diff --git a/src/backend/mir_interpreter/handlers/boxes.rs b/src/backend/mir_interpreter/handlers/boxes.rs
index 6f36321e..2738534d 100644
--- a/src/backend/mir_interpreter/handlers/boxes.rs
+++ b/src/backend/mir_interpreter/handlers/boxes.rs
@@ -212,7 +212,7 @@ impl MirInterpreter {
                 );
             }
         }
-        if self.try_handle_instance_box(dst, box_val, method, args)? {
+        if super::boxes_instance::try_handle_instance_box(self, dst, box_val, method, args)? {
             if method == "length" && std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
                 eprintln!("[vm-trace] length dispatch handler=instance_box");
             }
@@ -308,157 +308,6 @@ impl MirInterpreter {
         super::boxes_string::try_handle_string_box(self, dst, box_val, method, args)
     }
 
-    fn try_handle_instance_box(
-        &mut self,
-        dst: Option<ValueId>,
-        box_val: ValueId,
-        method: &str,
-        args: &[ValueId],
-    ) -> Result<bool, VMError> {
-        let recv_vm = self.reg_load(box_val)?;
-        let recv_box_any: Box<dyn NyashBox> = match recv_vm.clone() {
-            VMValue::BoxRef(b) => b.share_box(),
-            other => other.to_nyash_box(),
-        };
-        if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") && method == "toString" {
-            eprintln!("[vm-trace] instance-check recv_box_any.type={} args_len={}", recv_box_any.type_name(), args.len());
-        }
-        if let Some(inst) = recv_box_any.as_any().downcast_ref::<crate::instance_v2::InstanceBox>() {
-            // Development guard: ensure JsonScanner core fields have sensible defaults
-            if inst.class_name == "JsonScanner" {
-                // populate missing fields to avoid Void in comparisons inside is_eof/advance
-                if inst.get_field_ng("position").is_none() {
-                    let _ = inst.set_field_ng("position".to_string(), crate::value::NyashValue::Integer(0));
-                }
-                if inst.get_field_ng("length").is_none() {
-                    let _ = inst.set_field_ng("length".to_string(), crate::value::NyashValue::Integer(0));
-                }
-                if inst.get_field_ng("line").is_none() {
-                    let _ = inst.set_field_ng("line".to_string(), crate::value::NyashValue::Integer(1));
-                }
-                if inst.get_field_ng("column").is_none() {
-                    let _ = inst.set_field_ng("column".to_string(), crate::value::NyashValue::Integer(1));
-                }
-                if inst.get_field_ng("text").is_none() {
-                    let _ = inst.set_field_ng("text".to_string(), crate::value::NyashValue::String(String::new()));
-                }
-            }
-            // JsonNodeInstance narrow bridges removed: rely on builder rewrite and instance dispatch
-            // birth: do not short-circuit; allow dispatch to lowered function "Class.birth/arity"
-            if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") && method == "toString" {
-                eprintln!(
-                    "[vm-trace] instance-check downcast=ok class={} stringify_present={{class:{}, alt:{}}}",
-                    inst.class_name,
-                    self.functions.contains_key(&format!("{}.stringify/0", inst.class_name)),
-                    self.functions.contains_key(&format!("{}Instance.stringify/0", inst.class_name))
-                );
-            }
-            // Resolve lowered method function: "Class.method/arity"
-            let primary = format!("{}.{}{}", inst.class_name, method, format!("/{}", args.len()));
-            // Alternate naming: "ClassInstance.method/arity"
-            let alt = format!("{}Instance.{}{}", inst.class_name, method, format!("/{}", args.len()));
-            // Static method variant that takes 'me' explicitly as first arg: "Class.method/(arity+1)"
-            let static_variant = format!("{}.{}{}", inst.class_name, method, format!("/{}", args.len() + 1));
-            // Special-case: toString() → stringify/0 if present
-            // Prefer base class (strip trailing "Instance") stringify when available.
-            let (stringify_base, stringify_inst) = if method == "toString" && args.is_empty() {
-                let base = inst
-                    .class_name
-                    .strip_suffix("Instance")
-                    .map(|s| s.to_string());
-                let base_name = base.unwrap_or_else(|| inst.class_name.clone());
-                (
-                    Some(format!("{}.stringify/0", base_name)),
-                    Some(format!("{}.stringify/0", inst.class_name)),
-                )
-            } else { (None, None) };
-
-            if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                eprintln!(
-                    "[vm-trace] instance-dispatch class={} method={} arity={} candidates=[{}, {}, {}]",
-                    inst.class_name, method, args.len(), primary, alt, static_variant
-                );
-            }
-
-            // Prefer stringify for toString() if present (semantic alias). Try instance first, then base.
-            let func_opt = if let Some(ref sname) = stringify_inst {
-                self.functions.get(sname).cloned()
-            } else { None }
-            .or_else(|| stringify_base.as_ref().and_then(|n| self.functions.get(n).cloned()))
-            .or_else(|| self.functions.get(&primary).cloned())
-            .or_else(|| self.functions.get(&alt).cloned())
-            .or_else(|| self.functions.get(&static_variant).cloned());
-
-            if let Some(func) = func_opt {
-                if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                    eprintln!("[vm-trace] instance-dispatch hit -> {}", func.signature.name);
-                }
-                // Build argv: me + args (works for both instance and static(me, ...))
-                let mut argv: Vec<VMValue> = Vec::with_capacity(1 + args.len());
-                // Dev assert: forbid birth(me==Void)
-                if method == "birth" && crate::config::env::using_is_dev() {
-                    if matches!(recv_vm, VMValue::Void) {
-                        return Err(VMError::InvalidInstruction("Dev assert: birth(me==Void) is forbidden".into()));
-                    }
-                }
-                argv.push(recv_vm.clone());
-                for a in args { argv.push(self.reg_load(*a)?); }
-                let ret = self.exec_function_inner(&func, Some(&argv))?;
-                if let Some(d) = dst { self.regs.insert(d, ret); }
-                return Ok(true);
-            } else {
-                // Conservative fallback: search unique function by name tail ".method/arity"
-                let tail = format!(".{}{}", method, format!("/{}", args.len()));
-                let mut cands: Vec<String> = self
-                    .functions
-                    .keys()
-                    .filter(|k| k.ends_with(&tail))
-                    .cloned()
-                    .collect();
-                if !cands.is_empty() {
-                    // Always narrow by receiver class prefix (and optional "Instance" suffix)
-                    let recv_cls = inst.class_name.clone();
-                    let pref1 = format!("{}.", recv_cls);
-                    let pref2 = format!("{}Instance.", recv_cls);
-                    let filtered: Vec<String> = cands
-                        .into_iter()
-                        .filter(|k| k.starts_with(&pref1) || k.starts_with(&pref2))
-                        .collect();
-                    if filtered.len() == 1 {
-                        let fname = &filtered[0];
-                        if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                            eprintln!("[vm-trace] instance-dispatch fallback (scoped) -> {}", fname);
-                        }
-                        if let Some(func) = self.functions.get(fname).cloned() {
-                            let mut argv: Vec<VMValue> = Vec::with_capacity(1 + args.len());
-                            if method == "birth" && crate::config::env::using_is_dev() {
-                                if matches!(recv_vm, VMValue::Void) {
-                                    return Err(VMError::InvalidInstruction("Dev assert: birth(me==Void) is forbidden".into()));
-                                }
-                            }
-                            argv.push(recv_vm.clone());
-                            for a in args { argv.push(self.reg_load(*a)?); }
-                            let ret = self.exec_function_inner(&func, Some(&argv))?;
-                            if let Some(d) = dst { self.regs.insert(d, ret); }
-                            return Ok(true);
-                        }
-                    } else if filtered.len() > 1 {
-                        if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                            eprintln!("[vm-trace] instance-dispatch multiple candidates after narrowing: {:?}", filtered);
-                        }
-                        // Ambiguous: do not dispatch cross-class
-                    } else {
-                        // No same-class candidate: do not dispatch cross-class
-                        if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                            eprintln!("[vm-trace] instance-dispatch no same-class candidate for tail .{}{}", method, format!("/{}", args.len()));
-                        }
-                    }
-                }
-            }
-        }
-        Ok(false)
-    }
-
     // moved: try_handle_array_box → handlers/boxes_array.rs
     fn try_handle_array_box(
         &mut self,
diff --git a/src/backend/mir_interpreter/handlers/boxes_instance.rs b/src/backend/mir_interpreter/handlers/boxes_instance.rs
new file mode 100644
index 00000000..0c8fec9a
--- /dev/null
+++ b/src/backend/mir_interpreter/handlers/boxes_instance.rs
@@ -0,0 +1,153 @@
+use super::*;
+use crate::box_trait::NyashBox;
+
+pub(super) fn try_handle_instance_box(
+    this: &mut MirInterpreter,
+    dst: Option<ValueId>,
+    box_val: ValueId,
+    method: &str,
+    args: &[ValueId],
+) -> Result<bool, VMError> {
+    let recv_vm = this.reg_load(box_val)?;
+    let recv_box_any: Box<dyn NyashBox> = match recv_vm.clone() {
+        VMValue::BoxRef(b) => b.share_box(),
+        other => other.to_nyash_box(),
+    };
+    if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") && method == "toString" {
+        eprintln!("[vm-trace] instance-check recv_box_any.type={} args_len={}", recv_box_any.type_name(), args.len());
+    }
+    if let Some(inst) = recv_box_any.as_any().downcast_ref::<crate::instance_v2::InstanceBox>() {
+        // Development guard: ensure JsonScanner core fields have sensible defaults
+        if inst.class_name == "JsonScanner" {
+            // populate missing fields to avoid Void in comparisons inside is_eof/advance
+            if inst.get_field_ng("position").is_none() {
+                let _ = inst.set_field_ng("position".to_string(), crate::value::NyashValue::Integer(0));
+            }
+            if inst.get_field_ng("length").is_none() {
+                let _ = inst.set_field_ng("length".to_string(), crate::value::NyashValue::Integer(0));
+            }
+            if inst.get_field_ng("line").is_none() {
+                let _ = inst.set_field_ng("line".to_string(), crate::value::NyashValue::Integer(1));
+            }
+            if inst.get_field_ng("column").is_none() {
+                let _ = inst.set_field_ng("column".to_string(), crate::value::NyashValue::Integer(1));
+            }
+            if inst.get_field_ng("text").is_none() {
+                let _ = inst.set_field_ng("text".to_string(), crate::value::NyashValue::String(String::new()));
+            }
+        }
+        // JsonNodeInstance narrow bridges removed: rely on builder rewrite and instance dispatch
+        // birth: do not short-circuit; allow dispatch to lowered function "Class.birth/arity"
+        if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") && method == "toString" {
+            eprintln!(
+                "[vm-trace] instance-check downcast=ok class={} stringify_present={{class:{}, alt:{}}}",
+                inst.class_name,
+                this.functions.contains_key(&format!("{}.stringify/0", inst.class_name)),
+                this.functions.contains_key(&format!("{}Instance.stringify/0", inst.class_name))
+            );
+        }
+        // Resolve lowered method function: "Class.method/arity"
+        let primary = format!("{}.{}{}", inst.class_name, method, format!("/{}", args.len()));
+        // Alternate naming: "ClassInstance.method/arity"
+        let alt = format!("{}Instance.{}{}", inst.class_name, method, format!("/{}", args.len()));
+        // Static method variant that takes 'me' explicitly as first arg: "Class.method/(arity+1)"
+        let static_variant = format!("{}.{}{}", inst.class_name, method, format!("/{}", args.len() + 1));
+        // Special-case: toString() → stringify/0 if present
+        // Prefer base class (strip trailing "Instance") stringify when available.
+        let (stringify_base, stringify_inst) = if method == "toString" && args.is_empty() {
+            let base = inst
+                .class_name
+                .strip_suffix("Instance")
+                .map(|s| s.to_string());
+            let base_name = base.unwrap_or_else(|| inst.class_name.clone());
+            (
+                Some(format!("{}.stringify/0", base_name)),
+                Some(format!("{}.stringify/0", inst.class_name)),
+            )
+        } else { (None, None) };
+
+        if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
+            eprintln!(
+                "[vm-trace] instance-dispatch class={} method={} arity={} candidates=[{}, {}, {}]",
+                inst.class_name, method, args.len(), primary, alt, static_variant
+            );
+        }
+
+        // Prefer stringify for toString() if present (semantic alias). Try instance first, then base.
+        let func_opt = if let Some(ref sname) = stringify_inst {
+            this.functions.get(sname).cloned()
+        } else { None }
+        .or_else(|| stringify_base.as_ref().and_then(|n| this.functions.get(n).cloned()))
+        .or_else(|| this.functions.get(&primary).cloned())
+        .or_else(|| this.functions.get(&alt).cloned())
+        .or_else(|| this.functions.get(&static_variant).cloned());
+
+        if let Some(func) = func_opt {
+            if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
+                eprintln!("[vm-trace] instance-dispatch hit -> {}", func.signature.name);
+            }
+            // Build argv: me + args (works for both instance and static(me, ...))
+            let mut argv: Vec<VMValue> = Vec::with_capacity(1 + args.len());
+            // Dev assert: forbid birth(me==Void)
+            if method == "birth" && crate::config::env::using_is_dev() {
+                if matches!(recv_vm, VMValue::Void) {
+                    return Err(VMError::InvalidInstruction("Dev assert: birth(me==Void) is forbidden".into()));
+                }
+            }
+            argv.push(recv_vm.clone());
+            for a in args { argv.push(this.reg_load(*a)?); }
+            let ret = this.exec_function_inner(&func, Some(&argv))?;
+            if let Some(d) = dst { this.regs.insert(d, ret); }
+            return Ok(true);
+        } else {
+            // Conservative fallback: search unique function by name tail ".method/arity"
+            let tail = format!(".{}{}", method, format!("/{}", args.len()));
+            let mut cands: Vec<String> = this
+                .functions
+                .keys()
+                .filter(|k| k.ends_with(&tail))
+                .cloned()
+                .collect();
+            if !cands.is_empty() {
+                // Always narrow by receiver class prefix (and optional "Instance" suffix)
+                let recv_cls = inst.class_name.clone();
+                let pref1 = format!("{}.", recv_cls);
+                let pref2 = format!("{}Instance.", recv_cls);
+                let filtered: Vec<String> = cands
+                    .into_iter()
+                    .filter(|k| k.starts_with(&pref1) || k.starts_with(&pref2))
+                    .collect();
+                if filtered.len() == 1 {
+                    let fname = &filtered[0];
+                    if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
+                        eprintln!("[vm-trace] instance-dispatch fallback (scoped) -> {}", fname);
+                    }
+                    if let Some(func) = this.functions.get(fname).cloned() {
+                        let mut argv: Vec<VMValue> = Vec::with_capacity(1 + args.len());
+                        if method == "birth" && crate::config::env::using_is_dev() {
+                            if matches!(recv_vm, VMValue::Void) {
+                                return Err(VMError::InvalidInstruction("Dev assert: birth(me==Void) is forbidden".into()));
+                            }
+                        }
+                        argv.push(recv_vm.clone());
+                        for a in args { argv.push(this.reg_load(*a)?); }
+                        let ret = this.exec_function_inner(&func, Some(&argv))?;
+                        if let Some(d) = dst { this.regs.insert(d, ret); }
+                        return Ok(true);
+                    }
+                } else if filtered.len() > 1 {
+                    if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
+                        eprintln!("[vm-trace] instance-dispatch multiple candidates after narrowing: {:?}", filtered);
+                    }
+                    // Ambiguous: do not dispatch cross-class
+                } else {
+                    // No same-class candidate: do not dispatch cross-class
+                    if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
+                        eprintln!("[vm-trace] instance-dispatch no same-class candidate for tail .{}{}", method, format!("/{}", args.len()));
+                    }
+                }
+            }
+        }
+    }
+    Ok(false)
+}
diff --git a/src/backend/mir_interpreter/handlers/mod.rs b/src/backend/mir_interpreter/handlers/mod.rs
index ecc231d7..928911d1 100644
--- a/src/backend/mir_interpreter/handlers/mod.rs
+++ b/src/backend/mir_interpreter/handlers/mod.rs
@@ -6,6 +6,7 @@ mod boxes_array;
 mod boxes_string;
 mod boxes_map;
 mod boxes_object_fields;
+mod boxes_instance;
 mod calls;
 mod externals;
 mod memory;