diff --git a/src/user/user_objects.cc b/src/user/user_objects.cc
index 066b7159..4c084b22 100644
--- a/src/user/user_objects.cc
+++ b/src/user/user_objects.cc
@@ -845,6 +845,12 @@ mjCBody& mjCBody::operator+=(const mjCBody& other) {
 
 // attach frame to body
 mjCBody& mjCBody::operator+=(const mjCFrame& other) {
+  // append a copy of the attached spec
+  if (other.model != model && !model->FindSpec(mjs_getString(other.model->spec.modelname))) {
+    model->AppendSpec(mj_copySpec(&other.model->spec));
+  }
+
+  // create a copy of the subtree that contains the frame
   mjCBody* subtree = other.body;
   other.model->prefix = other.prefix;
   other.model->suffix = other.suffix;
@@ -1805,6 +1811,12 @@ mjCFrame& mjCFrame::operator=(const mjCFrame& other) {
 
 // attach body to frame
 mjCFrame& mjCFrame::operator+=(const mjCBody& other) {
+  // append a copy of the attached spec
+  if (other.model != model && !model->FindSpec(mjs_getString(other.model->spec.modelname))) {
+    model->AppendSpec(mj_copySpec(&other.model->spec));
+  }
+
+  // apply namespace and store keyframes in the source model
   other.model->prefix = other.prefix;
   other.model->suffix = other.suffix;
   other.model->StoreKeyframes(model);
diff --git a/test/user/user_api_test.cc b/test/user/user_api_test.cc
index 23521001..7922d1d8 100644
--- a/test/user/user_api_test.cc
+++ b/test/user/user_api_test.cc
@@ -624,7 +624,7 @@ TEST_F(PluginTest, TextureFromBuffer) {
 // -------------------------------- test attach --------------------------------
 
 static constexpr char xml_child[] = R"(
-  <mujoco>
+  <mujoco model="child">
     <default>
       <default class="cylinder">
         <geom type="cylinder" size=".1 1 0"/>
@@ -691,7 +691,7 @@ TEST_F(MujocoTest, AttachSame) {
   std::string field = "";
 
   static constexpr char xml_result[] = R"(
-  <mujoco>
+  <mujoco model="child">
     <default>
       <default class="cylinder">
         <geom type="cylinder" size=".1 1 0"/>
@@ -781,6 +781,9 @@ TEST_F(MujocoTest, AttachSame) {
   mjsBody* attached = mjs_attachBody(frame, body, "attached-", "-1");
   EXPECT_THAT(attached, mjs_findBody(parent, "attached-body-1"));
 
+  // check that the spec was not copied
+  EXPECT_THAT(mjs_findSpec(parent, "child"), IsNull());
+
   // compile new model
   mjModel* m_attached = mj_compile(parent, 0);
   EXPECT_THAT(m_attached, NotNull());
@@ -912,6 +915,9 @@ TEST_F(MujocoTest, AttachDifferent) {
   mjsBody* attached = mjs_attachBody(frame, body, "attached-", "-1");
   EXPECT_THAT(attached, mjs_findBody(parent, "attached-body-1"));
 
+  // check that the spec was copied
+  EXPECT_THAT(mjs_findSpec(parent, "child"), NotNull());
+
   // compile new model
   mjModel* m_attached = mj_compile(parent, 0);
   EXPECT_THAT(m_attached, NotNull());
@@ -1043,6 +1049,9 @@ TEST_F(MujocoTest, AttachFrame) {
   mjsFrame* attached = mjs_attachFrame(body, frame, "attached-", "-1");
   EXPECT_THAT(attached, mjs_findFrame(parent, "attached-pframe-1"));
 
+  // check that the spec was copied
+  EXPECT_THAT(mjs_findSpec(parent, "child"), NotNull());
+
   // compile new model
   mjModel* m_attached = mj_compile(parent, 0);
   EXPECT_THAT(m_attached, NotNull());
@@ -1073,7 +1082,7 @@ void TestDetachBody(bool compile) {
   std::string field = "";
 
   static constexpr char xml_result[] = R"(
-  <mujoco>
+  <mujoco model="child">
     <asset>
       <texture name="texture" type="2d" builtin="checker" width="32" height="32"/>
       <material name="material" texture="texture" texrepeat="1 1" texuniform="true"/>