diff --git a/optax/contrib/_sharding_test.py b/optax/contrib/_sharding_test.py
new file mode 100644
index 000000000..9eb278782
--- /dev/null
+++ b/optax/contrib/_sharding_test.py
@@ -0,0 +1,77 @@
+# Copyright 2026 DeepMind Technologies Limited. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+"""Sharding and dtype-stability tests for optax.contrib optimizers."""
+
+from absl.testing import absltest
+from absl.testing import parameterized
+import jax
+import jax.numpy as jnp
+import optax
+from optax._src import test_utils
+from optax._src import utils
+
+# Set device count before the JAX backend is initialized so that
+# jax.make_mesh((8,), ...) works in the tests below.
+jax.config.update('jax_num_cpu_devices', 8)
+
+OPTIMIZERS = {
+    'acprop': optax.contrib.acprop(1e-3),
+    'ademamix': optax.contrib.ademamix(1e-3),
+    'adopt': optax.contrib.adopt(1e-2),
+    'cocob': optax.contrib.cocob(),
+    'dadapt_adamw': optax.contrib.dadapt_adamw(1e-1),
+    'dog': optax.contrib.dog(1.0),
+    'dowg': optax.contrib.dowg(1.0),
+    'galore': optax.contrib.galore(1e-2, rank=4),
+    'madgrad': optax.contrib.madgrad(1e-2),
+    'muon': optax.contrib.muon(1e-2),
+    'prodigy': optax.contrib.prodigy(1e-1),
+    'schedule_free_adamw': optax.contrib.schedule_free_adamw(
+        1e-2, warmup_steps=5000
+    ),
+    'schedule_free_sgd': optax.contrib.schedule_free_sgd(
+        1e-2, warmup_steps=5000
+    ),
+    'simplified_ademamix': optax.contrib.simplified_ademamix(1e-3),
+}
+
+
+class ContribShardingTest(parameterized.TestCase):
+
+  @parameterized.named_parameters(OPTIMIZERS.items())
+  def test_state_sharding_type_stable(self, optimizer):
+    if utils.parse_version(jax.__version__) < utils.parse_version('0.7.2'):
+      self.skipTest('Skipping sharding-in-types test')
+
+    mesh = jax.make_mesh(
+        (8,), ('x',), axis_types=(jax.sharding.AxisType.Explicit,)
+    )
+    sharding = jax.sharding.NamedSharding(mesh, jax.P(None, 'x'))
+
+    with jax.set_mesh(mesh):
+      params = jnp.zeros(
+          (2, 8, 4), dtype=jnp.float16, out_sharding=sharding
+      )
+
+      state = optimizer.init(params)
+      _, state_after_update = optimizer.update(params, state, params)
+      test_utils.assert_trees_all_equal(
+          jax.tree.map(jax.typeof, state),
+          jax.tree.map(jax.typeof, state_after_update),
+      )
+
+
+if __name__ == '__main__':
+  absltest.main()