diff --git a/src/backend/vm.rs b/src/backend/vm.rs
index 4ef18b5c..d8c92d61 100644
--- a/src/backend/vm.rs
+++ b/src/backend/vm.rs
@@ -751,14 +751,14 @@ impl VM {
         if let Some(array_box) = box_value.as_any().downcast_ref::<crate::boxes::array::ArrayBox>() {
             match method {
                 "length" | "len" => {
-                    let items = array_box.items.lock().unwrap();
+                    let items = array_box.items.read().unwrap();
                     return Ok(Box::new(IntegerBox::new(items.len() as i64)));
                 },
                 "get" => {
                     // get(index) - get element at index
                     if let Some(index_box) = _args.get(0) {
                         if let Some(index_int) = index_box.as_any().downcast_ref::<IntegerBox>() {
-                            let items = array_box.items.lock().unwrap();
+                            let items = array_box.items.read().unwrap();
                             let index = index_int.value as usize;
                             if index < items.len() {
                                 return Ok(items[index].clone_box());
diff --git a/src/box_trait.rs b/src/box_trait.rs
index 844c475e..752b041a 100644
--- a/src/box_trait.rs
+++ b/src/box_trait.rs
@@ -176,7 +176,7 @@ impl StringBox {
     /// Join array elements using this string as delimiter
     pub fn join(&self, array_box: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
         if let Some(array) = array_box.as_any().downcast_ref::<ArrayBox>() {
-            let strings: Vec<String> = array.items.lock().unwrap()
+            let strings: Vec<String> = array.items.read().unwrap()
                 .iter()
                 .map(|element| element.to_string_box().value)
                 .collect();
diff --git a/src/boxes/array/mod.rs b/src/boxes/array/mod.rs
index 33853490..8566ea37 100644
--- a/src/boxes/array/mod.rs
+++ b/src/boxes/array/mod.rs
@@ -299,8 +299,8 @@ impl NyashBox for ArrayBox {
 
     fn equals(&self, other: &dyn NyashBox) -> BoolBox {
         if let Some(other_array) = other.as_any().downcast_ref::<ArrayBox>() {
-            let self_items = self.items.lock().unwrap();
-            let other_items = other_array.items.lock().unwrap();
+            let self_items = self.items.read().unwrap();
+            let other_items = other_array.items.read().unwrap();
             
             if self_items.len() != other_items.len() {
                 return BoolBox::new(false);
@@ -318,3 +318,14 @@ impl NyashBox for ArrayBox {
         }
     }
 }
+
+// Debug implementation for ArrayBox
+impl std::fmt::Debug for ArrayBox {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let items = self.items.read().unwrap();
+        f.debug_struct("ArrayBox")
+            .field("id", &self.base.id)
+            .field("length", &items.len())
+            .finish()
+    }
+}
diff --git a/src/boxes/buffer/mod.rs b/src/boxes/buffer/mod.rs
index 70788be1..4fa21ec5 100644
--- a/src/boxes/buffer/mod.rs
+++ b/src/boxes/buffer/mod.rs
@@ -216,3 +216,14 @@ impl NyashBox for BufferBox {
         }
     }
 }
+
+// Debug implementation for BufferBox  
+impl std::fmt::Debug for BufferBox {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let data = self.data.read().unwrap();
+        f.debug_struct("BufferBox")
+            .field("id", &self.base.id)
+            .field("length", &data.len())
+            .finish()
+    }
+}
diff --git a/src/boxes/debug_box.rs b/src/boxes/debug_box.rs
index 30e0abfd..caa4e627 100644
--- a/src/boxes/debug_box.rs
+++ b/src/boxes/debug_box.rs
@@ -100,7 +100,7 @@
  */
 
 use std::collections::HashMap;
-use std::sync::{Arc, Mutex};
+use std::sync::RwLock;
 use chrono::Local;
 use crate::box_trait::{BoxCore, BoxBase, next_box_id, NyashBox, StringBox, BoolBox, VoidBox};
 use crate::interpreter::RuntimeError;
@@ -110,10 +110,10 @@ use std::any::Any;
 #[derive(Debug, Clone)]
 pub struct DebugBox {
     base: BoxBase,
-    tracking_enabled: Arc<Mutex<bool>>,
-    tracked_boxes: Arc<Mutex<HashMap<String, TrackedBoxInfo>>>,
-    breakpoints: Arc<Mutex<Vec<String>>>,
-    call_stack: Arc<Mutex<Vec<CallInfo>>>,
+    tracking_enabled: RwLock<bool>,
+    tracked_boxes: RwLock<HashMap<String, TrackedBoxInfo>>,
+    breakpoints: RwLock<Vec<String>>,
+    call_stack: RwLock<Vec<CallInfo>>,
 }
 
 #[derive(Debug, Clone)]
@@ -135,34 +135,34 @@ impl DebugBox {
     pub fn new() -> Self {
         DebugBox {
             base: BoxBase::new(),
-            tracking_enabled: Arc::new(Mutex::new(false)),
-            tracked_boxes: Arc::new(Mutex::new(HashMap::new())),
-            breakpoints: Arc::new(Mutex::new(Vec::new())),
-            call_stack: Arc::new(Mutex::new(Vec::new())),
+            tracking_enabled: RwLock::new(false),
+            tracked_boxes: RwLock::new(HashMap::new()),
+            breakpoints: RwLock::new(Vec::new()),
+            call_stack: RwLock::new(Vec::new()),
         }
     }
 
     pub fn start_tracking(&self) -> Result<Box<dyn NyashBox>, RuntimeError> {
-        let mut enabled = self.tracking_enabled.lock().unwrap();
+        let mut enabled = self.tracking_enabled.write().unwrap();
         *enabled = true;
         println!("[DEBUG] Tracking started");
         Ok(Box::new(VoidBox::new()))
     }
 
     pub fn stop_tracking(&self) -> Result<Box<dyn NyashBox>, RuntimeError> {
-        let mut enabled = self.tracking_enabled.lock().unwrap();
+        let mut enabled = self.tracking_enabled.write().unwrap();
         *enabled = false;
         println!("[DEBUG] Tracking stopped");
         Ok(Box::new(VoidBox::new()))
     }
 
     pub fn track_box(&self, box_value: &dyn NyashBox, name: &str) -> Result<Box<dyn NyashBox>, RuntimeError> {
-        let enabled = self.tracking_enabled.lock().unwrap();
+        let enabled = self.tracking_enabled.read().unwrap();
         if !*enabled {
             return Ok(Box::new(VoidBox::new()));
         }
 
-        let mut tracked = self.tracked_boxes.lock().unwrap();
+        let mut tracked = self.tracked_boxes.write().unwrap();
         
         let info = TrackedBoxInfo {
             box_type: box_value.type_name().to_string(),
@@ -298,7 +298,7 @@ impl DebugBox {
     }
 
     pub fn is_tracking(&self) -> Result<Box<dyn NyashBox>, RuntimeError> {
-        let enabled = self.tracking_enabled.lock().unwrap();
+        let enabled = self.tracking_enabled.read().unwrap();
         Ok(Box::new(BoolBox::new(*enabled)))
     }
 
diff --git a/src/boxes/json/mod.rs b/src/boxes/json/mod.rs
index c8d31647..c7d86180 100644
--- a/src/boxes/json/mod.rs
+++ b/src/boxes/json/mod.rs
@@ -242,14 +242,14 @@ fn nyash_box_to_json_value(value: Box<dyn NyashBox>) -> Value {
     } else if let Some(string_box) = value.as_any().downcast_ref::<StringBox>() {
         Value::String(string_box.value.clone())
     } else if let Some(array_box) = value.as_any().downcast_ref::<ArrayBox>() {
-        let items = array_box.items.lock().unwrap();
+        let items = array_box.items.read().unwrap();
         let arr: Vec<Value> = items.iter()
             .map(|item| nyash_box_to_json_value(item.clone_box()))
             .collect();
         Value::Array(arr)
     } else if let Some(map_box) = value.as_any().downcast_ref::<MapBox>() {
         let data = map_box.get_data();
-        let map = data.lock().unwrap();
+        let map = data.read().unwrap();
         let mut obj = serde_json::Map::new();
         for (key, val) in map.iter() {
             obj.insert(key.clone(), nyash_box_to_json_value(val.clone_box()));
diff --git a/src/boxes/stream/mod.rs b/src/boxes/stream/mod.rs
index 32d0f73e..c7ae4292 100644
--- a/src/boxes/stream/mod.rs
+++ b/src/boxes/stream/mod.rs
@@ -6,36 +6,35 @@ use crate::box_trait::{NyashBox, StringBox, BoolBox, IntegerBox, BoxCore, BoxBas
 use crate::boxes::buffer::BufferBox;
 use crate::boxes::array::ArrayBox;
 use std::any::Any;
-use std::sync::{Arc, Mutex};
+use std::sync::RwLock;
 use std::io::{Read, Write, Result};
 
-#[derive(Debug, Clone)]
 pub struct NyashStreamBox {
-    buffer: Arc<Mutex<Vec<u8>>>,
-    position: Arc<Mutex<usize>>,
+    buffer: RwLock<Vec<u8>>,
+    position: RwLock<usize>,
     base: BoxBase,
 }
 
 impl NyashStreamBox {
     pub fn new() -> Self {
         NyashStreamBox {
-            buffer: Arc::new(Mutex::new(Vec::new())),
-            position: Arc::new(Mutex::new(0)),
+            buffer: RwLock::new(Vec::new()),
+            position: RwLock::new(0),
             base: BoxBase::new(),
         }
     }
     
     pub fn from_data(data: Vec<u8>) -> Self {
         NyashStreamBox {
-            buffer: Arc::new(Mutex::new(data)),
-            position: Arc::new(Mutex::new(0)),
+            buffer: RwLock::new(data),
+            position: RwLock::new(0),
             base: BoxBase::new(),
         }
     }
     
     pub fn read(&self, buf: &mut [u8]) -> Result<usize> {
-        let buffer = self.buffer.lock().unwrap();
-        let mut position = self.position.lock().unwrap();
+        let buffer = self.buffer.read().unwrap();
+        let mut position = self.position.write().unwrap();
         
         let available = buffer.len().saturating_sub(*position);
         let to_read = buf.len().min(available);
@@ -50,21 +49,21 @@ impl NyashStreamBox {
     }
     
     pub fn write(&self, buf: &[u8]) -> Result<()> {
-        let mut buffer = self.buffer.lock().unwrap();
+        let mut buffer = self.buffer.write().unwrap();
         buffer.extend_from_slice(buf);
         Ok(())
     }
     
     pub fn len(&self) -> usize {
-        self.buffer.lock().unwrap().len()
+        self.buffer.read().unwrap().len()
     }
     
     pub fn position(&self) -> usize {
-        *self.position.lock().unwrap()
+        *self.position.read().unwrap()
     }
     
     pub fn reset(&self) {
-        *self.position.lock().unwrap() = 0;
+        *self.position.write().unwrap() = 0;
     }
     
     /// ストリームに書き込み
@@ -75,7 +74,7 @@ impl NyashStreamBox {
             let array_data = buffer_box.readAll();
             // ArrayBoxをバイト配列に変換
             if let Some(array_box) = array_data.as_any().downcast_ref::<ArrayBox>() {
-                let items = array_box.items.lock().unwrap();
+                let items = array_box.items.read().unwrap();
                 let mut bytes = Vec::new();
                 for item in items.iter() {
                     if let Some(int_box) = item.as_any().downcast_ref::<IntegerBox>() {
@@ -142,8 +141,8 @@ impl NyashBox for NyashStreamBox {
     }
 
     fn to_string_box(&self) -> StringBox {
-        let buffer = self.buffer.lock().unwrap();
-        let position = self.position.lock().unwrap();
+        let buffer = self.buffer.read().unwrap();
+        let position = self.position.read().unwrap();
         StringBox::new(format!("NyashStreamBox({} bytes, pos: {})", buffer.len(), *position))
     }
 
@@ -155,10 +154,10 @@ impl NyashBox for NyashStreamBox {
 
     fn equals(&self, other: &dyn NyashBox) -> BoolBox {
         if let Some(other_stream) = other.as_any().downcast_ref::<NyashStreamBox>() {
-            let self_buffer = self.buffer.lock().unwrap();
-            let self_position = self.position.lock().unwrap();
-            let other_buffer = other_stream.buffer.lock().unwrap();
-            let other_position = other_stream.position.lock().unwrap();
+            let self_buffer = self.buffer.read().unwrap();
+            let self_position = self.position.read().unwrap();
+            let other_buffer = other_stream.buffer.read().unwrap();
+            let other_position = other_stream.position.read().unwrap();
             BoolBox::new(*self_buffer == *other_buffer && *self_position == *other_position)
         } else {
             BoolBox::new(false)
@@ -176,8 +175,8 @@ impl BoxCore for NyashStreamBox {
     }
 
     fn fmt_box(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        let buffer = self.buffer.lock().unwrap();
-        let position = self.position.lock().unwrap();
+        let buffer = self.buffer.read().unwrap();
+        let position = self.position.read().unwrap();
         write!(f, "NyashStreamBox({} bytes, pos: {})", buffer.len(), *position)
     }
     
@@ -190,6 +189,32 @@ impl BoxCore for NyashStreamBox {
     }
 }
 
+// Clone implementation for NyashStreamBox (needed since RwLock doesn't auto-derive Clone)
+impl Clone for NyashStreamBox {
+    fn clone(&self) -> Self {
+        let buffer = self.buffer.read().unwrap();
+        let position = self.position.read().unwrap();
+        NyashStreamBox {
+            buffer: RwLock::new(buffer.clone()),
+            position: RwLock::new(*position),
+            base: BoxBase::new(),
+        }
+    }
+}
+
+// Debug implementation for NyashStreamBox
+impl std::fmt::Debug for NyashStreamBox {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let buffer = self.buffer.read().unwrap();
+        let position = self.position.read().unwrap();
+        f.debug_struct("NyashStreamBox")
+            .field("id", &self.base.id)
+            .field("buffer_len", &buffer.len())
+            .field("position", &position)
+            .finish()
+    }
+}
+
 impl std::fmt::Display for NyashStreamBox {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         self.fmt_box(f)
diff --git a/src/boxes/string_box.rs b/src/boxes/string_box.rs
index e9a401f0..da9b74ae 100644
--- a/src/boxes/string_box.rs
+++ b/src/boxes/string_box.rs
@@ -117,7 +117,7 @@ impl StringBox {
     pub fn join(&self, array_box: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
         use crate::boxes::array::ArrayBox;
         if let Some(array) = array_box.as_any().downcast_ref::<ArrayBox>() {
-            let strings: Vec<String> = array.items.lock().unwrap()
+            let strings: Vec<String> = array.items.read().unwrap()
                 .iter()
                 .map(|element| element.to_string_box().value)
                 .collect();