Add reshard API in experimental. Currently for sharding_in_types we have 2 APIs: mesh_cast and reshard. Both work in sharding_in_types mode and affect the sharding of the aval. Following are the semantics of both:

jax/__init__.py

@@ -126,7 +126,6 @@
 from jax._src.api import value_and_grad as value_and_grad
 from jax._src.api import vjp as vjp
 from jax._src.api import vmap as vmap
-from jax._src.api import hidden_axes as hidden_axes
 from jax._src.sharding_impls import NamedSharding as NamedSharding
 from jax._src.sharding_impls import make_mesh as make_mesh
 

jax/_src/api.py

@@ -99,8 +99,6 @@
 map, unsafe_map = safe_map, map
 zip, unsafe_zip = safe_zip, zip
 
-hidden_axes = pjit.hidden_axes
-
 
 def _nan_check_posthook(fun, args, kwargs, output):
   """Hook function called by the C++ jit/pmap to perform NaN checking."""

jax/_src/lax/slicing.py

@@ -1883,7 +1883,8 @@ def _gather_sharding_rule(operand, indices, *, dimension_numbers,
                           slice_sizes, unique_indices, indices_are_sorted,
                           mode, fill_value):
   # TODO(yashkatariya): Write a proper gather sharding rule.
-  if mesh_lib.get_abstract_mesh()._are_all_axes_hidden:  # type: ignore
+  cur_mesh = mesh_lib.get_abstract_mesh()
+  if cur_mesh._are_all_axes_hidden or cur_mesh._are_all_axes_collective:  # type: ignore
     return None
   raise GatherShardingError(
       "Use `.at[...].get(out_sharding=)` to provide output PartitionSpec for"

jax/_src/lax/utils.py

@@ -57,7 +57,7 @@ def call_sharding_rule(prim, rule, num_out, *avals, **kwargs):
             f'sharding rule for {prim.name} is not implemented. Please file a'
             ' bug at https://github.com/jax-ml/jax/issues. You can work around'
             ' this error by dropping that operation into full hidden sharding'
-            ' mode via: `jax.hidden_axes(fun, out_shardings=...)`')
+            ' mode via: `jax.experimental.shard.hidden_axes(fun, out_shardings=...)`')
     return rule(*avals, **kwargs)
   return None if num_out is None else [None] * num_out
 

jax/_src/pjit.py

@@ -2684,17 +2684,11 @@ def _sharding_constraint_batcher(
 
 # TODO(yashkatariya): Make shardings optional.
 def mesh_cast(xs, out_shardings):
-  if isinstance(out_shardings, (NamedSharding, PartitionSpec)):
-    return tree_map(
-        lambda x: mesh_cast_p.bind(
-            x, src_sharding=x.sharding, dst_sharding=canonicalize_sharding(
-                out_shardings, check_mesh_consistency=False)), xs)
-
   x_flat, treedef = tree_flatten(xs)
   shardings_flat = flatten_axes("mesh_cast shardings", treedef, out_shardings)
   out_flat = [
       mesh_cast_p.bind(
-          x, src_sharding=x.sharding,
+          x, src_sharding=x.aval.sharding,
           dst_sharding=canonicalize_sharding(s, check_mesh_consistency=False))
       for x, s in safe_zip(x_flat, shardings_flat)
   ]
@@ -2779,6 +2773,49 @@ def _mesh_cast_hlo_lowering(ctx, x_node, *, src_sharding, dst_sharding):
 # batching.fancy_primitive_batchers[mesh_cast_p] = _mesh_cast_batcher
 # batching.skippable_batchers[mesh_cast_p] = lambda _: ()
 
+# -------------------- reshard ------------------------------------
+
+def reshard(xs, out_shardings):
+  x_flat, treedef = tree_flatten(xs)
+  shardings_flat = flatten_axes("reshard shardings", treedef, out_shardings)
+  out_flat = []
+  for x, s in safe_zip(x_flat, shardings_flat):
+    ds = canonicalize_sharding(s)
+    ds = ds.with_spec(ds.spec._normalized_spec(x.ndim))  # type: ignore
+    out_flat.append(reshard_p.bind(x, src_sharding=x.aval.sharding,
+                                   dst_sharding=ds))
+  return tree_unflatten(treedef, out_flat)
+
+reshard_p = core.Primitive('reshard')
+
+def _reshard_abstract_eval(aval, src_sharding, dst_sharding):
+  if src_sharding.mesh.abstract_mesh != dst_sharding.mesh.abstract_mesh:
+    raise ValueError(
+        f'Mesh of the input {src_sharding.mesh.abstract_mesh} does not'
+        ' equal the mesh of the target sharding'
+        f' {dst_sharding.mesh.abstract_mesh} for shape {aval.str_short()}')
+  return aval.update(sharding=dst_sharding)
+reshard_p.def_abstract_eval(_reshard_abstract_eval)
+
+def _reshard_impl(x, src_sharding, dst_sharding):
+  return dispatch.apply_primitive(reshard_p, x, src_sharding=src_sharding,
+                                  dst_sharding=dst_sharding)
+reshard_p.def_impl(_reshard_impl)
+
+def _reshard_transpose_rule(ct, _, src_sharding, dst_sharding):
+  return [reshard_p.bind(ct, src_sharding=dst_sharding,
+                         dst_sharding=src_sharding)]
+ad.deflinear2(reshard_p, _reshard_transpose_rule)
+
+def _reshard_hlo_lowering(ctx, x_node, *, src_sharding, dst_sharding):
+  aval, = ctx.avals_in
+  aval_out, = ctx.avals_out
+  proto = (dst_sharding._to_sdy_sharding(aval.ndim)
+           if config.use_shardy_partitioner.value else
+           dst_sharding._to_xla_hlo_sharding(aval.ndim).to_proto())
+  return [mlir.lower_sharding_under_shit(ctx, x_node, aval_out, proto)]
+mlir.register_lowering(reshard_p, _reshard_hlo_lowering)
+
 # -------------------- auto and user mode -------------------------
 
 def _get_new_mesh(axes: str | tuple[str, ...] | None,

jax/_src/sharding_impls.py

@@ -1766,11 +1766,13 @@ def canonicalize_sharding(sharding: NamedSharding | PartitionSpec | None,
     sharding = NamedSharding(mesh_lib.get_abstract_mesh(), sharding)  # type: ignore
   else:
     if (check_mesh_consistency and
-        sharding.mesh != mesh_lib.get_abstract_mesh()):
+        sharding.mesh.abstract_mesh != mesh_lib.get_abstract_mesh()):
       raise ValueError(
           f'Context mesh {mesh_lib.get_abstract_mesh()} should match the mesh'
-          f' of sharding {sharding.mesh}. This error occurs at source: '
-          f' {source_info_util.summarize(source_info_util.current())}')
+          f' of sharding {sharding.mesh.abstract_mesh}. This error occurs at'
+          f' source:  {source_info_util.summarize(source_info_util.current())}')
+    if isinstance(sharding.mesh, mesh_lib.Mesh):
+      sharding = NamedSharding(sharding.mesh.abstract_mesh, sharding.spec)
 
   for s in flatten_spec(sharding.spec):
     if sharding.mesh._name_to_type[s] in {

jax/experimental/jax2tf/tests/primitives_test.py

@@ -176,6 +176,8 @@ def test_primitive_coverage(self):
         continue
       if p.name == "mesh_cast":
         continue
+      if p.name == "reshard":
+        continue
       # TODO: Remove once tensorflow is 2.10.0 everywhere.
       if p.name == "optimization_barrier":
         continue

jax/experimental/shard.py

@@ -0,0 +1,18 @@
+# Copyright 2025 The JAX Authors.
+#
+# 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
+#
+#     https://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 ific language governing permissions and
+# limitations under the License.
+
+from jax._src.pjit import (
+    reshard as reshard,
+    hidden_axes as hidden_axes,
+)

tests/pjit_test.py

@@ -43,18 +43,19 @@
 from jax.sharding import PartitionSpec as P, Mesh
 from jax.experimental import multihost_utils
 from jax.experimental.shard_map import shard_map
-from jax.experimental.custom_partitioning import custom_partitioning, SdyShardingRule, BATCHING
+from jax.experimental.custom_partitioning import (
+    custom_partitioning, SdyShardingRule, BATCHING)
 from jax._src import array
 from jax._src.sharding import Sharding, common_devices_indices_map
 from jax._src import op_shardings
 from jax._src import sharding_impls
 from jax._src.sharding_impls import (
     AUTO, UNSPECIFIED, NamedSharding, GSPMDSharding, PositionalSharding,
     SingleDeviceSharding, parse_flatten_op_sharding)
-from jax._src.pjit import (pjit, mesh_cast, visible_axes,
-                           use_hidden_axes, use_visible_axes)
+from jax._src.pjit import (pjit, mesh_cast, hidden_axes, visible_axes,
+                           use_hidden_axes, use_visible_axes, reshard)
 from jax._src import mesh as mesh_lib
-from jax._src.mesh import set_abstract_mesh, get_abstract_mesh, AxisTypes
+from jax._src.mesh import AxisTypes
 from jax._src.interpreters import pxla
 from jax._src.lib.mlir import dialects
 from jax._src import xla_bridge
@@ -5905,24 +5906,6 @@ def f(x, y):
         ValueError, "For primitive dot_general, context mesh.*aval mesh"):
       f(arr, arr.T)
 
-  def test_mesh_cast_src_dst_mesh_mismatch(self):
-    np_inp = np.arange(16.).reshape(8, 2)
-    mesh = jtu.create_mesh((2, 1), ('x', 'y'),
-                           axis_types={mesh_lib.AxisTypes.Visible: ('x', 'y')})
-    mesh2 = jtu.create_mesh((2, 1), ('a', 'b'),
-                            axis_types={mesh_lib.AxisTypes.Visible: ('a', 'b')})
-    s = NamedSharding(mesh, P('x', 'y'))
-    arr = jax.device_put(np_inp, s)
-    f = lambda x: mesh_cast(x, NamedSharding(mesh2, P('a', 'b')))
-    with self.assertRaisesRegex(
-        ValueError, "Mesh shape of the input.*does not match"):
-      f(arr)
-
-    with mesh_lib.use_mesh(mesh):
-      with self.assertRaisesRegex(
-          ValueError, "Mesh shape of the input.*does not match"):
-        jax.jit(f)(arr)
-
   @jtu.with_user_mesh((2, 2), ('x', 'y'))
   def test_split(self, mesh):
     np_inp = np.arange(16.).reshape(8, 2)
@@ -5960,8 +5943,7 @@ def test_return_output_different_context(self, mesh):
 
     @jax.jit
     def f(x):
-      auto_mesh = get_abstract_mesh().update_axis_types({AxisTypes.Hidden: 'x'})
-      with set_abstract_mesh(auto_mesh):
+      with use_hidden_axes('x'):
         x = mesh_cast(x, P(None, None))
         return x
 
@@ -6048,7 +6030,7 @@ def test_auto_mode_mix(self, mesh):
     s = NamedSharding(mesh, P('x', 'y'))
     arr = jax.device_put(np_inp, s)
 
-    @partial(jax.hidden_axes, axes='x', out_shardings=P('x', None))
+    @partial(hidden_axes, axes='x', out_shardings=P('x', None))
     def h(y):
       self.assertEqual(y.sharding.spec, P(None, 'y'))
       z = jnp.sin(y)
@@ -6181,6 +6163,86 @@ def g(x, y):
     out = jax.jit(jax.grad(g))(embed, tok)
     self.assertEqual(out.sharding, embed.sharding)
 
+  @jtu.with_user_mesh((2, 2), ('x', 'y'))
+  def test_reshard_error(self, mesh):
+    np_inp = np.arange(16.).reshape(8, 2)
+    s = NamedSharding(mesh, P('x', 'y'))
+    arr = jax.device_put(np_inp, s)
+
+    def f(x):
+      y = reshard(x, P('x', None))
+      self.assertEqual(y.aval.sharding.spec, P('x', None))
+      return y
+
+    out = f(arr)
+    self.assertEqual(out.sharding, NamedSharding(mesh, P('x', None)))
+
+    f = jax.jit(f)
+
+    out = f(arr)
+    self.assertEqual(out.sharding, NamedSharding(mesh, P('x', None)))
+
+    lowered_text = f.lower(arr).as_text()
+    self.check_wsc_in_lowered(lowered_text)
+
+    def g(x):
+      y = f(x)
+      return jnp.sum(y)
+
+    out = jax.grad(g)(arr)
+    self.assertEqual(out.sharding, arr.sharding)
+
+    out = jax.jit(jax.grad(g))(arr)
+    self.assertEqual(out.sharding, arr.sharding)
+
+    @jax.jit
+    def h(x):
+      with use_hidden_axes('x'):
+        return reshard(x, P('y', None))
+
+    with self.assertRaisesRegex(
+        ValueError, 'Mesh of the input.*does not equal.*target sharding'):
+      h(arr)
+
+  @jtu.with_user_mesh((2, 2), ('x', 'y'))
+  def test_full_auto_outside_jit(self, mesh):
+    np_inp = np.arange(16.).reshape(8, 2)
+    s = NamedSharding(mesh, P('x', 'y'))
+    arr = jax.device_put(np_inp, s)
+
+    @jax.jit
+    def f(x):
+      y = x * 2
+      self.assertEqual(y.sharding.spec, P(None, None))
+      z = jnp.sin(y)
+      self.assertEqual(z.sharding.spec, P(None, None))
+      a = z @ z.T
+      self.assertEqual(a.sharding.spec, P(None, None))
+      return a
+
+    hf = hidden_axes(f, axes=('x', 'y'), out_shardings=P('x', 'y'))
+    out = hf(arr)
+    self.assertEqual(out.sharding, NamedSharding(mesh, P('x', 'y')))
+
+  @jtu.with_user_mesh((2, 2), ('x', 'y'),
+                      axis_types={AxisTypes.Hidden: ('x', 'y')})
+  def test_full_visible_outside_jit(self, mesh):
+    np_inp = np.arange(16.).reshape(8, 2)
+    s = NamedSharding(mesh, P('x', 'y'))
+    arr = jax.device_put(np_inp, s)
+
+    @jax.jit
+    def f(x):
+      y = x * 2
+      self.assertEqual(y.sharding.spec, P('x', 'y'))
+      z = jnp.sin(y)
+      self.assertEqual(z.sharding.spec, P('x', 'y'))
+      return z
+
+    hf = visible_axes(f, axes=('x', 'y'), in_shardings=P('x', 'y'))
+    out = hf(arr)  # doesn't crash
+    self.assertEqual(out.sharding, NamedSharding(mesh, P('x', 'y')))
+
 
 @jtu.pytest_mark_if_available('multiaccelerator')
 class PJitErrorTest(jtu.JaxTestCase):