diff --git a/3rd/cjson/src/cJSONw.cpp b/3rd/cjson/src/cJSONw.cpp
index f445c513..a70024fb 100644
--- a/3rd/cjson/src/cJSONw.cpp
+++ b/3rd/cjson/src/cJSONw.cpp
@@ -36,7 +36,7 @@ static cJSON* convert_python_object_to_cjson(PyObject* obj, VM* vm){
         case VM::tp_tuple.index: return convert_list_to_cjson<Tuple>(_CAST(Tuple&, obj), vm);
         default: break;
     }
-    vm->TypeError(fmt("unrecognized type ", _type_name(vm, obj_t).escape()));
+    vm->TypeError(_S("unrecognized type ", _type_name(vm, obj_t).escape()));
     PK_UNREACHABLE()
 }
 
@@ -110,7 +110,7 @@ void add_module_cjson(VM* vm){
             const char* start = cJSON_GetErrorPtr();
             const char* end = start;
             while(*end != '\0' && *end != '\n') end++;
-            vm->IOError(fmt("cjson: ", std::string_view(start, end-start)));
+            vm->IOError(_S("cjson: ", std::string_view(start, end-start)));
         }
         PyObject* output = convert_cjson_to_python_object(json, vm);
         cJSON_Delete(json);
diff --git a/3rd/lua_bridge/src/lua_bridge.cpp b/3rd/lua_bridge/src/lua_bridge.cpp
index 7b48243c..0a5bd040 100644
--- a/3rd/lua_bridge/src/lua_bridge.cpp
+++ b/3rd/lua_bridge/src/lua_bridge.cpp
@@ -285,7 +285,7 @@ void lua_push_from_python(VM* vm, PyObject* val){
         lua_rawgeti(_L, LUA_REGISTRYINDEX, func.r);
         return;
     }
-    vm->RuntimeError(fmt("unsupported python type: ", _type_name(vm, t).escape()));
+    vm->RuntimeError(_S("unsupported python type: ", _type_name(vm, t).escape()));
 }
 
 PyObject* lua_popx_to_python(VM* vm) {
@@ -321,7 +321,7 @@ PyObject* lua_popx_to_python(VM* vm) {
         default: {
             const char* type_name = lua_typename(_L, type);
             lua_pop(_L, 1);
-            vm->RuntimeError(fmt("unsupported lua type: '", type_name, "'"));
+            vm->RuntimeError(_S("unsupported lua type: '", type_name, "'"));
         }
     }
     PK_UNREACHABLE()
diff --git a/include/pocketpy/cffi.h b/include/pocketpy/cffi.h
index b8267baa..c2c72933 100644
--- a/include/pocketpy/cffi.h
+++ b/include/pocketpy/cffi.h
@@ -19,7 +19,7 @@ namespace pkpy {
     static PyObject* register_class(VM* vm, PyObject* mod, Type base=0) {   \
         std::string_view mod_name = PK_OBJ_GET(Str, mod->attr("__name__")).sv();   \
         if(mod_name != #mod) {                                                     \
-            Str msg = fmt("register_class() failed: ", mod_name, " != ", #mod);    \
+            Str msg = _S("register_class() failed: ", mod_name, " != ", #mod);    \
             throw std::runtime_error(msg.str());                            \
         }                                                                   \
         PyObject* type = vm->new_type_object(mod, #name, base);             \
diff --git a/include/pocketpy/namedict.h b/include/pocketpy/namedict.h
index dbf71286..0dff0104 100644
--- a/include/pocketpy/namedict.h
+++ b/include/pocketpy/namedict.h
@@ -291,7 +291,7 @@ struct NameDictImpl{
     V operator[](StrName key) const {
         V val = try_get_likely_found(key);
         if(val == default_invalid_value<V>()){
-            throw std::runtime_error(fmt("NameDict key not found: ", key.escape()).str());
+            throw std::runtime_error(_S("NameDict key not found: ", key.escape()).str());
         }
         return val;
     }
diff --git a/include/pocketpy/str.h b/include/pocketpy/str.h
index f227296a..d4c082e3 100644
--- a/include/pocketpy/str.h
+++ b/include/pocketpy/str.h
@@ -154,7 +154,7 @@ struct SStream{
 };
 
 template<typename... Args>
-Str fmt(Args&&... args) {
+Str _S(Args&&... args) {
     SStream ss;
     (ss << ... << args);
     return ss.str();
diff --git a/include/pocketpy/vm.h b/include/pocketpy/vm.h
index fb740b08..fe80e561 100644
--- a/include/pocketpy/vm.h
+++ b/include/pocketpy/vm.h
@@ -340,22 +340,22 @@ public:
     void RuntimeError(const Str& msg){ _builtin_error("RuntimeError", msg); }
     void ZeroDivisionError(const Str& msg){ _builtin_error("ZeroDivisionError", msg); }
     void ZeroDivisionError(){ _builtin_error("ZeroDivisionError", "division by zero"); }
-    void NameError(StrName name){ _builtin_error("NameError", fmt("name ", name.escape() + " is not defined")); }
-    void UnboundLocalError(StrName name){ _builtin_error("UnboundLocalError", fmt("local variable ", name.escape() + " referenced before assignment")); }
+    void NameError(StrName name){ _builtin_error("NameError", _S("name ", name.escape() + " is not defined")); }
+    void UnboundLocalError(StrName name){ _builtin_error("UnboundLocalError", _S("local variable ", name.escape() + " referenced before assignment")); }
     void KeyError(PyObject* obj){ _builtin_error("KeyError", obj); }
     void ImportError(const Str& msg){ _builtin_error("ImportError", msg); }
 
     void BinaryOptError(const char* op, PyObject* _0, PyObject* _1) {
         StrName name_0 = _type_name(vm, _tp(_0));
         StrName name_1 = _type_name(vm, _tp(_1));
-        TypeError(fmt("unsupported operand type(s) for ", op, ": ", name_0.escape(), " and ", name_1.escape()));
+        TypeError(_S("unsupported operand type(s) for ", op, ": ", name_0.escape(), " and ", name_1.escape()));
     }
 
     void AttributeError(PyObject* obj, StrName name){
         if(isinstance(obj, vm->tp_type)){
-            _builtin_error("AttributeError", fmt("type object ", _type_name(vm, PK_OBJ_GET(Type, obj)).escape(), " has no attribute ", name.escape()));
+            _builtin_error("AttributeError", _S("type object ", _type_name(vm, PK_OBJ_GET(Type, obj)).escape(), " has no attribute ", name.escape()));
         }else{
-            _builtin_error("AttributeError", fmt(_type_name(vm, _tp(obj)).escape(), " object has no attribute ", name.escape()));
+            _builtin_error("AttributeError", _S(_type_name(vm, _tp(obj)).escape(), " object has no attribute ", name.escape()));
         }
     }
     void AttributeError(const Str& msg){ _builtin_error("AttributeError", msg); }
diff --git a/src/ceval.cpp b/src/ceval.cpp
index 13bd98fc..21b909bc 100644
--- a/src/ceval.cpp
+++ b/src/ceval.cpp
@@ -546,7 +546,7 @@ __NEXT_STEP:;
     TARGET(GOTO) {
         StrName _name(byte.arg);
         int index = co->labels.try_get_likely_found(_name);
-        if(index < 0) RuntimeError(fmt("label ", _name.escape(), " not found"));
+        if(index < 0) RuntimeError(_S("label ", _name.escape(), " not found"));
         frame->jump_abs_break(index);
     } DISPATCH();
     /*****************************************/
@@ -680,7 +680,7 @@ __NEXT_STEP:;
                 _name = StrName::get(CAST(Str&, key).sv());
                 PyObject* value = _0->attr().try_get_likely_found(_name);
                 if(value == nullptr){
-                    ImportError(fmt("cannot import name ", _name.escape()));
+                    ImportError(_S("cannot import name ", _name.escape()));
                 }else{
                     frame->f_globals().set(_name, value);
                 }
diff --git a/src/cffi.cpp b/src/cffi.cpp
index 64f1fbdb..6685904a 100644
--- a/src/cffi.cpp
+++ b/src/cffi.cpp
@@ -16,7 +16,7 @@ namespace pkpy{
 
         vm->bind__repr__(PK_OBJ_GET(Type, type), [](VM* vm, PyObject* obj){
             VoidP& self = PK_OBJ_GET(VoidP, obj);
-            return VAR(fmt("<void* at ", self.hex(), ">"));
+            return VAR(_S("<void* at ", self.hex(), ">"));
         });
 
 #define BIND_CMP(name, op)  \
@@ -61,12 +61,12 @@ namespace pkpy{
                 if(s[i]>='0' && s[i]<='9') c += s[i]-'0';
                 else if(s[i]>='A' && s[i]<='F') c += s[i]-'A'+10;
                 else if(s[i]>='a' && s[i]<='f') c += s[i]-'a'+10;
-                else vm->ValueError(fmt("invalid hex char: '", s[i], "'"));
+                else vm->ValueError(_S("invalid hex char: '", s[i], "'"));
                 c <<= 4;
                 if(s[i+1]>='0' && s[i+1]<='9') c += s[i+1]-'0';
                 else if(s[i+1]>='A' && s[i+1]<='F') c += s[i+1]-'A'+10;
                 else if(s[i+1]>='a' && s[i+1]<='f') c += s[i+1]-'a'+10;
-                else vm->ValueError(fmt("invalid hex char: '", s[i+1], "'"));
+                else vm->ValueError(_S("invalid hex char: '", s[i+1], "'"));
                 buffer.p[i/2] = c;
             }
             return VAR_T(C99Struct, std::move(buffer));
@@ -235,7 +235,7 @@ void add_module_c(VM* vm){
     });                                                                 \
     vm->bind__repr__(type_t, [](VM* vm, PyObject* obj){                 \
         VoidP& self = _CAST(VoidP&, obj);                               \
-        return VAR(fmt("<", CNAME, "* at ", self.hex(), ">"));         \
+        return VAR(_S("<", CNAME, "* at ", self.hex(), ">"));         \
     });                                                                 \
 
     BIND_PRIMITIVE(char, "char")
diff --git a/src/compiler.cpp b/src/compiler.cpp
index 74c14eca..6d3251d9 100644
--- a/src/compiler.cpp
+++ b/src/compiler.cpp
@@ -133,7 +133,7 @@ namespace pkpy{
     void Compiler::consume(TokenIndex expected) {
         if (!match(expected)){
             SyntaxError(
-                fmt("expected '", TK_STR(expected), "', got '", TK_STR(curr().type), "'")
+                _S("expected '", TK_STR(expected), "', got '", TK_STR(curr().type), "'")
             );
         }
     }
diff --git a/src/frame.cpp b/src/frame.cpp
index 4e91f603..b5030ccb 100644
--- a/src/frame.cpp
+++ b/src/frame.cpp
@@ -34,7 +34,7 @@ namespace pkpy{
         PyObject* obj = _s->popx();         // pop exception object
         // get the stack size of the try block
         int _stack_size = co->blocks[block].base_stack_size;
-        if(stack_size() < _stack_size) throw std::runtime_error(fmt("invalid state: ", stack_size(), '<', _stack_size).str());
+        if(stack_size() < _stack_size) throw std::runtime_error(_S("invalid state: ", stack_size(), '<', _stack_size).str());
         _s->reset(actual_sp_base() + _locals.size() + _stack_size);          // rollback the stack   
         _s->push(obj);                                      // push exception object
         _next_ip = co->blocks[block].end;
diff --git a/src/linalg.cpp b/src/linalg.cpp
index 0852af2e..69cb1ce0 100644
--- a/src/linalg.cpp
+++ b/src/linalg.cpp
@@ -249,7 +249,7 @@ static Vec2 SmoothDamp(Vec2 current, Vec2 target, PyVec2& currentVelocity, float
                 for(int i=0; i<9; i++) mat.v[i] = CAST_F(args[1+i]);
                 return vm->heap.gcnew<PyMat3x3>(PK_OBJ_GET(Type, args[0]), mat);
             }
-            vm->TypeError(fmt("Mat3x3.__new__ takes 0 or 1 or 9 arguments, got ", args.size()-1));
+            vm->TypeError(_S("Mat3x3.__new__ takes 0 or 1 or 9 arguments, got ", args.size()-1));
             return vm->None;
         });
 
diff --git a/src/pocketpy.cpp b/src/pocketpy.cpp
index 29ff308e..21cd65b6 100644
--- a/src/pocketpy.cpp
+++ b/src/pocketpy.cpp
@@ -1273,7 +1273,7 @@ void init_builtins(VM* _vm) {
 
     _vm->bind__repr__(VM::tp_module, [](VM* vm, PyObject* _0) {
         const Str& path = CAST(Str&, _0->attr(__path__));
-        return VAR(fmt("<module ", path.escape(), ">"));
+        return VAR(_S("<module ", path.escape(), ">"));
     });
 
     // tp_property
@@ -1339,7 +1339,7 @@ void init_builtins(VM* _vm) {
 
     _vm->bind__repr__(VM::tp_exception, [](VM* vm, PyObject* _0) {
         Exception& self = _CAST(Exception&, _0);
-        return VAR(fmt(_type_name(vm, _0->type), '(', self.msg.escape(), ')'));
+        return VAR(_S(_type_name(vm, _0->type), '(', self.msg.escape(), ')'));
     });
 
     _vm->bind__str__(VM::tp_exception, [](VM* vm, PyObject* _0) {
@@ -1363,7 +1363,7 @@ void VM::post_init(){
 
     _all_types[tp_module].m__getattr__ = [](VM* vm, PyObject* obj, StrName name) -> PyObject*{
         const Str& path = CAST(Str&, obj->attr(__path__));
-        return vm->py_import(fmt(path, ".", name.sv()), false);
+        return vm->py_import(_S(path, ".", name.sv()), false);
     };
 
     bind_method<1>(tp_property, "setter", [](VM* vm, ArgsView args) {
diff --git a/src/pocketpy_c.cpp b/src/pocketpy_c.cpp
index dd779e5b..eecd21fb 100644
--- a/src/pocketpy_c.cpp
+++ b/src/pocketpy_c.cpp
@@ -8,7 +8,7 @@ typedef int (*LuaStyleFuncC)(VM*);
 #define PK_ASSERT_N_EXTRA_ELEMENTS(n) \
     int __ex_count = count_extra_elements(vm, n); \
     if(__ex_count < n){ \
-        Str msg = fmt("expected at least ", n, " elements, got ", __ex_count); \
+        Str msg = _S("expected at least ", n, " elements, got ", __ex_count); \
         pkpy_error(vm_handle, "StackError", pkpy_string(msg.c_str())); \
         return false; \
     }
diff --git a/src/repl.cpp b/src/repl.cpp
index 54291194..6d9b0941 100644
--- a/src/repl.cpp
+++ b/src/repl.cpp
@@ -3,7 +3,7 @@
 namespace pkpy {
     REPL::REPL(VM* vm) : vm(vm){
         vm->stdout_write("pocketpy " PK_VERSION " (" __DATE__ ", " __TIME__ ") ");
-        vm->stdout_write(fmt("[", sizeof(void*)*8, " bit] on ", kPlatformStrings[PK_SYS_PLATFORM], "\n"));
+        vm->stdout_write(_S("[", sizeof(void*)*8, " bit] on ", kPlatformStrings[PK_SYS_PLATFORM], "\n"));
         vm->stdout_write("https://github.com/pocketpy/pocketpy" "\n");
         vm->stdout_write("Type \"exit()\" to exit." "\n");
     }
diff --git a/src/vm.cpp b/src/vm.cpp
index 547b5692..9168139e 100644
--- a/src/vm.cpp
+++ b/src/vm.cpp
@@ -26,7 +26,7 @@ namespace pkpy{
                 if(!first) ss << ", ";
                 first = false;
                 if(!is_non_tagged_type(k, vm->tp_str)){
-                    vm->TypeError(fmt("json keys must be string, got ", _type_name(vm, vm->_tp(k))));
+                    vm->TypeError(_S("json keys must be string, got ", _type_name(vm, vm->_tp(k))));
                 }
                 ss << _CAST(Str&, k).escape(false) << ": ";
                 write_object(v);
@@ -55,7 +55,7 @@ namespace pkpy{
             }else if(obj_t == vm->tp_dict){
                 write_dict(_CAST(Dict&, obj));
             }else{
-                vm->TypeError(fmt("unrecognized type ", _type_name(vm, obj_t).escape()));
+                vm->TypeError(_S("unrecognized type ", _type_name(vm, obj_t).escape()));
             }
         }
 
@@ -198,7 +198,7 @@ namespace pkpy{
         PyObject* obj = heap._new<Type>(tp_type, _all_types.size());
         const PyTypeInfo& base_info = _all_types[base];
         if(!base_info.subclass_enabled){
-            TypeError(fmt("type ", base_info.name.escape(), " is not `subclass_enabled`"));
+            TypeError(_S("type ", base_info.name.escape(), " is not `subclass_enabled`"));
         }
         PyTypeInfo info{
             obj,
@@ -316,7 +316,7 @@ namespace pkpy{
                 out = _import_handler(filename.data, filename.size, &out_size);
             }
             if(out == nullptr){
-                if(throw_err) ImportError(fmt("module ", path.escape(), " not found"));
+                if(throw_err) ImportError(_S("module ", path.escape(), " not found"));
                 else return nullptr;
             }
             PK_ASSERT(out_size >= 0)
@@ -442,7 +442,7 @@ i64 VM::py_hash(PyObject* obj){
         has_custom_eq = f != _t(tp_object)->attr(__eq__);
     }
     if(has_custom_eq){
-        TypeError(fmt("unhashable type: ", ti->name.escape()));
+        TypeError(_S("unhashable type: ", ti->name.escape()));
         PK_UNREACHABLE()
     }else{
         return PK_BITS(obj);
@@ -545,7 +545,7 @@ PyObject* VM::new_module(Str name, Str package) {
     // we do not allow override in order to avoid memory leak
     // it is because Module objects are not garbage collected
     if(_modules.contains(name)){
-        throw std::runtime_error(fmt("module ", name.escape(), " already exists").str());
+        throw std::runtime_error(_S("module ", name.escape(), " already exists").str());
     }
     // set it into _modules
     _modules.set(name, obj);
@@ -557,20 +557,20 @@ static std::string _opcode_argstr(VM* vm, Bytecode byte, const CodeObject* co){
     switch(byte.op){
         case OP_LOAD_CONST: case OP_FORMAT_STRING: case OP_IMPORT_PATH:
             if(vm != nullptr){
-                argStr += fmt(" (", CAST(Str, vm->py_repr(co->consts[byte.arg])), ")").sv();
+                argStr += _S(" (", CAST(Str, vm->py_repr(co->consts[byte.arg])), ")").sv();
             }
             break;
         case OP_LOAD_NAME: case OP_LOAD_GLOBAL: case OP_LOAD_NONLOCAL: case OP_STORE_GLOBAL:
         case OP_LOAD_ATTR: case OP_LOAD_METHOD: case OP_STORE_ATTR: case OP_DELETE_ATTR:
         case OP_BEGIN_CLASS: case OP_GOTO:
         case OP_DELETE_GLOBAL: case OP_INC_GLOBAL: case OP_DEC_GLOBAL: case OP_STORE_CLASS_ATTR:
-            argStr += fmt(" (", StrName(byte.arg).sv(), ")").sv();
+            argStr += _S(" (", StrName(byte.arg).sv(), ")").sv();
             break;
         case OP_LOAD_FAST: case OP_STORE_FAST: case OP_DELETE_FAST: case OP_INC_FAST: case OP_DEC_FAST:
-            argStr += fmt(" (", co->varnames[byte.arg].sv(), ")").sv();
+            argStr += _S(" (", co->varnames[byte.arg].sv(), ")").sv();
             break;
         case OP_LOAD_FUNCTION:
-            argStr += fmt(" (", co->func_decls[byte.arg]->code->name, ")").sv();
+            argStr += _S(" (", co->func_decls[byte.arg]->code->name, ")").sv();
             break;
     }
     return argStr;
@@ -789,7 +789,7 @@ void VM::_prepare_py_call(PyObject** buffer, ArgsView args, ArgsView kwargs, con
     int decl_argc = decl->args.size();
 
     if(args.size() < decl_argc){
-        vm->TypeError(fmt(
+        vm->TypeError(_S(
             co->name, "() takes ", decl_argc, " positional arguments but ", args.size(), " were given"
         ));
     }
@@ -813,7 +813,7 @@ void VM::_prepare_py_call(PyObject** buffer, ArgsView args, ArgsView kwargs, con
             if(i >= args.size()) break;
             buffer[kv.index] = args[i++];
         }
-        if(i < args.size()) TypeError(fmt("too many arguments", " (", decl->code->name, ')'));
+        if(i < args.size()) TypeError(_S("too many arguments", " (", decl->code->name, ')'));
     }
     
     PyObject* vkwargs;
@@ -833,7 +833,7 @@ void VM::_prepare_py_call(PyObject** buffer, ArgsView args, ArgsView kwargs, con
         }else{
             // otherwise, set as **kwargs if possible
             if(vkwargs == nullptr){
-                TypeError(fmt(key.escape(), " is an invalid keyword argument for ", co->name, "()"));
+                TypeError(_S(key.escape(), " is an invalid keyword argument for ", co->name, "()"));
             }else{
                 Dict& dict = _CAST(Dict&, vkwargs);
                 dict.set(VAR(key.sv()), kwargs[j+1]);
@@ -897,12 +897,12 @@ PyObject* VM::vectorcall(int ARGC, int KWARGC, bool op_call){
         int co_nlocals = co->varnames.size();
         if(decl->is_simple){
             if(args.size() != decl->args.size()){
-                TypeError(fmt(
+                TypeError(_S(
                     co->name, "() takes ", decl->args.size(), " positional arguments but ", args.size(), " were given"
                 ));
             }
             if(!kwargs.empty()){
-                TypeError(fmt(co->name, "() takes no keyword arguments"));
+                TypeError(_S(co->name, "() takes no keyword arguments"));
             }
             s_data.reset(_base + co_nlocals);
             int i = 0;
@@ -1145,7 +1145,7 @@ void VM::setattr(PyObject* obj, StrName name, PyObject* value){
             if(prop.setter != vm->None){
                 call(prop.setter, obj, value);
             }else{
-                TypeError(fmt("readonly attribute: ", name.escape()));
+                TypeError(_S("readonly attribute: ", name.escape()));
             }
             return;
         }
@@ -1170,7 +1170,7 @@ PyObject* VM::bind(PyObject* obj, const char* sig, const char* docstring, Native
     CodeObject_ co;
     try{
         // fn(a, b, *c, d=1) -> None
-        co = compile(fmt("def ", sig, " : pass"), "<bind>", EXEC_MODE);
+        co = compile(_S("def ", sig, " : pass"), "<bind>", EXEC_MODE);
     }catch(const Exception&){
         throw std::runtime_error("invalid signature: " + std::string(sig));
     }
@@ -1337,7 +1337,7 @@ void Dict::_probe_1(PyObject *key, bool &ok, int &i) const{
 
 void NativeFunc::check_size(VM* vm, ArgsView args) const{
     if(args.size() != argc && argc != -1) {
-        vm->TypeError(fmt("expected ", argc, " arguments, got ", args.size()));
+        vm->TypeError(_S("expected ", argc, " arguments, got ", args.size()));
     }
 }