diff --git a/src/user/user_api.cc b/src/user/user_api.cc
index a73bd491..3dfc5db5 100644
--- a/src/user/user_api.cc
+++ b/src/user/user_api.cc
@@ -123,6 +123,25 @@ void* mjm_addFrame(mjmBody* bodyspec, void* parentframe) {
 
 
 
+// add actuator to model
+mjmActuator* mjm_addActuator(void* model, void* defspec) {
+  mjCModel* modelC = static_cast<mjCModel*>(model);
+  mjCDef* def = static_cast<mjCDef*>(defspec);
+  mjCActuator* actuator = modelC->AddActuator(def);
+  return &actuator->spec;
+}
+
+
+
+// add sensor to model
+mjmSensor* mjm_addSensor(void* model) {
+  mjCModel* modelC = static_cast<mjCModel*>(model);
+  mjCSensor* sensor = modelC->AddSensor();
+  return &sensor->spec;
+}
+
+
+
 // Add plugin to model.
 mjElement mjm_addPlugin(void* model) {
   mjCModel* modelC = static_cast<mjCModel*>(model);
diff --git a/src/user/user_api.h b/src/user/user_api.h
index bd429ef3..d56d9191 100644
--- a/src/user/user_api.h
+++ b/src/user/user_api.h
@@ -211,6 +211,59 @@ typedef struct _mjmLight {
 } mjmLight;
 
 
+typedef struct _mjmActuator {
+  mjElement element;              // compiler only, do not modify
+  mjString name;                  // name
+  mjString classname;             // class name
+  mjString info;                  // message appended to errors
+  int group;                      // group for visualization
+  int ctrllimited;                // are control limits defined: 0 false, 1 true, 2 auto
+  int forcelimited;               // are force limits defined: 0 false, 1 true, 2 auto
+  int actlimited;                 // are activation limits defined: 0 false, 1 true, 2 auto
+  int actdim;                     // dimension of associated activations
+  int plugin_actdim;              // actuator state size for plugins
+  mjtDyn dyntype;                 // dynamics type
+  mjtTrn trntype;                 // transmission type
+  mjtGain gaintype;               // gain type
+  mjtBias biastype;               // bias type
+  double dynprm[mjNDYN];          // dynamics parameters
+  double gainprm[mjNGAIN];        // gain parameters
+  double biasprm[mjNGAIN];        // bias parameters
+  mjtByte actearly;               // apply activations to qfrc instantly
+  double ctrlrange[2];            // control range
+  double forcerange[2];           // force range
+  double actrange[2];             // activation range
+  double lengthrange[2];          // length range
+  double gear[6];                 // length and transmitted force scaling
+  double cranklength;             // crank length, for slider-crank only
+  mjDouble userdata;              // user data
+  mjString target;                // transmission target name
+  mjString slidersite;            // site defining cylinder, for slider-crank only
+  mjString refsite;               // reference site, for site transmission only
+  mjmPlugin plugin;               // actuator plugin
+} mjmActuator;
+
+
+typedef struct _mjmSensor {
+  mjElement element;              // compiler only, do not modify
+  mjString name;                  // name
+  mjString classname;             // class name
+  mjString info;                  // message appended to errors
+  mjtSensor type;                 // type of sensor
+  mjtDataType datatype;           // data type for sensor measurement
+  mjtStage needstage;             // compute stage needed to simulate sensor
+  mjtObj objtype;                 // type of sensorized object
+  mjString objname;               // name of sensorized object
+  mjtObj reftype;                 // type of referenced object
+  mjString refname;               // name of referenced object
+  int dim;                        // number of scalar outputs
+  double cutoff;                  // cutoff for real and positive datatypes
+  double noise;                   // noise stdev
+  mjDouble userdata;              // user data
+  mjmPlugin plugin;               // sensor plugin
+} mjmSensor;
+
+
 //---------------------------------- Public API ----------------------------------------------------
 
 // Create model.
@@ -246,6 +299,12 @@ MJAPI mjmLight* mjm_addLight(mjmBody* body, void* defspec);
 // Add frame to body.
 MJAPI void* mjm_addFrame(mjmBody* body, void* parentframe);
 
+// Add actuator to model.
+MJAPI mjmActuator* mjm_addActuator(void* model, void* defspec);
+
+// Add sensor to model.
+MJAPI mjmSensor* mjm_addSensor(void* model);
+
 // Add plugin to model.
 MJAPI mjElement mjm_addPlugin(void* model);
 
@@ -306,6 +365,12 @@ MJAPI void mjm_defaultCamera(mjmCamera& camera);
 // Default light attributes.
 MJAPI void mjm_defaultLight(mjmLight& light);
 
+// Default actuator attributes.
+MJAPI void mjm_defaultActuator(mjmActuator& actuator);
+
+// Default sensor attributes.
+MJAPI void mjm_defaultSensor(mjmSensor& sensor);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/src/user/user_init.cc b/src/user/user_init.cc
index 1272a0f7..e2af855f 100644
--- a/src/user/user_init.cc
+++ b/src/user/user_init.cc
@@ -81,6 +81,8 @@ void mjm_defaultGeom(mjmGeom& geom) {
   geom.fluid_coefs[2] = 1.5;       // angular drag coefficient
   geom.fluid_coefs[3] = 1.0;       // kutta lift coefficient
   geom.fluid_coefs[4] = 1.0;       // magnus lift coefficient
+  geom.plugin.active = false;
+  geom.plugin.instance = nullptr;
 }
 
 
@@ -131,3 +133,44 @@ void mjm_defaultLight(mjmLight& light) {
 }
 
 
+
+// default actuator attributes
+void mjm_defaultActuator(mjmActuator& actuator) {
+  memset(&actuator, 0, sizeof(mjmActuator));
+  actuator.group = 0;
+  actuator.ctrllimited = 2;
+  actuator.forcelimited = 2;
+  actuator.actlimited = 2;
+  actuator.actdim = -1;
+  actuator.plugin_actdim = 0;
+  actuator.trntype = mjTRN_UNDEFINED;
+  actuator.dyntype = mjDYN_NONE;
+  actuator.gaintype = mjGAIN_FIXED;
+  actuator.biastype = mjBIAS_NONE;
+  actuator.actearly = 0;
+  actuator.gear[0] = 1;
+  actuator.dynprm[0] = 1;
+  actuator.gainprm[0] = 1;
+  actuator.cranklength = 0;
+  actuator.plugin.active = false;
+  actuator.plugin.instance = nullptr;
+}
+
+
+
+// default sensor attributes
+void mjm_defaultSensor(mjmSensor& sensor) {
+  memset(&sensor, 0, sizeof(mjmSensor));
+  sensor.type = mjSENS_TOUCH;
+  sensor.datatype = mjDATATYPE_REAL;
+  sensor.needstage = mjSTAGE_ACC;
+  sensor.objtype = mjOBJ_UNKNOWN;
+  sensor.reftype = mjOBJ_UNKNOWN;
+  sensor.cutoff = 0;
+  sensor.noise = 0;
+  sensor.dim = 0;
+}
+
+
+
+
diff --git a/src/user/user_model.cc b/src/user/user_model.cc
index d82c2922..b7c1f0f9 100644
--- a/src/user/user_model.cc
+++ b/src/user/user_model.cc
@@ -2298,7 +2298,7 @@ void mjCModel::CopyObjects(mjModel* m) {
     copyvec(m->actuator_forcerange + 2*i, pac->forcerange, 2);
     copyvec(m->actuator_actrange + 2*i, pac->actrange, 2);
     copyvec(m->actuator_lengthrange + 2*i, pac->lengthrange, 2);
-    copyvec(m->actuator_user+nuser_actuator*i, pac->userdata.data(), nuser_actuator);
+    copyvec(m->actuator_user+nuser_actuator*i, pac->get_userdata().data(), nuser_actuator);
   }
 
   // sensors
@@ -2318,7 +2318,7 @@ void mjCModel::CopyObjects(mjModel* m) {
     m->sensor_dim[i] = psen->dim;
     m->sensor_cutoff[i] = (mjtNum)psen->cutoff;
     m->sensor_noise[i] = (mjtNum)psen->noise;
-    copyvec(m->sensor_user+nuser_sensor*i, psen->userdata.data(), nuser_sensor);
+    copyvec(m->sensor_user+nuser_sensor*i, psen->get_userdata().data(), nuser_sensor);
 
     // calculate address and advance
     m->sensor_adr[i] = adr;
@@ -2920,13 +2920,13 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
   if (nuser_actuator == -1) {
     nuser_actuator = 0;
     for (int i=0; i<actuators.size(); i++) {
-      nuser_actuator = mjMAX(nuser_actuator, actuators[i]->userdata.size());
+      nuser_actuator = mjMAX(nuser_actuator, actuators[i]->spec_userdata_.size());
     }
   }
   if (nuser_sensor == -1) {
     nuser_sensor = 0;
     for (int i=0; i<sensors.size(); i++) {
-      nuser_sensor = mjMAX(nuser_sensor, sensors[i]->userdata.size());
+      nuser_sensor = mjMAX(nuser_sensor, sensors[i]->spec_userdata_.size());
     }
   }
 
@@ -3500,7 +3500,7 @@ bool mjCModel::CopyBack(const mjModel* m) {
     pa->cranklength = (double)m->actuator_cranklength[i];
 
     if (nuser_actuator) {
-      copyvec(pa->userdata.data(), m->actuator_user + nuser_actuator*i, nuser_actuator);
+      copyvec(pa->userdata_.data(), m->actuator_user + nuser_actuator*i, nuser_actuator);
     }
   }
 
@@ -3510,7 +3510,7 @@ bool mjCModel::CopyBack(const mjModel* m) {
     sensors[i]->noise = (double)m->sensor_noise[i];
 
     if (nuser_sensor) {
-      copyvec(sensors[i]->userdata.data(), m->sensor_user + nuser_sensor*i, nuser_sensor);
+      copyvec(sensors[i]->userdata_.data(), m->sensor_user + nuser_sensor*i, nuser_sensor);
     }
   }
 
diff --git a/src/user/user_objects.cc b/src/user/user_objects.cc
index 052a2eb9..3fb88a34 100644
--- a/src/user/user_objects.cc
+++ b/src/user/user_objects.cc
@@ -475,7 +475,7 @@ void mjCDef::Compile(const mjCModel* model) {
   site.userdata_.resize(model->nuser_site);
   camera.userdata_.resize(model->nuser_cam);
   tendon.userdata.resize(model->nuser_tendon);
-  actuator.userdata.resize(model->nuser_actuator);
+  actuator.userdata_.resize(model->nuser_actuator);
 }
 
 
@@ -3963,35 +3963,13 @@ void mjCWrap::Compile(void) {
 
 // initialize defaults
 mjCActuator::mjCActuator(mjCModel* _model, mjCDef* _def) {
-  // actuator defaults
-  group = 0;
-  ctrllimited = 2;
-  forcelimited = 2;
-  actlimited = 2;
-  actdim = -1;
-  plugin_actdim = 0;
-  trntype = mjTRN_UNDEFINED;
-  dyntype = mjDYN_NONE;
-  gaintype = mjGAIN_FIXED;
-  biastype = mjBIAS_NONE;
-  mjuu_zerovec(dynprm, mjNDYN);
-  mjuu_zerovec(gainprm, mjNGAIN);
-  mjuu_zerovec(biasprm, mjNBIAS);
-  mjuu_zerovec(ctrlrange, 2);
-  mjuu_zerovec(forcerange, 2);
-  mjuu_zerovec(actrange, 2);
-  mjuu_zerovec(lengthrange, 2);
-  mjuu_zerovec(gear, 6);
-  gear[0] = 1;
-  dynprm[0] = 1;
-  gainprm[0] = 1;
-  cranklength = 0;
-  target.clear();
-  slidersite.clear();
-  refsite.clear();
-  userdata.clear();
+  mjm_defaultActuator(spec);
 
   // clear private variables
+  spec_target_.clear();
+  spec_slidersite_.clear();
+  spec_refsite_.clear();
+  spec_userdata_.clear();
   trnid[0] = trnid[1] = -1;
 
   // reset to default if given
@@ -4003,23 +3981,59 @@ mjCActuator::mjCActuator(mjCModel* _model, mjCDef* _def) {
   model = _model;
   def = (_def ? _def : (_model ? _model->defaults[0] : 0));
 
+  // in case this actuator is not compiled
+  CopyFromSpec();
+
   // point to local (needs to be after defaults)
-  plugin.name = (mjString)&plugin_name;
-  plugin.instance_name = (mjString)&plugin_instance_name;
+  MakePointerLocal();
+}
+
+
+
+void mjCActuator::MakePointerLocal() {
+  spec.element = (mjElement)this;
+  spec.name = (mjString)&name;
+  spec.classname = (mjString)&classname;
+  spec.userdata = (mjDouble)&spec_userdata_;
+  spec.target = (mjString)&spec_target_;
+  spec.refsite = (mjString)&spec_refsite_;
+  spec.slidersite = (mjString)&spec_slidersite_;
+  spec.plugin.name = (mjString)&plugin_name;
+  spec.plugin.instance_name = (mjString)&plugin_instance_name;
+  spec.info = (mjString)&info;
+}
+
+
+
+void mjCActuator::CopyFromSpec() {
+  *static_cast<mjmActuator*>(this) = spec;
+  userdata_ = spec_userdata_;
+  target_ = spec_target_;
+  refsite_ = spec_refsite_;
+  slidersite_ = spec_slidersite_;
+  userdata = (mjDouble)&userdata_;
+  target = (mjString)&target_;
+  refsite = (mjString)&refsite_;
+  slidersite = (mjString)&slidersite_;
+  plugin.active = spec.plugin.active;
+  plugin.instance = spec.plugin.instance;
+  plugin.name = spec.plugin.name;
+  plugin.instance_name = spec.plugin.instance_name;
 }
 
 
 
 // compiler
 void mjCActuator::Compile(void) {
+  CopyFromSpec();
   mjCJoint* pjnt;
 
   // resize userdata
-  if (userdata.size() > model->nuser_actuator) {
+  if (userdata_.size() > model->nuser_actuator) {
     throw mjCError(this, "user has more values than nuser_actuator in actuator '%s' (id = %d)",
                    name.c_str(), id);
   }
-  userdata.resize(model->nuser_actuator);
+  userdata_.resize(model->nuser_actuator);
 
   // if limited is auto, set to 1 if range is specified, otherwise unlimited
   if (forcelimited==2) {
@@ -4104,7 +4118,7 @@ void mjCActuator::Compile(void) {
   }
 
   // check for missing target name
-  if (target.empty()) {
+  if (target_.empty()) {
     throw mjCError(this,
                    "missing transmission target for actuator '%s' (id = %d)", name.c_str(), id);
   }
@@ -4115,10 +4129,10 @@ void mjCActuator::Compile(void) {
   case mjTRN_JOINT:
   case mjTRN_JOINTINPARENT:
     // get joint
-    ptarget = model->FindObject(mjOBJ_JOINT, target);
+    ptarget = model->FindObject(mjOBJ_JOINT, target_);
     if (!ptarget) {
       throw mjCError(this,
-                     "unknown transmission target '%s' for actuator id = %d", target.c_str(), id);
+                     "unknown transmission target '%s' for actuator id = %d", target_.c_str(), id);
     }
     pjnt = (mjCJoint*) ptarget;
 
@@ -4132,12 +4146,12 @@ void mjCActuator::Compile(void) {
 
   case mjTRN_SLIDERCRANK:
     // get slidersite, copy in trnid[1]
-    if (slidersite.empty()) {
+    if (slidersite_.empty()) {
       throw mjCError(this, "missing base site for slider-crank '%s' (id = %d)", name.c_str(), id);
     }
-    ptarget = model->FindObject(mjOBJ_SITE, slidersite);
+    ptarget = model->FindObject(mjOBJ_SITE, slidersite_);
     if (!ptarget) {
-      throw mjCError(this, "base site '%s' not found for actuator %d", slidersite.c_str(), id);
+      throw mjCError(this, "base site '%s' not found for actuator %d", slidersite_.c_str(), id);
     }
     trnid[1] = ptarget->id;
 
@@ -4148,31 +4162,31 @@ void mjCActuator::Compile(void) {
     }
 
     // proceed with regular target
-    ptarget = model->FindObject(mjOBJ_SITE, target);
+    ptarget = model->FindObject(mjOBJ_SITE, target_);
     break;
 
   case mjTRN_TENDON:
     // get tendon
-    ptarget = model->FindObject(mjOBJ_TENDON, target);
+    ptarget = model->FindObject(mjOBJ_TENDON, target_);
     break;
 
   case mjTRN_SITE:
     // get refsite, copy into trnid[1]
-    if (!refsite.empty()) {
-      ptarget = model->FindObject(mjOBJ_SITE, refsite);
+    if (!refsite_.empty()) {
+      ptarget = model->FindObject(mjOBJ_SITE, refsite_);
       if (!ptarget) {
-        throw mjCError(this, "reference site '%s' not found for actuator %d", refsite.c_str(), id);
+        throw mjCError(this, "reference site '%s' not found for actuator %d", refsite_.c_str(), id);
       }
       trnid[1] = ptarget->id;
     }
 
     // proceed with regular site target
-    ptarget = model->FindObject(mjOBJ_SITE, target);
+    ptarget = model->FindObject(mjOBJ_SITE, target_);
     break;
 
   case mjTRN_BODY:
     // get body
-    ptarget = model->FindObject(mjOBJ_BODY, target);
+    ptarget = model->FindObject(mjOBJ_BODY, target_);
     break;
 
   default:
@@ -4181,7 +4195,7 @@ void mjCActuator::Compile(void) {
 
   // assign and check
   if (!ptarget) {
-    throw mjCError(this, "transmission target '%s' not found in actuator %d", target.c_str(), id);
+    throw mjCError(this, "transmission target '%s' not found in actuator %d", target_.c_str(), id);
   } else {
     trnid[0] = ptarget->id;
   }
@@ -4209,41 +4223,67 @@ void mjCActuator::Compile(void) {
 
 // initialize defaults
 mjCSensor::mjCSensor(mjCModel* _model) {
+  mjm_defaultSensor(spec);
+
   // set model
   model = _model;
 
-  // set sensor defaults (somewhat arbitrary)
-  type = mjSENS_TOUCH;
-  datatype = mjDATATYPE_REAL;
-  needstage = mjSTAGE_ACC;
-  objtype = mjOBJ_UNKNOWN;
-  objname.clear();
-  reftype = mjOBJ_UNKNOWN;
-  refname.clear();
-  cutoff = 0;
-  noise = 0;
-  userdata.clear();
-  dim = 0;
-
   // clear private variables
+  spec_objname_.clear();
+  spec_refname_.clear();
+  spec_userdata_.clear();
   obj = nullptr;
   refid = -1;
 
-  // point to local (needs to be after defaults)
-  plugin.name = (mjString)&plugin_name;
-  plugin.instance_name = (mjString)&plugin_instance_name;
+  // in case this sensor is not compiled
+  CopyFromSpec();
+
+  // point to local
+  MakePointerLocal();
+}
+
+
+
+void mjCSensor::MakePointerLocal() {
+  spec.element = (mjElement)this;
+  spec.name = (mjString)&name;
+  spec.classname = (mjString)&classname;
+  spec.userdata = (mjDouble)&spec_userdata_;
+  spec.objname = (mjString)&spec_objname_;
+  spec.refname = (mjString)&spec_refname_;
+  spec.plugin.name = (mjString)&plugin_name;
+  spec.plugin.instance_name = (mjString)&plugin_instance_name;
+  spec.info = (mjString)&info;
+}
+
+
+
+void mjCSensor::CopyFromSpec() {
+  *static_cast<mjmSensor*>(this) = spec;
+  userdata_ = spec_userdata_;
+  objname_ = spec_objname_;
+  refname_ = spec_refname_;
+  userdata = (mjDouble)&userdata_;
+  objname = (mjString)&objname_;
+  refname = (mjString)&refname_;
+  plugin.active = spec.plugin.active;
+  plugin.instance = spec.plugin.instance;
+  plugin.name = spec.plugin.name;
+  plugin.instance_name = spec.plugin.instance_name;
 }
 
 
 
 // compiler
 void mjCSensor::Compile(void) {
+  CopyFromSpec();
+
   // resize userdata
-  if (userdata.size() > model->nuser_sensor) {
+  if (userdata_.size() > model->nuser_sensor) {
     throw mjCError(this, "user has more values than nuser_sensor in sensor '%s' (id = %d)",
                    name.c_str(), id);
   }
-  userdata.resize(model->nuser_sensor);
+  userdata_.resize(model->nuser_sensor);
 
   // require non-negative noise
   if (noise<0) {
@@ -4258,14 +4298,14 @@ void mjCSensor::Compile(void) {
   // get objid from objtype and objname
   if (objtype!=mjOBJ_UNKNOWN) {
     // check for missing object name
-    if (objname.empty()) {
+    if (objname_.empty()) {
       throw mjCError(this,
                      "missing name of sensorized object in sensor '%s' (id = %d)",
                      name.c_str(), id);
     }
 
     // find name
-    obj = model->FindObject(objtype, objname);
+    obj = model->FindObject(objtype, objname_);
     if (!obj) {
       throw mjCError(this,
                      "unrecognized name of sensorized object in sensor '%s' (id = %d)",
@@ -4285,14 +4325,14 @@ void mjCSensor::Compile(void) {
   // get refid from reftype and refname
   if (reftype!=mjOBJ_UNKNOWN) {
     // check for missing object name
-    if (refname.empty()) {
+    if (refname_.empty()) {
       throw mjCError(this,
                      "missing name of reference frame object in sensor '%s' (id = %d)",
                      name.c_str(), id);
     }
 
     // find name
-    mjCBase* pref = model->FindObject(reftype, refname);
+    mjCBase* pref = model->FindObject(reftype, refname_);
     if (!pref) {
       throw mjCError(this,
                      "unrecognized name of reference frame object in sensor '%s' (id = %d)",
@@ -4351,7 +4391,7 @@ void mjCSensor::Compile(void) {
 
     // check for camera resolution for camera projection sensor
     if (type==mjSENS_CAMPROJECTION) {
-      mjCCamera* camref = (mjCCamera*) model->FindObject(mjOBJ_CAMERA, refname);
+      mjCCamera* camref = (mjCCamera*) model->FindObject(mjOBJ_CAMERA, refname_);
       if (!camref->resolution[0] || !camref->resolution[1]) {
         throw mjCError(this,
                        "camera projection sensor requires camera resolution '%s' (id = %d)",
diff --git a/src/user/user_objects.h b/src/user/user_objects.h
index 6ff9a30a..3cc25fbc 100644
--- a/src/user/user_objects.h
+++ b/src/user/user_objects.h
@@ -1141,43 +1141,41 @@ class mjCPlugin : public mjCBase {
 //------------------------- class mjCActuator ------------------------------------------------------
 // Describes an actuator
 
-class mjCActuator : public mjCBase {
+class mjCActuator : public mjCBase, private mjmActuator {
   friend class mjCDef;
   friend class mjCModel;
   friend class mjXWriter;
 
  public:
-  // variables set by user or API
-  int group;                      // group for visualization
-  int ctrllimited;                // are control limits defined: 0 false, 1 true, 2 auto
-  int forcelimited;               // are force limits defined: 0 false, 1 true, 2 auto
-  int actlimited;                 // are activation limits defined: 0 false, 1 true, 2 auto
-  int actdim;                     // dimension of associated activations
-  int plugin_actdim;              // actuator state size for plugins
-  mjtDyn dyntype;                 // dynamics type
-  mjtTrn trntype;                 // transmission type
-  mjtGain gaintype;               // gain type
-  mjtBias biastype;               // bias type
-  double dynprm[mjNDYN];          // dynamics parameters
-  double gainprm[mjNGAIN];        // gain parameters
-  double biasprm[mjNGAIN];        // bias parameters
-  bool actearly = false;          // apply activations to qfrc instantly
-  double ctrlrange[2];            // control range
-  double forcerange[2];           // force range
-  double actrange[2];             // activation range
-  double lengthrange[2];          // length range
-  double gear[6];                 // length and transmitted force scaling
-  double cranklength;             // crank length, for slider-crank only
-  std::vector<double> userdata;   // user data
-  std::string target;             // transmission target name
-  std::string slidersite;         // site defining cylinder, for slider-crank only
-  std::string refsite;            // reference site, for site transmission only
+  mjmActuator spec;
+  using mjCBase::name;
+  using mjCBase::classname;
+  using mjCBase::info;
+  using mjCBase::plugin;
+
+  // used by mjXWriter and mjCModel
+  const std::vector<double>& get_userdata() { return userdata_; }
+  const std::string& get_target() { return spec_target_; }
+  const std::string& get_slidersite() { return spec_slidersite_; }
+  const std::string& get_refsite() { return spec_refsite_; }
 
  private:
   mjCActuator(mjCModel* = 0, mjCDef* = 0);  // constructor
   void Compile(void);                       // compiler
+  void CopyFromSpec();
+  void MakePointerLocal();
 
   int trnid[2];                   // id of transmission target
+
+  // variable-size data
+  std::string target_;
+  std::string slidersite_;
+  std::string refsite_;
+  std::vector<double> userdata_;
+  std::string spec_target_;
+  std::string spec_slidersite_;
+  std::string spec_refsite_;
+  std::vector<double> spec_userdata_;
 };
 
 
@@ -1185,31 +1183,39 @@ class mjCActuator : public mjCBase {
 //------------------------- class mjCSensor --------------------------------------------------------
 // Describes a sensor
 
-class mjCSensor : public mjCBase {
+class mjCSensor : public mjCBase, private mjmSensor {
   friend class mjCDef;
   friend class mjCModel;
   friend class mjXWriter;
 
  public:
-  // variables set by user or API
-  mjtSensor type;                 // type of sensor
-  mjtDataType datatype;           // data type for sensor measurement
-  mjtStage needstage;             // compute stage needed to simulate sensor
-  mjtObj objtype;                 // type of sensorized object
-  std::string objname;            // name of sensorized object
-  mjtObj reftype;
-  std::string refname;
-  int dim;                        // number of scalar outputs
-  double cutoff;                  // cutoff for real and positive datatypes
-  double noise;                   // noise stdev
-  std::vector<double> userdata;   // user data
+  mjmSensor spec;
+  using mjCBase::name;
+  using mjCBase::classname;
+  using mjCBase::info;
+  using mjCBase::plugin;
+
+  // used by mjXWriter and mjCModel
+  const std::vector<double>& get_userdata() { return userdata_; }
+  const std::string& get_objname() { return spec_objname_; }
+  const std::string& get_refname() { return spec_refname_; }
 
  private:
   mjCSensor(mjCModel*);           // constructor
   void Compile(void);             // compiler
+  void CopyFromSpec();
+  void MakePointerLocal();
 
   mjCBase* obj;                   // sensorized object
   int refid;                      // id of reference frame
+
+  // variable-size data
+  std::string objname_;
+  std::string refname_;
+  std::vector<double> userdata_;
+  std::string spec_objname_;
+  std::string spec_refname_;
+  std::vector<double> spec_userdata_;
 };
 
 
diff --git a/src/xml/xml_native_reader.cc b/src/xml/xml_native_reader.cc
index 5fb376ca..4387868a 100644
--- a/src/xml/xml_native_reader.cc
+++ b/src/xml/xml_native_reader.cc
@@ -1853,12 +1853,16 @@ void mjXReader::OneTendon(XMLElement* elem, mjCTendon* pten) {
 
 
 // actuator element parser
-void mjXReader::OneActuator(XMLElement* elem, mjCActuator* pact) {
-  string text, type;
+void mjXReader::OneActuator(XMLElement* elem, mjmActuator* pact) {
+  string text, type, name, classname, target, slidersite, refsite;
 
   // common attributes
-  ReadAttrTxt(elem, "name", pact->name);
-  ReadAttrTxt(elem, "class", pact->classname);
+  if (ReadAttrTxt(elem, "name", name)) {
+    mjm_setString(pact->name, name.c_str());
+  }
+  if (ReadAttrTxt(elem, "class", classname)) {
+    mjm_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);
@@ -1871,27 +1875,33 @@ void mjXReader::OneActuator(XMLElement* elem, mjCActuator* pact) {
 
   // transmission target and type
   int cnt = 0;
-  if (ReadAttrTxt(elem, "joint", pact->target)) {
+  if (ReadAttrTxt(elem, "joint", target)) {
+    mjm_setString(pact->target, target.c_str());
     pact->trntype = mjTRN_JOINT;
     cnt++;
   }
-  if (ReadAttrTxt(elem, "jointinparent", pact->target)) {
+  if (ReadAttrTxt(elem, "jointinparent", target)) {
+    mjm_setString(pact->target, target.c_str());
     pact->trntype = mjTRN_JOINTINPARENT;
     cnt++;
   }
-  if (ReadAttrTxt(elem, "tendon", pact->target)) {
+  if (ReadAttrTxt(elem, "tendon", target)) {
+    mjm_setString(pact->target, target.c_str());
     pact->trntype = mjTRN_TENDON;
     cnt++;
   }
-  if (ReadAttrTxt(elem, "cranksite", pact->target)) {
+  if (ReadAttrTxt(elem, "cranksite", target)) {
+    mjm_setString(pact->target, target.c_str());
     pact->trntype = mjTRN_SLIDERCRANK;
     cnt++;
   }
-  if (ReadAttrTxt(elem, "site", pact->target)) {
+  if (ReadAttrTxt(elem, "site", target)) {
+    mjm_setString(pact->target, target.c_str());
     pact->trntype = mjTRN_SITE;
     cnt++;
   }
-  if (ReadAttrTxt(elem, "body", pact->target)) {
+  if (ReadAttrTxt(elem, "body", target)) {
+    mjm_setString(pact->target, target.c_str());
     pact->trntype = mjTRN_BODY;
     cnt++;
   }
@@ -1902,13 +1912,19 @@ void mjXReader::OneActuator(XMLElement* elem, mjCActuator* pact) {
 
   // slidercrank-specific parameters
   int r1 = ReadAttr(elem, "cranklength", 1, &pact->cranklength, text);
-  int r2 = ReadAttrTxt(elem, "slidersite", pact->slidersite);
+  int r2 = ReadAttrTxt(elem, "slidersite", slidersite);
+  if (r2) {
+    mjm_setString(pact->slidersite, slidersite.c_str());
+  }
   if ((r1 || r2) && pact->trntype!=mjTRN_SLIDERCRANK && pact->trntype!=mjTRN_UNDEFINED) {
     throw mjXError(elem, "cranklength and slidersite can only be used in slidercrank transmission");
   }
 
   // site-specific parameters (refsite)
-  int r3 = ReadAttrTxt(elem, "refsite", pact->refsite);
+  int r3 = ReadAttrTxt(elem, "refsite", refsite);
+  if (r3) {
+    mjm_setString(pact->refsite, refsite.c_str());
+  }
   if (r3 && pact->trntype!=mjTRN_SITE && pact->trntype!=mjTRN_UNDEFINED) {
     throw mjXError(elem, "refsite can only be used with site transmission");
   }
@@ -2103,9 +2119,14 @@ void mjXReader::OneActuator(XMLElement* elem, mjCActuator* pact) {
   }
 
   // read userdata
-  ReadVector(elem, "user", pact->userdata, text);
+  std::vector<double> userdata;
+  if (ReadVector(elem, "user", userdata, text)) {
+    mjm_setDouble(pact->userdata, userdata.data(), userdata.size());
+  }
 
-  GetXMLPos(elem, pact);
+  // write info
+  mjm_setString(pact->info,
+      std::string("line = " + std::to_string(elem->GetLineNum()) + ", column = -1").c_str());
 }
 
 
@@ -2571,7 +2592,7 @@ void mjXReader::Default(XMLElement* section, int parentid) {
              name=="cylinder"    ||
              name=="muscle"      ||
              name=="adhesion") {
-      OneActuator(elem, &def->actuator);
+      OneActuator(elem, &def->actuator.spec);
     }
 
     // copy into private attributes
@@ -2580,6 +2601,7 @@ void mjXReader::Default(XMLElement* section, int parentid) {
     mjm_finalize(def->site.spec.element);
     mjm_finalize(def->camera.spec.element);
     mjm_finalize(def->light.spec.element);
+    mjm_finalize(def->actuator.spec.element);
 
     // advance
     elem = elem->NextSiblingElement();
@@ -3423,7 +3445,7 @@ void mjXReader::Actuator(XMLElement* section) {
     }
 
     // create actuator and parse
-    mjCActuator* pact = model->AddActuator(def);
+    mjmActuator* pact = mjm_addActuator(model, def);
     OneActuator(elem, pact);
 
     // advance to next element
@@ -3436,133 +3458,136 @@ void mjXReader::Actuator(XMLElement* section) {
 // sensor section parser
 void mjXReader::Sensor(XMLElement* section) {
   int n;
-  string text;
   XMLElement* elem = section->FirstChildElement();
   while (elem) {
     // create sensor, get string type
-    mjCSensor* psen = model->AddSensor();
+    mjmSensor* psen = mjm_addSensor(model);
     string type = elem->Value();
-    string plugin_name = "";
-    string instance_name = "";
+    string text, name, objname, refname;
+    std::vector<double> userdata;
 
     // read name, noise, userdata
-    ReadAttrTxt(elem, "name", psen->name);
+    if (ReadAttrTxt(elem, "name", name)) {
+      mjm_setString(psen->name, name.c_str());
+    }
     ReadAttr(elem, "cutoff", 1, &psen->cutoff, text);
     ReadAttr(elem, "noise", 1, &psen->noise, text);
-    ReadVector(elem, "user", psen->userdata, text);
+    if (ReadVector(elem, "user", userdata, text)) {
+      mjm_setDouble(psen->userdata, userdata.data(), userdata.size());
+    }
 
     // common robotic sensors, attached to a site
     if (type=="touch") {
       psen->type = mjSENS_TOUCH;
       psen->objtype = mjOBJ_SITE;
-      ReadAttrTxt(elem, "site", psen->objname, true);
+      ReadAttrTxt(elem, "site", objname, true);
     } else if (type=="accelerometer") {
       psen->type = mjSENS_ACCELEROMETER;
       psen->objtype = mjOBJ_SITE;
-      ReadAttrTxt(elem, "site", psen->objname, true);
+      ReadAttrTxt(elem, "site", objname, true);
     } else if (type=="velocimeter") {
       psen->type = mjSENS_VELOCIMETER;
       psen->objtype = mjOBJ_SITE;
-      ReadAttrTxt(elem, "site", psen->objname, true);
+      ReadAttrTxt(elem, "site", objname, true);
     } else if (type=="gyro") {
       psen->type = mjSENS_GYRO;
       psen->objtype = mjOBJ_SITE;
-      ReadAttrTxt(elem, "site", psen->objname, true);
+      ReadAttrTxt(elem, "site", objname, true);
     } else if (type=="force") {
       psen->type = mjSENS_FORCE;
       psen->objtype = mjOBJ_SITE;
-      ReadAttrTxt(elem, "site", psen->objname, true);
+      ReadAttrTxt(elem, "site", objname, true);
     } else if (type=="torque") {
       psen->type = mjSENS_TORQUE;
       psen->objtype = mjOBJ_SITE;
-      ReadAttrTxt(elem, "site", psen->objname, true);
+      ReadAttrTxt(elem, "site", objname, true);
     } else if (type=="magnetometer") {
       psen->type = mjSENS_MAGNETOMETER;
       psen->objtype = mjOBJ_SITE;
-      ReadAttrTxt(elem, "site", psen->objname, true);
+      ReadAttrTxt(elem, "site", objname, true);
     } else if (type=="camprojection") {
       psen->type = mjSENS_CAMPROJECTION;
       psen->objtype = mjOBJ_SITE;
-      ReadAttrTxt(elem, "site", psen->objname, true);
-      ReadAttrTxt(elem, "camera", psen->refname, true);
+      ReadAttrTxt(elem, "site", objname, true);
+      ReadAttrTxt(elem, "camera", refname, true);
       psen->reftype = mjOBJ_CAMERA;
     } else if (type=="rangefinder") {
       psen->type = mjSENS_RANGEFINDER;
       psen->objtype = mjOBJ_SITE;
-      ReadAttrTxt(elem, "site", psen->objname, true);
+      ReadAttrTxt(elem, "site", objname, true);
     }
 
     // sensors related to scalar joints, tendons, actuators
     else if (type=="jointpos") {
       psen->type = mjSENS_JOINTPOS;
       psen->objtype = mjOBJ_JOINT;
-      ReadAttrTxt(elem, "joint", psen->objname, true);
+      ReadAttrTxt(elem, "joint", objname, true);
     } else if (type=="jointvel") {
       psen->type = mjSENS_JOINTVEL;
       psen->objtype = mjOBJ_JOINT;
-      ReadAttrTxt(elem, "joint", psen->objname, true);
+      ReadAttrTxt(elem, "joint", objname, true);
     } else if (type=="tendonpos") {
       psen->type = mjSENS_TENDONPOS;
       psen->objtype = mjOBJ_TENDON;
-      ReadAttrTxt(elem, "tendon", psen->objname, true);
+      ReadAttrTxt(elem, "tendon", objname, true);
     } else if (type=="tendonvel") {
       psen->type = mjSENS_TENDONVEL;
       psen->objtype = mjOBJ_TENDON;
-      ReadAttrTxt(elem, "tendon", psen->objname, true);
+      ReadAttrTxt(elem, "tendon", objname, true);
     } else if (type=="actuatorpos") {
       psen->type = mjSENS_ACTUATORPOS;
       psen->objtype = mjOBJ_ACTUATOR;
-      ReadAttrTxt(elem, "actuator", psen->objname, true);
+      ReadAttrTxt(elem, "actuator", objname, true);
     } else if (type=="actuatorvel") {
       psen->type = mjSENS_ACTUATORVEL;
       psen->objtype = mjOBJ_ACTUATOR;
-      ReadAttrTxt(elem, "actuator", psen->objname, true);
+      ReadAttrTxt(elem, "actuator", objname, true);
     } else if (type=="actuatorfrc") {
       psen->type = mjSENS_ACTUATORFRC;
       psen->objtype = mjOBJ_ACTUATOR;
-      ReadAttrTxt(elem, "actuator", psen->objname, true);
+      ReadAttrTxt(elem, "actuator", objname, true);
     } else if (type=="jointactuatorfrc") {
       psen->type = mjSENS_JOINTACTFRC;
       psen->objtype = mjOBJ_JOINT;
-      ReadAttrTxt(elem, "joint", psen->objname, true);
+      ReadAttrTxt(elem, "joint", objname, true);
     }
 
     // sensors related to ball joints
     else if (type=="ballquat") {
       psen->type = mjSENS_BALLQUAT;
       psen->objtype = mjOBJ_JOINT;
-      ReadAttrTxt(elem, "joint", psen->objname, true);
+      ReadAttrTxt(elem, "joint", objname, true);
     } else if (type=="ballangvel") {
       psen->type = mjSENS_BALLANGVEL;
       psen->objtype = mjOBJ_JOINT;
-      ReadAttrTxt(elem, "joint", psen->objname, true);
+      ReadAttrTxt(elem, "joint", objname, true);
     }
 
     // joint and tendon limit sensors
     else if (type=="jointlimitpos") {
       psen->type = mjSENS_JOINTLIMITPOS;
       psen->objtype = mjOBJ_JOINT;
-      ReadAttrTxt(elem, "joint", psen->objname, true);
+      ReadAttrTxt(elem, "joint", objname, true);
     } else if (type=="jointlimitvel") {
       psen->type = mjSENS_JOINTLIMITVEL;
       psen->objtype = mjOBJ_JOINT;
-      ReadAttrTxt(elem, "joint", psen->objname, true);
+      ReadAttrTxt(elem, "joint", objname, true);
     } else if (type=="jointlimitfrc") {
       psen->type = mjSENS_JOINTLIMITFRC;
       psen->objtype = mjOBJ_JOINT;
-      ReadAttrTxt(elem, "joint", psen->objname, true);
+      ReadAttrTxt(elem, "joint", objname, true);
     } else if (type=="tendonlimitpos") {
       psen->type = mjSENS_TENDONLIMITPOS;
       psen->objtype = mjOBJ_TENDON;
-      ReadAttrTxt(elem, "tendon", psen->objname, true);
+      ReadAttrTxt(elem, "tendon", objname, true);
     } else if (type=="tendonlimitvel") {
       psen->type = mjSENS_TENDONLIMITVEL;
       psen->objtype = mjOBJ_TENDON;
-      ReadAttrTxt(elem, "tendon", psen->objname, true);
+      ReadAttrTxt(elem, "tendon", objname, true);
     } else if (type=="tendonlimitfrc") {
       psen->type = mjSENS_TENDONLIMITFRC;
       psen->objtype = mjOBJ_TENDON;
-      ReadAttrTxt(elem, "tendon", psen->objname, true);
+      ReadAttrTxt(elem, "tendon", objname, true);
     }
 
     // sensors attached to an object with spatial frame: (x)body, geom, site, camera
@@ -3570,10 +3595,10 @@ void mjXReader::Sensor(XMLElement* section) {
       psen->type = mjSENS_FRAMEPOS;
       ReadAttrTxt(elem, "objtype", text, true);
       psen->objtype = (mjtObj)mju_str2Type(text.c_str());
-      ReadAttrTxt(elem, "objname", psen->objname, true);
+      ReadAttrTxt(elem, "objname", objname, true);
       if (ReadAttrTxt(elem, "reftype", text)) {
         psen->reftype = (mjtObj)mju_str2Type(text.c_str());
-        ReadAttrTxt(elem, "refname", psen->refname, true);
+        ReadAttrTxt(elem, "refname", refname, true);
       } else if (ReadAttrTxt(elem, "refname", text)) {
         throw mjXError(elem, "refname '%s' given but reftype is missing", text.c_str());
       }
@@ -3581,10 +3606,10 @@ void mjXReader::Sensor(XMLElement* section) {
       psen->type = mjSENS_FRAMEQUAT;
       ReadAttrTxt(elem, "objtype", text, true);
       psen->objtype = (mjtObj)mju_str2Type(text.c_str());
-      ReadAttrTxt(elem, "objname", psen->objname, true);
+      ReadAttrTxt(elem, "objname", objname, true);
       if (ReadAttrTxt(elem, "reftype", text)) {
         psen->reftype = (mjtObj)mju_str2Type(text.c_str());
-        ReadAttrTxt(elem, "refname", psen->refname, true);
+        ReadAttrTxt(elem, "refname", refname, true);
       } else if (ReadAttrTxt(elem, "refname", text)) {
         throw mjXError(elem, "refname '%s' given but reftype is missing", text.c_str());
       }
@@ -3592,10 +3617,10 @@ void mjXReader::Sensor(XMLElement* section) {
       psen->type = mjSENS_FRAMEXAXIS;
       ReadAttrTxt(elem, "objtype", text, true);
       psen->objtype = (mjtObj)mju_str2Type(text.c_str());
-      ReadAttrTxt(elem, "objname", psen->objname, true);
+      ReadAttrTxt(elem, "objname", objname, true);
       if (ReadAttrTxt(elem, "reftype", text)) {
         psen->reftype = (mjtObj)mju_str2Type(text.c_str());
-        ReadAttrTxt(elem, "refname", psen->refname, true);
+        ReadAttrTxt(elem, "refname", refname, true);
       } else if (ReadAttrTxt(elem, "refname", text)) {
         throw mjXError(elem, "refname '%s' given but reftype is missing", text.c_str());
       }
@@ -3603,10 +3628,10 @@ void mjXReader::Sensor(XMLElement* section) {
       psen->type = mjSENS_FRAMEYAXIS;
       ReadAttrTxt(elem, "objtype", text, true);
       psen->objtype = (mjtObj)mju_str2Type(text.c_str());
-      ReadAttrTxt(elem, "objname", psen->objname, true);
+      ReadAttrTxt(elem, "objname", objname, true);
       if (ReadAttrTxt(elem, "reftype", text)) {
         psen->reftype = (mjtObj)mju_str2Type(text.c_str());
-        ReadAttrTxt(elem, "refname", psen->refname, true);
+        ReadAttrTxt(elem, "refname", refname, true);
       } else if (ReadAttrTxt(elem, "refname", text)) {
         throw mjXError(elem, "refname '%s' given but reftype is missing", text.c_str());
       }
@@ -3614,10 +3639,10 @@ void mjXReader::Sensor(XMLElement* section) {
       psen->type = mjSENS_FRAMEZAXIS;
       ReadAttrTxt(elem, "objtype", text, true);
       psen->objtype = (mjtObj)mju_str2Type(text.c_str());
-      ReadAttrTxt(elem, "objname", psen->objname, true);
+      ReadAttrTxt(elem, "objname", objname, true);
       if (ReadAttrTxt(elem, "reftype", text)) {
         psen->reftype = (mjtObj)mju_str2Type(text.c_str());
-        ReadAttrTxt(elem, "refname", psen->refname, true);
+        ReadAttrTxt(elem, "refname", refname, true);
       } else if (ReadAttrTxt(elem, "refname", text)) {
         throw mjXError(elem, "refname '%s' given but reftype is missing", text.c_str());
       }
@@ -3625,10 +3650,10 @@ void mjXReader::Sensor(XMLElement* section) {
       psen->type = mjSENS_FRAMELINVEL;
       ReadAttrTxt(elem, "objtype", text, true);
       psen->objtype = (mjtObj)mju_str2Type(text.c_str());
-      ReadAttrTxt(elem, "objname", psen->objname, true);
+      ReadAttrTxt(elem, "objname", objname, true);
       if (ReadAttrTxt(elem, "reftype", text)) {
         psen->reftype = (mjtObj)mju_str2Type(text.c_str());
-        ReadAttrTxt(elem, "refname", psen->refname, true);
+        ReadAttrTxt(elem, "refname", refname, true);
       } else if (ReadAttrTxt(elem, "refname", text)) {
         throw mjXError(elem, "refname '%s' given but reftype is missing", text.c_str());
       }
@@ -3636,10 +3661,10 @@ void mjXReader::Sensor(XMLElement* section) {
       psen->type = mjSENS_FRAMEANGVEL;
       ReadAttrTxt(elem, "objtype", text, true);
       psen->objtype = (mjtObj)mju_str2Type(text.c_str());
-      ReadAttrTxt(elem, "objname", psen->objname, true);
+      ReadAttrTxt(elem, "objname", objname, true);
       if (ReadAttrTxt(elem, "reftype", text)) {
         psen->reftype = (mjtObj)mju_str2Type(text.c_str());
-        ReadAttrTxt(elem, "refname", psen->refname, true);
+        ReadAttrTxt(elem, "refname", refname, true);
       } else if (ReadAttrTxt(elem, "refname", text)) {
         throw mjXError(elem, "refname '%s' given but reftype is missing", text.c_str());
       }
@@ -3647,27 +3672,27 @@ void mjXReader::Sensor(XMLElement* section) {
       psen->type = mjSENS_FRAMELINACC;
       ReadAttrTxt(elem, "objtype", text, true);
       psen->objtype = (mjtObj)mju_str2Type(text.c_str());
-      ReadAttrTxt(elem, "objname", psen->objname, true);
+      ReadAttrTxt(elem, "objname", objname, true);
     } else if (type=="frameangacc") {
       psen->type = mjSENS_FRAMEANGACC;
       ReadAttrTxt(elem, "objtype", text, true);
       psen->objtype = (mjtObj)mju_str2Type(text.c_str());
-      ReadAttrTxt(elem, "objname", psen->objname, true);
+      ReadAttrTxt(elem, "objname", objname, true);
     }
 
     // sensors related to kinematic subtrees; attached to a body (which is the subtree root)
     else if (type=="subtreecom") {
       psen->type = mjSENS_SUBTREECOM;
       psen->objtype = mjOBJ_BODY;
-      ReadAttrTxt(elem, "body", psen->objname, true);
+      ReadAttrTxt(elem, "body", objname, true);
     } else if (type=="subtreelinvel") {
       psen->type = mjSENS_SUBTREELINVEL;
       psen->objtype = mjOBJ_BODY;
-      ReadAttrTxt(elem, "body", psen->objname, true);
+      ReadAttrTxt(elem, "body", objname, true);
     } else if (type=="subtreeangmom") {
       psen->type = mjSENS_SUBTREEANGMOM;
       psen->objtype = mjOBJ_BODY;
-      ReadAttrTxt(elem, "body", psen->objname, true);
+      ReadAttrTxt(elem, "body", objname, true);
     }
 
     // global sensors
@@ -3679,14 +3704,14 @@ void mjXReader::Sensor(XMLElement* section) {
     // user-defined sensor
     else if (type=="user") {
       psen->type = mjSENS_USER;
-      bool objname_given = ReadAttrTxt(elem, "objname", psen->objname);
+      bool objname_given = ReadAttrTxt(elem, "objname", objname);
       if (ReadAttrTxt(elem, "objtype", text)) {
         if (!objname_given) {
           throw mjXError(elem, "objtype '%s' given but objname is missing", text.c_str());
         }
         psen->objtype = (mjtObj)mju_str2Type(text.c_str());
       } else if (objname_given) {
-        throw mjXError(elem, "objname '%s' given but objtype is missing", psen->objname.c_str());
+        throw mjXError(elem, "objname '%s' given but objtype is missing", objname.c_str());
       }
       ReadAttrInt(elem, "dim", &psen->dim, true);
 
@@ -3701,38 +3726,39 @@ void mjXReader::Sensor(XMLElement* section) {
 
     else if (type=="plugin") {
       psen->type = mjSENS_PLUGIN;
-      ReadAttrTxt(elem, "plugin", plugin_name);
-      ReadAttrTxt(elem, "instance", instance_name);
-      mjm_setString(psen->plugin.name, plugin_name.c_str());
-      mjm_setString(psen->plugin.instance_name, instance_name.c_str());
-      if (instance_name.empty()) {
-        psen->plugin.instance = mjm_addPlugin(model);
-      } else {
-        model->hasImplicitPluginElem = true;
-      }
-      ReadPluginConfigs(elem, (mjCPlugin*)psen->plugin.instance);
+      OnePlugin(elem, &psen->plugin);
       ReadAttrTxt(elem, "objtype", text);
       psen->objtype = (mjtObj)mju_str2Type(text.c_str());
-      ReadAttrTxt(elem, "objname", psen->objname);
-      if (psen->objtype != mjOBJ_UNKNOWN && psen->objname.empty()) {
+      ReadAttrTxt(elem, "objname", objname);
+      if (psen->objtype != mjOBJ_UNKNOWN && objname.empty()) {
         throw mjXError(elem, "objtype is specified but objname is not");
       }
-      if (psen->objtype == mjOBJ_UNKNOWN && !psen->objname.empty()) {
+      if (psen->objtype == mjOBJ_UNKNOWN && !objname.empty()) {
         throw mjXError(elem, "objname is specified but objtype is not");
       }
       if (ReadAttrTxt(elem, "reftype", text)) {
         psen->reftype = (mjtObj)mju_str2Type(text.c_str());
       }
-      ReadAttrTxt(elem, "refname", psen->refname);
-      if (psen->reftype != mjOBJ_UNKNOWN && psen->refname.empty()) {
+      ReadAttrTxt(elem, "refname", refname);
+      if (psen->reftype != mjOBJ_UNKNOWN && refname.empty()) {
         throw mjXError(elem, "reftype is specified but refname is not");
       }
-      if (psen->reftype == mjOBJ_UNKNOWN && !psen->refname.empty()) {
+      if (psen->reftype == mjOBJ_UNKNOWN && !refname.empty()) {
         throw mjXError(elem, "refname is specified but reftype is not");
       }
     }
 
-    GetXMLPos(elem, psen);
+    if (!objname.empty()) {
+      mjm_setString(psen->objname, objname.c_str());
+    }
+
+    if (!refname.empty()) {
+      mjm_setString(psen->refname, refname.c_str());
+    }
+
+    // write info
+    mjm_setString(psen->info,
+        std::string("line = " + std::to_string(elem->GetLineNum()) + ", column = -1").c_str());
 
     // advance to next element
     elem = elem->NextSiblingElement();
diff --git a/src/xml/xml_native_reader.h b/src/xml/xml_native_reader.h
index 38387d12..ce69a21b 100644
--- a/src/xml/xml_native_reader.h
+++ b/src/xml/xml_native_reader.h
@@ -65,7 +65,7 @@ class mjXReader : public mjXBase {
   void OnePair(tinyxml2::XMLElement* elem, mjCPair* ppair);
   void OneEquality(tinyxml2::XMLElement* elem, mjCEquality* pequality);
   void OneTendon(tinyxml2::XMLElement* elem, mjCTendon* ptendon);
-  void OneActuator(tinyxml2::XMLElement* elem, mjCActuator* pactuator);
+  void OneActuator(tinyxml2::XMLElement* elem, mjmActuator* pactuator);
   void OneComposite(tinyxml2::XMLElement* elem, mjmBody* pbody, mjCDef* def);
   void OneFlexcomp(tinyxml2::XMLElement* elem, mjmBody* pbody);
   void OnePlugin(tinyxml2::XMLElement* elem, mjmPlugin* plugin);
diff --git a/src/xml/xml_native_writer.cc b/src/xml/xml_native_writer.cc
index 4d9955a0..408dcd1a 100644
--- a/src/xml/xml_native_writer.cc
+++ b/src/xml/xml_native_writer.cc
@@ -664,29 +664,29 @@ void mjXWriter::OneActuator(XMLElement* elem, mjCActuator* pact, mjCDef* def) {
     // transmission target
     switch (pact->trntype) {
     case mjTRN_JOINT:
-      WriteAttrTxt(elem, "joint", pact->target);
+      WriteAttrTxt(elem, "joint", pact->get_target());
       break;
 
     case mjTRN_JOINTINPARENT:
-      WriteAttrTxt(elem, "jointinparent", pact->target);
+      WriteAttrTxt(elem, "jointinparent", pact->get_target());
       break;
 
     case mjTRN_TENDON:
-      WriteAttrTxt(elem, "tendon", pact->target);
+      WriteAttrTxt(elem, "tendon", pact->get_target());
       break;
 
     case mjTRN_SLIDERCRANK:
-      WriteAttrTxt(elem, "cranksite", pact->target);
-      WriteAttrTxt(elem, "slidersite", pact->slidersite);
+      WriteAttrTxt(elem, "cranksite", pact->get_target());
+      WriteAttrTxt(elem, "slidersite", pact->get_slidersite());
       break;
 
     case mjTRN_SITE:
-      WriteAttrTxt(elem, "site", pact->target);
-      WriteAttrTxt(elem, "refsite", pact->refsite);
+      WriteAttrTxt(elem, "site", pact->get_target());
+      WriteAttrTxt(elem, "refsite", pact->get_refsite());
       break;
 
     case mjTRN_BODY:
-      WriteAttrTxt(elem, "body", pact->target);
+      WriteAttrTxt(elem, "body", pact->get_target());
       break;
 
     default:        // SHOULD NOT OCCUR
@@ -747,9 +747,9 @@ void mjXWriter::OneActuator(XMLElement* elem, mjCActuator* pact, mjCDef* def) {
 
   // userdata
   if (writingdefaults) {
-    WriteVector(elem, "user", pact->userdata);
+    WriteVector(elem, "user", pact->get_userdata());
   } else {
-    WriteVector(elem, "user", pact->userdata, def->actuator.userdata);
+    WriteVector(elem, "user", pact->get_userdata(), def->actuator.get_userdata());
   }
 }
 
@@ -1748,171 +1748,171 @@ void mjXWriter::Sensor(XMLElement* root) {
     // common robotic sensors, attached to a site
     case mjSENS_TOUCH:
       elem = InsertEnd(section, "touch");
-      WriteAttrTxt(elem, "site", psen->objname);
+      WriteAttrTxt(elem, "site", psen->get_objname());
       break;
     case mjSENS_ACCELEROMETER:
       elem = InsertEnd(section, "accelerometer");
-      WriteAttrTxt(elem, "site", psen->objname);
+      WriteAttrTxt(elem, "site", psen->get_objname());
       break;
     case mjSENS_VELOCIMETER:
       elem = InsertEnd(section, "velocimeter");
-      WriteAttrTxt(elem, "site", psen->objname);
+      WriteAttrTxt(elem, "site", psen->get_objname());
       break;
     case mjSENS_GYRO:
       elem = InsertEnd(section, "gyro");
-      WriteAttrTxt(elem, "site", psen->objname);
+      WriteAttrTxt(elem, "site", psen->get_objname());
       break;
     case mjSENS_FORCE:
       elem = InsertEnd(section, "force");
-      WriteAttrTxt(elem, "site", psen->objname);
+      WriteAttrTxt(elem, "site", psen->get_objname());
       break;
     case mjSENS_TORQUE:
       elem = InsertEnd(section, "torque");
-      WriteAttrTxt(elem, "site", psen->objname);
+      WriteAttrTxt(elem, "site", psen->get_objname());
       break;
     case mjSENS_MAGNETOMETER:
       elem = InsertEnd(section, "magnetometer");
-      WriteAttrTxt(elem, "site", psen->objname);
+      WriteAttrTxt(elem, "site", psen->get_objname());
       break;
     case mjSENS_RANGEFINDER:
       elem = InsertEnd(section, "rangefinder");
-      WriteAttrTxt(elem, "site", psen->objname);
+      WriteAttrTxt(elem, "site", psen->get_objname());
       break;
     case mjSENS_CAMPROJECTION:
       elem = InsertEnd(section, "camprojection");
-      WriteAttrTxt(elem, "site", psen->objname);
-      WriteAttrTxt(elem, "camera", psen->refname);
+      WriteAttrTxt(elem, "site", psen->get_objname());
+      WriteAttrTxt(elem, "camera", psen->get_refname());
       break;
 
     // sensors related to scalar joints, tendons, actuators
     case mjSENS_JOINTPOS:
       elem = InsertEnd(section, "jointpos");
-      WriteAttrTxt(elem, "joint", psen->objname);
+      WriteAttrTxt(elem, "joint", psen->get_objname());
       break;
     case mjSENS_JOINTVEL:
       elem = InsertEnd(section, "jointvel");
-      WriteAttrTxt(elem, "joint", psen->objname);
+      WriteAttrTxt(elem, "joint", psen->get_objname());
       break;
     case mjSENS_TENDONPOS:
       elem = InsertEnd(section, "tendonpos");
-      WriteAttrTxt(elem, "tendon", psen->objname);
+      WriteAttrTxt(elem, "tendon", psen->get_objname());
       break;
     case mjSENS_TENDONVEL:
       elem = InsertEnd(section, "tendonvel");
-      WriteAttrTxt(elem, "tendon", psen->objname);
+      WriteAttrTxt(elem, "tendon", psen->get_objname());
       break;
     case mjSENS_ACTUATORPOS:
       elem = InsertEnd(section, "actuatorpos");
-      WriteAttrTxt(elem, "actuator", psen->objname);
+      WriteAttrTxt(elem, "actuator", psen->get_objname());
       break;
     case mjSENS_ACTUATORVEL:
       elem = InsertEnd(section, "actuatorvel");
-      WriteAttrTxt(elem, "actuator", psen->objname);
+      WriteAttrTxt(elem, "actuator", psen->get_objname());
       break;
     case mjSENS_ACTUATORFRC:
       elem = InsertEnd(section, "actuatorfrc");
-      WriteAttrTxt(elem, "actuator", psen->objname);
+      WriteAttrTxt(elem, "actuator", psen->get_objname());
       break;
     case mjSENS_JOINTACTFRC:
       elem = InsertEnd(section, "jointactuatorfrc");
-      WriteAttrTxt(elem, "joint", psen->objname);
+      WriteAttrTxt(elem, "joint", psen->get_objname());
       break;
 
     // sensors related to ball joints
     case mjSENS_BALLQUAT:
       elem = InsertEnd(section, "ballquat");
-      WriteAttrTxt(elem, "joint", psen->objname);
+      WriteAttrTxt(elem, "joint", psen->get_objname());
       break;
     case mjSENS_BALLANGVEL:
       elem = InsertEnd(section, "ballangvel");
-      WriteAttrTxt(elem, "joint", psen->objname);
+      WriteAttrTxt(elem, "joint", psen->get_objname());
       break;
 
     // joint and tendon limit sensors
     case mjSENS_JOINTLIMITPOS:
       elem = InsertEnd(section, "jointlimitpos");
-      WriteAttrTxt(elem, "joint", psen->objname);
+      WriteAttrTxt(elem, "joint", psen->get_objname());
       break;
     case mjSENS_JOINTLIMITVEL:
       elem = InsertEnd(section, "jointlimitvel");
-      WriteAttrTxt(elem, "joint", psen->objname);
+      WriteAttrTxt(elem, "joint", psen->get_objname());
       break;
     case mjSENS_JOINTLIMITFRC:
       elem = InsertEnd(section, "jointlimitfrc");
-      WriteAttrTxt(elem, "joint", psen->objname);
+      WriteAttrTxt(elem, "joint", psen->get_objname());
       break;
     case mjSENS_TENDONLIMITPOS:
       elem = InsertEnd(section, "tendonlimitpos");
-      WriteAttrTxt(elem, "tendon", psen->objname);
+      WriteAttrTxt(elem, "tendon", psen->get_objname());
       break;
     case mjSENS_TENDONLIMITVEL:
       elem = InsertEnd(section, "tendonlimitvel");
-      WriteAttrTxt(elem, "tendon", psen->objname);
+      WriteAttrTxt(elem, "tendon", psen->get_objname());
       break;
     case mjSENS_TENDONLIMITFRC:
       elem = InsertEnd(section, "tendonlimitfrc");
-      WriteAttrTxt(elem, "tendon", psen->objname);
+      WriteAttrTxt(elem, "tendon", psen->get_objname());
       break;
 
     // sensors attached to an object with spatial frame: (x)body, geom, site, camera
     case mjSENS_FRAMEPOS:
       elem = InsertEnd(section, "framepos");
       WriteAttrTxt(elem, "objtype", mju_type2Str(psen->objtype));
-      WriteAttrTxt(elem, "objname", psen->objname);
+      WriteAttrTxt(elem, "objname", psen->get_objname());
       break;
     case mjSENS_FRAMEQUAT:
       elem = InsertEnd(section, "framequat");
       WriteAttrTxt(elem, "objtype", mju_type2Str(psen->objtype));
-      WriteAttrTxt(elem, "objname", psen->objname);
+      WriteAttrTxt(elem, "objname", psen->get_objname());
       break;
     case mjSENS_FRAMEXAXIS:
       elem = InsertEnd(section, "framexaxis");
       WriteAttrTxt(elem, "objtype", mju_type2Str(psen->objtype));
-      WriteAttrTxt(elem, "objname", psen->objname);
+      WriteAttrTxt(elem, "objname", psen->get_objname());
       break;
     case mjSENS_FRAMEYAXIS:
       elem = InsertEnd(section, "frameyaxis");
       WriteAttrTxt(elem, "objtype", mju_type2Str(psen->objtype));
-      WriteAttrTxt(elem, "objname", psen->objname);
+      WriteAttrTxt(elem, "objname", psen->get_objname());
       break;
     case mjSENS_FRAMEZAXIS:
       elem = InsertEnd(section, "framezaxis");
       WriteAttrTxt(elem, "objtype", mju_type2Str(psen->objtype));
-      WriteAttrTxt(elem, "objname", psen->objname);
+      WriteAttrTxt(elem, "objname", psen->get_objname());
       break;
     case mjSENS_FRAMELINVEL:
       elem = InsertEnd(section, "framelinvel");
       WriteAttrTxt(elem, "objtype", mju_type2Str(psen->objtype));
-      WriteAttrTxt(elem, "objname", psen->objname);
+      WriteAttrTxt(elem, "objname", psen->get_objname());
       break;
     case mjSENS_FRAMEANGVEL:
       elem = InsertEnd(section, "frameangvel");
       WriteAttrTxt(elem, "objtype", mju_type2Str(psen->objtype));
-      WriteAttrTxt(elem, "objname", psen->objname);
+      WriteAttrTxt(elem, "objname", psen->get_objname());
       break;
     case mjSENS_FRAMELINACC:
       elem = InsertEnd(section, "framelinacc");
       WriteAttrTxt(elem, "objtype", mju_type2Str(psen->objtype));
-      WriteAttrTxt(elem, "objname", psen->objname);
+      WriteAttrTxt(elem, "objname", psen->get_objname());
       break;
     case mjSENS_FRAMEANGACC:
       elem = InsertEnd(section, "frameangacc");
       WriteAttrTxt(elem, "objtype", mju_type2Str(psen->objtype));
-      WriteAttrTxt(elem, "objname", psen->objname);
+      WriteAttrTxt(elem, "objname", psen->get_objname());
       break;
 
     // sensors related to kinematic subtrees; attached to a body (which is the subtree root)
     case mjSENS_SUBTREECOM:
       elem = InsertEnd(section, "subtreecom");
-      WriteAttrTxt(elem, "body", psen->objname);
+      WriteAttrTxt(elem, "body", psen->get_objname());
       break;
     case mjSENS_SUBTREELINVEL:
       elem = InsertEnd(section, "subtreelinvel");
-      WriteAttrTxt(elem, "body", psen->objname);
+      WriteAttrTxt(elem, "body", psen->get_objname());
       break;
     case mjSENS_SUBTREEANGMOM:
       elem = InsertEnd(section, "subtreeangmom");
-      WriteAttrTxt(elem, "body", psen->objname);
+      WriteAttrTxt(elem, "body", psen->get_objname());
       break;
 
     // global sensors
@@ -1926,35 +1926,16 @@ void mjXWriter::Sensor(XMLElement* root) {
       elem = InsertEnd(section, "plugin");
       if (psen->objtype != mjOBJ_UNKNOWN) {
         WriteAttrTxt(elem, "objtype", mju_type2Str(psen->objtype));
-        WriteAttrTxt(elem, "objname", psen->objname);
-      }
-      instance_name = std::string(mjm_getString(psen->plugin.instance_name));
-      plugin_name = std::string(mjm_getString(psen->plugin.name));
-      if (!instance_name.empty()) {
-        WriteAttrTxt(elem, "instance", instance_name);
-      } else {
-        WriteAttrTxt(elem, "plugin", plugin_name);
-        const mjpPlugin* plugin = mjp_getPluginAtSlot(
-            ((mjCPlugin*)psen->plugin.instance)->plugin_slot);
-        const char* c = &((mjCPlugin*)psen->plugin.instance)->flattened_attributes[0];
-        for (int i = 0; i < plugin->nattribute; ++i) {
-          std::string value(c);
-          if (!value.empty()) {
-            XMLElement* config_elem = InsertEnd(elem, "config");
-            WriteAttrTxt(config_elem, "key", plugin->attributes[i]);
-            WriteAttrTxt(config_elem, "value", value);
-            c += value.size();
-          }
-          ++c;
-        }
+        WriteAttrTxt(elem, "objname", psen->get_objname());
       }
+      OnePlugin(elem, psen);
       break;
 
     // user-defined sensor
     case mjSENS_USER:
       elem = InsertEnd(section, "user");
       if (mju_type2Str(psen->objtype)) WriteAttrTxt(elem, "objtype", mju_type2Str(psen->objtype));
-      WriteAttrTxt(elem, "objname", psen->objname);
+      WriteAttrTxt(elem, "objname", psen->get_objname());
       WriteAttrInt(elem, "dim", psen->dim);
       WriteAttrKey(elem, "needstage", stage_map, stage_sz, (int)psen->needstage);
       WriteAttrKey(elem, "datatype", datatype_map, datatype_sz, (int)psen->datatype);
@@ -1970,12 +1951,12 @@ void mjXWriter::Sensor(XMLElement* root) {
     if (psen->type != mjSENS_PLUGIN) {
       WriteAttr(elem, "noise", 1, &psen->noise, &zero);
     }
-    WriteVector(elem, "user", psen->userdata);
+    WriteVector(elem, "user", psen->get_userdata());
 
     // add reference if present
     if (psen->reftype != mjOBJ_UNKNOWN && psen->type != mjSENS_CAMPROJECTION) {
       WriteAttrTxt(elem, "reftype", mju_type2Str(psen->reftype));
-      WriteAttrTxt(elem, "refname", psen->refname);
+      WriteAttrTxt(elem, "refname", psen->get_refname());
     }
   }