diff --git a/src/mir/builder.rs b/src/mir/builder.rs
index 88afd982..4bb41aa9 100644
--- a/src/mir/builder.rs
+++ b/src/mir/builder.rs
@@ -88,7 +88,7 @@ pub(crate) mod type_registry;
 mod types; // types::annotation / inference（型注釈/推論の箱: 推論は後段）
 mod utils;
 mod vars; // variables/scope helpers // small loop helpers (header/exit context) // TypeRegistryBox（型情報管理の一元化）
-pub(crate) mod repl_session; // Phase 288 P2: REPL session state (VMValue-based persistence)
+// Phase 288 Box化: repl_session moved to src/runner/repl/repl_session.rs
 
 // Unified member property kinds for computed/once/birth_once
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
diff --git a/src/runner/mod.rs b/src/runner/mod.rs
index 137eb0da..db5c4a49 100644
--- a/src/runner/mod.rs
+++ b/src/runner/mod.rs
@@ -34,6 +34,7 @@ pub mod modes;
 mod pipe_io;
 mod pipeline;
 mod plugins;
+mod repl;  // Phase 288: REPL module
 mod selfhost;
 mod stage1_bridge;
 mod tasks;
@@ -51,8 +52,6 @@ use nyash_rust::runtime;
 /// Main execution coordinator
 pub struct NyashRunner {
     config: CliConfig,
-    /// Phase 288 P2: REPL session - stores runtime values across evaluations
-    repl_session: std::cell::RefCell<Option<crate::mir::builder::repl_session::ReplSessionBox>>,
 }
 
 /// Minimal task runner: read hako.toml (preferred) or nyash.toml [env]/[tasks], run the named task via shell
@@ -72,7 +71,6 @@ impl NyashRunner {
 
         Self {
             config,
-            repl_session: std::cell::RefCell::new(None), // Phase 288 P2
         }
     }
 
@@ -94,120 +92,6 @@ impl NyashRunner {
         build::run_build_mvp_impl(self, cfg_path)
     }
 
-    /// Phase 288 P1: Run REPL mode
-    fn run_repl(&self) -> ! {
-        use std::io::{self, Write};
-
-        println!("Nyash REPL v1.0 - Phase 288 MVP");
-        println!("Type .help for commands, .exit to quit");
-
-        let stdin = io::stdin();
-        let mut line_buf = String::new();
-
-        loop {
-            print!(">>> ");
-            io::stdout().flush().unwrap();
-
-            line_buf.clear();
-            match stdin.read_line(&mut line_buf) {
-                Ok(0) => break,  // EOF
-                Ok(_) => {
-                    let line = line_buf.trim();
-
-                    // REPL commands
-                    match line {
-                        ".exit" | ".quit" => break,
-                        ".help" => {
-                            println!("Commands:");
-                            println!("  .exit / .quit - Exit REPL");
-                            println!("  .reset - Clear session");
-                            println!("  .help - Show this help");
-                            continue;
-                        }
-                        ".reset" => {
-                            // Phase 288 P3: Reset session
-                            let mut session_ref = self.repl_session.borrow_mut();
-                            if let Some(ref mut session) = *session_ref {
-                                session.reset();
-                                println!("Session reset");
-                            } else {
-                                println!("Session reset (no active session)");
-                            }
-                            continue;
-                        }
-                        "" => continue,
-                        _ => {}
-                    }
-
-                    // Evaluate line
-                    match self.eval_repl_line(line) {
-                        Ok(result) => {
-                            if !result.is_empty() {
-                                println!("{}", result);
-                            }
-                        }
-                        Err(e) => eprintln!("Error: {}", e),
-                    }
-                }
-                Err(e) => {
-                    eprintln!("Input error: {}", e);
-                    break;
-                }
-            }
-        }
-
-        println!("Goodbye!");
-        std::process::exit(0);
-    }
-
-    /// Phase 288 P2: Evaluate REPL line (VMValue persistence)
-    fn eval_repl_line(&self, line: &str) -> Result<String, String> {
-        use crate::parser::NyashParser;
-        use crate::mir::MirCompiler;
-        use crate::backend::mir_interpreter::MirInterpreter;
-        use crate::mir::builder::repl_session::ReplSessionBox;
-
-        // Initialize session on first use
-        {
-            let mut session_ref = self.repl_session.borrow_mut();
-            if session_ref.is_none() {
-                *session_ref = Some(ReplSessionBox::new());
-            }
-        }
-
-        // Parse (minimal wrapper for REPL context - use Main for VM entry point)
-        let code = format!("static box Main {{ main() {{ {} }} }}", line);
-        let ast = NyashParser::parse_from_string(&code)
-            .map_err(|e| format!("Parse error: {}", e))?;
-
-        // Compile with REPL mode flag (暗黙 local 許可)
-        let mut compiler = MirCompiler::new();
-        compiler.set_repl_mode(true);
-
-        let mir_result = compiler.compile_with_source(ast, Some("<repl>"))
-            .map_err(|e| format!("Compile error: {}", e))?;
-
-        // Execute
-        let mut vm = MirInterpreter::new();
-        let result_box = vm.execute_module(&mir_result.module)
-            .map_err(|e| format!("Runtime error: {}", e))?;
-
-        // Phase 288 P3: Convert to VMValue and store in session
-        use crate::backend::VMValue;
-        let vm_value = VMValue::from_nyash_box(result_box);
-
-        {
-            let mut session_ref = self.repl_session.borrow_mut();
-            if let Some(ref mut session) = *session_ref {
-                session.set_last_value(vm_value);
-                session.eval_count += 1;
-            }
-        }
-
-        // Phase 288 P3: print() output already displayed via ExternCall
-        // Expression auto-display deferred to Phase 288.1
-        Ok(String::new())
-    }
 }
 
 #[cfg(not(feature = "jit-direct-only"))]
@@ -221,7 +105,7 @@ impl NyashRunner {
         }
         // Phase 288 P1: REPL mode
         if self.config.repl {
-            self.run_repl();  // never returns
+            repl::run_repl(self.config.clone());  // never returns
         }
         let groups = self.config.as_groups();
 
diff --git a/src/runner/repl/mod.rs b/src/runner/repl/mod.rs
new file mode 100644
index 00000000..49da8276
--- /dev/null
+++ b/src/runner/repl/mod.rs
@@ -0,0 +1,22 @@
+//! REPL Module - Box-First Architecture
+//!
+//! Phase 288: Box化モジュール化
+//! - ReplRunnerBox: REPL実行器の完全隔離
+//! - ReplSessionBox: セッション状態の管理
+//!
+//! 公開API: run_repl() のみ
+
+mod repl_runner;
+mod repl_session;
+
+use repl_runner::ReplRunnerBox;
+use crate::cli::CliConfig;
+
+/// Phase 288: REPL モード起動（公開API）
+///
+/// REPL ループを開始し、プログラムは終了しない（never returns）。
+/// `.exit` コマンドで終了する。
+pub(crate) fn run_repl(config: CliConfig) -> ! {
+    let runner = ReplRunnerBox::new(config);
+    runner.run()
+}
diff --git a/src/runner/repl/repl_runner.rs b/src/runner/repl/repl_runner.rs
new file mode 100644
index 00000000..7c9a5cd8
--- /dev/null
+++ b/src/runner/repl/repl_runner.rs
@@ -0,0 +1,143 @@
+//! ReplRunnerBox - REPL execution engine
+//!
+//! Box-First Design: Complete isolation of REPL execution logic
+//! Phase 288 P1-P3
+//!
+//! Responsibilities:
+//! - REPL loop management (.exit, .help, .reset commands)
+//! - Line evaluation (parse → compile → execute)
+//! - Session state management via ReplSessionBox
+
+use super::repl_session::ReplSessionBox;
+use crate::cli::CliConfig;
+use std::cell::RefCell;
+
+/// Phase 288: REPL実行器（箱理論モジュール化）
+pub(super) struct ReplRunnerBox {
+    #[allow(dead_code)]
+    config: CliConfig,
+    session: RefCell<Option<ReplSessionBox>>,
+}
+
+impl ReplRunnerBox {
+    pub(super) fn new(config: CliConfig) -> Self {
+        Self {
+            config,
+            session: RefCell::new(None),
+        }
+    }
+
+    /// REPL ループ（メインエントリーポイント）
+    pub(super) fn run(&self) -> ! {
+        use std::io::{self, Write};
+
+        println!("Nyash REPL v1.0 - Phase 288 MVP");
+        println!("Type .help for commands, .exit to quit");
+
+        let stdin = io::stdin();
+        let mut line_buf = String::new();
+
+        loop {
+            print!(">>> ");
+            io::stdout().flush().unwrap();
+
+            line_buf.clear();
+            match stdin.read_line(&mut line_buf) {
+                Ok(0) => break,  // EOF
+                Ok(_) => {
+                    let line = line_buf.trim();
+
+                    // REPL commands
+                    match line {
+                        ".exit" | ".quit" => break,
+                        ".help" => {
+                            println!("Commands:");
+                            println!("  .exit / .quit - Exit REPL");
+                            println!("  .reset - Clear session");
+                            println!("  .help - Show this help");
+                            continue;
+                        }
+                        ".reset" => {
+                            // Phase 288 P3: Reset session
+                            let mut session_ref = self.session.borrow_mut();
+                            if let Some(ref mut session) = *session_ref {
+                                session.reset();
+                                println!("Session reset");
+                            } else {
+                                println!("Session reset (no active session)");
+                            }
+                            continue;
+                        }
+                        "" => continue,
+                        _ => {}
+                    }
+
+                    // Evaluate line
+                    match self.eval_line(line) {
+                        Ok(result) => {
+                            if !result.is_empty() {
+                                println!("{}", result);
+                            }
+                        }
+                        Err(e) => eprintln!("Error: {}", e),
+                    }
+                }
+                Err(e) => {
+                    eprintln!("Input error: {}", e);
+                    break;
+                }
+            }
+        }
+
+        println!("Goodbye!");
+        std::process::exit(0);
+    }
+
+    /// 1行評価（内部メソッド）
+    fn eval_line(&self, line: &str) -> Result<String, String> {
+        use crate::parser::NyashParser;
+        use crate::mir::MirCompiler;
+        use crate::backend::mir_interpreter::MirInterpreter;
+
+        // Initialize session on first use
+        {
+            let mut session_ref = self.session.borrow_mut();
+            if session_ref.is_none() {
+                *session_ref = Some(ReplSessionBox::new());
+            }
+        }
+
+        // Parse (minimal wrapper for REPL context - use Main for VM entry point)
+        let code = format!("static box Main {{ main() {{ {} }} }}", line);
+        let ast = NyashParser::parse_from_string(&code)
+            .map_err(|e| format!("Parse error: {}", e))?;
+
+        // Compile with REPL mode flag (暗黙 local 許可)
+        let mut compiler = MirCompiler::new();
+        compiler.set_repl_mode(true);
+
+        let mir_result = compiler.compile_with_source(ast, Some("<repl>"))
+            .map_err(|e| format!("Compile error: {}", e))?;
+
+        // Execute
+        let mut vm = MirInterpreter::new();
+        let result_box = vm.execute_module(&mir_result.module)
+            .map_err(|e| format!("Runtime error: {}", e))?;
+
+        // Phase 288 P3: Convert to VMValue and store in session
+        use crate::backend::VMValue;
+        let vm_value = VMValue::from_nyash_box(result_box);
+
+        {
+            let mut session_ref = self.session.borrow_mut();
+            if let Some(ref mut session) = *session_ref {
+                session.set_last_value(vm_value);
+                session.eval_count += 1;
+            }
+        }
+
+        // Phase 288 P3: print() output already displayed via ExternCall
+        // Expression auto-display deferred to Phase 288.1
+        Ok(String::new())
+    }
+}
diff --git a/src/mir/builder/repl_session.rs b/src/runner/repl/repl_session.rs
similarity index 75%
rename from src/mir/builder/repl_session.rs
rename to src/runner/repl/repl_session.rs
index 8ae13381..40f007b2 100644
--- a/src/mir/builder/repl_session.rs
+++ b/src/runner/repl/repl_session.rs
@@ -11,7 +11,7 @@ use std::collections::BTreeMap;
 
 /// REPL session context - isolated from file mode
 #[derive(Debug, Default)]
-pub struct ReplSessionBox {
+pub(super) struct ReplSessionBox {
     /// Session-level variables (runtime values, persists across evaluations)
     pub variables: BTreeMap<String, VMValue>,
 
@@ -23,31 +23,34 @@ pub struct ReplSessionBox {
 }
 
 impl ReplSessionBox {
-    pub fn new() -> Self {
+    pub(super) fn new() -> Self {
         Self::default()
     }
 
     /// REPL set: 変数に実行時の値を保存
-    pub fn set(&mut self, name: String, value: VMValue) {
+    #[allow(dead_code)]
+    pub(super) fn set(&mut self, name: String, value: VMValue) {
         self.variables.insert(name, value);
     }
 
     /// REPL get: 変数の実行時の値を取得（未定義は None）
-    pub fn get(&self, name: &str) -> Option<&VMValue> {
+    #[allow(dead_code)]
+    pub(super) fn get(&self, name: &str) -> Option<&VMValue> {
         self.variables.get(name)
     }
 
     /// セッションに変数が存在するか確認
-    pub fn has(&self, name: &str) -> bool {
+    #[allow(dead_code)]
+    pub(super) fn has(&self, name: &str) -> bool {
         self.variables.contains_key(name)
     }
 
-    pub fn set_last_value(&mut self, value: VMValue) {
+    pub(super) fn set_last_value(&mut self, value: VMValue) {
         self.last_value = Some(value.clone());
         self.variables.insert("_".to_string(), value);
     }
 
-    pub fn reset(&mut self) {
+    pub(super) fn reset(&mut self) {
         self.variables.clear();
         self.last_value = None;
         self.eval_count = 0;