diff --git a/src/interpreter/core.rs b/src/interpreter/core.rs
index 37163ee5..f37768a8 100644
--- a/src/interpreter/core.rs
+++ b/src/interpreter/core.rs
@@ -547,12 +547,29 @@ impl NyashInterpreter {
     
     /// local変数を宣言（関数内でのみ有効）
     pub(super) fn declare_local_variable(&mut self, name: &str, value: Box<dyn NyashBox>) {
-        self.local_vars.insert(name.to_string(), Arc::from(value));
+        // Pass-by-share for plugin handle types; by-value (clone) semantics can be applied at call sites
+        #[allow(unused_mut)]
+        let mut store_value = value;
+        #[cfg(all(feature = "plugins", not(target_arch = "wasm32")))]
+        {
+            if store_value.as_any().downcast_ref::<crate::runtime::plugin_loader_v2::PluginBoxV2>().is_some() {
+                store_value = store_value.share_box();
+            }
+        }
+        self.local_vars.insert(name.to_string(), Arc::from(store_value));
     }
     
     /// outbox変数を宣言（static関数内で所有権移転）
     pub(super) fn declare_outbox_variable(&mut self, name: &str, value: Box<dyn NyashBox>) {
-        self.outbox_vars.insert(name.to_string(), Arc::from(value));
+        #[allow(unused_mut)]
+        let mut store_value = value;
+        #[cfg(all(feature = "plugins", not(target_arch = "wasm32")))]
+        {
+            if store_value.as_any().downcast_ref::<crate::runtime::plugin_loader_v2::PluginBoxV2>().is_some() {
+                store_value = store_value.share_box();
+            }
+        }
+        self.outbox_vars.insert(name.to_string(), Arc::from(store_value));
     }
     
     /// local変数スタックを保存・復元（関数呼び出し時）
diff --git a/src/interpreter/statements.rs b/src/interpreter/statements.rs
index 28df28ed..cbc3e9e4 100644
--- a/src/interpreter/statements.rs
+++ b/src/interpreter/statements.rs
@@ -282,7 +282,22 @@ impl NyashInterpreter {
                     }
                 }
                 
-                self.set_variable(name, val.clone_box())?;
+                // Assign-by-share for plugin handle types; clone for others
+                let assigned = {
+                    #[cfg(all(feature = "plugins", not(target_arch = "wasm32")))]
+                    {
+                        if val.as_any().downcast_ref::<crate::runtime::plugin_loader_v2::PluginBoxV2>().is_some() {
+                            val.share_box()
+                        } else {
+                            val.clone_box()
+                        }
+                    }
+                    #[cfg(any(not(feature = "plugins"), target_arch = "wasm32"))]
+                    {
+                        val.clone_box()
+                    }
+                };
+                self.set_variable(name, assigned)?;
                 Ok(val)
             }
             
@@ -330,7 +345,18 @@ impl NyashInterpreter {
                     // 🚨 フィールド差し替え時の自動finiは削除（Nyashの明示的哲学）
                     // プログラマーが必要なら明示的にfini()を呼ぶべき
                     
-                    instance.set_field(field, Arc::from(val.clone_box()))
+                    // Store-by-share for plugin handle types; clone for others
+                    let stored = {
+                        #[cfg(all(feature = "plugins", not(target_arch = "wasm32")))]
+                        {
+                            if val.as_any().downcast_ref::<crate::runtime::plugin_loader_v2::PluginBoxV2>().is_some() {
+                                val.share_box()
+                            } else { val.clone_box() }
+                        }
+                        #[cfg(any(not(feature = "plugins"), target_arch = "wasm32"))]
+                        { val.clone_box() }
+                    };
+                    instance.set_field(field, Arc::from(stored))
                         .map_err(|e| RuntimeError::InvalidOperation { message: e })?;
                     Ok(val)
                 } else {
@@ -354,7 +380,17 @@ impl NyashInterpreter {
                     // 🚨 フィールド差し替え時の自動finiは削除（Nyashの明示的哲学）
                     // プログラマーが必要なら明示的にfini()を呼ぶべき
                     
-                    instance.set_field(field, Arc::from(val.clone_box()))
+                    let stored = {
+                        #[cfg(all(feature = "plugins", not(target_arch = "wasm32")))]
+                        {
+                            if val.as_any().downcast_ref::<crate::runtime::plugin_loader_v2::PluginBoxV2>().is_some() {
+                                val.share_box()
+                            } else { val.clone_box() }
+                        }
+                        #[cfg(any(not(feature = "plugins"), target_arch = "wasm32"))]
+                        { val.clone_box() }
+                    };
+                    instance.set_field(field, Arc::from(stored))
                         .map_err(|e| RuntimeError::InvalidOperation { message: e })?;
                     Ok(val)
                 } else {
@@ -378,7 +414,17 @@ impl NyashInterpreter {
                     // 🚨 フィールド差し替え時の自動finiは削除（Nyashの明示的哲学）
                     // プログラマーが必要なら明示的にfini()を呼ぶべき
                     
-                    instance.set_field(field, Arc::from(val.clone_box()))
+                    let stored = {
+                        #[cfg(all(feature = "plugins", not(target_arch = "wasm32")))]
+                        {
+                            if val.as_any().downcast_ref::<crate::runtime::plugin_loader_v2::PluginBoxV2>().is_some() {
+                                val.share_box()
+                            } else { val.clone_box() }
+                        }
+                        #[cfg(any(not(feature = "plugins"), target_arch = "wasm32"))]
+                        { val.clone_box() }
+                    };
+                    instance.set_field(field, Arc::from(stored))
                         .map_err(|e| RuntimeError::InvalidOperation { message: e })?;
                     Ok(val)
                 } else {
diff --git a/tests/e2e_plugin_counterbox.rs b/tests/e2e_plugin_counterbox.rs
index a6a6b2f0..fad48bac 100644
--- a/tests/e2e_plugin_counterbox.rs
+++ b/tests/e2e_plugin_counterbox.rs
@@ -50,7 +50,7 @@ v2
 }
 
 #[test]
-fn e2e_counter_assignment_clones_not_shares() {
+fn e2e_counter_assignment_shares_handle() {
     if !try_init_plugins() { return; }
 
     let code = r#"
@@ -66,10 +66,9 @@ v
 
     match interpreter.execute(ast) {
         Ok(result) => {
-            // Current semantics: assignment clones (not shares), so c remains 0
-            assert_eq!(result.to_string_box().value, "0");
+            // New semantics: plugin handle assign shares, so c reflects x.inc()
+            assert_eq!(result.to_string_box().value, "1");
         }
         Err(e) => panic!("Counter assignment test failed: {:?}", e),
     }
 }
-