diff --git a/src/user/user_mesh.cc b/src/user/user_mesh.cc
index db09b8fb..c6c5d9de 100644
--- a/src/user/user_mesh.cc
+++ b/src/user/user_mesh.cc
@@ -2120,6 +2120,14 @@ void mjCSkin::PointToLocal() {
 
 
 
+void mjCSkin::NameSpace(const mjCModel* m) {
+  for (int i=0; i<(int)spec_bodyname_.size(); i++) {
+    spec_bodyname_[i] = m->prefix + spec_bodyname_[i] + m->suffix;
+  }
+}
+
+
+
 void mjCSkin::CopyFromSpec() {
   *static_cast<mjmSkin*>(this) = spec;
   file_ = spec_file_;
@@ -2163,6 +2171,20 @@ mjCSkin::~mjCSkin() {
 
 
 
+void mjCSkin::ResolveReferences(const mjCModel* m) {
+  size_t nbone = bodyname_.size();
+  bodyid.resize(nbone);
+  for (int i=0; i<nbone; i++) {
+    mjCBase* pbody = m->FindObject(mjOBJ_BODY, bodyname_[i]);
+    if (!pbody) {
+      throw mjCError(this, "unknown body '%s' in skin", bodyname_[i].c_str());
+    }
+    bodyid[i] = pbody->id;
+  }
+}
+
+
+
 // compiler
 void mjCSkin::Compile(const mjVFS* vfs) {
   CopyFromSpec();
@@ -2243,14 +2265,7 @@ void mjCSkin::Compile(const mjVFS* vfs) {
   }
 
   // resolve body names
-  bodyid.resize(nbone);
-  for (int i=0; i<nbone; i++) {
-    mjCBase* pbody = model->FindObject(mjOBJ_BODY, bodyname_[i]);
-    if (!pbody) {
-      throw mjCError(this, "unknown body '%s' in skin", bodyname_[i].c_str());
-    }
-    bodyid[i] = pbody->id;
-  }
+  ResolveReferences(model);
 
   // resolve material name
   mjCBase* pmat = model->FindObject(mjOBJ_MATERIAL, material_);
@@ -2510,6 +2525,15 @@ void mjCFlex::PointToLocal() {
 }
 
 
+
+void mjCFlex::NameSpace(const mjCModel* m) {
+  for (int i=0; i<(int)spec_vertbody_.size(); i++) {
+    spec_vertbody_[i] = m->prefix + spec_vertbody_[i] + m->suffix;
+  }
+}
+
+
+
 void mjCFlex::CopyFromSpec() {
   *static_cast<mjmFlex*>(this) = spec;
   spec.info = (mjString)&info;
@@ -2542,6 +2566,18 @@ void mjCFlex::DelTexcoord() {
 }
 
 
+void mjCFlex::ResolveReferences(const mjCModel* m) {
+  for (int i=0; i<(int)vertbody_.size(); i++) {
+    mjCBase* pbody = m->FindObject(mjOBJ_BODY, vertbody_[i]);
+    if (pbody) {
+      vertbodyid.push_back(pbody->id);
+    } else {
+        throw mjCError(this, "unkown body '%s' in flex", vertbody_[i].c_str());
+    }
+  }
+}
+
+
 // compiler
 void mjCFlex::Compile(const mjVFS* vfs) {
   CopyFromSpec();
@@ -2603,14 +2639,7 @@ void mjCFlex::Compile(const mjVFS* vfs) {
   }
 
   // resolve body ids
-  for (int i=0; i<(int)vertbody_.size(); i++) {
-    mjCBase* pbody = model->FindObject(mjOBJ_BODY, vertbody_[i]);
-    if (pbody) {
-      vertbodyid.push_back(pbody->id);
-    } else {
-        throw mjCError(this, "unkown body '%s' in flex", vertbody_[i].c_str());
-    }
-  }
+  ResolveReferences(model);
 
   // process elements
   for (int e=0; e<(int)elem_.size()/(dim+1); e++) {
diff --git a/src/user/user_model.cc b/src/user/user_model.cc
index 8728b5e6..0cd073f6 100644
--- a/src/user/user_model.cc
+++ b/src/user/user_model.cc
@@ -120,6 +120,8 @@ mjCModel::mjCModel() {
   tuples.clear();
   keys.clear();
   defaults.clear();
+  prefix = "";
+  suffix = "";
   Clear();
 
   //------------------------ master default set
@@ -180,101 +182,66 @@ mjCModel& mjCModel::operator=(const mjCModel& other) {
 
 
 
+// copy vector of elements from another model to this model
+template <class T>
+void mjCModel::CopyList(std::vector<T*>& dest,
+                        const std::vector<T*>& source,
+                        std::map<mjCDef*, int>& def_map,
+                        const std::vector<mjCDef*>& defaults) {
+  // loop over the elements from the other model
+  for (T* element : source) {
+    try {
+      // try to find the referenced object in this model
+      element->NameSpace(element->model);
+      element->CopyFromSpec();
+      element->ResolveReferences(this);
+    } catch (mjCError err) {
+      // if not present, skip the element
+      continue;
+    }
+    // copy the element from the other model to this model
+    dest.push_back(new T(*element));
+    dest.back()->model = this;
+    dest.back()->def = defaults[def_map[element->def]];
+  }
+}
+
+
+
 mjCModel& mjCModel::operator+=(const mjCModel& other) {
   if (this != &other) {
+    // create global lists
+    MakeLists(bodies[0]);
+    CreateObjectLists();
+    ProcessLists();
+
+    // copy all elements not in the tree
     std::map<mjCDef*, int> def_map;
     for (int i = 0; i < other.defaults.size(); i++) {
       defaults.push_back(new mjCDef(*other.defaults[i]));
       def_map[other.defaults[i]] = i;
     }
-    for (const mjCFlex* flex : other.flexes) {
-      flexes.push_back(new mjCFlex(*flex));
-      flexes.back()->model = this;
-      flexes.back()->def = defaults[def_map[flex->def]];
-    }
-    for (const mjCMesh* mesh : other.meshes) {
-      meshes.push_back(new mjCMesh(*mesh));
-      meshes.back()->model = this;
-      meshes.back()->def = defaults[def_map[mesh->def]];
-    }
-    for (const mjCSkin* skin : other.skins) {
-      skins.push_back(new mjCSkin(*skin));
-      skins.back()->model = this;
-      skins.back()->def = defaults[def_map[skin->def]];
-    }
-    for (const mjCHField* hfield : other.hfields) {
-      hfields.push_back(new mjCHField(*hfield));
-      hfields.back()->model = this;
-      hfields.back()->def = defaults[def_map[hfield->def]];
-    }
-    for (const mjCTexture* texture : other.textures) {
-      textures.push_back(new mjCTexture(*texture));
-      textures.back()->model = this;
-      textures.back()->def = defaults[def_map[texture->def]];
-    }
-    for (const mjCMaterial* material : other.materials) {
-      materials.push_back(new mjCMaterial(*material));
-      materials.back()->model = this;
-      materials.back()->def = defaults[def_map[material->def]];
-    }
-    for (const mjCPair* pair : other.pairs) {
-      pairs.push_back(new mjCPair(*pair));
-      pairs.back()->model = this;
-      pairs.back()->def = defaults[def_map[pair->def]];
-    }
-    for (const mjCBodyPair* exclude : other.excludes) {
-      excludes.push_back(new mjCBodyPair(*exclude));
-      excludes.back()->model = this;
-      excludes.back()->def = defaults[def_map[exclude->def]];
-    }
-    for (const mjCEquality* equality : other.equalities) {
-      equalities.push_back(new mjCEquality(*equality));
-      equalities.back()->model = this;
-      equalities.back()->def = defaults[def_map[equality->def]];
-    }
-    for (const mjCTendon* tendon : other.tendons) {
-      tendons.push_back(new mjCTendon(*tendon));
-      tendons.back()->SetModel(this);
-      tendons.back()->def = defaults[def_map[tendon->def]];
-    }
-    for (const mjCActuator* actuator : other.actuators) {
-      actuators.push_back(new mjCActuator(*actuator));
-      actuators.back()->model = this;
-      actuators.back()->def = defaults[def_map[actuator->def]];
-    }
-    for (const mjCSensor* sensor : other.sensors) {
-      sensors.push_back(new mjCSensor(*sensor));
-      sensors.back()->model = this;
-      sensors.back()->def = defaults[def_map[sensor->def]];
-    }
-    for (const mjCNumeric* numeric : other.numerics) {
-      numerics.push_back(new mjCNumeric(*numeric));
-      numerics.back()->model = this;
-      numerics.back()->def = defaults[def_map[numeric->def]];
-    }
-    for (const mjCText* text : other.texts) {
-      texts.push_back(new mjCText(*text));
-      texts.back()->model = this;
-      texts.back()->def = defaults[def_map[text->def]];
-    }
-    for (const mjCTuple* tuple : other.tuples) {
-      tuples.push_back(new mjCTuple(*tuple));
-      tuples.back()->model = this;
-      tuples.back()->def = defaults[def_map[tuple->def]];
-    }
-    for (const mjCKey* key : other.keys) {
-      keys.push_back(new mjCKey(*key));
-      keys.back()->model = this;
-      keys.back()->def = defaults[def_map[key->def]];
-    }
+    CopyList(flexes, other.flexes, def_map, defaults);
+    CopyList(meshes, other.meshes, def_map, defaults);
+    CopyList(skins, other.skins, def_map, defaults);
+    CopyList(hfields, other.hfields, def_map, defaults);
+    CopyList(textures, other.textures, def_map, defaults);
+    CopyList(materials, other.materials, def_map, defaults);
+    CopyList(pairs, other.pairs, def_map, defaults);
+    CopyList(excludes, other.excludes, def_map, defaults);
+    CopyList(tendons, other.tendons, def_map, defaults);
+    CopyList(equalities, other.equalities, def_map, defaults);
+    CopyList(actuators, other.actuators, def_map, defaults);
+    CopyList(sensors, other.sensors, def_map, defaults);
+    CopyList(numerics, other.numerics, def_map, defaults);
+    CopyList(texts, other.texts, def_map, defaults);
+    CopyList(tuples, other.tuples, def_map, defaults);
+    CopyList(keys, other.keys, def_map, defaults);
 
     // plugins are global
     plugins = other.plugins;
     active_plugins = other.active_plugins;
 
-    // create global lists
-    MakeLists(bodies[0]);
-
     // update defaults for the copied objects
     for (int i = 1; i < other.bodies.size(); i++) {
       bodies[i]->def = defaults[def_map[other.bodies[i]->def]];
@@ -296,7 +263,7 @@ mjCModel& mjCModel::operator+=(const mjCModel& other) {
     }
 
     // cast children to mjCBase
-    CreateObjectLists();
+
 
     // restore to the same state as other
     if (!compiled) {
@@ -670,28 +637,28 @@ mjCBase* mjCModel::GetObject(mjtObj type, int id) {
 //------------------------ API FOR ACCESS TO PRIVATE VARIABLES -------------------------------------
 
 // compiled flag
-bool mjCModel::IsCompiled(void) {
+bool mjCModel::IsCompiled(void) const {
   return compiled;
 }
 
 
 
 // get reference of error object
-const mjCError& mjCModel::GetError(void) {
+const mjCError& mjCModel::GetError(void) const {
   return errInfo;
 }
 
 
 
 // pointer to world body
-mjCBody* mjCModel::GetWorld(void) {
+mjCBody* mjCModel::GetWorld(void) const {
   return bodies[0];
 }
 
 
 
 // find default class name in array
-mjCDef* mjCModel::FindDef(string name) {
+mjCDef* mjCModel::FindDef(string name) const {
   for (int i=0; i<(int)defaults.size(); i++) {
     if (defaults[i]->name==name) {
       return defaults[i];
@@ -754,7 +721,7 @@ static T* findobject(std::string_view name, const vector<T*>& list, const mjKeyM
 }
 
 // find object in global lists given string type and name
-mjCBase* mjCModel::FindObject(mjtObj type, string name) {
+mjCBase* mjCModel::FindObject(mjtObj type, string name) const {
   if (!object_lists[type]) {
     return nullptr;
   }
@@ -764,7 +731,7 @@ mjCBase* mjCModel::FindObject(mjtObj type, string name) {
 
 
 // find body by name
-mjCBody* mjCModel::FindBody(mjCBody* body, std::string name) {
+mjCBody* mjCModel::FindBody(mjCBody* body, std::string name) const {
   if (body->name == name) {
     return body;
   }
@@ -782,7 +749,7 @@ mjCBody* mjCModel::FindBody(mjCBody* body, std::string name) {
 
 
 // find frame by name
-mjCFrame* mjCModel::FindFrame(mjCBody* body, std::string name) {
+mjCFrame* mjCModel::FindFrame(mjCBody* body, std::string name) const{
   for (auto frame : body->frames) {
     if (frame->name == name) {
       return frame;
@@ -802,7 +769,7 @@ mjCFrame* mjCModel::FindFrame(mjCBody* body, std::string name) {
 
 
 // detect null pose
-bool mjCModel::IsNullPose(const mjtNum* pos, const mjtNum* quat) {
+bool mjCModel::IsNullPose(const mjtNum* pos, const mjtNum* quat) const {
   bool result = true;
 
   // check position if given
@@ -2913,6 +2880,21 @@ static void processlist(mjListKeyMap& ids, vector<T*>& list,
 }
 
 
+
+// set object ids, check for repeated names
+void mjCModel::ProcessLists() {
+  for (int i = 0; i < mjNOBJECT; i++) {
+    if (i != mjOBJ_XBODY && object_lists[i]) {
+      processlist(ids, *object_lists[i], (mjtObj) i);
+    }
+  }
+
+  // check repeated names in meta elements
+  processlist(ids, frames, mjOBJ_FRAME);
+}
+
+
+
 // error handler for low-level engine
 static thread_local std::jmp_buf error_jmp_buf;
 static thread_local char errortext[500] = "";
@@ -3046,14 +3028,7 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
   CheckEmptyNames();
 
   // set object ids, check for repeated names
-  for (int i = 0; i < mjNOBJECT; i++) {
-    if (i != mjOBJ_XBODY && object_lists[i]) {
-      processlist(ids, *object_lists[i], (mjtObj) i);
-    }
-  }
-
-  // check repeated names in meta elements
-  processlist(ids, frames, mjOBJ_FRAME);
+  ProcessLists();
 
   // delete visual assets
   if (discardvisual) {
diff --git a/src/user/user_model.h b/src/user/user_model.h
index d6c1933f..167d3840 100644
--- a/src/user/user_model.h
+++ b/src/user/user_model.h
@@ -34,6 +34,11 @@ typedef std::map<std::string, int, std::less<> > mjKeyMap;
 typedef std::array<mjKeyMap, mjNOBJECT> mjListKeyMap;
 
 class mjCModel_ {
+ public:
+  // attach namespaces
+  std::string prefix;
+  std::string suffix;
+
  protected:
   bool compiled;      // already compiled flag
 
@@ -185,6 +190,12 @@ class mjCModel : public mjCModel_, private mjSpec {
   mjCKey* AddKey();
   mjCPlugin* AddPlugin();
 
+  // copy vector of elements to this model
+  template <class T> void CopyList(std::vector<T*>& dest,
+                                   const std::vector<T*>& source,
+                                   std::map<mjCDef*, int>& def_map,
+                                   const std::vector<mjCDef*>& defaults);
+
   // delete elements marked as discard=true
   template <class T> void Delete(std::vector<T*>& elements,
                                  const std::vector<bool>& discard);
@@ -197,15 +208,15 @@ class mjCModel : public mjCModel_, private mjSpec {
   mjCBase* GetObject(mjtObj type, int id);  // pointer to specified object
 
   // API for access to other variables
-  bool IsCompiled();                                       // is model already compiled
-  const mjCError& GetError(void);                          // get reference of error object
-  mjCBody* GetWorld();                                     // pointer to world body
-  mjCDef* FindDef(std::string name);                       // find default class name
-  mjCDef* AddDef(std::string name, int parentid);          // add default class to array
-  mjCBase* FindObject(mjtObj type, std::string name);      // find object given type and name
-  mjCBody* FindBody(mjCBody* body, std::string name);      // find body given name
-  mjCFrame* FindFrame(mjCBody* body, std::string name);    // find frame given name
-  bool IsNullPose(const mjtNum* pos, const mjtNum* quat);  // detect null pose
+  bool IsCompiled() const;                                       // is model already compiled
+  const mjCError& GetError(void) const;                          // get reference of error object
+  mjCBody* GetWorld() const;                                     // pointer to world body
+  mjCDef* FindDef(std::string name) const;                       // find default class name
+  mjCDef* AddDef(std::string name, int parentid);                // add default class to array
+  mjCBase* FindObject(mjtObj type, std::string name) const;      // find object given type and name
+  mjCBody* FindBody(mjCBody* body, std::string name) const;      // find body given name
+  mjCFrame* FindFrame(mjCBody* body, std::string name) const;    // find frame given name
+  bool IsNullPose(const mjtNum* pos, const mjtNum* quat) const;  // detect null pose
 
   // accessors
   std::string get_meshdir(void) const { return meshdir_; }
@@ -289,6 +300,9 @@ class mjCModel : public mjCModel_, private mjSpec {
   // create mjCBase lists from children lists
   void CreateObjectLists();
 
+  // populate objects ids
+  void ProcessLists();
+
   mjListKeyMap ids;   // map from object names to ids
   mjCError errInfo;   // last error info
   bool plugin_owner;  // this class allocated the plugins
diff --git a/src/user/user_objects.cc b/src/user/user_objects.cc
index e4f3c744..8e2a1d78 100644
--- a/src/user/user_objects.cc
+++ b/src/user/user_objects.cc
@@ -793,15 +793,45 @@ mjCBody::~mjCBody() {
 
 
 // apply prefix and suffix, propagate to children
-void mjCBody::SetNameSpace() {
+void mjCBody::NameSpace(const mjCModel* m) {
   if (!name.empty()) {
     name = prefix + name + suffix;
   }
 
+  for (auto& joint : joints) {
+    if (!joint->name.empty()) {
+      joint->name = prefix + joint->name + suffix;
+    }
+  }
+
+  for (auto& geom : geoms) {
+    if (!geom->name.empty()) {
+      geom->name = prefix + geom->name + suffix;
+    }
+  }
+
+  for (auto& site : sites) {
+    if (!site->name.empty()) {
+      site->name = prefix + site->name + suffix;
+    }
+  }
+
+  for (auto& camera : cameras) {
+    if (!camera->name.empty()) {
+      camera->name = prefix + camera->name + suffix;
+    }
+  }
+
+  for (auto& light : lights) {
+    if (!light->name.empty()) {
+      light->name = prefix + light->name + suffix;
+    }
+  }
+
   for (auto& body : bodies) {
-    body->prefix = prefix;
-    body->suffix = suffix;
-    body->SetNameSpace();
+    body->prefix = m->prefix;
+    body->suffix = m->suffix;
+    body->NameSpace(m);
   }
 }
 
@@ -1368,8 +1398,10 @@ mjCFrame& mjCFrame::operator=(const mjCFrame& other) {
 // attach body to frame
 mjCFrame& mjCFrame::operator+=(const mjCBody& other) {
   mjCBody* subtree = new mjCBody(other, model);
+  other.model->prefix = subtree->prefix;
+  other.model->suffix = subtree->suffix;
   subtree->SetFrame(this);
-  subtree->SetNameSpace();
+  subtree->NameSpace(other.model);
 
   // add to tree
   *body += *subtree;
@@ -3777,8 +3809,8 @@ mjCPair& mjCPair::operator=(const mjCPair& other) {
     this->spec = other.spec;
     *static_cast<mjCPair_*>(this) = static_cast<const mjCPair_&>(other);
     *static_cast<mjmPair*>(this) = static_cast<const mjmPair&>(other);
-    this->geom1 = other.geom1;
-    this->geom2 = other.geom2;
+    this->geom1 = nullptr;
+    this->geom2 = nullptr;
   }
   PointToLocal();
   return *this;
@@ -3797,6 +3829,16 @@ void mjCPair::PointToLocal() {
 
 
 
+void mjCPair::NameSpace(const mjCModel* m) {
+  if (!name.empty()) {
+    name = m->prefix + name + m->suffix;
+  }
+  spec_geomname1_ = m->prefix + spec_geomname1_ + m->suffix;
+  spec_geomname2_ = m->prefix + spec_geomname2_ + m->suffix;
+}
+
+
+
 void mjCPair::CopyFromSpec() {
   *static_cast<mjmPair*>(this) = spec;
   geomname1_ = spec_geomname1_;
@@ -3807,31 +3849,19 @@ void mjCPair::CopyFromSpec() {
 
 
 
-// compiler
-void mjCPair::Compile(void) {
-  CopyFromSpec();
-
-  // check condim
-  if (condim!=1 && condim!=3 && condim!=4 && condim!=6) {
-    throw mjCError(this, "invalid condim in collision %d", "", id);
-  }
-
+void mjCPair::ResolveReferences(const mjCModel* m) {
   // find geom 1
-  geom1 = (mjCGeom*)model->FindObject(mjOBJ_GEOM, geomname1_);
+  geom1 = (mjCGeom*)m->FindObject(mjOBJ_GEOM, geomname1_);
   if (!geom1) {
     throw mjCError(this, "geom '%s' not found in collision %d", geomname1_.c_str(), id);
   }
 
   // find geom 2
-  geom2 = (mjCGeom*)model->FindObject(mjOBJ_GEOM, geomname2_);
+  geom2 = (mjCGeom*)m->FindObject(mjOBJ_GEOM, geomname2_);
   if (!geom2) {
     throw mjCError(this, "geom '%s' not found in collision %d", geomname2_.c_str(), id);
   }
 
-  // mark geoms as not visual
-  geom1->SetNotVisual();
-  geom2->SetNotVisual();
-
   // swap if body1 > body2
   if (geom1->body->id > geom2->body->id) {
     string nametmp = geomname1_;
@@ -3845,6 +3875,25 @@ void mjCPair::Compile(void) {
 
   // get geom ids and body signature
   signature = ((geom1->body->id)<<16) + geom2->body->id;
+}
+
+
+
+// compiler
+void mjCPair::Compile(void) {
+  CopyFromSpec();
+
+  // check condim
+  if (condim!=1 && condim!=3 && condim!=4 && condim!=6) {
+    throw mjCError(this, "invalid condim in collision %d", "", id);
+  }
+
+  // find geoms
+  ResolveReferences(model);
+
+  // mark geoms as not visual
+  geom1->SetNotVisual();
+  geom2->SetNotVisual();
 
   // set undefined margin: max
   if (!mjuu_defined(margin)) {
@@ -3989,6 +4038,16 @@ void mjCBodyPair::PointToLocal() {
 
 
 
+void mjCBodyPair::NameSpace(const mjCModel* m) {
+  if (!name.empty()) {
+    name = m->prefix + name + m->suffix;
+  }
+  spec_bodyname1_ = m->prefix + spec_bodyname1_ + m->suffix;
+  spec_bodyname2_ = m->prefix + spec_bodyname2_ + m->suffix;
+}
+
+
+
 void mjCBodyPair::CopyFromSpec() {
   *static_cast<mjmExclude*>(this) = spec;
   bodyname1_ = spec_bodyname1_;
@@ -3999,18 +4058,15 @@ void mjCBodyPair::CopyFromSpec() {
 
 
 
-// compiler
-void mjCBodyPair::Compile(void) {
-  CopyFromSpec();
-
+void mjCBodyPair::ResolveReferences(const mjCModel* m) {
   // find body 1
-  mjCBody* pb1 = (mjCBody*)model->FindObject(mjOBJ_BODY, bodyname1_);
+  mjCBody* pb1 = (mjCBody*)m->FindObject(mjOBJ_BODY, bodyname1_);
   if (!pb1) {
     throw mjCError(this, "body '%s' not found in bodypair %d", bodyname1_.c_str(), id);
   }
 
   // find body 2
-  mjCBody* pb2 = (mjCBody*)model->FindObject(mjOBJ_BODY, bodyname2_);
+  mjCBody* pb2 = (mjCBody*)m->FindObject(mjOBJ_BODY, bodyname2_);
   if (!pb2) {
     throw mjCError(this, "body '%s' not found in bodypair %d", bodyname2_.c_str(), id);
   }
@@ -4034,6 +4090,16 @@ void mjCBodyPair::Compile(void) {
 
 
 
+// compiler
+void mjCBodyPair::Compile(void) {
+  CopyFromSpec();
+
+  // find bodies
+  ResolveReferences(model);
+}
+
+
+
 //------------------ class mjCEquality implementation ----------------------------------------------
 
 // initialize default constraint
@@ -4091,6 +4157,17 @@ void mjCEquality::PointToLocal() {
 }
 
 
+
+void mjCEquality::NameSpace(const mjCModel* m) {
+  if (!name.empty()) {
+    name = m->prefix + name + m->suffix;
+  }
+  spec_name1_ = m->prefix + spec_name1_ + m->suffix;
+  spec_name2_ = m->prefix + spec_name2_ + m->suffix;
+}
+
+
+
 void mjCEquality::CopyFromSpec() {
   *static_cast<mjmEquality*>(this) = spec;
   name1_ = spec_name1_;
@@ -4100,10 +4177,8 @@ void mjCEquality::CopyFromSpec() {
 }
 
 
-// compiler
-void mjCEquality::Compile(void) {
-  CopyFromSpec();
 
+void mjCEquality::ResolveReferences(const mjCModel* m) {
   mjtObj objtype;
   mjCBase *px1, *px2;
   mjtJoint jt1, jt2;
@@ -4122,7 +4197,7 @@ void mjCEquality::Compile(void) {
   }
 
   // find object 1, get id
-  px1 = model->FindObject(objtype, name1_);
+  px1 = m->FindObject(objtype, name1_);
   if (!px1) {
     throw mjCError(this, "unknown element '%s' in equality constraint %d", name1_.c_str(), id);
   }
@@ -4130,7 +4205,7 @@ void mjCEquality::Compile(void) {
 
   // find object 2, get id
   if (!name2_.empty()) {
-    px2 = model->FindObject(objtype, name2_);
+    px2 = m->FindObject(objtype, name2_);
     if (!px2) {
       throw mjCError(this, "unknown element '%s' in equality constraint %d", name2_.c_str(), id);
     }
@@ -4153,11 +4228,6 @@ void mjCEquality::Compile(void) {
     obj2id = 0;
   }
 
-  // make sure flex is not rigid
-  if (type==mjEQ_FLEX && model->flexes[obj1id]->rigid) {
-    throw mjCError(this, "rigid flex '%s' in equality constraint %d", name1_.c_str(), id);
-  }
-
   // make sure the two objects are different
   if (obj1id==obj2id) {
     throw mjCError(this, "element '%s' is repeated in equality constraint %d", name1_.c_str(), id);
@@ -4177,6 +4247,21 @@ void mjCEquality::Compile(void) {
 
 
 
+// compiler
+void mjCEquality::Compile(void) {
+  CopyFromSpec();
+
+  // find objects
+  ResolveReferences(model);
+
+  // make sure flex is not rigid
+  if (type==mjEQ_FLEX && model->flexes[obj1id]->rigid) {
+    throw mjCError(this, "rigid flex '%s' in equality constraint %d", name1_.c_str(), id);
+  }
+}
+
+
+
 //------------------ class mjCTendon implementation ------------------------------------------------
 
 // constructor
@@ -4243,6 +4328,17 @@ void mjCTendon::PointToLocal() {
 
 
 
+void mjCTendon::NameSpace(const mjCModel* m) {
+  if (!name.empty()) {
+    name = m->prefix + name + m->suffix;
+  }
+  for (int i=0; i<path.size(); i++) {
+    path[i]->NameSpace(model);
+  }
+}
+
+
+
 void mjCTendon::CopyFromSpec() {
   *static_cast<mjmTendon*>(this) = spec;
   material_ = spec_material_;
@@ -4361,6 +4457,14 @@ mjCWrap* mjCTendon::GetWrap(int id) {
 
 
 
+void mjCTendon::ResolveReferences(const mjCModel* m) {
+  for (int i=0; i<path.size(); i++) {
+    path[i]->ResolveReferences(m);
+  }
+}
+
+
+
 // compiler
 void mjCTendon::Compile(void) {
   CopyFromSpec();
@@ -4395,9 +4499,7 @@ void mjCTendon::Compile(void) {
   }
 
   // compile objects in path
-  for (int i=0; i<sz; i++) {
-    path[i]->Compile();
-  }
+  ResolveReferences(model);
 
   // check path
   for (int i=0; i<sz; i++) {
@@ -4536,15 +4638,21 @@ void mjCWrap::PointToLocal() {
 
 
 
-// compiler
-void mjCWrap::Compile(void) {
+void mjCWrap::NameSpace(const mjCModel* m) {
+  name = m->prefix + name + m->suffix;
+  sidesite = m->prefix + sidesite + m->suffix;
+}
+
+
+
+void mjCWrap::ResolveReferences(const mjCModel* m) {
   mjCBase *pside;
 
   // handle wrap object types
   switch (type) {
   case mjWRAP_JOINT:                          // joint
     // find joint by name
-    obj = model->FindObject(mjOBJ_JOINT, name);
+    obj = m->FindObject(mjOBJ_JOINT, name);
     if (!obj) {
       throw mjCError(this,
                      "joint '%s' not found in tendon %d, wrap %d",
@@ -4555,7 +4663,7 @@ void mjCWrap::Compile(void) {
 
   case mjWRAP_SPHERE:                         // geom (cylinder type set here)
     // find geom by name
-    obj = model->FindObject(mjOBJ_GEOM, name);
+    obj = m->FindObject(mjOBJ_GEOM, name);
     if (!obj) {
       throw mjCError(this,
                      "geom '%s' not found in tendon %d, wrap %d",
@@ -4574,7 +4682,7 @@ void mjCWrap::Compile(void) {
     // process side site
     if (!sidesite.empty()) {
       // find site by name
-      pside = model->FindObject(mjOBJ_SITE, sidesite);
+      pside = m->FindObject(mjOBJ_SITE, sidesite);
       if (!pside) {
         throw mjCError(this,
                        "side site '%s' not found in tendon %d, wrap %d",
@@ -4598,7 +4706,7 @@ void mjCWrap::Compile(void) {
 
   case mjWRAP_SITE:                           // site
     // find site by name
-    obj = model->FindObject(mjOBJ_SITE, name);
+    obj = m->FindObject(mjOBJ_SITE, name);
     if (!obj) {
       throw mjCError(this, "site '%s' not found in wrap %d", name.c_str(), id);
     }
@@ -4618,6 +4726,7 @@ mjCActuator::mjCActuator(mjCModel* _model, mjCDef* _def) {
   mjm_defaultActuator(spec);
 
   // clear private variables
+  ptarget = nullptr;
   spec_target_.clear();
   spec_slidersite_.clear();
   spec_refsite_.clear();
@@ -4653,6 +4762,7 @@ mjCActuator& mjCActuator::operator=(const mjCActuator& other) {
     this->spec = other.spec;
     *static_cast<mjCActuator_*>(this) = static_cast<const mjCActuator_&>(other);
     *static_cast<mjmActuator*>(this) = static_cast<const mjmActuator&>(other);
+    ptarget = nullptr;
   }
   PointToLocal();
   return *this;
@@ -4681,6 +4791,17 @@ void mjCActuator::PointToLocal() {
 
 
 
+void mjCActuator::NameSpace(const mjCModel* m) {
+  if (!name.empty()) {
+    name = m->prefix + name + m->suffix;
+  }
+  spec_target_ = m->prefix + spec_target_ + m->suffix;
+  spec_refsite_ = m->prefix + spec_refsite_ + m->suffix;
+  spec_slidersite_ = m->prefix + spec_slidersite_ + m->suffix;
+}
+
+
+
 void mjCActuator::CopyFromSpec() {
   *static_cast<mjmActuator*>(this) = spec;
   userdata_ = spec_userdata_;
@@ -4699,31 +4820,13 @@ void mjCActuator::CopyFromSpec() {
 
 
 
-// compiler
-void mjCActuator::Compile(void) {
-  CopyFromSpec();
+void mjCActuator::ResolveReferences(const mjCModel* m) {
   mjCJoint* pjnt;
-
-  // resize userdata
-  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);
-
-  // check for missing target name
-  if (target_.empty()) {
-    throw mjCError(this,
-                   "missing transmission target for actuator '%s' (id = %d)", name.c_str(), id);
-  }
-
-  // find transmission target in object arrays
-  mjCBase* ptarget = 0;
   switch (trntype) {
   case mjTRN_JOINT:
   case mjTRN_JOINTINPARENT:
     // get joint
-    ptarget = model->FindObject(mjOBJ_JOINT, target_);
+    ptarget = m->FindObject(mjOBJ_JOINT, target_);
     if (!ptarget) {
       throw mjCError(this,
                      "unknown transmission target '%s' for actuator id = %d", target_.c_str(), id);
@@ -4743,7 +4846,7 @@ void mjCActuator::Compile(void) {
     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 = m->FindObject(mjOBJ_SITE, slidersite_);
     if (!ptarget) {
       throw mjCError(this, "base site '%s' not found for actuator %d", slidersite_.c_str(), id);
     }
@@ -4756,18 +4859,18 @@ void mjCActuator::Compile(void) {
     }
 
     // proceed with regular target
-    ptarget = model->FindObject(mjOBJ_SITE, target_);
+    ptarget = m->FindObject(mjOBJ_SITE, target_);
     break;
 
   case mjTRN_TENDON:
     // get tendon
-    ptarget = model->FindObject(mjOBJ_TENDON, target_);
+    ptarget = m->FindObject(mjOBJ_TENDON, target_);
     break;
 
   case mjTRN_SITE:
     // get refsite, copy into trnid[1]
     if (!refsite_.empty()) {
-      ptarget = model->FindObject(mjOBJ_SITE, refsite_);
+      ptarget = m->FindObject(mjOBJ_SITE, refsite_);
       if (!ptarget) {
         throw mjCError(this, "reference site '%s' not found for actuator %d", refsite_.c_str(), id);
       }
@@ -4775,12 +4878,12 @@ void mjCActuator::Compile(void) {
     }
 
     // proceed with regular site target
-    ptarget = model->FindObject(mjOBJ_SITE, target_);
+    ptarget = m->FindObject(mjOBJ_SITE, target_);
     break;
 
   case mjTRN_BODY:
     // get body
-    ptarget = model->FindObject(mjOBJ_BODY, target_);
+    ptarget = m->FindObject(mjOBJ_BODY, target_);
     break;
 
   default:
@@ -4793,6 +4896,29 @@ void mjCActuator::Compile(void) {
   } else {
     trnid[0] = ptarget->id;
   }
+}
+
+
+
+// compiler
+void mjCActuator::Compile(void) {
+  CopyFromSpec();
+
+  // resize userdata
+  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);
+
+  // check for missing target name
+  if (target_.empty()) {
+    throw mjCError(this,
+                   "missing transmission target for actuator '%s' (id = %d)", name.c_str(), id);
+  }
+
+  // find transmission target in object arrays
+  ResolveReferences(model);
 
   // handle inheritrange
   if (gaintype == mjGAIN_FIXED && biastype == mjBIAS_AFFINE &&
@@ -4812,7 +4938,7 @@ void mjCActuator::Compile(void) {
 
     const double* target_range;
     if (trntype == mjTRN_JOINT) {
-      pjnt = (mjCJoint*) ptarget;
+      mjCJoint* pjnt = (mjCJoint*) ptarget;
       if (pjnt->spec.type != mjJNT_HINGE && pjnt->spec.type != mjJNT_SLIDE) {
         throw mjCError(this, "inheritrange can only be used with hinge and slide joints, "
                        "actuator '%s' (id = %d)", name.c_str(), id);
@@ -4950,6 +5076,7 @@ mjCSensor::mjCSensor(mjCModel* _model) {
   spec_refname_.clear();
   spec_userdata_.clear();
   obj = nullptr;
+  ref = nullptr;
   refid = -1;
 
   // in case this sensor is not compiled
@@ -4972,6 +5099,8 @@ mjCSensor& mjCSensor::operator=(const mjCSensor& other) {
     this->spec = other.spec;
     *static_cast<mjCSensor_*>(this) = static_cast<const mjCSensor_&>(other);
     *static_cast<mjmSensor*>(this) = static_cast<const mjmSensor&>(other);
+    obj = nullptr;
+    ref = nullptr;
   }
   PointToLocal();
   return *this;
@@ -4993,6 +5122,16 @@ void mjCSensor::PointToLocal() {
 
 
 
+void mjCSensor::NameSpace(const mjCModel* m) {
+  if (!name.empty()) {
+    name = m->prefix + name + m->suffix;
+  }
+  spec_objname_ = m->prefix + spec_objname_ + m->suffix;
+  spec_refname_ = m->prefix + spec_refname_ + m->suffix;
+}
+
+
+
 void mjCSensor::CopyFromSpec() {
   *static_cast<mjmSensor*>(this) = spec;
   userdata_ = spec_userdata_;
@@ -5009,27 +5148,7 @@ void mjCSensor::CopyFromSpec() {
 
 
 
-// compiler
-void mjCSensor::Compile(void) {
-  CopyFromSpec();
-
-  // resize userdata
-  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);
-
-  // require non-negative noise
-  if (noise<0) {
-    throw mjCError(this, "negative noise in sensor '%s' (id = %d)", name.c_str(), id);
-  }
-
-  // require non-negative cutoff
-  if (cutoff<0) {
-    throw mjCError(this, "negative cutoff in sensor '%s' (id = %d)", name.c_str(), id);
-  }
-
+void mjCSensor::ResolveReferences(const mjCModel* m) {
   // get objid from objtype and objname
   if (objtype!=mjOBJ_UNKNOWN) {
     // check for missing object name
@@ -5040,7 +5159,7 @@ void mjCSensor::Compile(void) {
     }
 
     // find name
-    obj = model->FindObject(objtype, objname_);
+    obj = m->FindObject(objtype, objname_);
     if (!obj) {
       throw mjCError(this,
                      "unrecognized name of sensorized object in sensor '%s' (id = %d)",
@@ -5067,8 +5186,8 @@ void mjCSensor::Compile(void) {
     }
 
     // find name
-    mjCBase* pref = model->FindObject(reftype, refname_);
-    if (!pref) {
+    ref = m->FindObject(reftype, refname_);
+    if (!ref) {
       throw mjCError(this,
                      "unrecognized name of reference frame object in sensor '%s' (id = %d)",
                      name.c_str(), id);
@@ -5083,8 +5202,35 @@ void mjCSensor::Compile(void) {
     }
 
     // get sensorized object id
-    refid = pref->id;
+    refid = ref->id;
   }
+}
+
+
+
+// compiler
+void mjCSensor::Compile(void) {
+  CopyFromSpec();
+
+  // resize userdata
+  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);
+
+  // require non-negative noise
+  if (noise<0) {
+    throw mjCError(this, "negative noise in sensor '%s' (id = %d)", name.c_str(), id);
+  }
+
+  // require non-negative cutoff
+  if (cutoff<0) {
+    throw mjCError(this, "negative cutoff in sensor '%s' (id = %d)", name.c_str(), id);
+  }
+
+  // Find referenced object
+  ResolveReferences(model);
 
   // process according to sensor type
   switch (type) {
@@ -5126,7 +5272,7 @@ 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*)ref;
       if (!camref->resolution[0] || !camref->resolution[1]) {
         throw mjCError(this,
                        "camera projection sensor requires camera resolution '%s' (id = %d)",
@@ -5619,6 +5765,17 @@ void mjCTuple::PointToLocal() {
 
 
 
+void mjCTuple::NameSpace(const mjCModel* m) {
+  if (!name.empty()) {
+    name = m->prefix + name + m->suffix;
+  }
+  for (int i=0; i<spec_objname_.size(); i++) {
+    spec_objname_[i] = m->prefix + spec_objname_[i] + m->suffix;
+  }
+}
+
+
+
 void mjCTuple::CopyFromSpec() {
   *static_cast<mjmTuple*>(this) = spec;
   objtype_ = spec_objtype_;
@@ -5644,10 +5801,7 @@ mjCTuple::~mjCTuple() {
 
 
 
-// compiler
-void mjCTuple::Compile(void) {
-  CopyFromSpec();
-
+void mjCTuple::ResolveReferences(const mjCModel* m) {
   // check for empty tuple
   if (objtype_.empty()) {
     throw mjCError(this, "tuple '%s' (id = %d) is empty", name.c_str(), id);
@@ -5665,7 +5819,7 @@ void mjCTuple::Compile(void) {
   // find objects, fill in ids
   for (int i=0; i<objtype_.size(); i++) {
     // find object by type and name
-    mjCBase* res = model->FindObject(objtype_[i], objname_[i]);
+    mjCBase* res = m->FindObject(objtype_[i], objname_[i]);
     if (!res) {
       throw mjCError(this, "unrecognized object '%s' in tuple %d", objname_[i].c_str(), id);
     }
@@ -5682,6 +5836,14 @@ void mjCTuple::Compile(void) {
 
 
 
+// compiler
+void mjCTuple::Compile(void) {
+  CopyFromSpec();
+  ResolveReferences(model);
+}
+
+
+
 //------------------ class mjCKey implementation ---------------------------------------------------
 
 // constructor
diff --git a/src/user/user_objects.h b/src/user/user_objects.h
index 50ae9f2e..1a4cd0c6 100644
--- a/src/user/user_objects.h
+++ b/src/user/user_objects.h
@@ -198,6 +198,12 @@ class mjCBase : public mjCBase_ {
   // Copy spec into private attributes
   virtual void CopyFromSpec() {}
 
+  // Throws an error if any of the references is missing
+  virtual void ResolveReferences(const mjCModel* m) {}
+
+  // Appends prefix and suffix to reference
+  virtual void NameSpace(const mjCModel* m) {}
+
   // Copy assignment
   mjCBase& operator=(const mjCBase& other);
 
@@ -284,7 +290,7 @@ class mjCBody : public mjCBody_, private mjmBody {
   mjCBase* FindObject(mjtObj type, std::string name, bool recursive = true);
 
   // Propagate suffix and prefix to the whole tree
-  void SetNameSpace();
+  void NameSpace(const mjCModel* m);
 
   // set explicitinertial to true
   void MakeInertialExplicit();
@@ -682,6 +688,8 @@ class mjCFlex: public mjCFlex_, private mjmFlex {
 
   void CopyFromSpec(void);
   void PointToLocal(void);
+  void ResolveReferences(const mjCModel* m);
+  void NameSpace(const mjCModel* m);
 
   // used by mjXWriter and mjCModel
   const std::string& get_material() { return material_; }
@@ -937,6 +945,8 @@ class mjCSkin: public mjCSkin_, private mjmSkin {
   mjCSkin& operator=(const mjCSkin& other);  // copy assignment
   ~mjCSkin();                                 // destructor
 
+  void ResolveReferences(const mjCModel* m);
+  void NameSpace(const mjCModel* m);
   void Compile(const mjVFS* vfs);             // compiler
   void LoadSKN(mjResource* resource);         // load skin in SKN BIN format
 };
@@ -1115,6 +1125,8 @@ class mjCPair : public mjCPair_, private mjmPair {
 
   void CopyFromSpec();
   void PointToLocal();
+  void ResolveReferences(const mjCModel* m);
+  void NameSpace(const mjCModel* m);
 
   std::string get_geomname1() { return geomname1_; }
   std::string get_geomname2() { return geomname2_; }
@@ -1162,6 +1174,8 @@ class mjCBodyPair : public mjCBodyPair_, private mjmExclude {
 
   void CopyFromSpec();
   void PointToLocal();
+  void ResolveReferences(const mjCModel* m);
+  void NameSpace(const mjCModel* m);
 
   std::string get_bodyname1() const { return bodyname1_; }
   std::string get_bodyname2() const { return bodyname2_; }
@@ -1207,6 +1221,8 @@ class mjCEquality : public mjCEquality_, private mjmEquality {
 
   void CopyFromSpec();
   void PointToLocal();
+  void ResolveReferences(const mjCModel* m);
+  void NameSpace(const mjCModel* m);
 
  private:
   mjCEquality(mjCModel* = 0, mjCDef* = 0);           // constructor
@@ -1264,6 +1280,8 @@ class mjCTendon : public mjCTendon_, private mjmTendon {
 
   void CopyFromSpec();
   void PointToLocal();
+  void ResolveReferences(const mjCModel* m);
+  void NameSpace(const mjCModel* m);
   void SetModel(mjCModel* _model);
 
   bool is_limited() const;
@@ -1299,6 +1317,9 @@ class mjCWrap : public mjCWrap_, private mjmWrap {
   using mjCBase::info;
 
   void PointToLocal();
+  void ResolveReferences(const mjCModel* m);
+  void NameSpace(const mjCModel* m);
+
   mjCBase* obj;                   // wrap object pointer
 
  private:
@@ -1306,8 +1327,6 @@ class mjCWrap : public mjCWrap_, private mjmWrap {
   mjCWrap(const mjCWrap& other);             // copy constructor
   mjCWrap& operator=(const mjCWrap& other);  // copy assignment
 
-  void Compile(void);                 // compiler
-
   mjCTendon* tendon;              // tendon owning this wrap
 };
 
@@ -1393,6 +1412,10 @@ class mjCActuator : public mjCActuator_, private mjmActuator {
   void Compile(void);                       // compiler
   void CopyFromSpec();
   void PointToLocal();
+  void ResolveReferences(const mjCModel* m);
+  void NameSpace(const mjCModel* m);
+
+  mjCBase* ptarget;  // transmission target
 };
 
 
@@ -1403,7 +1426,6 @@ class mjCActuator : public mjCActuator_, private mjmActuator {
 class mjCSensor_ : public mjCBase {
  protected:
   int refid;                      // id of reference frame
-  mjCBase* obj;                   // sensorized object
 
   // variable-size data
   std::string plugin_name;
@@ -1440,6 +1462,11 @@ class mjCSensor : public mjCSensor_, private mjmSensor {
   void Compile(void);             // compiler
   void CopyFromSpec();
   void PointToLocal();
+  void ResolveReferences(const mjCModel* m);
+  void NameSpace(const mjCModel* m);
+
+  mjCBase* obj;                   // sensorized object
+  mjCBase* ref;                   // sensorized reference
 };
 
 
@@ -1533,6 +1560,8 @@ class mjCTuple : public mjCTuple_, private mjmTuple {
 
   void PointToLocal();
   void CopyFromSpec();
+  void ResolveReferences(const mjCModel* m);
+  void NameSpace(const mjCModel* m);
 
  private:
   mjCTuple(mjCModel*);                         // constructor
diff --git a/test/user/user_api_test.cc b/test/user/user_api_test.cc
index d6aa0f87..e0a83d5f 100644
--- a/test/user/user_api_test.cc
+++ b/test/user/user_api_test.cc
@@ -215,13 +215,27 @@ TEST_F(MujocoTest, Attach) {
   static constexpr char xml_child[] = R"(
   <mujoco>
     <worldbody>
-      <body name="cylinder">
-        <joint type="hinge"/>
+      <body name="body">
+        <joint type="hinge" name="hinge"/>
         <geom type="cylinder" size=".1 1 0"/>
           <body name="named"/>
           <body/>
       </body>
+      <body name="discard"/>
     </worldbody>
+    <sensor>
+      <framepos name="keep" objtype="body" objname="body"/>
+      <framepos name="discard" objtype="body" objname="discard"/>
+    </sensor>
+    <tendon>
+      <fixed name="fixed">
+        <joint joint="hinge" coef="2"/>
+      </fixed>
+    </tendon>
+    <actuator>
+      <position joint="hinge"/>
+      <position tendon="fixed"/>
+    </actuator>
   </mujoco>)";
 
   static constexpr char xml_result[] = R"(
@@ -231,8 +245,8 @@ TEST_F(MujocoTest, Attach) {
         <freejoint/>
         <geom size=".1"/>
         <frame name="frame" pos=".1 0 0" euler="0 90 0">
-          <body name="attached-cylinder-1">
-            <joint type="hinge"/>
+          <body name="attached-body-1">
+            <joint type="hinge" name="attached-hinge-1"/>
             <geom type="cylinder" size=".1 1 0"/>
               <body name="attached-named-1"/>
               <body/>
@@ -240,6 +254,18 @@ TEST_F(MujocoTest, Attach) {
         </frame>
       </body>
     </worldbody>
+    <sensor>
+      <framepos name="attached-keep-1" objtype="body" objname="attached-body-1"/>
+    </sensor>
+    <tendon>
+      <fixed name="attached-fixed-1">
+        <joint joint="attached-hinge-1" coef="2"/>
+      </fixed>
+    </tendon>
+    <actuator>
+      <position joint="attached-hinge-1"/>
+      <position tendon="attached-fixed-1"/>
+    </actuator>
   </mujoco>)";
 
   // model with one free sphere and a frame
@@ -255,7 +281,7 @@ TEST_F(MujocoTest, Attach) {
   EXPECT_THAT(child, NotNull()) << er.data();
 
   // get subtree
-  mjmBody* body = mjm_findBody(child, "cylinder");
+  mjmBody* body = mjm_findBody(child, "body");
   EXPECT_THAT(body, NotNull());
 
   // attach child to parent frame
@@ -267,7 +293,7 @@ TEST_F(MujocoTest, Attach) {
   EXPECT_THAT(m_attached, NotNull());
 
   // check full name stored in mjModel
-  EXPECT_STREQ(mj_id2name(m_attached, mjOBJ_BODY, 2), "attached-cylinder-1");
+  EXPECT_STREQ(mj_id2name(m_attached, mjOBJ_BODY, 2), "attached-body-1");
 
   // check body 2 is attached to body 1
   EXPECT_THAT(m_attached->body_parentid[2], 1);