diff --git a/Cargo.toml b/Cargo.toml
index 14bbc765..910107e8 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -111,7 +111,7 @@ js-sys = "0.3"
 
 # WASM backend dependencies (Phase 8)
 wabt = "0.10"
-wasmtime = "18.0"
+wasmtime = "35.0.0"
 
 # GUI フレームワーク - only when gui feature is enabled
 egui = { version = "0.29", optional = true }
diff --git a/docs/CURRENT_TASK.md b/docs/CURRENT_TASK.md
index 92245bd0..f3013a50 100644
--- a/docs/CURRENT_TASK.md
+++ b/docs/CURRENT_TASK.md
@@ -1,4 +1,56 @@
-# 🎯 現在のタスク (2025-08-16 警告削減100%完了!)
+# 🎯 現在のタスク (2025-08-16 Phase 9.77 WASM緊急復旧開始！)
+
+## 🚨 **Phase 9.77進行中: WASM緊急復旧作業**
+
+### ✅ **Task 1.1完了: BoxCall命令実装** 
+- **BoxCall実装**: toString(), print(), equals(), clone(), log()メソッド完全実装 ✅
+- **codegen.rs修正**: generate_box_call関数とヘルパー関数5個追加 ✅
+- **パターンマッチ追加**: MirInstruction::BoxCall対応 ✅
+- **ビルド成功**: コンパイルエラーなし ✅
+
+### ✅ **Task 1.2完了: wasmtimeバージョン統一 + RuntimeImports**
+- **wasmtime更新**: 18.0 → 35.0.0 完了 ✅
+- **RuntimeImports追加**: box_to_string, box_print, box_equals, box_clone 実装済み ✅
+- **ビルド成功**: バージョン互換性問題解決 ✅
+
+### 🔄 **Task 1.3進行中: WASM出力UTF-8エラー修正**
+**現状**: 「Generated WASM is not valid UTF-8」エラーが継続
+```bash
+❌ Generated WASM is not valid UTF-8
+```
+
+**実装済み修正**:
+- wabt::wat2wasm呼び出し修正（wat_source.as_bytes()使用）
+- UTF-8検証とデバッグ出力追加
+- しかしエラーメッセージの発生元が不明
+
+**調査結果**:
+- エラーメッセージがソースコード内に見つからない
+- wabt crateまたは外部ツールから発生している可能性
+- 簡単なテストケース（`local result = 42`）でも同じエラー
+
+### 📋 **Phase 9.77緊急復旧計画**
+詳細: [phase_9_77_wasm_emergency.md](docs/予定/native-plan/issues/phase_9_77_wasm_emergency.md)
+
+**Task 1.1**: ✅ BoxCall命令実装（完了）
+**Task 1.2**: ✅ wasmtimeバージョン統一 + RuntimeImports（完了）
+**Task 1.3**: 🔄 WASM出力エラー修正（進行中）
+
+### 🎯 **Copilotへの引き継ぎ事項**
+1. **UTF-8エラーの原因特定**
+   - エラーメッセージ「Generated WASM is not valid UTF-8」の発生元調査
+   - wabt::wat2wasm以外の場所でエラーが出ている可能性
+   - runner.rsやmain.rsでのエラー処理確認
+
+2. **デバッグアプローチ**
+   - WAT生成内容の詳細確認（デバッグ出力は実装済み）
+   - WASM生成パイプライン全体の調査
+   - 最小再現テストケースでの検証
+
+3. **修正候補**
+   - エラーメッセージの発生元を特定してから対応
+   - WAT形式の検証強化
+   - バイナリ出力処理の見直し
 
 ## 🎉 **Phase 9.75j完了: 警告削減100%達成!**
 
diff --git a/local_tests/debug_wasm_generation.nyash b/local_tests/debug_wasm_generation.nyash
new file mode 100644
index 00000000..581832e2
--- /dev/null
+++ b/local_tests/debug_wasm_generation.nyash
@@ -0,0 +1,8 @@
+// 🧪 最も単純なWASM生成テスト
+// 変数操作も避けて、純粋な計算のみ
+
+static box Main {
+    main() {
+        return "成功"
+    }
+}
\ No newline at end of file
diff --git a/local_tests/test_mir_wasm_minimal.nyash b/local_tests/test_mir_wasm_minimal.nyash
new file mode 100644
index 00000000..92d73ecc
--- /dev/null
+++ b/local_tests/test_mir_wasm_minimal.nyash
@@ -0,0 +1,17 @@
+// 🧪 MIR → WASM変換テスト（最小版）
+// メソッド呼び出しを避けて基本演算のみテスト
+
+static box Main {
+    init { result }
+    
+    main() {
+        // 基本演算のみ（メソッド呼び出しなし）
+        local a = 10
+        local b = 20
+        local sum = a + b
+        local product = sum * 3
+        
+        me.result = product
+        return "計算完了"
+    }
+}
\ No newline at end of file
diff --git a/local_tests/test_mir_wasm_simple.nyash b/local_tests/test_mir_wasm_simple.nyash
new file mode 100644
index 00000000..0f7269f1
--- /dev/null
+++ b/local_tests/test_mir_wasm_simple.nyash
@@ -0,0 +1,28 @@
+// 🧪 MIR → WASM変換テスト用シンプルプログラム
+// 基本的な演算とBox操作をテスト
+
+static box Main {
+    init { console, result }
+    
+    main() {
+        me.console = new ConsoleBox()
+        me.console.log("🚀 MIR → WASM変換テスト開始")
+        
+        // 基本演算テスト
+        local a = 10
+        local b = 20
+        local sum = a + b
+        
+        me.console.log("計算結果: " + sum.toString())
+        
+        // StringBox操作テスト
+        local greeting = "Hello"
+        local name = "WASM"
+        local message = greeting + " " + name + "!"
+        
+        me.console.log("メッセージ: " + message)
+        
+        me.result = sum
+        return "MIR → WASM変換テスト完了"
+    }
+}
\ No newline at end of file
diff --git a/local_tests/test_simple_wasm.nyash b/local_tests/test_simple_wasm.nyash
new file mode 100644
index 00000000..da18925c
--- /dev/null
+++ b/local_tests/test_simple_wasm.nyash
@@ -0,0 +1 @@
+local result = 42
diff --git a/src/backend/wasm/codegen.rs b/src/backend/wasm/codegen.rs
index a2d0e3df..9da43a40 100644
--- a/src/backend/wasm/codegen.rs
+++ b/src/backend/wasm/codegen.rs
@@ -403,6 +403,11 @@ impl WasmCodegen {
                 Ok(instructions)
             },
             
+            // Phase 9.77: BoxCall Implementation - Critical Box method calls
+            MirInstruction::BoxCall { dst, box_val, method, args, effects: _ } => {
+                self.generate_box_call(*dst, *box_val, method, args)
+            },
+            
             // Unsupported instructions
             _ => Err(WasmError::UnsupportedInstruction(
                 format!("Instruction not yet supported: {:?}", instruction)
@@ -570,6 +575,117 @@ impl WasmCodegen {
             .copied()
             .ok_or_else(|| WasmError::CodegenError(format!("Local variable not found for ValueId: {:?}", value_id)))
     }
+    
+    /// Phase 9.77: Generate BoxCall method invocation
+    /// Implements critical Box methods: toString, print, equals, clone
+    fn generate_box_call(&mut self, dst: Option<ValueId>, box_val: ValueId, method: &str, args: &[ValueId]) -> Result<Vec<String>, WasmError> {
+        match method {
+            "toString" => self.generate_to_string_call(dst, box_val),
+            "print" => self.generate_print_call(dst, box_val),
+            "equals" => self.generate_equals_call(dst, box_val, args),
+            "clone" => self.generate_clone_call(dst, box_val),
+            "log" => self.generate_log_call(dst, box_val, args),
+            _ => Err(WasmError::UnsupportedInstruction(
+                format!("Unsupported BoxCall method: {}", method)
+            ))
+        }
+    }
+    
+    /// Generate toString() method call - Box → String conversion
+    fn generate_to_string_call(&mut self, dst: Option<ValueId>, box_val: ValueId) -> Result<Vec<String>, WasmError> {
+        let Some(dst) = dst else {
+            return Err(WasmError::CodegenError("toString() requires destination".to_string()));
+        };
+        
+        Ok(vec![
+            format!(";; toString() implementation for ValueId({})", box_val.as_u32()),
+            format!("local.get ${}", self.get_local_index(box_val)?),
+            "call $box_to_string".to_string(),
+            format!("local.set ${}", self.get_local_index(dst)?),
+        ])
+    }
+    
+    /// Generate print() method call - Basic output
+    fn generate_print_call(&mut self, dst: Option<ValueId>, box_val: ValueId) -> Result<Vec<String>, WasmError> {
+        let mut instructions = vec![
+            format!(";; print() implementation for ValueId({})", box_val.as_u32()),
+            format!("local.get ${}", self.get_local_index(box_val)?),
+            "call $box_print".to_string(),
+        ];
+        
+        // Store void result if destination is provided
+        if let Some(dst) = dst {
+            instructions.extend(vec![
+                "i32.const 0".to_string(), // Void result
+                format!("local.set ${}", self.get_local_index(dst)?),
+            ]);
+        }
+        
+        Ok(instructions)
+    }
+    
+    /// Generate equals() method call - Box comparison
+    fn generate_equals_call(&mut self, dst: Option<ValueId>, box_val: ValueId, args: &[ValueId]) -> Result<Vec<String>, WasmError> {
+        let Some(dst) = dst else {
+            return Err(WasmError::CodegenError("equals() requires destination".to_string()));
+        };
+        
+        if args.len() != 1 {
+            return Err(WasmError::CodegenError(
+                format!("equals() expects 1 argument, got {}", args.len())
+            ));
+        }
+        
+        Ok(vec![
+            format!(";; equals() implementation for ValueId({}) == ValueId({})", box_val.as_u32(), args[0].as_u32()),
+            format!("local.get ${}", self.get_local_index(box_val)?),
+            format!("local.get ${}", self.get_local_index(args[0])?),
+            "call $box_equals".to_string(),
+            format!("local.set ${}", self.get_local_index(dst)?),
+        ])
+    }
+    
+    /// Generate clone() method call - Box duplication
+    fn generate_clone_call(&mut self, dst: Option<ValueId>, box_val: ValueId) -> Result<Vec<String>, WasmError> {
+        let Some(dst) = dst else {
+            return Err(WasmError::CodegenError("clone() requires destination".to_string()));
+        };
+        
+        Ok(vec![
+            format!(";; clone() implementation for ValueId({})", box_val.as_u32()),
+            format!("local.get ${}", self.get_local_index(box_val)?),
+            "call $box_clone".to_string(),
+            format!("local.set ${}", self.get_local_index(dst)?),
+        ])
+    }
+    
+    /// Generate log() method call - Console logging (ConsoleBox.log)
+    fn generate_log_call(&mut self, dst: Option<ValueId>, box_val: ValueId, args: &[ValueId]) -> Result<Vec<String>, WasmError> {
+        let mut instructions = vec![
+            format!(";; log() implementation for ValueId({})", box_val.as_u32()),
+        ];
+        
+        // Load box_val (ConsoleBox instance)
+        instructions.push(format!("local.get ${}", self.get_local_index(box_val)?));
+        
+        // Load all arguments
+        for arg in args {
+            instructions.push(format!("local.get ${}", self.get_local_index(*arg)?));
+        }
+        
+        // Call console log function
+        instructions.push("call $console_log".to_string());
+        
+        // Store void result if destination is provided
+        if let Some(dst) = dst {
+            instructions.extend(vec![
+                "i32.const 0".to_string(), // Void result
+                format!("local.set ${}", self.get_local_index(dst)?),
+            ]);
+        }
+        
+        Ok(instructions)
+    }
 }
 
 #[cfg(test)]
diff --git a/src/backend/wasm/mod.rs b/src/backend/wasm/mod.rs
index b18ae8c9..40f503e0 100644
--- a/src/backend/wasm/mod.rs
+++ b/src/backend/wasm/mod.rs
@@ -61,9 +61,36 @@ impl WasmBackend {
         // Generate WAT (WebAssembly Text) first for debugging
         let wat_text = self.compile_to_wat(mir_module)?;
         
-        // Convert WAT to WASM binary using wabt
-        wabt::wat2wasm(&wat_text)
-            .map_err(|e| WasmError::WasmValidationError(format!("WAT to WASM conversion failed: {}", e)))
+        // Phase 9.77 Task 1.3: Fix UTF-8 encoding error in WAT→WASM conversion
+        self.wat_to_wasm(&wat_text)
+    }
+    
+    /// Convert WAT text to WASM binary with proper UTF-8 handling
+    fn wat_to_wasm(&self, wat_source: &str) -> Result<Vec<u8>, WasmError> {
+        // Debug: Print WAT source for analysis
+        eprintln!("🔍 WAT Source Debug (length: {}):", wat_source.len());
+        eprintln!("WAT Content:\n{}", wat_source);
+        
+        // UTF-8 validation to prevent encoding errors
+        if !wat_source.is_ascii() {
+            eprintln!("❌ WAT source contains non-ASCII characters");
+            return Err(WasmError::WasmValidationError(
+                "WAT source contains non-ASCII characters".to_string()
+            ));
+        }
+        
+        eprintln!("✅ WAT source is ASCII-compatible");
+        
+        // Convert to bytes as required by wabt::wat2wasm
+        eprintln!("🔄 Converting WAT to WASM bytes...");
+        let wasm_bytes = wabt::wat2wasm(wat_source.as_bytes())
+            .map_err(|e| {
+                eprintln!("❌ wabt::wat2wasm failed: {}", e);
+                WasmError::WasmValidationError(format!("WAT to WASM conversion failed: {}", e))
+            })?;
+        
+        eprintln!("✅ WASM conversion successful, {} bytes generated", wasm_bytes.len());
+        Ok(wasm_bytes)
     }
     
     /// Compile MIR module to WAT text format (for debugging)
diff --git a/src/backend/wasm/runtime.rs b/src/backend/wasm/runtime.rs
index ea284bbc..df73b82c 100644
--- a/src/backend/wasm/runtime.rs
+++ b/src/backend/wasm/runtime.rs
@@ -89,6 +89,40 @@ impl RuntimeImports {
             result: None,
         });
         
+        // Phase 9.77: BoxCall runtime functions
+        
+        // box_to_string - Convert any Box to string representation
+        self.imports.push(ImportFunction {
+            module: "env".to_string(),
+            name: "box_to_string".to_string(),
+            params: vec!["i32".to_string()], // box_ptr
+            result: Some("i32".to_string()), // string_box_ptr
+        });
+        
+        // box_print - Print any Box to console
+        self.imports.push(ImportFunction {
+            module: "env".to_string(),
+            name: "box_print".to_string(),
+            params: vec!["i32".to_string()], // box_ptr
+            result: None,
+        });
+        
+        // box_equals - Compare two Boxes for equality
+        self.imports.push(ImportFunction {
+            module: "env".to_string(),
+            name: "box_equals".to_string(),
+            params: vec!["i32".to_string(), "i32".to_string()], // box1_ptr, box2_ptr
+            result: Some("i32".to_string()), // bool result
+        });
+        
+        // box_clone - Clone a Box
+        self.imports.push(ImportFunction {
+            module: "env".to_string(),
+            name: "box_clone".to_string(),
+            params: vec!["i32".to_string()], // box_ptr
+            result: Some("i32".to_string()), // cloned_box_ptr
+        });
+        
         // Future: env.file_read, env.file_write for file I/O
         // Future: env.http_request for network access
     }