diff --git a/source/tests/common/test_mixins.py b/source/tests/common/test_mixins.py
new file mode 100644
index 0000000000..e311baf5cf
--- /dev/null
+++ b/source/tests/common/test_mixins.py
@@ -0,0 +1,77 @@
+# SPDX-License-Identifier: LGPL-3.0-or-later
+"""Shared test mixins and utilities used by both pt and pt_expt tests.
+
+These are kept in ``tests.common`` so that importing them does NOT trigger
+``tests.pt.__init__`` (which sets ``torch.set_default_device("cuda:9999999")``
+and pushes a ``DeviceContext`` onto the mode stack, breaking pt_expt tests
+on CPU-only machines).
+"""
+
+import numpy as np
+
+
+class TestCaseSingleFrameWithNlist:
+    """Mixin providing a small 2-frame, 2-type test system with neighbor list."""
+
+    def setUp(self) -> None:
+        # nloc == 3, nall == 4
+        self.nloc = 3
+        self.nall = 4
+        self.nf, self.nt = 2, 2
+        self.coord_ext = np.array(
+            [
+                [0, 0, 0],
+                [0, 1, 0],
+                [0, 0, 1],
+                [0, -2, 0],
+            ],
+            dtype=np.float64,
+        ).reshape([1, self.nall, 3])
+        self.atype_ext = np.array([0, 0, 1, 0], dtype=int).reshape([1, self.nall])
+        self.mapping = np.array([0, 1, 2, 0], dtype=int).reshape([1, self.nall])
+        # sel = [5, 2]
+        self.sel = [5, 2]
+        self.sel_mix = [7]
+        self.natoms = [3, 3, 2, 1]
+        self.nlist = np.array(
+            [
+                [1, 3, -1, -1, -1, 2, -1],
+                [0, -1, -1, -1, -1, 2, -1],
+                [0, 1, -1, -1, -1, -1, -1],
+            ],
+            dtype=int,
+        ).reshape([1, self.nloc, sum(self.sel)])
+        self.rcut = 2.2
+        self.rcut_smth = 0.4
+        # permutations
+        self.perm = np.array([2, 0, 1, 3], dtype=np.int32)
+        inv_perm = np.array([1, 2, 0, 3], dtype=np.int32)
+        # permute the coord and atype
+        self.coord_ext = np.concatenate(
+            [self.coord_ext, self.coord_ext[:, self.perm, :]], axis=0
+        ).reshape(self.nf, self.nall * 3)
+        self.atype_ext = np.concatenate(
+            [self.atype_ext, self.atype_ext[:, self.perm]], axis=0
+        )
+        self.mapping = np.concatenate(
+            [self.mapping, self.mapping[:, self.perm]], axis=0
+        )
+
+        # permute the nlist
+        nlist1 = self.nlist[:, self.perm[: self.nloc], :]
+        mask = nlist1 == -1
+        nlist1 = inv_perm[nlist1]
+        nlist1 = np.where(mask, -1, nlist1)
+        self.nlist = np.concatenate([self.nlist, nlist1], axis=0)
+        self.atol = 1e-12
+
+
+def get_tols(prec):
+    """Return (rtol, atol) for a given precision string."""
+    if prec in ["single", "float32"]:
+        rtol, atol = 0.0, 1e-4
+    elif prec in ["double", "float64"]:
+        rtol, atol = 0.0, 1e-12
+    else:
+        raise ValueError(f"unknown prec {prec}")
+    return rtol, atol
diff --git a/source/tests/pt/model/test_env_mat.py b/source/tests/pt/model/test_env_mat.py
index 391f50da03..0c99311ca3 100644
--- a/source/tests/pt/model/test_env_mat.py
+++ b/source/tests/pt/model/test_env_mat.py
@@ -21,58 +21,9 @@
 dtype = env.GLOBAL_PT_FLOAT_PRECISION
 
 
-class TestCaseSingleFrameWithNlist:
-    def setUp(self) -> None:
-        # nloc == 3, nall == 4
-        self.nloc = 3
-        self.nall = 4
-        self.nf, self.nt = 2, 2
-        self.coord_ext = np.array(
-            [
-                [0, 0, 0],
-                [0, 1, 0],
-                [0, 0, 1],
-                [0, -2, 0],
-            ],
-            dtype=np.float64,
-        ).reshape([1, self.nall, 3])
-        self.atype_ext = np.array([0, 0, 1, 0], dtype=int).reshape([1, self.nall])
-        self.mapping = np.array([0, 1, 2, 0], dtype=int).reshape([1, self.nall])
-        # sel = [5, 2]
-        self.sel = [5, 2]
-        self.sel_mix = [7]
-        self.natoms = [3, 3, 2, 1]
-        self.nlist = np.array(
-            [
-                [1, 3, -1, -1, -1, 2, -1],
-                [0, -1, -1, -1, -1, 2, -1],
-                [0, 1, -1, -1, -1, -1, -1],
-            ],
-            dtype=int,
-        ).reshape([1, self.nloc, sum(self.sel)])
-        self.rcut = 2.2
-        self.rcut_smth = 0.4
-        # permutations
-        self.perm = np.array([2, 0, 1, 3], dtype=np.int32)
-        inv_perm = np.array([1, 2, 0, 3], dtype=np.int32)
-        # permute the coord and atype
-        self.coord_ext = np.concatenate(
-            [self.coord_ext, self.coord_ext[:, self.perm, :]], axis=0
-        ).reshape(self.nf, self.nall * 3)
-        self.atype_ext = np.concatenate(
-            [self.atype_ext, self.atype_ext[:, self.perm]], axis=0
-        )
-        self.mapping = np.concatenate(
-            [self.mapping, self.mapping[:, self.perm]], axis=0
-        )
-
-        # permute the nlist
-        nlist1 = self.nlist[:, self.perm[: self.nloc], :]
-        mask = nlist1 == -1
-        nlist1 = inv_perm[nlist1]
-        nlist1 = np.where(mask, -1, nlist1)
-        self.nlist = np.concatenate([self.nlist, nlist1], axis=0)
-        self.atol = 1e-12
+from ...common.test_mixins import (
+    TestCaseSingleFrameWithNlist,
+)
 
 
 class TestCaseSingleFrameWithNlistWithVirtual:
diff --git a/source/tests/pt/model/test_mlp.py b/source/tests/pt/model/test_mlp.py
index e6dde660cb..5f067cb0e6 100644
--- a/source/tests/pt/model/test_mlp.py
+++ b/source/tests/pt/model/test_mlp.py
@@ -24,17 +24,9 @@
     PRECISION_DICT,
 )
 
-
-def get_tols(prec):
-    if prec in ["single", "float32"]:
-        rtol, atol = 0.0, 1e-4
-    elif prec in ["double", "float64"]:
-        rtol, atol = 0.0, 1e-12
-    # elif prec in ["half", "float16"]:
-    #   rtol, atol=1e-2, 0
-    else:
-        raise ValueError(f"unknown prec {prec}")
-    return rtol, atol
+from ...common.test_mixins import (
+    get_tols,
+)
 
 
 class TestMLPLayer(unittest.TestCase):
diff --git a/source/tests/pt_expt/conftest.py b/source/tests/pt_expt/conftest.py
index afda179a82..f2a9b07a6a 100644
--- a/source/tests/pt_expt/conftest.py
+++ b/source/tests/pt_expt/conftest.py
@@ -1,28 +1,14 @@
 # SPDX-License-Identifier: LGPL-3.0-or-later
 """Conftest for pt_expt tests.
 
-Clears any leaked ``torch.utils._device.DeviceContext`` modes that may
-have been left on the torch function mode stack by ``make_fx`` or other
-tracing utilities during test collection.  A stale ``DeviceContext``
-silently reroutes ``torch.tensor(...)`` calls (without an explicit
-``device=``) to a fake CUDA device, causing spurious "no NVIDIA driver"
-errors on CPU-only machines.
-
-The leak is triggered when pytest collects descriptor test modules that
-import ``make_fx``.  A ``DeviceContext(cuda:127)`` ends up on the
-``torch.overrides`` function mode stack and is never popped.
-
-Our own code (``display_if_exist`` in ``deepmd/dpmodel/loss/loss.py``)
-is already fixed to pass ``device=`` explicitly.  However, PyTorch's
-``Adam._init_group`` (``torch/optim/adam.py``) contains::
-
-    torch.tensor(0.0, dtype=_get_scalar_dtype())  # no device=
-
-on the ``capturable=False, fused=False`` path (the default).  This is
-a PyTorch bug — the ``capturable=True`` branch correctly uses
-``device=p.device`` but the default branch omits it.  We cannot fix
-PyTorch internals, so this fixture works around the issue by popping
-leaked ``DeviceContext`` modes before each test.
+Safety net: pops any leaked ``torch.utils._device.DeviceContext`` modes
+from the torch function mode stack before each test.
+
+The primary leak source was ``source/tests/pt/__init__.py`` which calls
+``torch.set_default_device("cuda:9999999")``; pt_expt tests previously
+imported shared mixins from ``tests.pt.model``, triggering that init.
+This was fixed by moving the shared mixins to ``tests.common.test_mixins``
+so pt_expt tests no longer import from the ``tests.pt`` package.
 """
 
 import pytest
@@ -50,20 +36,13 @@ def _pop_device_contexts() -> list:
 
 @pytest.fixture(autouse=True, scope="session")
 def _clear_leaked_device_context_session():
-    """Pop any stale DeviceContext once at session start.
-
-    This runs before any setUpClass, preventing CUDA init errors
-    in tests that call trainer.run() during class setup.
-    """
+    """Pop any stale DeviceContext once at session start."""
     _pop_device_contexts()
     yield
 
 
 @pytest.fixture(autouse=True)
 def _clear_leaked_device_context():
-    """Pop any stale ``DeviceContext`` before each test, restore after."""
-    popped = _pop_device_contexts()
+    """Pop any stale ``DeviceContext`` before each test (safety net)."""
+    _pop_device_contexts()
     yield
-    # Restore in reverse order so the stack is back to its original state.
-    for ctx in reversed(popped):
-        ctx.__enter__()
diff --git a/source/tests/pt_expt/descriptor/test_dpa1.py b/source/tests/pt_expt/descriptor/test_dpa1.py
index 9827ca6679..0524c6c98a 100644
--- a/source/tests/pt_expt/descriptor/test_dpa1.py
+++ b/source/tests/pt_expt/descriptor/test_dpa1.py
@@ -18,10 +18,8 @@
     PRECISION_DICT,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
-)
-from ...pt.model.test_mlp import (
     get_tols,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/descriptor/test_dpa2.py b/source/tests/pt_expt/descriptor/test_dpa2.py
index 47ea4c0811..a3794052f4 100644
--- a/source/tests/pt_expt/descriptor/test_dpa2.py
+++ b/source/tests/pt_expt/descriptor/test_dpa2.py
@@ -22,10 +22,8 @@
     PRECISION_DICT,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
-)
-from ...pt.model.test_mlp import (
     get_tols,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/descriptor/test_dpa3.py b/source/tests/pt_expt/descriptor/test_dpa3.py
index 4aeec0dbad..7cdcd6ced7 100644
--- a/source/tests/pt_expt/descriptor/test_dpa3.py
+++ b/source/tests/pt_expt/descriptor/test_dpa3.py
@@ -21,10 +21,8 @@
     PRECISION_DICT,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
-)
-from ...pt.model.test_mlp import (
     get_tols,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/descriptor/test_hybrid.py b/source/tests/pt_expt/descriptor/test_hybrid.py
index 41a87273d5..3185733ddf 100644
--- a/source/tests/pt_expt/descriptor/test_hybrid.py
+++ b/source/tests/pt_expt/descriptor/test_hybrid.py
@@ -24,10 +24,8 @@
     PRECISION_DICT,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
-)
-from ...pt.model.test_mlp import (
     get_tols,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/descriptor/test_se_atten_v2.py b/source/tests/pt_expt/descriptor/test_se_atten_v2.py
index 5d71095af4..01ead9e179 100644
--- a/source/tests/pt_expt/descriptor/test_se_atten_v2.py
+++ b/source/tests/pt_expt/descriptor/test_se_atten_v2.py
@@ -18,10 +18,8 @@
     PRECISION_DICT,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
-)
-from ...pt.model.test_mlp import (
     get_tols,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/descriptor/test_se_e2_a.py b/source/tests/pt_expt/descriptor/test_se_e2_a.py
index 841ef776ff..0efb90b1bb 100644
--- a/source/tests/pt_expt/descriptor/test_se_e2_a.py
+++ b/source/tests/pt_expt/descriptor/test_se_e2_a.py
@@ -21,10 +21,8 @@
     PairExcludeMask,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
-)
-from ...pt.model.test_mlp import (
     get_tols,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/descriptor/test_se_r.py b/source/tests/pt_expt/descriptor/test_se_r.py
index c9aeaffac4..0bac2251b4 100644
--- a/source/tests/pt_expt/descriptor/test_se_r.py
+++ b/source/tests/pt_expt/descriptor/test_se_r.py
@@ -18,10 +18,8 @@
     PRECISION_DICT,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
-)
-from ...pt.model.test_mlp import (
     get_tols,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/descriptor/test_se_t.py b/source/tests/pt_expt/descriptor/test_se_t.py
index 80a32d03a0..3da6ef02f3 100644
--- a/source/tests/pt_expt/descriptor/test_se_t.py
+++ b/source/tests/pt_expt/descriptor/test_se_t.py
@@ -18,10 +18,8 @@
     PRECISION_DICT,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
-)
-from ...pt.model.test_mlp import (
     get_tols,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/descriptor/test_se_t_tebd.py b/source/tests/pt_expt/descriptor/test_se_t_tebd.py
index 0f50e6000e..bb4b1dc80d 100644
--- a/source/tests/pt_expt/descriptor/test_se_t_tebd.py
+++ b/source/tests/pt_expt/descriptor/test_se_t_tebd.py
@@ -18,10 +18,8 @@
     PRECISION_DICT,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
-)
-from ...pt.model.test_mlp import (
     get_tols,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/fitting/test_dipole_fitting.py b/source/tests/pt_expt/fitting/test_dipole_fitting.py
index f5ac7ba177..959b23d6ea 100644
--- a/source/tests/pt_expt/fitting/test_dipole_fitting.py
+++ b/source/tests/pt_expt/fitting/test_dipole_fitting.py
@@ -17,7 +17,7 @@
     env,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/fitting/test_dos_fitting.py b/source/tests/pt_expt/fitting/test_dos_fitting.py
index 3fe06a8618..340088e672 100644
--- a/source/tests/pt_expt/fitting/test_dos_fitting.py
+++ b/source/tests/pt_expt/fitting/test_dos_fitting.py
@@ -17,7 +17,7 @@
     env,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/fitting/test_ener_fitting.py b/source/tests/pt_expt/fitting/test_ener_fitting.py
index 63ae82ab9a..fe55bd628a 100644
--- a/source/tests/pt_expt/fitting/test_ener_fitting.py
+++ b/source/tests/pt_expt/fitting/test_ener_fitting.py
@@ -14,7 +14,7 @@
     env,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/fitting/test_invar_fitting.py b/source/tests/pt_expt/fitting/test_invar_fitting.py
index 30cbe84401..cf54ea500b 100644
--- a/source/tests/pt_expt/fitting/test_invar_fitting.py
+++ b/source/tests/pt_expt/fitting/test_invar_fitting.py
@@ -15,7 +15,7 @@
     env,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/fitting/test_polar_fitting.py b/source/tests/pt_expt/fitting/test_polar_fitting.py
index 1c150f7154..e11b4455e5 100644
--- a/source/tests/pt_expt/fitting/test_polar_fitting.py
+++ b/source/tests/pt_expt/fitting/test_polar_fitting.py
@@ -17,7 +17,7 @@
     env,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/fitting/test_property_fitting.py b/source/tests/pt_expt/fitting/test_property_fitting.py
index ca3dbc11af..19177be849 100644
--- a/source/tests/pt_expt/fitting/test_property_fitting.py
+++ b/source/tests/pt_expt/fitting/test_property_fitting.py
@@ -17,7 +17,7 @@
     env,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/loss/test_ener.py b/source/tests/pt_expt/loss/test_ener.py
index 37d7d4c703..9bc81f119f 100644
--- a/source/tests/pt_expt/loss/test_ener.py
+++ b/source/tests/pt_expt/loss/test_ener.py
@@ -23,7 +23,7 @@
     PRECISION_DICT,
 )
 
-from ...pt.model.test_mlp import (
+from ...common.test_mixins import (
     get_tols,
 )
 from ...seed import (
diff --git a/source/tests/pt_expt/test_change_bias.py b/source/tests/pt_expt/test_change_bias.py
index 16974cc653..50d114af28 100644
--- a/source/tests/pt_expt/test_change_bias.py
+++ b/source/tests/pt_expt/test_change_bias.py
@@ -123,12 +123,6 @@ class TestChangeBias(unittest.TestCase):
 
     @classmethod
     def setUpClass(cls) -> None:
-        from .conftest import (
-            _pop_device_contexts,
-        )
-
-        _pop_device_contexts()
-
         data_dir = os.path.join(EXAMPLE_DIR, "data")
         if not os.path.isdir(data_dir):
             raise unittest.SkipTest(f"Example data not found: {data_dir}")
diff --git a/source/tests/pt_expt/utils/test_exclusion_mask.py b/source/tests/pt_expt/utils/test_exclusion_mask.py
index 6f836913af..cc0671c117 100644
--- a/source/tests/pt_expt/utils/test_exclusion_mask.py
+++ b/source/tests/pt_expt/utils/test_exclusion_mask.py
@@ -12,7 +12,7 @@
     PairExcludeMask,
 )
 
-from ...pt.model.test_env_mat import (
+from ...common.test_mixins import (
     TestCaseSingleFrameWithNlist,
 )