Fix explicit MJCF default class scope (#3687)

han-xudong · web-flow · commit ee84505ae44f · 2026-08-01T18:25:32.000Z
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -139,6 +139,7 @@
 - Fix `SolverFeatherstone` BALL joints to apply passive `joint_damping` on all three angular DOFs.
 - Fix `eval_ik()` and `SolverSemiImplicit` rounding small float32 revolute-joint angles to zero. (#3434)
 - Fix excessive memory usage when importing MJCF or URDF models containing many visual-only shapes with self-collisions disabled.
+- Prevent explicit MJCF classes from retaining unrelated `childclass` defaults.
 - Fix `FastKitchenG1` ASV metrics to build the kitchen scene instead of a plain G1 model.
 - Fix the `diffsim_bear` example crashing with its default CUDA configuration and diverging after a few training iterations.
 - Fix masked PID state reset to execute on the integral-state device. (#3447)
diff --git a/newton/_src/utils/import_mjcf.py b/newton/_src/utils/import_mjcf.py
@@ -513,9 +513,9 @@ def is_ignored_class(name: str) -> bool:
         return any(re.match(pattern, name) for pattern in ignore_classes)
 
     def resolve_class_defaults(element_class: str | None, ambient_defaults: dict) -> dict:
-        """Merge an element's default class (pre-resolved by resolve_defaults) over the ambient defaults."""
+        """Resolve an explicit default class or retain the ambient defaults."""
         if element_class is not None and element_class in class_defaults:
-            return merge_attrib(ambient_defaults, class_defaults[element_class])
+            return class_defaults[element_class]
         return ambient_defaults
 
     def resolve_element_attrib(element, tag: str, ambient_defaults: dict | None = None) -> dict:
diff --git a/newton/tests/test_import_mjcf.py b/newton/tests/test_import_mjcf.py
@@ -4816,6 +4816,33 @@ def test_default_inheritance(self):
         else:
             self.fail("Model should have mujoco.condim attribute")
 
+    def test_explicit_class_replaces_childclass(self):
+        """Use an explicit class without retaining sibling childclass defaults."""
+        mjcf = """
+<mujoco>
+    <default>
+        <default class="ambient">
+            <geom friction="0.7 0.1 0.01"/>
+        </default>
+        <default class="explicit">
+            <geom size="0.2"/>
+        </default>
+    </default>
+    <worldbody>
+        <body childclass="ambient">
+            <geom class="explicit"/>
+        </body>
+    </worldbody>
+</mujoco>
+"""
+        builder = newton.ModelBuilder()
+        builder.add_mjcf(mjcf)
+
+        self.assertAlmostEqual(builder.shape_scale[0][0], 0.2)
+        self.assertAlmostEqual(builder.shape_material_mu[0], 1.0)
+        self.assertAlmostEqual(builder.shape_material_mu_torsional[0], 0.005)
+        self.assertAlmostEqual(builder.shape_material_mu_rolling[0], 0.0001)
+
 
 class TestImportMjcfActuatorsFrames(unittest.TestCase):
     def test_actuatorfrcrange_parsing(self):
