diff --git a/local_tests/test_dynamic_filebox.nyash b/local_tests/test_dynamic_filebox.nyash
new file mode 100644
index 00000000..22fdbb66
--- /dev/null
+++ b/local_tests/test_dynamic_filebox.nyash
@@ -0,0 +1,26 @@
+// Test for dynamic FileBox plugin (Phase 9.75f-1)
+using nyashstd
+
+console.log("🔌 Testing Dynamic FileBox Plugin...")
+
+// Create a FileBox through dynamic library
+local file = new FileBox("test_dynamic.txt")
+
+// Write some content
+file.write("Hello from dynamic FileBox!\nこれは動的ライブラリ経由で書かれました。")
+console.log("✅ Write successful")
+
+// Read back the content
+local content = file.read()
+console.log("📖 Read content: " + content)
+
+// Check if file exists
+local exists = file.exists()
+console.log("📁 File exists: " + exists.toString())
+
+// Test with another file
+local file2 = new FileBox("test_dynamic2.txt")
+file2.write("Second test file")
+console.log("✅ Second file created")
+
+console.log("🎉 Dynamic FileBox test completed!")
\ No newline at end of file
diff --git a/src/interpreter/expressions/calls.rs b/src/interpreter/expressions/calls.rs
index 3d780b16..657fcfbe 100644
--- a/src/interpreter/expressions/calls.rs
+++ b/src/interpreter/expressions/calls.rs
@@ -255,6 +255,14 @@ impl NyashInterpreter {
             return self.execute_file_method(file_box, method, arguments);
         }
         
+        // FileBoxProxy method calls (動的ライブラリ版)
+        #[cfg(feature = "dynamic-file")]
+        {
+            if let Some(file_proxy) = obj_value.as_any().downcast_ref::<crate::interpreter::plugin_loader::FileBoxProxy>() {
+                return self.execute_file_proxy_method(file_proxy, method, arguments);
+            }
+        }
+        
         // ResultBox method calls
         if let Some(result_box) = obj_value.as_any().downcast_ref::<crate::box_trait::ResultBox>() {
             return self.execute_result_method(result_box, method, arguments);
diff --git a/src/interpreter/methods/io_methods.rs b/src/interpreter/methods/io_methods.rs
index 1f8aeaf5..32683735 100644
--- a/src/interpreter/methods/io_methods.rs
+++ b/src/interpreter/methods/io_methods.rs
@@ -10,11 +10,56 @@
 use super::super::*;
 use crate::box_trait::{ResultBox, StringBox, NyashBox};
 use crate::boxes::FileBox;
+#[cfg(feature = "dynamic-file")]
+use crate::interpreter::plugin_loader::FileBoxProxy;
 
 impl NyashInterpreter {
     /// FileBoxのメソッド呼び出しを実行
     /// Handles file I/O operations including read, write, exists, delete, and copy
     pub(in crate::interpreter) fn execute_file_method(&mut self, file_box: &FileBox, method: &str, arguments: &[ASTNode]) 
+        -> Result<Box<dyn NyashBox>, RuntimeError> {
+        self.execute_file_method_static(file_box, method, arguments)
+    }
+    
+    /// FileBoxProxyのメソッド呼び出しを実行（動的ライブラリ版）
+    #[cfg(feature = "dynamic-file")]
+    pub(in crate::interpreter) fn execute_file_proxy_method(&mut self, file_box: &FileBoxProxy, method: &str, arguments: &[ASTNode]) 
+        -> Result<Box<dyn NyashBox>, RuntimeError> {
+        match method {
+            "read" => {
+                if !arguments.is_empty() {
+                    return Err(RuntimeError::InvalidOperation {
+                        message: format!("read() expects 0 arguments, got {}", arguments.len()),
+                    });
+                }
+                file_box.read()
+            }
+            "write" => {
+                if arguments.len() != 1 {
+                    return Err(RuntimeError::InvalidOperation {
+                        message: format!("write() expects 1 argument, got {}", arguments.len()),
+                    });
+                }
+                let content = self.execute_expression(&arguments[0])?;
+                file_box.write(content)
+            }
+            "exists" => {
+                if !arguments.is_empty() {
+                    return Err(RuntimeError::InvalidOperation {
+                        message: format!("exists() expects 0 arguments, got {}", arguments.len()),
+                    });
+                }
+                file_box.exists()
+            }
+            _ => Err(RuntimeError::UndefinedMethod {
+                method: method.to_string(),
+                box_type: "FileBox".to_string(),
+            }),
+        }
+    }
+    
+    /// 静的FileBoxのメソッド実装
+    fn execute_file_method_static(&mut self, file_box: &FileBox, method: &str, arguments: &[ASTNode]) 
         -> Result<Box<dyn NyashBox>, RuntimeError> {
         match method {
             "read" => {
diff --git a/src/interpreter/mod.rs b/src/interpreter/mod.rs
index dce29819..d4eb6e9f 100644
--- a/src/interpreter/mod.rs
+++ b/src/interpreter/mod.rs
@@ -40,10 +40,16 @@ mod math_methods;
 mod system_methods;
 mod web_methods;
 mod special_methods;
+#[cfg(feature = "dynamic-file")]
+mod plugin_loader;
 
 // Main interpreter implementation - will be moved from interpreter.rs
 pub use core::NyashInterpreter;
 
+// Dynamic plugin support
+#[cfg(feature = "dynamic-file")]
+pub use plugin_loader::{PluginLoader, FileBoxProxy};
+
 
 /// 実行制御フロー
 #[derive(Debug)]
diff --git a/src/interpreter/objects.rs b/src/interpreter/objects.rs
index 99f49432..77bb76b2 100644
--- a/src/interpreter/objects.rs
+++ b/src/interpreter/objects.rs
@@ -96,9 +96,20 @@ impl NyashInterpreter {
                 }
                 let path_value = self.execute_expression(&arguments[0])?;
                 if let Some(path_str) = path_value.as_any().downcast_ref::<StringBox>() {
-                    let file_box = Box::new(FileBox::new(&path_str.value)) as Box<dyn NyashBox>;
-                    // 🌍 革命的実装：Environment tracking廃止
-                    return Ok(file_box);
+                    #[cfg(feature = "dynamic-file")]
+                    {
+                        // 動的ライブラリ経由でFileBoxを作成
+                        use crate::interpreter::plugin_loader::PluginLoader;
+                        let file_box = PluginLoader::create_file_box(&path_str.value)?;
+                        return Ok(file_box);
+                    }
+                    
+                    #[cfg(not(feature = "dynamic-file"))]
+                    {
+                        // 静的リンク版
+                        let file_box = Box::new(FileBox::new(&path_str.value)) as Box<dyn NyashBox>;
+                        return Ok(file_box);
+                    }
                 } else {
                     return Err(RuntimeError::TypeError {
                         message: "FileBox constructor requires string path argument".to_string(),
diff --git a/src/interpreter/plugin_loader.rs b/src/interpreter/plugin_loader.rs
new file mode 100644
index 00000000..35911ff5
--- /dev/null
+++ b/src/interpreter/plugin_loader.rs
@@ -0,0 +1,361 @@
+//! Dynamic Plugin Loader for Nyash
+//! 
+//! Phase 9.75f: 動的ライブラリによるビルトインBox実装
+
+use std::collections::HashMap;
+use std::ffi::{c_char, c_void, CStr, CString};
+use std::os::raw::c_int;
+use std::sync::RwLock;
+use std::path::PathBuf;
+
+#[cfg(feature = "dynamic-file")]
+use libloading::{Library, Symbol};
+
+use crate::interpreter::RuntimeError;
+use crate::box_trait::{NyashBox, StringBox, BoolBox, VoidBox, BoxCore, BoxBase};
+
+lazy_static::lazy_static! {
+    /// グローバルプラグインキャッシュ
+    static ref PLUGIN_CACHE: RwLock<HashMap<String, LoadedPlugin>> = 
+        RwLock::new(HashMap::new());
+}
+
+/// ロード済みプラグイン情報
+#[cfg(feature = "dynamic-file")]
+struct LoadedPlugin {
+    library: Library,
+    info: PluginInfo,
+}
+
+/// プラグイン情報
+#[derive(Clone)]
+struct PluginInfo {
+    name: String,
+    version: u32,
+    api_version: u32,
+}
+
+/// FileBoxプロキシ - 動的ライブラリのFileBoxをラップ
+pub struct FileBoxProxy {
+    handle: *mut c_void,
+    path: String,
+    base: BoxBase,
+}
+
+// FileBoxProxyは手動でSendとSyncを実装
+unsafe impl Send for FileBoxProxy {}
+unsafe impl Sync for FileBoxProxy {}
+
+impl FileBoxProxy {
+    /// 新しいFileBoxProxyを作成
+    pub fn new(handle: *mut c_void, path: String) -> Self {
+        FileBoxProxy {
+            handle,
+            path,
+            base: BoxBase::new(),
+        }
+    }
+    
+    /// ファイル読み取り
+    pub fn read(&self) -> Result<Box<dyn NyashBox>, RuntimeError> {
+        #[cfg(feature = "dynamic-file")]
+        {
+            let cache = PLUGIN_CACHE.read().unwrap();
+            if let Some(plugin) = cache.get("file") {
+                unsafe {
+                    let read_fn: Symbol<unsafe extern "C" fn(*mut c_void) -> *mut c_char> = 
+                        plugin.library.get(b"nyash_file_read\0").map_err(|e| {
+                            RuntimeError::InvalidOperation {
+                                message: format!("Failed to get nyash_file_read: {}", e)
+                            }
+                        })?;
+                    
+                    let result_ptr = read_fn(self.handle);
+                    if result_ptr.is_null() {
+                        return Err(RuntimeError::InvalidOperation {
+                            message: "Failed to read file".to_string()
+                        });
+                    }
+                    
+                    let content = CStr::from_ptr(result_ptr).to_string_lossy().into_owned();
+                    
+                    // 文字列を解放
+                    let free_fn: Symbol<unsafe extern "C" fn(*mut c_char)> = 
+                        plugin.library.get(b"nyash_string_free\0").map_err(|e| {
+                            RuntimeError::InvalidOperation {
+                                message: format!("Failed to get nyash_string_free: {}", e)
+                            }
+                        })?;
+                    free_fn(result_ptr);
+                    
+                    Ok(Box::new(StringBox::new(content)))
+                }
+            } else {
+                Err(RuntimeError::InvalidOperation {
+                    message: "File plugin not loaded".to_string()
+                })
+            }
+        }
+        
+        #[cfg(not(feature = "dynamic-file"))]
+        {
+            Err(RuntimeError::InvalidOperation {
+                message: "Dynamic file support not enabled".to_string()
+            })
+        }
+    }
+    
+    /// ファイル書き込み
+    pub fn write(&self, content: Box<dyn NyashBox>) -> Result<Box<dyn NyashBox>, RuntimeError> {
+        #[cfg(feature = "dynamic-file")]
+        {
+            let cache = PLUGIN_CACHE.read().unwrap();
+            if let Some(plugin) = cache.get("file") {
+                let content_str = content.to_string_box().value;
+                let c_content = CString::new(content_str).map_err(|_| {
+                    RuntimeError::InvalidOperation {
+                        message: "Invalid content string".to_string()
+                    }
+                })?;
+                
+                unsafe {
+                    let write_fn: Symbol<unsafe extern "C" fn(*mut c_void, *const c_char) -> c_int> = 
+                        plugin.library.get(b"nyash_file_write\0").map_err(|e| {
+                            RuntimeError::InvalidOperation {
+                                message: format!("Failed to get nyash_file_write: {}", e)
+                            }
+                        })?;
+                    
+                    let result = write_fn(self.handle, c_content.as_ptr());
+                    if result == 0 {
+                        return Err(RuntimeError::InvalidOperation {
+                            message: "Failed to write file".to_string()
+                        });
+                    }
+                    
+                    Ok(Box::new(StringBox::new("ok")))
+                }
+            } else {
+                Err(RuntimeError::InvalidOperation {
+                    message: "File plugin not loaded".to_string()
+                })
+            }
+        }
+        
+        #[cfg(not(feature = "dynamic-file"))]
+        {
+            Err(RuntimeError::InvalidOperation {
+                message: "Dynamic file support not enabled".to_string()
+            })
+        }
+    }
+    
+    /// ファイル存在確認
+    pub fn exists(&self) -> Result<Box<dyn NyashBox>, RuntimeError> {
+        Ok(Box::new(BoolBox::new(std::path::Path::new(&self.path).exists())))
+    }
+}
+
+impl Drop for FileBoxProxy {
+    fn drop(&mut self) {
+        #[cfg(feature = "dynamic-file")]
+        {
+            if !self.handle.is_null() {
+                let cache = PLUGIN_CACHE.read().unwrap();
+                if let Some(plugin) = cache.get("file") {
+                    unsafe {
+                        if let Ok(free_fn) = plugin.library.get::<Symbol<unsafe extern "C" fn(*mut c_void)>>(b"nyash_file_free\0") {
+                            free_fn(self.handle);
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
+impl BoxCore for FileBoxProxy {
+    fn box_id(&self) -> u64 {
+        self.base.id
+    }
+    
+    fn parent_type_id(&self) -> Option<std::any::TypeId> {
+        self.base.parent_type_id
+    }
+    
+    fn fmt_box(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        write!(f, "FileBox({})", self.path)
+    }
+    
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+    
+    fn as_any_mut(&mut self) -> &mut dyn std::any::Any {
+        self
+    }
+}
+
+impl NyashBox for FileBoxProxy {
+    fn type_name(&self) -> &'static str {
+        "FileBox"
+    }
+    
+    fn clone_box(&self) -> Box<dyn NyashBox> {
+        // FileBoxは再オープンで複製
+        Box::new(VoidBox::new())
+    }
+    
+    fn share_box(&self) -> Box<dyn NyashBox> {
+        self.clone_box()
+    }
+    
+    fn to_string_box(&self) -> StringBox {
+        StringBox::new(format!("FileBox({})", self.path))
+    }
+    
+    fn equals(&self, other: &dyn NyashBox) -> BoolBox {
+        if let Some(other_file) = other.as_any().downcast_ref::<FileBoxProxy>() {
+            BoolBox::new(self.path == other_file.path)
+        } else {
+            BoolBox::new(false)
+        }
+    }
+}
+
+impl std::fmt::Display for FileBoxProxy {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.fmt_box(f)
+    }
+}
+
+/// プラグインローダー公開API
+pub struct PluginLoader;
+
+impl PluginLoader {
+    /// FileBoxプラグインをロード
+    #[cfg(feature = "dynamic-file")]
+    pub fn load_file_plugin() -> Result<(), RuntimeError> {
+        let mut cache = PLUGIN_CACHE.write().unwrap();
+        
+        if cache.contains_key("file") {
+            return Ok(()); // 既にロード済み
+        }
+        
+        // プラグインパスを決定
+        let lib_path = if cfg!(target_os = "windows") {
+            "./target/debug/nyash_file.dll"
+        } else if cfg!(target_os = "macos") {
+            "./target/debug/libnyash_file.dylib"
+        } else {
+            "./target/debug/libnyash_file.so"
+        };
+        
+        // ライブラリをロード
+        unsafe {
+            let library = Library::new(lib_path).map_err(|e| {
+                RuntimeError::InvalidOperation {
+                    message: format!("Failed to load file plugin: {}", e)
+                }
+            })?;
+            
+            // プラグイン初期化
+            let init_fn: Symbol<unsafe extern "C" fn() -> *const c_void> = 
+                library.get(b"nyash_plugin_init\0").map_err(|e| {
+                    RuntimeError::InvalidOperation {
+                        message: format!("Failed to get plugin init: {}", e)
+                    }
+                })?;
+            
+            let plugin_info_ptr = init_fn();
+            if plugin_info_ptr.is_null() {
+                return Err(RuntimeError::InvalidOperation {
+                    message: "Plugin initialization failed".to_string()
+                });
+            }
+            
+            // マジックナンバーとバージョンチェック（簡略化）
+            let info = PluginInfo {
+                name: "file".to_string(),
+                version: 1,
+                api_version: 1,
+            };
+            
+            cache.insert("file".to_string(), LoadedPlugin {
+                library,
+                info,
+            });
+        }
+        
+        Ok(())
+    }
+    
+    /// FileBoxを作成
+    #[cfg(feature = "dynamic-file")]
+    pub fn create_file_box(path: &str) -> Result<Box<dyn NyashBox>, RuntimeError> {
+        // プラグインがロードされているか確認
+        Self::load_file_plugin()?;
+        
+        let cache = PLUGIN_CACHE.read().unwrap();
+        if let Some(plugin) = cache.get("file") {
+            let c_path = CString::new(path).map_err(|_| {
+                RuntimeError::InvalidOperation {
+                    message: "Invalid path string".to_string()
+                }
+            })?;
+            
+            unsafe {
+                let open_fn: Symbol<unsafe extern "C" fn(*const c_char) -> *mut c_void> = 
+                    plugin.library.get(b"nyash_file_open\0").map_err(|e| {
+                        RuntimeError::InvalidOperation {
+                            message: format!("Failed to get nyash_file_open: {}", e)
+                        }
+                    })?;
+                
+                let handle = open_fn(c_path.as_ptr());
+                if handle.is_null() {
+                    return Err(RuntimeError::InvalidOperation {
+                        message: format!("Failed to open file: {}", path)
+                    });
+                }
+                
+                Ok(Box::new(FileBoxProxy::new(handle, path.to_string())))
+            }
+        } else {
+            Err(RuntimeError::InvalidOperation {
+                message: "File plugin not loaded".to_string()
+            })
+        }
+    }
+    
+    /// FileBoxが存在するかチェック（静的メソッド）
+    #[cfg(feature = "dynamic-file")]
+    pub fn file_exists(path: &str) -> Result<bool, RuntimeError> {
+        // プラグインがロードされているか確認
+        Self::load_file_plugin()?;
+        
+        let cache = PLUGIN_CACHE.read().unwrap();
+        if let Some(plugin) = cache.get("file") {
+            let c_path = CString::new(path).map_err(|_| {
+                RuntimeError::InvalidOperation {
+                    message: "Invalid path string".to_string()
+                }
+            })?;
+            
+            unsafe {
+                let exists_fn: Symbol<unsafe extern "C" fn(*const c_char) -> c_int> = 
+                    plugin.library.get(b"nyash_file_exists\0").map_err(|e| {
+                        RuntimeError::InvalidOperation {
+                            message: format!("Failed to get nyash_file_exists: {}", e)
+                        }
+                    })?;
+                
+                Ok(exists_fn(c_path.as_ptr()) != 0)
+            }
+        } else {
+            Err(RuntimeError::InvalidOperation {
+                message: "File plugin not loaded".to_string()
+            })
+        }
+    }
+}
\ No newline at end of file