py
+++ b/src/llvm_py/instructions/boxcall.py
@@ -4,6 +4,10 @@ Core of Nyash's "Everything is Box" philosophy
 """
 
 import llvmlite.ir as ir
+
+# Phase 287 P5: Universal slot constants (SSOT)
+UNIVERSAL_SLOT_TOSTRING = 0  # toString/stringify/str (all types)
+TOSTRING_METHODS = ("toString", "stringify", "str")
 from typing import Dict, List, Optional, Any
 from instructions.safepoint import insert_automatic_safepoint
 from naming_helper import encode_static_method
@@ -93,7 +97,7 @@ def lower_boxcall(
 
     # Phase 287 P4: Universal slot #0 handling (toString/stringify/str)
     # SSOT: toString is ALWAYS slot #0, works on ALL types including primitives
-    if method_id == 0 and method_name in ("toString", "stringify", "str"):
+    if method_id == UNIVERSAL_SLOT_TOSTRING and method_name in TOSTRING_METHODS:
         import os, sys
         if os.environ.get('NYASH_LLVM_TRACE_SLOT') == '1':
             print(f"[llvm-py/slot] Universal slot #0: {method_name} on box_vid={box_vid}", file=sys.stderr)
diff --git a/src/mir/builder/rewrite/known.rs b/src/mir/builder/rewrite/known.rs
index 8c21d707..1bf29d1c 100644
--- a/src/mir/builder/rewrite/known.rs
+++ b/src/mir/builder/rewrite/known.rs
@@ -1,5 +1,42 @@
 use super::super::{MirBuilder, ValueId};
 
+/// Phase 287 P5: Guard against rewriting primitive type toString/stringify/str
+///
+/// Returns true if should block rewrite (primitive type detected)
+/// SSOT: toString/stringify/str use universal slot #0, not user-defined methods
+fn should_block_primitive_str_rewrite(
+    builder: &MirBuilder,
+    object_value: ValueId,
+    method: &str,
+) -> bool {
+    // Only guard toString/stringify/str methods
+    if !(method == "toString" || method == "stringify" || method == "str") {
+        return false;
+    }
+
+    let Some(recv_type) = builder.type_ctx.value_types.get(&object_value) else {
+        return false;
+    };
+
+    use crate::mir::MirType;
+    let is_primitive = match recv_type {
+        MirType::Integer | MirType::Float | MirType::Bool | MirType::String => true,
+        MirType::Box(name) if name == "IntegerBox" || name == "FloatBox"
+            || name == "BoolBox" || name == "StringBox" => true,
+        _ => false,
+    };
+
+    if is_primitive {
+        if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
+            eprintln!("[P287-GUARD] Primitive type {:?} uses universal slot #0 for {}, not user-defined method",
+                recv_type, method);
+        }
+        true
+    } else {
+        false
+    }
+}
+
 /// Gate: whether instance→function rewrite is enabled.
 fn rewrite_enabled() -> bool {
     // New primary flag (P4): NYASH_REWRITE_KNOWN_DEFAULT (default ON; allow explicit OFF)
@@ -53,23 +90,9 @@ pub(crate) fn try_known_rewrite(
         return None;
     }
 
-    // Phase 287 P4: Don't rewrite for primitive types (toString/stringify/str only)
-    // (should use universal slot toString[#0], not user-defined static methods)
-    if method == "toString" || method == "stringify" || method == "str" {
-        if let Some(recv_type) = builder.type_ctx.value_types.get(&object_value) {
-            use crate::mir::MirType;
-            let is_primitive = match recv_type {
-                MirType::Integer | MirType::Float | MirType::Bool | MirType::String => true,
-                MirType::Box(name) if name == "IntegerBox" || name == "FloatBox" || name == "BoolBox" || name == "StringBox" => true,
-                _ => false,
-            };
-            if is_primitive {
-                if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
-                    eprintln!("[P287-GUARD] try_known_rewrite: BLOCKED primitive type {:?} for {}", recv_type, method);
-                }
-                return None;
-            }
-        }
+    // Phase 287 P5: Unified primitive guard (toString/stringify/str use universal slot #0)
+    if should_block_primitive_str_rewrite(builder, object_value, method) {
+        return None;
     }
 
     // Policy gates（従来互換）
@@ -148,23 +171,9 @@ pub(crate) fn try_known_rewrite_to_dst(
         return None;
     }
 
-    // Phase 287 P4: Don't rewrite for primitive types (toString/stringify/str only)
-    // (should use universal slot toString[#0], not user-defined static methods)
-    if method == "toString" || method == "stringify" || method == "str" {
-        if let Some(recv_type) = builder.type_ctx.value_types.get(&object_value) {
-            use crate::mir::MirType;
-            let is_primitive = match recv_type {
-                MirType::Integer | MirType::Float | MirType::Bool | MirType::String => true,
-                MirType::Box(name) if name == "IntegerBox" || name == "FloatBox" || name == "BoolBox" || name == "StringBox" => true,
-                _ => false,
-            };
-            if is_primitive {
-                if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
-                    eprintln!("[P287-GUARD] try_known_rewrite_to_dst: BLOCKED primitive type {:?} for {}", recv_type, method);
-                }
-                return None;
-            }
-        }
+    // Phase 287 P5: Unified primitive guard (toString/stringify/str use universal slot #0)
+    if should_block_primitive_str_rewrite(builder, object_value, method) {
+        return None;
     }
 
     let allow_userbox_rewrite =
@@ -248,23 +257,9 @@ pub(crate) fn try_unique_suffix_rewrite(
         return None;
     }
 
-    // Phase 287 P4: Don't rewrite for primitive types (toString/stringify/str only)
-    // (should use universal slot toString[#0], not user-defined static methods)
-    if method == "toString" || method == "stringify" || method == "str" {
-        if let Some(recv_type) = builder.type_ctx.value_types.get(&object_value) {
-            use crate::mir::MirType;
-            let is_primitive = match recv_type {
-                MirType::Integer | MirType::Float | MirType::Bool | MirType::String => true,
-                MirType::Box(name) if name == "IntegerBox" || name == "FloatBox" || name == "BoolBox" || name == "StringBox" => true,
-                _ => false,
-            };
-            if is_primitive {
-                if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
-                    eprintln!("[P287-GUARD] try_unique_suffix_rewrite: BLOCKED primitive type {:?} for {}", recv_type, method);
-                }
-                return None;
-            }
-        }
+    // Phase 287 P5: Unified primitive guard (toString/stringify/str use universal slot #0)
+    if should_block_primitive_str_rewrite(builder, object_value, method) {
+        return None;
     }
 
     // unified
@@ -324,23 +319,9 @@ pub(crate) fn try_unique_suffix_rewrite_to_dst(
         return None;
     }
 
-    // Phase 287 P4: Don't rewrite for primitive types (toString/stringify/str only)
-    // (should use universal slot toString[#0], not user-defined static methods)
-    if method == "toString" || method == "stringify" || method == "str" {
-        if let Some(recv_type) = builder.type_ctx.value_types.get(&object_value) {
-            use crate::mir::MirType;
-            let is_primitive = match recv_type {
-                MirType::Integer | MirType::Float | MirType::Bool | MirType::String => true,
-                MirType::Box(name) if name == "IntegerBox" || name == "FloatBox" || name == "BoolBox" || name == "StringBox" => true,
-                _ => false,
-            };
-            if is_primitive {
-                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 {:?} for {}", recv_type, method);
-                }
-                return None;
-            }
-        }
+    // Phase 287 P5: Unified primitive guard (toString/stringify/str use universal slot #0)
+    if should_block_primitive_str_rewrite(builder, object_value, method) {
+        return None;
     }
 
     let _name_const = match crate::mir::builder::name_const::make_name_const_result(builder, &fname)
diff --git a/src/mir/builder/rewrite/special.rs b/src/mir/builder/rewrite/special.rs
index b527484c..0772dfb1 100644
--- a/src/mir/builder/rewrite/special.rs
+++ b/src/mir/builder/rewrite/special.rs
@@ -1,5 +1,12 @@
 use super::super::MirBuilder;
 
+/// Phase 287 P5: Trace helper for toString normalization debugging
+fn trace_tostring(msg: impl std::fmt::Display) {
+    if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
+        eprintln!("[P287-BOXCALL] {}", msg);
+    }
+}
+
 /// Phase 287 P4: toString/stringify/str → BoxCall(slot #0) normalization (SSOT)
 ///
 /// Root cause fix: toString is a universal slot [#0] method that should ALWAYS use BoxCall,
@@ -23,9 +30,7 @@ pub(crate) fn try_early_str_like_to_dst(
         return None;
     }
 
-    if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
-        eprintln!("[P287-BOXCALL] Normalizing {method} to BoxCall(slot #0) for object_value={:?}", object_value);
-    }
+    trace_tostring(format!("Normalizing {method} to BoxCall(slot #0) for object_value={:?}", object_value));
 
     // Phase 287 P4: ALWAYS emit BoxCall for toString/stringify/str (universal slot #0)
     // Do NOT rewrite to Global or Method - BoxCall is the SSOT for display methods
@@ -48,9 +53,7 @@ pub(crate) fn try_early_str_like_to_dst(
         super::super::MirType::Box("StringBox".to_string()),
     );
 
-    if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
-        eprintln!("[P287-BOXCALL] Emitted BoxCall for {method} -> dst={:?}", actual_dst);
-    }
+    trace_tostring(format!("Emitted BoxCall for {method} -> dst={:?}", actual_dst));
 
     Some(Ok(actual_dst))
 }