diff --git a/src/ezmsg/sigproc/concat.py b/src/ezmsg/sigproc/concat.py
index 38f4fb3..10ef674 100644
--- a/src/ezmsg/sigproc/concat.py
+++ b/src/ezmsg/sigproc/concat.py
@@ -393,6 +393,12 @@ class ConcatSettings(ez.Settings):
     new_key: str | None = None
     """Output AxisArray key. If None, uses the key from signal A."""
 
+    auto_coerce_backend: bool = False
+    """If True, silently coerce signal B to signal A's array namespace when the
+    two inputs are on mismatched backends (e.g. MLX vs numpy). Defaults to False
+    (strict): a backend mismatch raises a clear error instead, since it is almost
+    always an upstream bug and silent coercion hides device<->host copies."""
+
 
 @dataclass
 class ConcatState:
@@ -450,6 +456,18 @@ def _concat(self, a: AxisArray, b: AxisArray) -> AxisArray:
         new_axis = concat_dim not in a.dims
 
         xp = get_namespace(a.data)
+        xp_b = get_namespace(b.data)
+        if xp_b is not xp:
+            if self.settings.auto_coerce_backend:
+                b = replace(b, data=xp.asarray(b.data))
+            else:
+                raise TypeError(
+                    f"Concat received inputs on mismatched backends: "
+                    f"a.data namespace={xp.__name__}, b.data namespace={xp_b.__name__}. "
+                    f"Coerce both inputs to one backend upstream "
+                    f"(e.g., via ezmsg.sigproc.asarray.AsArrayTransformer) before merging, "
+                    f"or set ConcatSettings(auto_coerce_backend=True) to coerce B to A's backend."
+                )
 
         # expand_dims for new-axis concatenation.
         if new_axis:
diff --git a/src/ezmsg/sigproc/resample.py b/src/ezmsg/sigproc/resample.py
index 5fec60d..4c98d67 100644
--- a/src/ezmsg/sigproc/resample.py
+++ b/src/ezmsg/sigproc/resample.py
@@ -7,6 +7,7 @@
 import ezmsg.core as ez
 import numpy as np
 import scipy.interpolate
+from array_api_compat import get_namespace
 from ezmsg.baseproc import (
     BaseConsumerUnit,
     BaseStatefulProcessor,
@@ -235,6 +236,13 @@ def __next__(self) -> AxisArray:
         # Calculate output
         resampled_data = f(xnew)
 
+        # scipy.interpolate.interp1d always returns numpy. Coerce back to the
+        # source's namespace so downstream merges with same-backend streams
+        # don't see a backend mismatch.
+        src_xp = get_namespace(src_axarr.data)
+        if get_namespace(resampled_data) is not src_xp:
+            resampled_data = src_xp.asarray(resampled_data)
+
         # Create output message
         if hasattr(ref_ax, "data"):
             out_ax = replace(ref_ax, data=xnew)
diff --git a/tests/unit/test_concat.py b/tests/unit/test_concat.py
index d3ebda1..d4ccdc1 100644
--- a/tests/unit/test_concat.py
+++ b/tests/unit/test_concat.py
@@ -366,6 +366,65 @@ def _msg_mx(np_data, ch_labels):
         np.testing.assert_array_equal(np.asarray(result.data[:, 2:]), 3.0)
 
 
+class TestBackendMismatch:
+    """Concat across mismatched array backends should fail loudly (default)
+    or coerce on opt-in, instead of raising a cryptic MLX-internals error."""
+
+    @staticmethod
+    def _msg_mx(np_data, ch_labels):
+        import mlx.core as mx
+
+        return AxisArray(
+            data=mx.array(np_data),
+            dims=["time", "ch"],
+            axes=frozendict(
+                {
+                    "time": AxisArray.TimeAxis(fs=100.0, offset=0.0),
+                    "ch": CoordinateAxis(data=np.array(ch_labels), dims=["ch"], unit="label"),
+                }
+            ),
+            key="",
+        )
+
+    @requires_mlx
+    def test_mismatch_raises_clear_error(self):
+        """MLX a + numpy b -> clear, named backend-mismatch error."""
+        n = 5
+        msg_a = self._msg_mx(np.ones((n, 2), dtype=np.float32), ["A0", "A1"])
+        msg_b = _make_msg(np.ones((n, 2), dtype=np.float32) * 2, ch_labels=["B0", "B1"])
+
+        proc = ConcatProcessor(ConcatSettings(axis="feature"))
+        with pytest.raises(TypeError, match="mismatched backends"):
+            proc._concat(msg_a, msg_b)
+
+    @requires_mlx
+    def test_auto_coerce_backend_coerces_b_to_a(self):
+        """auto_coerce_backend=True coerces b onto a's namespace instead of raising."""
+        import mlx.core as mx
+
+        n = 5
+        msg_a = self._msg_mx(np.ones((n, 2), dtype=np.float32), ["A0", "A1"])
+        msg_b = _make_msg(np.ones((n, 2), dtype=np.float32) * 2, ch_labels=["B0", "B1"])
+
+        proc = ConcatProcessor(ConcatSettings(axis="feature", auto_coerce_backend=True))
+        result = proc._concat(msg_a, msg_b)
+
+        assert isinstance(result.data, mx.array)
+        assert result.data.shape == (n, 2, 2)
+        np.testing.assert_array_equal(np.asarray(result.data[:, :, 0]), 1.0)
+        np.testing.assert_array_equal(np.asarray(result.data[:, :, 1]), 2.0)
+
+    def test_same_backend_numpy_unaffected(self):
+        """Matched numpy inputs are never coerced or rejected (default settings)."""
+        n = 5
+        msg_a = _make_msg(np.ones((n, 2)), ch_labels=["A0", "A1"])
+        msg_b = _make_msg(np.ones((n, 2)) * 2, ch_labels=["B0", "B1"])
+
+        result = ConcatProcessor(ConcatSettings(axis="feature"))._concat(msg_a, msg_b)
+        assert isinstance(result.data, np.ndarray)
+        assert result.data.shape == (n, 2, 2)
+
+
 class TestAssertIdenticalSharedAxes:
     def test_identical_passes(self):
         settings = ConcatSettings(axis="feature", assert_identical_shared_axes=True)
diff --git a/tests/unit/test_resample.py b/tests/unit/test_resample.py
index 0a9825c..0f8953d 100644
--- a/tests/unit/test_resample.py
+++ b/tests/unit/test_resample.py
@@ -7,6 +7,7 @@
 from ezmsg.util.messages.chunker import array_chunker
 
 from ezmsg.sigproc.resample import ResampleProcessor
+from tests.helpers.util import requires_mlx
 
 
 @pytest.fixture
@@ -128,6 +129,51 @@ async def test_resample_project(irregular_messages):
     print(result)
 
 
+@requires_mlx
+def test_resample_preserves_mlx_backend():
+    """ResampleProcessor must keep the source's array namespace.
+
+    scipy.interpolate.interp1d always returns numpy, so without the
+    namespace coercion an MLX input silently downgrades to numpy and a
+    downstream same-backend merge then fails with a cryptic error.
+    """
+    import mlx.core as mx
+    from array_api_compat import get_namespace
+
+    np.random.seed(0)
+    fs = 128.0
+    n = 200
+    tvec = np.sort(np.arange(n) / fs + np.random.normal(0, 0.001, n))
+    ch_ax = AxisArray.CoordinateAxis(data=np.arange(3).astype(str), dims=["ch"], unit="label")
+
+    def _mk(a: int, b: int) -> AxisArray:
+        return AxisArray(
+            data=mx.array(np.random.randn(b - a, 3).astype(np.float32)),
+            dims=["time", "ch"],
+            axes={
+                "time": AxisArray.CoordinateAxis(data=tvec[a:b], dims=["time"], unit="s"),
+                "ch": ch_ax,
+            },
+            key="mlx_stream",
+        )
+
+    resample = ResampleProcessor(resample_rate=100.0, buffer_duration=4.0)
+    mlx_ns = get_namespace(mx.array([1.0]))
+    seen_output = False
+    for i in range(0, n, 40):
+        resample(_mk(i, min(i + 40, n)))
+        result = next(resample)
+        if result.data.shape[0] == 0:
+            continue
+        seen_output = True
+        assert isinstance(result.data, mx.array), (
+            f"Expected mx.array out, got {type(result.data).__name__}"
+        )
+        assert get_namespace(result.data) is mlx_ns
+
+    assert seen_output, "Resampler never produced a non-empty output to check."
+
+
 @pytest.mark.asyncio
 async def test_resample_no_input():
     """Test calling next() on ResampleProcessor before receiving any input messages."""