Fix put_data crash when nefc=0 for dense models (#1174)

kevinzakka · web-flow · commit 42a7c5622737 · 2026-02-22T13:26:36.000-08:00
Tendons with frictionloss cause MuJoCo to pre-allocate efc_J with
size nv even when nefc=0, causing reshape((0, nv)) to fail with
ValueError. Short-circuit to np.zeros((0, nv)) before the
sparse/dense branch.
diff --git a/mujoco_warp/_src/io.py b/mujoco_warp/_src/io.py
@@ -889,7 +889,9 @@ def put_data(
 
   efc = types.Constraint(**efc_kwargs)
 
-  if mujoco.mj_isSparse(mjm):
+  if mjd.nefc == 0:
+    efc_j = np.zeros((0, mjm.nv))
+  elif mujoco.mj_isSparse(mjm):
     efc_j = np.zeros((mjd.nefc, mjm.nv))
     mujoco.mju_sparse2dense(efc_j, mjd.efc_J, mjd.efc_J_rownnz, mjd.efc_J_rowadr, mjd.efc_J_colind)
   else:
diff --git a/mujoco_warp/_src/io_test.py b/mujoco_warp/_src/io_test.py
@@ -1350,6 +1350,44 @@ def test_render_context_with_textures(self):
     self.assertTrue(rc.use_textures, "use_textures")
     self.assertEqual(rc.textures.shape, (mjm.ntex,), "textures")
 
+  def test_put_data_nefc_zero_dense(self):
+    """put_data succeeds for dense models with nefc=0 and non-empty efc_J."""
+    # A tendon with frictionloss causes MuJoCo to pre-allocate efc_J with
+    # size nv even when nefc=0, causing reshape((0, nv)) to fail.
+    mjm = mujoco.MjModel.from_xml_string("""
+      <mujoco>
+        <worldbody>
+          <body pos="0 0 1">
+            <freejoint/>
+            <geom type="box" size="0.1 0.1 0.1" mass="1.0"/>
+            <site name="s1" pos="0 0 0.1"/>
+            <body pos="0.3 0 0">
+              <joint type="hinge" axis="0 0 1"/>
+              <geom type="sphere" size="0.05" mass="0.2"/>
+              <site name="s2" pos="0 0 -0.05"/>
+            </body>
+          </body>
+        </worldbody>
+        <tendon>
+          <spatial limited="true" range="0 0.5"
+            damping="2.0" stiffness="10.0" frictionloss="0.5">
+            <site site="s1"/>
+            <site site="s2"/>
+          </spatial>
+        </tendon>
+      </mujoco>
+    """)
+    mjd = mujoco.MjData(mjm)
+    mujoco.mj_forward(mjm, mjd)
+
+    self.assertFalse(mujoco.mj_isSparse(mjm))
+    self.assertEqual(mjd.nefc, 0)
+
+    m = mjwarp.put_model(mjm)
+    d = mjwarp.put_data(mjm, mjd)
+
+    self.assertEqual(d.efc.J.shape[2], m.nv_pad)
+
 
 if __name__ == "__main__":
   wp.init()
