diff --git a/doc/includes/references.h b/doc/includes/references.h
index 15345e994c..6cbecfd432 100644
--- a/doc/includes/references.h
+++ b/doc/includes/references.h
@@ -1743,7 +1743,6 @@ typedef struct mjsFrame_ {         // frame specification
 typedef struct mjsJoint_ {         // joint specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
   mjtJoint type;                   // joint type
 
   // kinematics
@@ -1781,7 +1780,6 @@ typedef struct mjsJoint_ {         // joint specification
 typedef struct mjsGeom_ {          // geom specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // classname
   mjtGeom type;                    // geom type
 
   // frame, size
@@ -1828,7 +1826,6 @@ typedef struct mjsGeom_ {          // geom specification
 typedef struct mjsSite_ {          // site specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
 
   // frame, size
   double pos[3];                   // position
@@ -1850,7 +1847,6 @@ typedef struct mjsSite_ {          // site specification
 typedef struct mjsCamera_ {        // camera specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
 
   // extrinsics
   double pos[3];                   // position
@@ -1878,7 +1874,6 @@ typedef struct mjsCamera_ {        // camera specification
 typedef struct mjsLight_ {         // light specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
 
   // frame
   double pos[3];                   // position
@@ -1904,7 +1899,6 @@ typedef struct mjsLight_ {         // light specification
 typedef struct mjsFlex_ {          // flex specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
 
   // contact properties
   int contype;                     // contact type
@@ -1943,7 +1937,6 @@ typedef struct mjsFlex_ {          // flex specification
 typedef struct mjsMesh_ {          // mesh specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
   mjString* content_type;          // content type of file
   mjString* file;                  // mesh file
   double refpos[3];                // reference position
@@ -1974,7 +1967,6 @@ typedef struct mjsHField_ {        // height field specification
 typedef struct mjsSkin_ {          // skin specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
   mjString* file;                  // skin file
   mjString* material;              // name of material used for rendering
   float rgba[4];                   // rgba when material is omitted
@@ -1999,7 +1991,6 @@ typedef struct mjsSkin_ {          // skin specification
 typedef struct mjsTexture_ {       // texture specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
   mjtTexture type;                 // texture type
 
   // method 1: builtin
@@ -2031,7 +2022,6 @@ typedef struct mjsTexture_ {       // texture specification
 typedef struct mjsMaterial_ {      // material specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
   mjString* texture;               // name of texture (empty: none)
   mjtByte texuniform;              // make texture cube uniform
   float texrepeat[2];              // texture repetition for 2D mapping
@@ -2047,7 +2037,6 @@ typedef struct mjsMaterial_ {      // material specification
 typedef struct mjsPair_ {          // pair specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
   mjString* geomname1;             // name of geom 1
   mjString* geomname2;             // name of geom 2
 
@@ -2071,7 +2060,6 @@ typedef struct mjsExclude_ {       // exclude specification
 typedef struct mjsEquality_ {      // equality specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
   mjtEq type;                      // constraint type
   double data[mjNEQDATA];          // type-dependent data
   mjtByte active;                  // is equality initially active
@@ -2084,7 +2072,6 @@ typedef struct mjsEquality_ {      // equality specification
 typedef struct mjsTendon_ {        // tendon specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
 
   // stiffness, damping, friction
   double stiffness;                // stiffness coefficient
@@ -2118,7 +2105,6 @@ typedef struct mjsWrap_ {          // wrapping object specification
 typedef struct mjsActuator_ {      // actuator specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
 
   // gain, bias
   mjtGain gaintype;                // gain type
@@ -2160,7 +2146,6 @@ typedef struct mjsActuator_ {      // actuator specification
 typedef struct mjsSensor_ {        // sensor specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
 
   // sensor definition
   mjtSensor type;                  // type of sensor
diff --git a/include/mujoco/mjspec.h b/include/mujoco/mjspec.h
index 91f284c7c6..d9ed53fa20 100644
--- a/include/mujoco/mjspec.h
+++ b/include/mujoco/mjspec.h
@@ -222,7 +222,6 @@ typedef struct mjsFrame_ {         // frame specification
 typedef struct mjsJoint_ {         // joint specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
   mjtJoint type;                   // joint type
 
   // kinematics
@@ -262,7 +261,6 @@ typedef struct mjsJoint_ {         // joint specification
 typedef struct mjsGeom_ {          // geom specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // classname
   mjtGeom type;                    // geom type
 
   // frame, size
@@ -311,7 +309,6 @@ typedef struct mjsGeom_ {          // geom specification
 typedef struct mjsSite_ {          // site specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
 
   // frame, size
   double pos[3];                   // position
@@ -335,7 +332,6 @@ typedef struct mjsSite_ {          // site specification
 typedef struct mjsCamera_ {        // camera specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
 
   // extrinsics
   double pos[3];                   // position
@@ -365,7 +361,6 @@ typedef struct mjsCamera_ {        // camera specification
 typedef struct mjsLight_ {         // light specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
 
   // frame
   double pos[3];                   // position
@@ -393,7 +388,6 @@ typedef struct mjsLight_ {         // light specification
 typedef struct mjsFlex_ {          // flex specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
 
   // contact properties
   int contype;                     // contact type
@@ -434,7 +428,6 @@ typedef struct mjsFlex_ {          // flex specification
 typedef struct mjsMesh_ {          // mesh specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
   mjString* content_type;          // content type of file
   mjString* file;                  // mesh file
   double refpos[3];                // reference position
@@ -470,7 +463,6 @@ typedef struct mjsHField_ {        // height field specification
 typedef struct mjsSkin_ {          // skin specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
   mjString* file;                  // skin file
   mjString* material;              // name of material used for rendering
   float rgba[4];                   // rgba when material is omitted
@@ -497,7 +489,6 @@ typedef struct mjsSkin_ {          // skin specification
 typedef struct mjsTexture_ {       // texture specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
   mjtTexture type;                 // texture type
 
   // method 1: builtin
@@ -531,7 +522,6 @@ typedef struct mjsTexture_ {       // texture specification
 typedef struct mjsMaterial_ {      // material specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
   mjString* texture;               // name of texture (empty: none)
   mjtByte texuniform;              // make texture cube uniform
   float texrepeat[2];              // texture repetition for 2D mapping
@@ -549,7 +539,6 @@ typedef struct mjsMaterial_ {      // material specification
 typedef struct mjsPair_ {          // pair specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
   mjString* geomname1;             // name of geom 1
   mjString* geomname2;             // name of geom 2
 
@@ -577,7 +566,6 @@ typedef struct mjsExclude_ {       // exclude specification
 typedef struct mjsEquality_ {      // equality specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
   mjtEq type;                      // constraint type
   double data[mjNEQDATA];          // type-dependent data
   mjtByte active;                  // is equality initially active
@@ -592,7 +580,6 @@ typedef struct mjsEquality_ {      // equality specification
 typedef struct mjsTendon_ {        // tendon specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
 
   // stiffness, damping, friction
   double stiffness;                // stiffness coefficient
@@ -630,7 +617,6 @@ typedef struct mjsWrap_ {          // wrapping object specification
 typedef struct mjsActuator_ {      // actuator specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
 
   // gain, bias
   mjtGain gaintype;                // gain type
@@ -674,7 +660,6 @@ typedef struct mjsActuator_ {      // actuator specification
 typedef struct mjsSensor_ {        // sensor specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* classname;             // class name
 
   // sensor definition
   mjtSensor type;                  // type of sensor
diff --git a/introspect/structs.py b/introspect/structs.py
index f24f8dd1b3..3361ba2812 100644
--- a/introspect/structs.py
+++ b/introspect/structs.py
@@ -8719,13 +8719,6 @@
                  ),
                  doc='name',
              ),
-             StructFieldDecl(
-                 name='classname',
-                 type=PointerType(
-                     inner_type=ValueType(name='mjString'),
-                 ),
-                 doc='class name',
-             ),
              StructFieldDecl(
                  name='type',
                  type=ValueType(name='mjtJoint'),
@@ -8893,13 +8886,6 @@
                  ),
                  doc='name',
              ),
-             StructFieldDecl(
-                 name='classname',
-                 type=PointerType(
-                     inner_type=ValueType(name='mjString'),
-                 ),
-                 doc='classname',
-             ),
              StructFieldDecl(
                  name='type',
                  type=ValueType(name='mjtGeom'),
@@ -9108,13 +9094,6 @@
                  ),
                  doc='name',
              ),
-             StructFieldDecl(
-                 name='classname',
-                 type=PointerType(
-                     inner_type=ValueType(name='mjString'),
-                 ),
-                 doc='class name',
-             ),
              StructFieldDecl(
                  name='pos',
                  type=ArrayType(
@@ -9212,13 +9191,6 @@
                  ),
                  doc='name',
              ),
-             StructFieldDecl(
-                 name='classname',
-                 type=PointerType(
-                     inner_type=ValueType(name='mjString'),
-                 ),
-                 doc='class name',
-             ),
              StructFieldDecl(
                  name='pos',
                  type=ArrayType(
@@ -9358,13 +9330,6 @@
                  ),
                  doc='name',
              ),
-             StructFieldDecl(
-                 name='classname',
-                 type=PointerType(
-                     inner_type=ValueType(name='mjString'),
-                 ),
-                 doc='class name',
-             ),
              StructFieldDecl(
                  name='pos',
                  type=ArrayType(
@@ -9483,13 +9448,6 @@
                  ),
                  doc='name',
              ),
-             StructFieldDecl(
-                 name='classname',
-                 type=PointerType(
-                     inner_type=ValueType(name='mjString'),
-                 ),
-                 doc='class name',
-             ),
              StructFieldDecl(
                  name='contype',
                  type=ValueType(name='int'),
@@ -9665,13 +9623,6 @@
                  ),
                  doc='name',
              ),
-             StructFieldDecl(
-                 name='classname',
-                 type=PointerType(
-                     inner_type=ValueType(name='mjString'),
-                 ),
-                 doc='class name',
-             ),
              StructFieldDecl(
                  name='content_type',
                  type=PointerType(
@@ -9862,13 +9813,6 @@
                  ),
                  doc='name',
              ),
-             StructFieldDecl(
-                 name='classname',
-                 type=PointerType(
-                     inner_type=ValueType(name='mjString'),
-                 ),
-                 doc='class name',
-             ),
              StructFieldDecl(
                  name='file',
                  type=PointerType(
@@ -9985,13 +9929,6 @@
                  ),
                  doc='name',
              ),
-             StructFieldDecl(
-                 name='classname',
-                 type=PointerType(
-                     inner_type=ValueType(name='mjString'),
-                 ),
-                 doc='class name',
-             ),
              StructFieldDecl(
                  name='type',
                  type=ValueType(name='mjtTexture'),
@@ -10121,13 +10058,6 @@
                  ),
                  doc='name',
              ),
-             StructFieldDecl(
-                 name='classname',
-                 type=PointerType(
-                     inner_type=ValueType(name='mjString'),
-                 ),
-                 doc='class name',
-             ),
              StructFieldDecl(
                  name='texture',
                  type=PointerType(
@@ -10214,13 +10144,6 @@
                  ),
                  doc='name',
              ),
-             StructFieldDecl(
-                 name='classname',
-                 type=PointerType(
-                     inner_type=ValueType(name='mjString'),
-                 ),
-                 doc='class name',
-             ),
              StructFieldDecl(
                  name='geomname1',
                  type=PointerType(
@@ -10352,13 +10275,6 @@
                  ),
                  doc='name',
              ),
-             StructFieldDecl(
-                 name='classname',
-                 type=PointerType(
-                     inner_type=ValueType(name='mjString'),
-                 ),
-                 doc='class name',
-             ),
              StructFieldDecl(
                  name='type',
                  type=ValueType(name='mjtEq'),
@@ -10435,13 +10351,6 @@
                  ),
                  doc='name',
              ),
-             StructFieldDecl(
-                 name='classname',
-                 type=PointerType(
-                     inner_type=ValueType(name='mjString'),
-                 ),
-                 doc='class name',
-             ),
              StructFieldDecl(
                  name='stiffness',
                  type=ValueType(name='double'),
@@ -10596,13 +10505,6 @@
                  ),
                  doc='name',
              ),
-             StructFieldDecl(
-                 name='classname',
-                 type=PointerType(
-                     inner_type=ValueType(name='mjString'),
-                 ),
-                 doc='class name',
-             ),
              StructFieldDecl(
                  name='gaintype',
                  type=ValueType(name='mjtGain'),
@@ -10793,13 +10695,6 @@
                  ),
                  doc='name',
              ),
-             StructFieldDecl(
-                 name='classname',
-                 type=PointerType(
-                     inner_type=ValueType(name='mjString'),
-                 ),
-                 doc='class name',
-             ),
              StructFieldDecl(
                  name='type',
                  type=ValueType(name='mjtSensor'),
diff --git a/src/user/user_mesh.cc b/src/user/user_mesh.cc
index e83660e3ce..39f9e18c93 100644
--- a/src/user/user_mesh.cc
+++ b/src/user/user_mesh.cc
@@ -195,7 +195,6 @@ mjCMesh& mjCMesh::operator=(const mjCMesh& other) {
 void mjCMesh::PointToLocal() {
   spec.element = static_cast<mjsElement*>(this);
   spec.name = &name;
-  spec.classname = &classname;
   spec.file = &spec_file_;
   spec.content_type = &spec_content_type_;
   spec.uservert = &spec_vert_;
@@ -1982,7 +1981,6 @@ mjCSkin& mjCSkin::operator=(const mjCSkin& other) {
 void mjCSkin::PointToLocal() {
   spec.element = static_cast<mjsElement*>(this);
   spec.name = &name;
-  spec.classname = &classname;
   spec.file = &spec_file_;
   spec.material = &spec_material_;
   spec.vert = &spec_vert_;
@@ -2394,7 +2392,6 @@ mjCFlex& mjCFlex::operator=(const mjCFlex& other) {
 void mjCFlex::PointToLocal() {
   spec.element = static_cast<mjsElement*>(this);
   spec.name = &name;
-  spec.classname = &classname;
   spec.material = &spec_material_;
   spec.vertbody = &spec_vertbody_;
   spec.vert = &spec_vert_;
diff --git a/src/user/user_objects.cc b/src/user/user_objects.cc
index 4f10402a0d..76a6179f76 100644
--- a/src/user/user_objects.cc
+++ b/src/user/user_objects.cc
@@ -1686,6 +1686,7 @@ mjCJoint::mjCJoint(mjCModel* _model, mjCDef* _def) {
   // set model, def
   model = _model;
   def = (_def ? _def : (_model ? _model->Defaults(0) : 0));
+  classname = def ? def->name : "";
 
   // point to local
   PointToLocal();
@@ -1726,7 +1727,6 @@ bool mjCJoint::is_actfrclimited() const { return islimited(actfrclimited, actfrc
 void mjCJoint::PointToLocal() {
   spec.element = static_cast<mjsElement*>(this);
   spec.name = &name;
-  spec.classname = &classname;
   spec.userdata = &spec_userdata_;
   spec.info = &info;
 }
@@ -1894,6 +1894,7 @@ mjCGeom::mjCGeom(mjCModel* _model, mjCDef* _def) {
   // set model, def
   model = _model;
   def = (_def ? _def : (_model ? _model->Defaults(0) : 0));
+  classname = def ? def->name : "";
 
   // point to local
   PointToLocal();
@@ -1927,7 +1928,6 @@ void mjCGeom::PointToLocal(void) {
   spec.element = static_cast<mjsElement*>(this);
   spec.name = &name;
   spec.info = &info;
-  spec.classname = &classname;
   spec.userdata = &spec_userdata_;
   spec.material = &spec_material_;
   spec.meshname = &spec_meshname_;
@@ -2537,6 +2537,7 @@ mjCSite::mjCSite(mjCModel* _model, mjCDef* _def) {
   // set model, def
   model = _model;
   def = (_def ? _def : (_model ? _model->Defaults(0) : 0));
+  classname = def ? def->name : "";
 }
 
 
@@ -2563,7 +2564,6 @@ void mjCSite::PointToLocal() {
   spec.element = static_cast<mjsElement*>(this);
   spec.name = &name;
   spec.info = &info;
-  spec.classname = &classname;
   spec.material = &spec_material_;
   spec.userdata = &spec_userdata_;
 }
@@ -2683,6 +2683,7 @@ mjCCamera::mjCCamera(mjCModel* _model, mjCDef* _def) {
   // set model, def
   model = _model;
   def = (_def ? _def : (_model ? _model->Defaults(0) : 0));
+  classname = def ? def->name : "";
 
   // point to local
   PointToLocal();
@@ -2714,7 +2715,6 @@ mjCCamera& mjCCamera::operator=(const mjCCamera& other) {
 void mjCCamera::PointToLocal() {
   spec.element = static_cast<mjsElement*>(this);
   spec.name = &name;
-  spec.classname = &classname;
   spec.userdata = &spec_userdata_;
   spec.targetbody = &spec_targetbody_;
   spec.info = &info;
@@ -2837,6 +2837,7 @@ mjCLight::mjCLight(mjCModel* _model, mjCDef* _def) {
   // set model, def
   model = _model;
   def = (_def ? _def : (_model ? _model->Defaults(0) : 0));
+  classname = def ? def->name : "";
 
   PointToLocal();
   CopyFromSpec();
@@ -2865,7 +2866,6 @@ mjCLight& mjCLight::operator=(const mjCLight& other) {
 void mjCLight::PointToLocal() {
   spec.element = static_cast<mjsElement*>(this);
   spec.name = &name;
-  spec.classname = &classname;
   spec.targetbody = &spec_targetbody_;
   spec.info = &info;
 }
@@ -3194,7 +3194,6 @@ mjCTexture& mjCTexture::operator=(const mjCTexture& other) {
 void mjCTexture::PointToLocal() {
   spec.element = static_cast<mjsElement*>(this);
   spec.name = &name;
-  spec.classname = &classname;
   spec.file = &spec_file_;
   spec.content_type = &spec_content_type_;
   spec.cubefiles = &spec_cubefiles_;
@@ -3873,6 +3872,7 @@ mjCMaterial::mjCMaterial(mjCModel* _model, mjCDef* _def) {
 
   model = _model;
   def = (_def ? _def : (_model ? _model->Defaults(0) : 0));
+  classname = def ? def->name : "";
 
   PointToLocal();
 
@@ -3903,7 +3903,6 @@ mjCMaterial& mjCMaterial::operator=(const mjCMaterial& other) {
 void mjCMaterial::PointToLocal() {
   spec.element = static_cast<mjsElement*>(this);
   spec.name = &name;
-  spec.classname = &classname;
   spec.texture = &spec_texture_;
   spec.info = &info;
 }
@@ -3960,6 +3959,7 @@ mjCPair::mjCPair(mjCModel* _model, mjCDef* _def) {
   // set model, def
   model = _model;
   def = (_def ? _def : (_model ? _model->Defaults(0) : 0));
+  classname = def ? def->name : "";
 
   // point to local
   PointToLocal();
@@ -3993,7 +3993,6 @@ mjCPair& mjCPair::operator=(const mjCPair& other) {
 void mjCPair::PointToLocal() {
   spec.element = static_cast<mjsElement*>(this);
   spec.name = &name;
-  spec.classname = &classname;
   spec.geomname1 = &spec_geomname1_;
   spec.geomname2 = &spec_geomname2_;
   spec.info = &info;
@@ -4321,6 +4320,7 @@ mjCEquality::mjCEquality(mjCModel* _model, mjCDef* _def) {
   // set model, def
   model = _model;
   def = (_def ? _def : (_model ? _model->Defaults(0) : 0));
+  classname = def ? def->name : "";
 
   // point to local
   PointToLocal();
@@ -4352,7 +4352,6 @@ mjCEquality& mjCEquality::operator=(const mjCEquality& other) {
 void mjCEquality::PointToLocal() {
   spec.element = static_cast<mjsElement*>(this);
   spec.name = &name;
-  spec.classname = &classname;
   spec.name1 = &spec_name1_;
   spec.name2 = &spec_name2_;
   spec.info = &info;
@@ -4483,6 +4482,7 @@ mjCTendon::mjCTendon(mjCModel* _model, mjCDef* _def) {
   // set model, def
   model = _model;
   def = (_def ? _def : (_model ? _model->Defaults(0) : 0));
+  classname = def ? def->name : "";
 
   // point to local
   PointToLocal();
@@ -4521,7 +4521,6 @@ bool mjCTendon::is_limited() const { return islimited(limited, range); }
 void mjCTendon::PointToLocal() {
   spec.element = static_cast<mjsElement*>(this);
   spec.name = &name;
-  spec.classname = &classname;
   spec.material = &spec_material_;
   spec.userdata = &spec_userdata_;
   spec.info = &info;
@@ -4961,6 +4960,7 @@ mjCActuator::mjCActuator(mjCModel* _model, mjCDef* _def) {
   // set model, def
   model = _model;
   def = (_def ? _def : (_model ? _model->Defaults(0) : 0));
+  classname = def ? def->name : "";
 
   // in case this actuator is not compiled
   CopyFromSpec();
@@ -5002,7 +5002,6 @@ bool mjCActuator::is_actlimited() const { return islimited(actlimited, actrange)
 void mjCActuator::PointToLocal() {
   spec.element = static_cast<mjsElement*>(this);
   spec.name = &name;
-  spec.classname = &classname;
   spec.userdata = &spec_userdata_;
   spec.target = &spec_target_;
   spec.refsite = &spec_refsite_;
@@ -5325,7 +5324,6 @@ mjCSensor& mjCSensor::operator=(const mjCSensor& other) {
 void mjCSensor::PointToLocal() {
   spec.element = static_cast<mjsElement*>(this);
   spec.name = &name;
-  spec.classname = &classname;
   spec.userdata = &spec_userdata_;
   spec.objname = &spec_objname_;
   spec.refname = &spec_refname_;
diff --git a/src/user/user_objects.h b/src/user/user_objects.h
index 9c50b48367..e6545e5776 100644
--- a/src/user/user_objects.h
+++ b/src/user/user_objects.h
@@ -300,7 +300,6 @@ class mjCBody : public mjCBody_, private mjsBody {
 
   // inherited
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   // used by mjXWriter and mjCModel
@@ -362,7 +361,6 @@ class mjCFrame : public mjCFrame_, private mjsFrame {
 
   mjsFrame spec;
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   void CopyFromSpec(void);
@@ -409,7 +407,6 @@ class mjCJoint : public mjCJoint_, private mjsJoint {
 
   mjsJoint spec;
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   void CopyFromSpec(void);
@@ -510,7 +507,6 @@ class mjCGeom : public mjCGeom_, private mjsGeom {
   void NameSpace(const mjCModel* m);
 
   // inherited
-  using mjCBase::classname;
   using mjCBase::info;
 };
 
@@ -548,7 +544,6 @@ class mjCSite : public mjCSite_, private mjsSite {
 
   // use strings from mjCBase rather than mjStrings from mjsSite
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   // used by mjXWriter and mjCModel
@@ -591,7 +586,6 @@ class mjCCamera : public mjCCamera_, private mjsCamera {
 
   mjsCamera spec;
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   // used by mjXWriter and mjCModel
@@ -631,7 +625,6 @@ class mjCLight : public mjCLight_, private mjsLight {
 
   mjsLight spec;
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   // used by mjXWriter and mjCModel
@@ -694,7 +687,6 @@ class mjCFlex: public mjCFlex_, private mjsFlex {
 
   mjsFlex spec;
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   void CopyFromSpec(void);
@@ -794,7 +786,6 @@ class mjCMesh: public mjCMesh_, private mjsMesh {
 
   mjsMesh spec;
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   void CopyFromSpec(void);
@@ -925,7 +916,6 @@ class mjCSkin: public mjCSkin_, private mjsSkin {
 
   mjsSkin spec;
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   const std::string& get_file() const { return file_; }
@@ -1027,7 +1017,6 @@ class mjCTexture : public mjCTexture_, private mjsTexture {
 
   mjsTexture spec;
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   void CopyFromSpec(void);
@@ -1082,7 +1071,6 @@ class mjCMaterial : public mjCMaterial_, private mjsMaterial {
 
   mjsMaterial spec;
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   void CopyFromSpec();
@@ -1123,7 +1111,6 @@ class mjCPair : public mjCPair_, private mjsPair {
 
   mjsPair spec;
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   void CopyFromSpec();
@@ -1220,7 +1207,6 @@ class mjCEquality : public mjCEquality_, private mjsEquality {
 
   mjsEquality spec;
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   void CopyFromSpec();
@@ -1261,7 +1247,6 @@ class mjCTendon : public mjCTendon_, private mjsTendon {
 
   mjsTendon spec;
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   void set_material(std::string _material) { material_ = _material; }
@@ -1394,7 +1379,6 @@ class mjCActuator : public mjCActuator_, private mjsActuator {
 
   mjsActuator spec;
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   // used by mjXWriter and mjCModel
@@ -1453,7 +1437,6 @@ class mjCSensor : public mjCSensor_, private mjsSensor {
 
   mjsSensor spec;
   using mjCBase::name;
-  using mjCBase::classname;
   using mjCBase::info;
 
   // used by mjXWriter and mjCModel
diff --git a/src/xml/xml_native_reader.cc b/src/xml/xml_native_reader.cc
index 6e685c76cd..fd005c5e78 100644
--- a/src/xml/xml_native_reader.cc
+++ b/src/xml/xml_native_reader.cc
@@ -1289,16 +1289,13 @@ void mjXReader::Statistic(XMLElement* section) {
 
 // flex element parser
 void mjXReader::OneFlex(XMLElement* elem, mjsFlex* pflex) {
-  string text, name, classname, material;
+  string text, name, material;
   int n;
 
   // read attributes
   if (ReadAttrTxt(elem, "name", name)) {
     mjs_setString(pflex->name, name.c_str());
   }
-  if (ReadAttrTxt(elem, "classname", classname)) {
-    mjs_setString(pflex->classname, classname.c_str());
-  }
   if (ReadAttrTxt(elem, "material", material)) {
     mjs_setString(pflex->material, material.c_str());
   }
@@ -1364,15 +1361,12 @@ void mjXReader::OneFlex(XMLElement* elem, mjsFlex* pflex) {
 // mesh element parser
 void mjXReader::OneMesh(XMLElement* elem, mjsMesh* pmesh) {
   int n;
-  string text, name, classname, content_type;
+  string text, name, content_type;
 
   // read attributes
   if (ReadAttrTxt(elem, "name", name)) {
     mjs_setString(pmesh->name, name.c_str());
   }
-  if (ReadAttrTxt(elem, "class", classname)) {
-    mjs_setString(pmesh->classname, classname.c_str());
-  }
   if (ReadAttrTxt(elem, "content_type", content_type)) {
     mjs_setString(pmesh->content_type, content_type.c_str());
   }
@@ -1526,16 +1520,13 @@ void mjXReader::OneSkin(XMLElement* elem, mjsSkin* pskin) {
 
 // material element parser
 void mjXReader::OneMaterial(XMLElement* elem, mjsMaterial* pmat) {
-  string text, name, classname, texture;
+  string text, name, texture;
   int n;
 
   // read attributes
   if (ReadAttrTxt(elem, "name", name)) {
     mjs_setString(pmat->name, name.c_str());
   }
-  if (ReadAttrTxt(elem, "class", classname)) {
-    mjs_setString(pmat->classname, classname.c_str());
-  }
   if (ReadAttrTxt(elem, "texture", texture)) {
     mjs_setString(pmat->texture, texture.c_str());
   }
@@ -1559,7 +1550,7 @@ void mjXReader::OneMaterial(XMLElement* elem, mjsMaterial* pmat) {
 
 // joint element parser
 void mjXReader::OneJoint(XMLElement* elem, mjsJoint* pjoint) {
-  string text, name, classname;
+  string text, name;
   std::vector<double> userdata;
   int n;
 
@@ -1567,9 +1558,6 @@ void mjXReader::OneJoint(XMLElement* elem, mjsJoint* pjoint) {
   if (ReadAttrTxt(elem, "name", name)) {
     mjs_setString(pjoint->name, name.c_str());
   }
-  if (ReadAttrTxt(elem, "class", classname)) {
-    mjs_setString(pjoint->classname, classname.c_str());
-  }
   if (MapValue(elem, "type", &n, joint_map, joint_sz)) {
     pjoint->type = (mjtJoint)n;
   }
@@ -1609,7 +1597,7 @@ void mjXReader::OneJoint(XMLElement* elem, mjsJoint* pjoint) {
 
 // geom element parser
 void mjXReader::OneGeom(XMLElement* elem, mjsGeom* pgeom) {
-  string text, name, classname;
+  string text, name;
   std::vector<double> userdata;
   std::string hfieldname, meshname, material;
   int n;
@@ -1618,9 +1606,6 @@ void mjXReader::OneGeom(XMLElement* elem, mjsGeom* pgeom) {
   if (ReadAttrTxt(elem, "name", name)) {
     mjs_setString(pgeom->name, name.c_str());
   }
-  if (ReadAttrTxt(elem, "class", classname)) {
-    mjs_setString(pgeom->classname, classname.c_str());
-  }
   if (MapValue(elem, "type", &n, geom_map, mjNGEOMTYPES)) {
     pgeom->type = (mjtGeom)n;
   }
@@ -1685,7 +1670,7 @@ void mjXReader::OneGeom(XMLElement* elem, mjsGeom* pgeom) {
 // site element parser
 void mjXReader::OneSite(XMLElement* elem, mjsSite* site) {
   int n;
-  string text, name, classname;
+  string text, name;
   std::vector<double> userdata;
   std::string material;
 
@@ -1693,9 +1678,6 @@ void mjXReader::OneSite(XMLElement* elem, mjsSite* site) {
   if (ReadAttrTxt(elem, "name", name)) {
     mjs_setString(site->name, name.c_str());
   }
-  if (ReadAttrTxt(elem, "class", classname)) {
-    mjs_setString(site->classname, classname.c_str());
-  }
   if (MapValue(elem, "type", &n, geom_map, mjNGEOMTYPES)) {
     site->type = (mjtGeom)n;
   }
@@ -1722,16 +1704,13 @@ void mjXReader::OneSite(XMLElement* elem, mjsSite* site) {
 // camera element parser
 void mjXReader::OneCamera(XMLElement* elem, mjsCamera* pcam) {
   int n;
-  string text, name, classname, targetbody;
+  string text, name, targetbody;
   std::vector<double> userdata;
 
   // read attributes
   if (ReadAttrTxt(elem, "name", name)) {
     mjs_setString(pcam->name, name.c_str());
   }
-  if (ReadAttrTxt(elem, "class", classname)) {
-    mjs_setString(pcam->classname, classname.c_str());
-  }
   if (ReadAttrTxt(elem, "target", targetbody)) {
     mjs_setString(pcam->targetbody, targetbody.c_str());
   }
@@ -1780,15 +1759,12 @@ void mjXReader::OneCamera(XMLElement* elem, mjsCamera* pcam) {
 // light element parser
 void mjXReader::OneLight(XMLElement* elem, mjsLight* plight) {
   int n;
-  string text, name, classname, targetbody;
+  string text, name, targetbody;
 
   // read attributes
   if (ReadAttrTxt(elem, "name", name)) {
     mjs_setString(plight->name, name.c_str());
   }
-  if (ReadAttrTxt(elem, "class", classname)) {
-    mjs_setString(plight->classname, classname.c_str());
-  }
   if (ReadAttrTxt(elem, "target", targetbody)) {
     mjs_setString(plight->targetbody, targetbody.c_str());
   }
@@ -1822,13 +1798,10 @@ void mjXReader::OneLight(XMLElement* elem, mjsLight* plight) {
 
 // pair element parser
 void mjXReader::OnePair(XMLElement* elem, mjsPair* ppair) {
-  string text, name, classname, geomname1, geomname2;
+  string text, name, geomname1, geomname2;
 
   // regular only
   if (!readingdefaults) {
-    if (ReadAttrTxt(elem, "class", classname)) {
-      mjs_setString(ppair->classname, classname.c_str());
-    }
     if (ReadAttrTxt(elem, "geom1", geomname1)) {
       mjs_setString(ppair->geomname1, geomname1.c_str());
     }
@@ -1858,7 +1831,7 @@ void mjXReader::OnePair(XMLElement* elem, mjsPair* ppair) {
 // equality element parser
 void mjXReader::OneEquality(XMLElement* elem, mjsEquality* pequality) {
   int n;
-  string text, name1, name2, name, classname;
+  string text, name1, name2, name;
 
   // read type (bad keywords already detected by schema)
   text = elem->Value();
@@ -1869,9 +1842,6 @@ void mjXReader::OneEquality(XMLElement* elem, mjsEquality* pequality) {
     if (ReadAttrTxt(elem, "name", name)) {
       mjs_setString(pequality->name, name.c_str());
     }
-    if (ReadAttrTxt(elem, "class", classname)) {
-      mjs_setString(pequality->classname, classname.c_str());
-    };
 
     switch (pequality->type) {
     case mjEQ_CONNECT:
@@ -1935,16 +1905,13 @@ void mjXReader::OneEquality(XMLElement* elem, mjsEquality* pequality) {
 
 // tendon element parser
 void mjXReader::OneTendon(XMLElement* elem, mjsTendon* pten) {
-  string text, name, classname, material;
+  string text, name, material;
   std::vector<double> userdata;
 
   // read attributes
   if (ReadAttrTxt(elem, "name", name)) {
     mjs_setString(pten->name, name.c_str());
   }
-  if (ReadAttrTxt(elem, "class", classname)) {
-    mjs_setString(pten->classname, classname.c_str());
-  }
   ReadAttrInt(elem, "group", &pten->group);
   if (ReadAttrTxt(elem, "material", material)) {
     mjs_setString(pten->material, material.c_str());
@@ -1979,15 +1946,12 @@ void mjXReader::OneTendon(XMLElement* elem, mjsTendon* pten) {
 
 // actuator element parser
 void mjXReader::OneActuator(XMLElement* elem, mjsActuator* pact) {
-  string text, type, name, classname, target, slidersite, refsite;
+  string text, type, name, target, slidersite, refsite;
 
   // common attributes
   if (ReadAttrTxt(elem, "name", name)) {
     mjs_setString(pact->name, name.c_str());
   }
-  if (ReadAttrTxt(elem, "class", classname)) {
-    mjs_setString(pact->classname, classname.c_str());
-  }
   ReadAttrInt(elem, "group", &pact->group);
   MapValue(elem, "ctrllimited", &pact->ctrllimited, TFAuto_map, 3);
   MapValue(elem, "forcelimited", &pact->forcelimited, TFAuto_map, 3);
diff --git a/src/xml/xml_native_writer.cc b/src/xml/xml_native_writer.cc
index b3db60afa7..7b3bf798b6 100644
--- a/src/xml/xml_native_writer.cc
+++ b/src/xml/xml_native_writer.cc
@@ -18,6 +18,7 @@
 #include <cstddef>
 #include <cstdio>
 #include <string>
+#include <string_view>
 #include <unordered_set>
 #include <vector>
 
@@ -198,7 +199,9 @@ void mjXWriter::OneMesh(XMLElement* elem, const mjCMesh* pmesh, mjCDef* def) {
   // regular
   if (!writingdefaults) {
     WriteAttrTxt(elem, "name", pmesh->name);
-    WriteAttrTxt(elem, "class", pmesh->classname);
+    if (pmesh->classname != "main") {
+      WriteAttrTxt(elem, "class", pmesh->classname);
+    }
     WriteAttrTxt(elem, "content_type", pmesh->get_content_type());
     WriteAttrTxt(elem, "file", pmesh->get_file());
 
@@ -295,7 +298,9 @@ void mjXWriter::OneMaterial(XMLElement* elem, const mjCMaterial* pmat, mjCDef* d
   // regular
   if (!writingdefaults) {
     WriteAttrTxt(elem, "name", pmat->name);
-    WriteAttrTxt(elem, "class", pmat->classname);
+    if (pmat->classname != "main") {
+      WriteAttrTxt(elem, "class", pmat->classname);
+    }
   }
 
   // defaults and regular
@@ -316,13 +321,16 @@ void mjXWriter::OneMaterial(XMLElement* elem, const mjCMaterial* pmat, mjCDef* d
 
 
 // write joint
-void mjXWriter::OneJoint(XMLElement* elem, const mjCJoint* pjoint, mjCDef* def) {
+void mjXWriter::OneJoint(XMLElement* elem, const mjCJoint* pjoint, mjCDef* def,
+                         std::string_view classname) {
   double zero = 0;
 
   // regular
   if (!writingdefaults) {
     WriteAttrTxt(elem, "name", pjoint->name);
-    WriteAttrTxt(elem, "class", pjoint->classname);
+    if (classname != pjoint->classname && pjoint->classname != "main") {
+      WriteAttrTxt(elem, "class", pjoint->classname);
+    }
     if (pjoint->type != mjJNT_FREE) {
       WriteAttr(elem, "pos", 3, pjoint->pos);
     }
@@ -364,17 +372,18 @@ void mjXWriter::OneJoint(XMLElement* elem, const mjCJoint* pjoint, mjCDef* def)
   }
 }
 
-
-
 // write geom
-void mjXWriter::OneGeom(XMLElement* elem, const mjCGeom* pgeom, mjCDef* def) {
+void mjXWriter::OneGeom(XMLElement* elem, const mjCGeom* pgeom, mjCDef* def,
+                        std::string_view classname) {
   double unitq[4] = {1, 0, 0, 0};
   double mass = 0;
 
   // regular
   if (!writingdefaults) {
     WriteAttrTxt(elem, "name", pgeom->name);
-    WriteAttrTxt(elem, "class", pgeom->classname);
+    if (classname != pgeom->classname && pgeom->classname != "main") {
+      WriteAttrTxt(elem, "class", pgeom->classname);
+    }
     if (mjGEOMINFO[pgeom->type]) {
       WriteAttr(elem, "size", mjGEOMINFO[pgeom->type], pgeom->size, def->Geom().size);
     }
@@ -459,16 +468,17 @@ void mjXWriter::OneGeom(XMLElement* elem, const mjCGeom* pgeom, mjCDef* def) {
   }
 }
 
-
-
 // write site
-void mjXWriter::OneSite(XMLElement* elem, const mjCSite* psite, mjCDef* def) {
+void mjXWriter::OneSite(XMLElement* elem, const mjCSite* psite, mjCDef* def,
+                        std::string_view classname) {
   double unitq[4] = {1, 0, 0, 0};
 
   // regular
   if (!writingdefaults) {
     WriteAttrTxt(elem, "name", psite->name);
-    WriteAttrTxt(elem, "class", psite->classname);
+    if (classname != psite->classname && psite->classname != "main") {
+      WriteAttrTxt(elem, "class", psite->classname);
+    }
     WriteAttr(elem, "pos", 3, psite->pos);
     WriteAttr(elem, "quat", 4, psite->quat, unitq);
     if (mjGEOMINFO[psite->type]) {
@@ -494,16 +504,17 @@ void mjXWriter::OneSite(XMLElement* elem, const mjCSite* psite, mjCDef* def) {
   }
 }
 
-
-
 // write camera
-void mjXWriter::OneCamera(XMLElement* elem, const mjCCamera* pcam, mjCDef* def) {
+void mjXWriter::OneCamera(XMLElement* elem, const mjCCamera* pcam, mjCDef* def,
+                          std::string_view classname) {
   double unitq[4] = {1, 0, 0, 0};
 
   // regular
   if (!writingdefaults) {
     WriteAttrTxt(elem, "name", pcam->name);
-    WriteAttrTxt(elem, "class", pcam->classname);
+    if (classname != pcam->classname && pcam->classname != "main") {
+      WriteAttrTxt(elem, "class", pcam->classname);
+    }
     WriteAttrTxt(elem, "target", pcam->get_targetbody());
     WriteAttr(elem, "pos", 3, pcam->pos);
     WriteAttr(elem, "quat", 4, pcam->quat, unitq);
@@ -534,14 +545,15 @@ void mjXWriter::OneCamera(XMLElement* elem, const mjCCamera* pcam, mjCDef* def)
   }
 }
 
-
-
 // write light
-void mjXWriter::OneLight(XMLElement* elem, const mjCLight* plight, mjCDef* def) {
+void mjXWriter::OneLight(XMLElement* elem, const mjCLight* plight, mjCDef* def,
+                         std::string_view classname) {
   // regular
   if (!writingdefaults) {
     WriteAttrTxt(elem, "name", plight->name);
-    WriteAttrTxt(elem, "class", plight->classname);
+    if (classname != plight->classname && plight->classname != "main") {
+      WriteAttrTxt(elem, "class", plight->classname);
+    }
     WriteAttrTxt(elem, "target", plight->get_targetbody());
     WriteAttr(elem, "pos", 3, plight->pos);
     WriteAttr(elem, "dir", 3, plight->dir);
@@ -561,13 +573,13 @@ void mjXWriter::OneLight(XMLElement* elem, const mjCLight* plight, mjCDef* def)
   WriteAttrKey(elem, "mode", camlight_map, camlight_sz, plight->mode, def->Light().mode);
 }
 
-
-
 // write pair
 void mjXWriter::OnePair(XMLElement* elem, const mjCPair* ppair, mjCDef* def) {
   // regular
   if (!writingdefaults) {
-    WriteAttrTxt(elem, "class", ppair->classname);
+    if (ppair->classname != "main") {
+      WriteAttrTxt(elem, "class", ppair->classname);
+    }
     WriteAttrTxt(elem, "geom1", ppair->get_geomname1());
     WriteAttrTxt(elem, "geom2", ppair->get_geomname2());
   }
@@ -591,7 +603,9 @@ void mjXWriter::OneEquality(XMLElement* elem, const mjCEquality* peq, mjCDef* de
   // regular
   if (!writingdefaults) {
     WriteAttrTxt(elem, "name", peq->name);
-    WriteAttrTxt(elem, "class", peq->classname);
+    if (peq->classname != "main") {
+      WriteAttrTxt(elem, "class", peq->classname);
+    }
 
     switch (peq->type) {
     case mjEQ_CONNECT:
@@ -644,7 +658,9 @@ void mjXWriter::OneTendon(XMLElement* elem, const mjCTendon* pten, mjCDef* def)
   // regular
   if (!writingdefaults) {
     WriteAttrTxt(elem, "name", pten->name);
-    WriteAttrTxt(elem, "class", pten->classname);
+    if (pten->classname != "main") {
+      WriteAttrTxt(elem, "class", pten->classname);
+    }
   }
 
   // defaults and regular
@@ -691,7 +707,9 @@ void mjXWriter::OneActuator(XMLElement* elem, const mjCActuator* pact, mjCDef* d
   // regular
   if (!writingdefaults) {
     WriteAttrTxt(elem, "name", pact->name);
-    WriteAttrTxt(elem, "class", pact->classname);
+    if (pact->classname != "main") {
+      WriteAttrTxt(elem, "class", pact->classname);
+    }
 
     // transmission target
     switch (pact->trntype) {
@@ -1515,7 +1533,7 @@ XMLElement* mjXWriter::OneFrame(XMLElement* elem, mjCFrame* frame) {
 
 
 // recursive body and frame writer
-void mjXWriter::Body(XMLElement* elem, mjCBody* body) {
+void mjXWriter::Body(XMLElement* elem, mjCBody* body, std::string_view childclass) {
   double unitq[4] = {1, 0, 0, 0};
 
   if (!body) {
@@ -1557,31 +1575,51 @@ void mjXWriter::Body(XMLElement* elem, mjCBody* body) {
   // write joints
   for (int i=0; i<body->joints.size(); i++) {
     XMLElement* celem = OneFrame(elem, body->joints[i]->frame);
-    OneJoint(InsertEnd(celem, "joint"), body->joints[i], body->joints[i]->def);
+    std::string classname = body->joints[i]->frame && !body->joints[i]->frame->classname.empty()
+                                ? body->joints[i]->frame->classname
+                                : body->classname;
+    OneJoint(InsertEnd(celem, "joint"), body->joints[i], body->joints[i]->def,
+             classname.empty() ? childclass : classname);
   }
 
   // write geoms
   for (int i=0; i<body->geoms.size(); i++) {
     XMLElement* celem = OneFrame(elem, body->geoms[i]->frame);
-    OneGeom(InsertEnd(celem, "geom"), body->geoms[i], body->geoms[i]->def);
+    std::string classname = body->geoms[i]->frame && !body->geoms[i]->frame->classname.empty()
+                                ? body->geoms[i]->frame->classname
+                                : body->classname;
+    OneGeom(InsertEnd(celem, "geom"), body->geoms[i], body->geoms[i]->def,
+            classname.empty() ? childclass : classname);
   }
 
   // write sites
   for (int i=0; i<body->sites.size(); i++) {
     XMLElement* celem = OneFrame(elem, body->sites[i]->frame);
-    OneSite(InsertEnd(celem, "site"), body->sites[i], body->sites[i]->def);
+    std::string classname = body->sites[i]->frame && !body->sites[i]->frame->classname.empty()
+                                ? body->sites[i]->frame->classname
+                                : body->classname;
+    OneSite(InsertEnd(celem, "site"), body->sites[i], body->sites[i]->def,
+            classname.empty() ? childclass : classname);
   }
 
   // write cameras
   for (int i=0; i<body->cameras.size(); i++) {
     XMLElement* celem = OneFrame(elem, body->cameras[i]->frame);
-    OneCamera(InsertEnd(celem, "camera"), body->cameras[i], body->cameras[i]->def);
+    std::string classname = body->cameras[i]->frame && !body->cameras[i]->frame->classname.empty()
+                                ? body->cameras[i]->frame->classname
+                                : body->classname;
+    OneCamera(InsertEnd(celem, "camera"), body->cameras[i], body->cameras[i]->def,
+              classname.empty() ? childclass : classname);
   }
 
   // write lights
   for (int i=0; i<body->lights.size(); i++) {
     XMLElement* celem = OneFrame(elem, body->lights[i]->frame);
-    OneLight(InsertEnd(celem, "light"), body->lights[i], body->lights[i]->def);
+    std::string classname = body->lights[i]->frame && !body->lights[i]->frame->classname.empty()
+                                ? body->lights[i]->frame->classname
+                                : body->classname;
+    OneLight(InsertEnd(celem, "light"), body->lights[i], body->lights[i]->def,
+             classname.empty() ? childclass : classname);
   }
 
   // write plugin
@@ -1592,7 +1630,10 @@ void mjXWriter::Body(XMLElement* elem, mjCBody* body) {
   // write child bodies recursively
   for (int i=0; i<body->bodies.size(); i++) {
     XMLElement* celem = OneFrame(elem, body->bodies[i]->frame);
-    Body(InsertEnd(celem, "body"), body->bodies[i]);
+    std::string classname = body->bodies[i]->frame && !body->bodies[i]->frame->classname.empty()
+                                ? body->bodies[i]->frame->classname
+                                : body->classname;
+    Body(InsertEnd(celem, "body"), body->bodies[i], classname.empty() ? childclass : classname);
   }
 }
 
diff --git a/src/xml/xml_native_writer.h b/src/xml/xml_native_writer.h
index c6791acd57..f0d6cceccd 100644
--- a/src/xml/xml_native_writer.h
+++ b/src/xml/xml_native_writer.h
@@ -17,6 +17,7 @@
 
 #include <cstdlib>
 #include <string>
+#include <string_view>
 
 #include <mujoco/mjspec.h>
 #include "user/user_objects.h"
@@ -49,7 +50,6 @@ class mjXWriter : public mjXBase {
   void Extension(tinyxml2::XMLElement* root);                             // extension section
   void Custom(tinyxml2::XMLElement* root);                                // custom section
   void Asset(tinyxml2::XMLElement* root);                                 // asset section
-  void Body(tinyxml2::XMLElement* elem, mjCBody* body);                   // body/world section
   void Contact(tinyxml2::XMLElement* root);                               // contact section
   void Deformable(tinyxml2::XMLElement* root);                            // deformable section
   void Equality(tinyxml2::XMLElement* root);                              // equality section
@@ -58,16 +58,24 @@ class mjXWriter : public mjXBase {
   void Sensor(tinyxml2::XMLElement* root);                                // sensor section
   void Keyframe(tinyxml2::XMLElement* root);                              // keyframe section
 
+  // body/world section
+  void Body(tinyxml2::XMLElement* elem, mjCBody* body, std::string_view childclass = "");
+
   // single element writers, used in defaults and main body
   void OneFlex(tinyxml2::XMLElement* elem, const mjCFlex* pflex);
   void OneMesh(tinyxml2::XMLElement* elem, const mjCMesh* pmesh,         mjCDef* def);
   void OneSkin(tinyxml2::XMLElement* elem, const mjCSkin* pskin);
   void OneMaterial(tinyxml2::XMLElement* elem, const mjCMaterial* pmaterial, mjCDef* def);
-  void OneJoint(tinyxml2::XMLElement* elem, const mjCJoint* pjoint,       mjCDef* def);
-  void OneGeom(tinyxml2::XMLElement* elem, const mjCGeom* pgeom,         mjCDef* def);
-  void OneSite(tinyxml2::XMLElement* elem, const mjCSite* psite,         mjCDef* def);
-  void OneCamera(tinyxml2::XMLElement* elem, const mjCCamera* pcamera,     mjCDef* def);
-  void OneLight(tinyxml2::XMLElement* elem, const mjCLight* plight,       mjCDef* def);
+  void OneJoint(tinyxml2::XMLElement* elem, const mjCJoint* pjoint, mjCDef* def,
+                std::string_view classname = "");
+  void OneGeom(tinyxml2::XMLElement* elem, const mjCGeom* pgeom, mjCDef* def,
+               std::string_view classname = "");
+  void OneSite(tinyxml2::XMLElement* elem, const mjCSite* psite, mjCDef* def,
+               std::string_view classname = "");
+  void OneCamera(tinyxml2::XMLElement* elem, const mjCCamera* pcamera,
+                 mjCDef* def, std::string_view classname = "");
+  void OneLight(tinyxml2::XMLElement* elem, const mjCLight* plight, mjCDef* def,
+                std::string_view classname = "");
   void OnePair(tinyxml2::XMLElement* elem, const mjCPair* ppair,         mjCDef* def);
   void OneEquality(tinyxml2::XMLElement* elem, const mjCEquality* pequality, mjCDef* def);
   void OneTendon(tinyxml2::XMLElement* elem, const mjCTendon* ptendon,     mjCDef* def);