diff --git a/doc/changelog.rst b/doc/changelog.rst
index ab19efef..67a6748e 100644
--- a/doc/changelog.rst
+++ b/doc/changelog.rst
@@ -2,19 +2,6 @@
 Changelog
 =========
 
-Upcoming version (not yet released)
------------------------------------
-
-MJX
-^^^
-- Promote ``ten_length`` to the public MJX API. Add Warp support for ``mjx.tendon``.
-
-.. admonition:: Breaking API changes
-   :class: attention
-
-   - ``ten_length`` was moved from ``mjx.Data._impl.ten_length`` to a public field ``mjx.Data.ten_length``.
-
-
 Version 3.3.5 (August 8, 2025)
 -----------------------------------
 
diff --git a/mjx/mujoco/mjx/_src/constraint.py b/mjx/mujoco/mjx/_src/constraint.py
index 042d2b72..410783f6 100644
--- a/mjx/mujoco/mjx/_src/constraint.py
+++ b/mjx/mujoco/mjx/_src/constraint.py
@@ -322,8 +322,8 @@ def _efc_equality_tendon(m: Model, d: Data) -> Optional[_Efc]:
 
   inv1, inv2 = m.tendon_invweight0[obj1id], m.tendon_invweight0[obj2id]
   jac1, jac2 = d._impl.ten_J[obj1id], d._impl.ten_J[obj2id]
-  pos1 = d.ten_length[obj1id] - m.tendon_length0[obj1id]
-  pos2 = d.ten_length[obj2id] - m.tendon_length0[obj2id]
+  pos1 = d._impl.ten_length[obj1id] - m.tendon_length0[obj1id]
+  pos2 = d._impl.ten_length[obj2id] - m.tendon_length0[obj2id]
   invweight = inv1 + inv2 * (obj2id > -1)
 
   return rows(
@@ -436,7 +436,7 @@ def _efc_limit_tendon(m: Model, d: Data) -> Optional[_Efc]:
   length, j, range_, margin, invweight, solref, solimp = jax.tree_util.tree_map(
       lambda x: x[tendon_id],
       (
-          d.ten_length,
+          d._impl.ten_length,
           d._impl.ten_J,
           m.tendon_range,
           m.tendon_margin,
diff --git a/mjx/mujoco/mjx/_src/io.py b/mjx/mujoco/mjx/_src/io.py
index d42333d0..c3600c2c 100644
--- a/mjx/mujoco/mjx/_src/io.py
+++ b/mjx/mujoco/mjx/_src/io.py
@@ -576,7 +576,6 @@ def _make_data_public_fields(m: types.Model) -> Dict[str, Any]:
       'qfrc_constraint': (m.nv, float_),
       'qfrc_inverse': (m.nv, float_),
       'cvel': (m.nbody, 6, float_),
-      'ten_length': (m.ntendon, float_),
   }
   zero_fields = {
       k: np.zeros(v[:-1], dtype=v[-1]) for k, v in zero_fields.items()
@@ -638,6 +637,7 @@ def _make_data_jax(
       'ten_wrapadr': (m.ntendon, np.int32),
       'ten_wrapnum': (m.ntendon, np.int32),
       'ten_J': (m.ntendon, m.nv, float_),
+      'ten_length': (m.ntendon, float_),
       'wrap_obj': (m.nwrap, 2, np.int32),
       'wrap_xpos': (m.nwrap, 6, float_),
       'actuator_length': (m.nu, float_),
@@ -742,6 +742,7 @@ def _make_data_c(
       'ten_J_rowadr': (m.ntendon, np.int32),
       'ten_J_colind': (m.ntendon, m.nv, np.int32),
       'ten_J': (m.ntendon, m.nv, float_),
+      'ten_length': (m.ntendon, float_),
       'ten_wrapadr': (m.ntendon, np.int32),
       'ten_wrapnum': (m.ntendon, np.int32),
       'wrap_obj': (m.nwrap, 2, np.int32),
diff --git a/mjx/mujoco/mjx/_src/io_test.py b/mjx/mujoco/mjx/_src/io_test.py
index 8c0954b3..dba9c429 100644
--- a/mjx/mujoco/mjx/_src/io_test.py
+++ b/mjx/mujoco/mjx/_src/io_test.py
@@ -478,7 +478,7 @@ class DataIOTest(parameterized.TestCase):
     np.testing.assert_allclose(dx.site_xmat.reshape((1, 9)), d.site_xmat)
 
     # tendon data is correct
-    np.testing.assert_allclose(dx.ten_length, d.ten_length)
+    np.testing.assert_allclose(dx._impl.ten_length, d.ten_length)
     np.testing.assert_equal(dx._impl.ten_wrapadr, np.zeros((1,)))
     np.testing.assert_equal(dx._impl.ten_wrapnum, np.zeros((1,)))
     np.testing.assert_equal(dx._impl.wrap_obj, np.zeros((2, 2)))
diff --git a/mjx/mujoco/mjx/_src/passive.py b/mjx/mujoco/mjx/_src/passive.py
index 156ef4e9..f7acdcd2 100644
--- a/mjx/mujoco/mjx/_src/passive.py
+++ b/mjx/mujoco/mjx/_src/passive.py
@@ -75,7 +75,7 @@ def _spring_damper(m: Model, d: Data) -> jax.Array:
   qfrc -= m.dof_damping * d.qvel
 
   # tendon-level spring-dampers
-  below, above = m.tendon_lengthspring.T - d.ten_length
+  below, above = m.tendon_lengthspring.T - d._impl.ten_length
   frc_spring = jp.where(below > 0, m.tendon_stiffness * below, 0)
   frc_spring = jp.where(above < 0, m.tendon_stiffness * above, frc_spring)
   frc_damper = -m.tendon_damping * d._impl.ten_velocity
diff --git a/mjx/mujoco/mjx/_src/sensor.py b/mjx/mujoco/mjx/_src/sensor.py
index 7d7fc6ed..31f7f030 100644
--- a/mjx/mujoco/mjx/_src/sensor.py
+++ b/mjx/mujoco/mjx/_src/sensor.py
@@ -22,6 +22,7 @@ from mujoco.mjx._src import math
 from mujoco.mjx._src import ray
 from mujoco.mjx._src import smooth
 from mujoco.mjx._src import support
+from mujoco.mjx._src.types import Impl
 from mujoco.mjx._src.types import Data
 from mujoco.mjx._src.types import DataJAX
 from mujoco.mjx._src.types import DisableBit
@@ -173,7 +174,7 @@ def sensor_pos(m: Model, d: Data) -> Data:
     elif sensor_type == SensorType.JOINTPOS:
       sensor = d.qpos[m.jnt_qposadr[objid]]
     elif sensor_type == SensorType.TENDONPOS:
-      sensor = d.ten_length[objid]
+      sensor = d._impl.ten_length[objid]
     elif sensor_type == SensorType.ACTUATORPOS:
       sensor = d._impl.actuator_length[objid]
     elif sensor_type == SensorType.BALLQUAT:
diff --git a/mjx/mujoco/mjx/_src/smooth.py b/mjx/mujoco/mjx/_src/smooth.py
index 1303f357..8f19f6f1 100644
--- a/mjx/mujoco/mjx/_src/smooth.py
+++ b/mjx/mujoco/mjx/_src/smooth.py
@@ -26,7 +26,6 @@ from mujoco.mjx._src.types import Data
 from mujoco.mjx._src.types import DataJAX
 from mujoco.mjx._src.types import DisableBit
 from mujoco.mjx._src.types import EqType
-from mujoco.mjx._src.types import Impl
 from mujoco.mjx._src.types import JointType
 from mujoco.mjx._src.types import Model
 from mujoco.mjx._src.types import ModelJAX
@@ -34,16 +33,11 @@ from mujoco.mjx._src.types import ObjType
 from mujoco.mjx._src.types import TrnType
 from mujoco.mjx._src.types import WrapType
 # pylint: enable=g-importing-member
-import mujoco.mjx.warp as mjxw
 import numpy as np
 
 
 def kinematics(m: Model, d: Data) -> Data:
   """Converts position/velocity from generalized coordinates to maximal."""
-  if m.impl == Impl.WARP and d.impl == Impl.WARP and mjxw.WARP_INSTALLED:
-    from mujoco.mjx.warp import smooth as mjxw_smooth  # pylint: disable=g-import-not-at-top  # pytype: disable=import-error
-    return mjxw_smooth.kinematics(m, d)
-
   def fn(carry, jnt_typs, jnt_pos, jnt_axis, qpos, qpos0, pos, quat):
     # calculate joint anchors, axes, body pos and quat in global frame
     # also normalize qpos while we're at it
@@ -850,10 +844,6 @@ def rne_postconstraint(m: Model, d: Data) -> Data:
 
 def tendon(m: Model, d: Data) -> Data:
   """Computes tendon lengths and moments."""
-  if m.impl == Impl.WARP and d.impl == Impl.WARP and mjxw.WARP_INSTALLED:
-    from mujoco.mjx.warp import smooth as mjxw_smooth  # pylint: disable=g-import-not-at-top  # pytype: disable=import-error
-    return mjxw_smooth.tendon(m, d)
-
   if not isinstance(m._impl, ModelJAX) or not isinstance(d._impl, DataJAX):
     raise ValueError('tendon requires JAX backend implementation.')
 
@@ -1101,7 +1091,7 @@ def tendon(m: Model, d: Data) -> Data:
 
   # assemble length and moment
   ten_length = (
-      jp.zeros_like(d.ten_length).at[tendon_id_jnt].set(length_jnt)
+      jp.zeros_like(d._impl.ten_length).at[tendon_id_jnt].set(length_jnt)
   )
   ten_length = ten_length.at[tendon_id_site].add(length_site)
   ten_length = ten_length.at[tendon_id_geom].add(length_geom)
@@ -1171,7 +1161,7 @@ def tendon(m: Model, d: Data) -> Data:
   ).reshape((m.nwrap, 2))
 
   return d.tree_replace({
-      'ten_length': ten_length,
+      '_impl.ten_length': ten_length,
       '_impl.ten_J': ten_moment,
       '_impl.ten_wrapadr': jp.array(ten_wrapadr, dtype=int),
       '_impl.ten_wrapnum': jp.array(ten_wrapnum, dtype=int),
@@ -1273,7 +1263,7 @@ def transmission(m: Model, d: Data) -> Data:
       wrench = jp.concatenate((frame_xmat @ gear[:3], frame_xmat @ gear[3:]))
       moment = jac @ wrench
     elif trntype == TrnType.TENDON:
-      length = d.ten_length[trnid[0]] * gear[:1]
+      length = d._impl.ten_length[trnid[0]] * gear[:1]
       moment = d._impl.ten_J[trnid[0]] * gear[0]
     else:
       raise RuntimeError(f'unrecognized trntype: {TrnType(trntype)}')
diff --git a/mjx/mujoco/mjx/_src/smooth_test.py b/mjx/mujoco/mjx/_src/smooth_test.py
index 4f9621a4..061087f9 100644
--- a/mjx/mujoco/mjx/_src/smooth_test.py
+++ b/mjx/mujoco/mjx/_src/smooth_test.py
@@ -122,7 +122,7 @@ class SmoothTest(absltest.TestCase):
     # tendon
     dx = jax.jit(mjx.tendon)(mx, mjx.put_data(m, d))
     _assert_attr_eq(d, dx._impl, 'ten_J')
-    _assert_attr_eq(d, dx, 'ten_length')
+    _assert_attr_eq(d, dx._impl, 'ten_length')
     # transmission
     dx = jax.jit(mjx.transmission)(mx, dx)
     _assert_attr_eq(d, dx._impl, 'actuator_length')
@@ -394,7 +394,7 @@ class TendonTest(parameterized.TestCase):
     mujoco.mj_forward(m, d)
     dx = jax.jit(mjx.forward)(mx, dx)
 
-    _assert_eq(d.ten_length, dx.ten_length, 'ten_length')
+    _assert_eq(d.ten_length, dx._impl.ten_length, 'ten_length')
     _assert_eq(d.ten_J, dx._impl.ten_J, 'ten_J')
     _assert_eq(d.ten_wrapnum, dx._impl.ten_wrapnum, 'ten_wrapnum')
     _assert_eq(d.ten_wrapadr, dx._impl.ten_wrapadr, 'ten_wrapadr')
diff --git a/mjx/mujoco/mjx/_src/types.py b/mjx/mujoco/mjx/_src/types.py
index 0425477f..a71f934c 100644
--- a/mjx/mujoco/mjx/_src/types.py
+++ b/mjx/mujoco/mjx/_src/types.py
@@ -998,6 +998,7 @@ class DataC(PyTreeNode):
   ten_J_rowadr: jax.Array  # pylint:disable=invalid-name
   ten_J_colind: jax.Array  # pylint:disable=invalid-name
   ten_J: jax.Array  # pylint:disable=invalid-name
+  ten_length: jax.Array
   wrap_obj: jax.Array
   wrap_xpos: jax.Array
   actuator_length: jax.Array
@@ -1069,6 +1070,7 @@ class DataJAX(PyTreeNode):
   ten_wrapadr: jax.Array
   ten_wrapnum: jax.Array
   ten_J: jax.Array  # pylint:disable=invalid-name
+  ten_length: jax.Array
   wrap_obj: jax.Array
   wrap_xpos: jax.Array
   actuator_length: jax.Array
@@ -1131,7 +1133,6 @@ class Data(PyTreeNode):
   ximat: jax.Array
   xanchor: jax.Array
   xaxis: jax.Array
-  ten_length: jax.Array
   geom_xpos: jax.Array
   geom_xmat: jax.Array
   site_xpos: jax.Array
diff --git a/mjx/mujoco/mjx/warp/forward.py b/mjx/mujoco/mjx/warp/forward.py
index c1ec298e..1daac020 100644
--- a/mjx/mujoco/mjx/warp/forward.py
+++ b/mjx/mujoco/mjx/warp/forward.py
@@ -1113,7 +1113,7 @@ def _forward_jax_impl(m: types.Model, d: types.Data):
       'ten_Jdot': d._impl.ten_Jdot.shape,
       'ten_actfrc': d._impl.ten_actfrc.shape,
       'ten_bias_coef': d._impl.ten_bias_coef.shape,
-      'ten_length': d.ten_length.shape,
+      'ten_length': d._impl.ten_length.shape,
       'ten_velocity': d._impl.ten_velocity.shape,
       'ten_wrapadr': d._impl.ten_wrapadr.shape,
       'ten_wrapnum': d._impl.ten_wrapnum.shape,
@@ -1777,7 +1777,7 @@ def _forward_jax_impl(m: types.Model, d: types.Data):
       d._impl.ten_Jdot,
       d._impl.ten_actfrc,
       d._impl.ten_bias_coef,
-      d.ten_length,
+      d._impl.ten_length,
       d._impl.ten_velocity,
       d._impl.ten_wrapadr,
       d._impl.ten_wrapnum,
@@ -1956,7 +1956,7 @@ def _forward_jax_impl(m: types.Model, d: types.Data):
       '_impl.ten_Jdot': out[99],
       '_impl.ten_actfrc': out[100],
       '_impl.ten_bias_coef': out[101],
-      'ten_length': out[102],
+      '_impl.ten_length': out[102],
       '_impl.ten_velocity': out[103],
       '_impl.ten_wrapadr': out[104],
       '_impl.ten_wrapnum': out[105],
@@ -3176,7 +3176,7 @@ def _step_jax_impl(m: types.Model, d: types.Data):
       'ten_Jdot': d._impl.ten_Jdot.shape,
       'ten_actfrc': d._impl.ten_actfrc.shape,
       'ten_bias_coef': d._impl.ten_bias_coef.shape,
-      'ten_length': d.ten_length.shape,
+      'ten_length': d._impl.ten_length.shape,
       'ten_velocity': d._impl.ten_velocity.shape,
       'ten_wrapadr': d._impl.ten_wrapadr.shape,
       'ten_wrapnum': d._impl.ten_wrapnum.shape,
@@ -3866,7 +3866,7 @@ def _step_jax_impl(m: types.Model, d: types.Data):
       d._impl.ten_Jdot,
       d._impl.ten_actfrc,
       d._impl.ten_bias_coef,
-      d.ten_length,
+      d._impl.ten_length,
       d._impl.ten_velocity,
       d._impl.ten_wrapadr,
       d._impl.ten_wrapnum,
@@ -4057,7 +4057,7 @@ def _step_jax_impl(m: types.Model, d: types.Data):
       '_impl.ten_Jdot': out[111],
       '_impl.ten_actfrc': out[112],
       '_impl.ten_bias_coef': out[113],
-      'ten_length': out[114],
+      '_impl.ten_length': out[114],
       '_impl.ten_velocity': out[115],
       '_impl.ten_wrapadr': out[116],
       '_impl.ten_wrapnum': out[117],
diff --git a/mjx/mujoco/mjx/warp/forward_test.py b/mjx/mujoco/mjx/warp/forward_test.py
index ff890658..5fae3d22 100644
--- a/mjx/mujoco/mjx/warp/forward_test.py
+++ b/mjx/mujoco/mjx/warp/forward_test.py
@@ -142,7 +142,7 @@ class ForwardTest(parameterized.TestCase):
       if m.ncam:
         tu.assert_attr_eq(dx, d, 'cam_xpos')
         tu.assert_eq(dx.cam_xmat, d.cam_xmat.reshape((-1, 3, 3)), 'cam_xmat')
-      tu.assert_attr_eq(dx, d, 'ten_length')
+      tu.assert_attr_eq(dx._impl, d, 'ten_length')
       tu.assert_attr_eq(dx._impl, d, 'ten_J')
       tu.assert_attr_eq(dx._impl, d, 'ten_wrapadr')
       tu.assert_attr_eq(dx._impl, d, 'ten_wrapnum')
diff --git a/mjx/mujoco/mjx/warp/smooth.py b/mjx/mujoco/mjx/warp/smooth.py
index 3b881891..cd0e4138 100644
--- a/mjx/mujoco/mjx/warp/smooth.py
+++ b/mjx/mujoco/mjx/warp/smooth.py
@@ -285,213 +285,3 @@ def kinematics(m: types.Model, d: types.Data):
 def kinematics_vmap(unused_axis_size, is_batched, m, d):
   d = kinematics(m, d)
   return d, is_batched[1]
-
-
-_m = mjwarp.Model(
-    **{f.name: None for f in dataclasses.fields(mjwarp.Model) if f.init}
-)
-_d = mjwarp.Data(
-    **{f.name: None for f in dataclasses.fields(mjwarp.Data) if f.init}
-)
-_o = mjwarp.Option(
-    **{f.name: None for f in dataclasses.fields(mjwarp.Option) if f.init}
-)
-_s = mjwarp.Statistic(
-    **{f.name: None for f in dataclasses.fields(mjwarp.Statistic) if f.init}
-)
-_c = mjwarp.Contact(
-    **{f.name: None for f in dataclasses.fields(mjwarp.Contact) if f.init}
-)
-_e = mjwarp.Constraint(
-    **{f.name: None for f in dataclasses.fields(mjwarp.Constraint) if f.init}
-)
-
-
-@ffi.format_args_for_warp
-def _tendon_shim(
-    # Model
-    nworld: int,
-    body_parentid: wp.array(dtype=int),
-    body_rootid: wp.array(dtype=int),
-    dof_bodyid: wp.array(dtype=int),
-    geom_bodyid: wp.array(dtype=int),
-    geom_size: wp.array2d(dtype=wp.vec3),
-    jnt_dofadr: wp.array(dtype=int),
-    jnt_qposadr: wp.array(dtype=int),
-    ntendon: int,
-    nv: int,
-    site_bodyid: wp.array(dtype=int),
-    tendon_adr: wp.array(dtype=int),
-    tendon_geom_adr: wp.array(dtype=int),
-    tendon_jnt_adr: wp.array(dtype=int),
-    tendon_num: wp.array(dtype=int),
-    tendon_site_pair_adr: wp.array(dtype=int),
-    wrap_geom_adr: wp.array(dtype=int),
-    wrap_jnt_adr: wp.array(dtype=int),
-    wrap_objid: wp.array(dtype=int),
-    wrap_prm: wp.array(dtype=float),
-    wrap_pulley_scale: wp.array(dtype=float),
-    wrap_site_pair_adr: wp.array(dtype=int),
-    wrap_type: wp.array(dtype=int),
-    # Data
-    cdof: wp.array2d(dtype=wp.spatial_vector),
-    geom_xmat: wp.array2d(dtype=wp.mat33),
-    geom_xpos: wp.array2d(dtype=wp.vec3),
-    qpos: wp.array2d(dtype=float),
-    site_xpos: wp.array2d(dtype=wp.vec3),
-    subtree_com: wp.array2d(dtype=wp.vec3),
-    ten_J: wp.array3d(dtype=float),
-    ten_length: wp.array2d(dtype=float),
-    ten_wrapadr: wp.array2d(dtype=int),
-    ten_wrapnum: wp.array2d(dtype=int),
-    wrap_geom_xpos: wp.array2d(dtype=wp.spatial_vector),
-    wrap_obj: wp.array2d(dtype=wp.vec2i),
-    wrap_xpos: wp.array2d(dtype=wp.spatial_vector),
-):
-  _m.stat = _s
-  _m.opt = _o
-  _d.efc = _e
-  _d.contact = _c
-  _m.body_parentid = body_parentid
-  _m.body_rootid = body_rootid
-  _m.dof_bodyid = dof_bodyid
-  _m.geom_bodyid = geom_bodyid
-  _m.geom_size = geom_size
-  _m.jnt_dofadr = jnt_dofadr
-  _m.jnt_qposadr = jnt_qposadr
-  _m.ntendon = ntendon
-  _m.nv = nv
-  _m.site_bodyid = site_bodyid
-  _m.tendon_adr = tendon_adr
-  _m.tendon_geom_adr = tendon_geom_adr
-  _m.tendon_jnt_adr = tendon_jnt_adr
-  _m.tendon_num = tendon_num
-  _m.tendon_site_pair_adr = tendon_site_pair_adr
-  _m.wrap_geom_adr = wrap_geom_adr
-  _m.wrap_jnt_adr = wrap_jnt_adr
-  _m.wrap_objid = wrap_objid
-  _m.wrap_prm = wrap_prm
-  _m.wrap_pulley_scale = wrap_pulley_scale
-  _m.wrap_site_pair_adr = wrap_site_pair_adr
-  _m.wrap_type = wrap_type
-  _d.cdof = cdof
-  _d.geom_xmat = geom_xmat
-  _d.geom_xpos = geom_xpos
-  _d.qpos = qpos
-  _d.site_xpos = site_xpos
-  _d.subtree_com = subtree_com
-  _d.ten_J = ten_J
-  _d.ten_length = ten_length
-  _d.ten_wrapadr = ten_wrapadr
-  _d.ten_wrapnum = ten_wrapnum
-  _d.wrap_geom_xpos = wrap_geom_xpos
-  _d.wrap_obj = wrap_obj
-  _d.wrap_xpos = wrap_xpos
-  _d.nworld = nworld
-  mjwarp.tendon(_m, _d)
-
-
-def _tendon_jax_impl(m: types.Model, d: types.Data):
-  output_dims = {
-      'cdof': d._impl.cdof.shape,
-      'geom_xmat': d.geom_xmat.shape,
-      'geom_xpos': d.geom_xpos.shape,
-      'qpos': d.qpos.shape,
-      'site_xpos': d.site_xpos.shape,
-      'subtree_com': d.subtree_com.shape,
-      'ten_J': d._impl.ten_J.shape,
-      'ten_length': d.ten_length.shape,
-      'ten_wrapadr': d._impl.ten_wrapadr.shape,
-      'ten_wrapnum': d._impl.ten_wrapnum.shape,
-      'wrap_geom_xpos': d._impl.wrap_geom_xpos.shape,
-      'wrap_obj': d._impl.wrap_obj.shape,
-      'wrap_xpos': d._impl.wrap_xpos.shape,
-  }
-  jf = ffi.jax_callable_variadic_tuple(
-      _tendon_shim,
-      num_outputs=13,
-      output_dims=output_dims,
-      vmap_method=None,
-      in_out_argnames={
-          'cdof',
-          'geom_xmat',
-          'geom_xpos',
-          'qpos',
-          'site_xpos',
-          'subtree_com',
-          'ten_J',
-          'ten_length',
-          'ten_wrapadr',
-          'ten_wrapnum',
-          'wrap_geom_xpos',
-          'wrap_obj',
-          'wrap_xpos',
-      },
-  )
-  out = jf(
-      d.qpos.shape[0],
-      m.body_parentid,
-      m.body_rootid,
-      m.dof_bodyid,
-      m.geom_bodyid,
-      m.geom_size,
-      m.jnt_dofadr,
-      m.jnt_qposadr,
-      m.ntendon,
-      m.nv,
-      m.site_bodyid,
-      m.tendon_adr,
-      m._impl.tendon_geom_adr,
-      m._impl.tendon_jnt_adr,
-      m.tendon_num,
-      m._impl.tendon_site_pair_adr,
-      m._impl.wrap_geom_adr,
-      m._impl.wrap_jnt_adr,
-      m.wrap_objid,
-      m.wrap_prm,
-      m._impl.wrap_pulley_scale,
-      m._impl.wrap_site_pair_adr,
-      m.wrap_type,
-      d._impl.cdof,
-      d.geom_xmat,
-      d.geom_xpos,
-      d.qpos,
-      d.site_xpos,
-      d.subtree_com,
-      d._impl.ten_J,
-      d.ten_length,
-      d._impl.ten_wrapadr,
-      d._impl.ten_wrapnum,
-      d._impl.wrap_geom_xpos,
-      d._impl.wrap_obj,
-      d._impl.wrap_xpos,
-  )
-  d = d.tree_replace({
-      '_impl.cdof': out[0],
-      'geom_xmat': out[1],
-      'geom_xpos': out[2],
-      'qpos': out[3],
-      'site_xpos': out[4],
-      'subtree_com': out[5],
-      '_impl.ten_J': out[6],
-      'ten_length': out[7],
-      '_impl.ten_wrapadr': out[8],
-      '_impl.ten_wrapnum': out[9],
-      '_impl.wrap_geom_xpos': out[10],
-      '_impl.wrap_obj': out[11],
-      '_impl.wrap_xpos': out[12],
-  })
-  return d
-
-
-@jax.custom_batching.custom_vmap
-@ffi.marshal_jax_warp_callable
-def tendon(m: types.Model, d: types.Data):
-  return _tendon_jax_impl(m, d)
-
-
-@tendon.def_vmap
-@ffi.marshal_custom_vmap
-def tendon_vmap(unused_axis_size, is_batched, m, d):
-  d = tendon(m, d)
-  return d, is_batched[1]
diff --git a/mjx/mujoco/mjx/warp/smooth_test.py b/mjx/mujoco/mjx/warp/smooth_test.py
index c83574fe..6d9bc349 100644
--- a/mjx/mujoco/mjx/warp/smooth_test.py
+++ b/mjx/mujoco/mjx/warp/smooth_test.py
@@ -19,7 +19,6 @@ import os
 import tempfile
 
 from absl.testing import absltest
-from absl.testing import parameterized
 import jax
 from jax import numpy as jp
 import mujoco
@@ -39,7 +38,7 @@ except ImportError:
 _FORCE_TEST = os.environ.get('MJX_WARP_FORCE_TEST', '0') == '1'
 
 
-class SmoothTest(parameterized.TestCase):
+class SmoothTest(absltest.TestCase):
 
   def setUp(self):
     super().setUp()
@@ -238,5 +237,6 @@ class SmoothTest(parameterized.TestCase):
       tu.assert_attr_eq(d, dx, 'site_xpos')
       tu.assert_eq(d.site_xmat.reshape((-1, 3, 3)), dx.site_xmat, 'site_xmat')
 
+
 if __name__ == '__main__':
   absltest.main()
diff --git a/mjx/mujoco/mjx/warp/test_util.py b/mjx/mujoco/mjx/warp/test_util.py
index 7521e094..b5c25f3c 100644
--- a/mjx/mujoco/mjx/warp/test_util.py
+++ b/mjx/mujoco/mjx/warp/test_util.py
@@ -152,8 +152,7 @@ def _mjx_efc(dx, worldid: int):
     return 0, empty, empty, np.zeros((0, dx.qvel.shape[0])), empty, empty
   efc_pos = select(dx._impl.efc__pos[:nefc])
   efc_type = select(dx._impl.efc__type[:nefc])
-  efc_d = select(dx._impl.efc__D[:nefc])
-  keys_sorted = np.lexsort((-efc_pos, efc_type, efc_d))
+  keys_sorted = np.lexsort((-efc_pos, efc_type))
   keys = keys[keys_sorted]
 
   nefc = len(keys)
@@ -179,7 +178,7 @@ def _mj_efc(d):
   else:
     efc_j = d.efc_J.reshape((-1, d.qvel.shape[0]))
 
-  keys = np.lexsort((-d.efc_pos, d.efc_type, d.efc_D))
+  keys = np.lexsort((-d.efc_pos, d.efc_type))
   type_ = d.efc_type[keys]
   pos = d.efc_pos[keys]
   efc_j = efc_j[keys]
diff --git a/mjx/mujoco/mjx/warp/types.py b/mjx/mujoco/mjx/warp/types.py
index 2d70fb60..e86b691d 100644
--- a/mjx/mujoco/mjx/warp/types.py
+++ b/mjx/mujoco/mjx/warp/types.py
@@ -361,6 +361,7 @@ class DataWarp(PyTreeNode):
   ten_Jdot: jax.Array
   ten_actfrc: jax.Array
   ten_bias_coef: jax.Array
+  ten_length: jax.Array
   ten_velocity: jax.Array
   ten_wrapadr: jax.Array
   ten_wrapnum: jax.Array