diff --git a/steptronoss/data/nextable/nextable.py b/steptronoss/data/nextable/nextable.py
index 827a48e..2d1255a 100644
--- a/steptronoss/data/nextable/nextable.py
+++ b/steptronoss/data/nextable/nextable.py
@@ -86,9 +86,10 @@ def fast_step(self):
 
     def __next__(self):
         for i in range(self.dp_size):
-            example = next(self.nextable)
             if i == self.dp_rank:
-                my_batch = example
+                my_batch = next(self.nextable)
+            else:
+                self.nextable.fast_step()
         return my_batch
 
     def __len__(self):
diff --git a/tests/test_nextable.py b/tests/test_nextable.py
new file mode 100644
index 0000000..c905000
--- /dev/null
+++ b/tests/test_nextable.py
@@ -0,0 +1,182 @@
+import importlib.util
+import os
+
+import pytest
+
+# nextable.py itself has no torch dependency, but the package __init__.py
+# imports async_accelerate which requires torch.  Load the module directly
+# so these pure-Python tests can run without torch installed.
+_spec = importlib.util.spec_from_file_location(
+    "nextable",
+    os.path.join(os.path.dirname(__file__), os.pardir, "steptronoss", "data", "nextable", "nextable.py"),
+)
+_mod = importlib.util.module_from_spec(_spec)
+_spec.loader.exec_module(_mod)
+
+DPMux = _mod.DPMux
+LazyUpdateNextable = _mod.LazyUpdateNextable
+Nextable = _mod.Nextable
+
+pytestmark = pytest.mark.cpu
+
+
+class SimpleNextable(LazyUpdateNextable):
+    """A minimal LazyUpdateNextable backed by a fixed list that loops."""
+
+    def __init__(self, items: list):
+        self._items = items
+        self._idx = 0
+
+    def get(self):
+        return self._items[self._idx % len(self._items)]
+
+    def update(self):
+        self._idx += 1
+
+    def state_dict(self):
+        return {"idx": self._idx}
+
+    def load_state_dict(self, state):
+        self._idx = state["idx"]
+
+    def __len__(self):
+        return len(self._items)
+
+
+class TestDPMuxBasic:
+    def test_each_rank_gets_correct_item(self):
+        items = [10, 20, 30, 40]
+        dp_size = 4
+        for rank in range(dp_size):
+            source = SimpleNextable(items)
+            mux = DPMux(source, dp_size=dp_size, dp_rank=rank)
+            assert next(mux) == items[rank]
+
+    def test_sequential_batches_across_steps(self):
+        items = list(range(8))
+        dp_size = 2
+        source = SimpleNextable(items)
+        mux = DPMux(source, dp_size=dp_size, dp_rank=0)
+
+        # rank 0 should get items 0, 2, 4, 6
+        results = [next(mux) for _ in range(4)]
+        assert results == [0, 2, 4, 6]
+
+    def test_rank1_gets_odd_items(self):
+        items = list(range(8))
+        dp_size = 2
+        source = SimpleNextable(items)
+        mux = DPMux(source, dp_size=dp_size, dp_rank=1)
+
+        results = [next(mux) for _ in range(4)]
+        assert results == [1, 3, 5, 7]
+
+    def test_dp_size_1_is_passthrough(self):
+        items = [10, 20, 30]
+        source = SimpleNextable(items)
+        mux = DPMux(source, dp_size=1, dp_rank=0)
+
+        results = [next(mux) for _ in range(3)]
+        assert results == items
+
+
+class TestDPMuxStateDict:
+    def test_state_roundtrip(self):
+        items = list(range(12))
+        source = SimpleNextable(items)
+        mux = DPMux(source, dp_size=3, dp_rank=1)
+
+        # Advance a few steps
+        for _ in range(2):
+            next(mux)
+
+        state = mux.state_dict()
+
+        # Create a fresh mux and restore
+        source2 = SimpleNextable(items)
+        mux2 = DPMux(source2, dp_size=3, dp_rank=1)
+        mux2.load_state_dict(state)
+
+        # Both should produce the same items going forward
+        seq1 = [next(mux) for _ in range(3)]
+        seq2 = [next(mux2) for _ in range(3)]
+        assert seq1 == seq2
+
+    def test_state_dict_reflects_underlying_source(self):
+        items = list(range(6))
+        source = SimpleNextable(items)
+        mux = DPMux(source, dp_size=2, dp_rank=0)
+
+        next(mux)  # consumes 2 items from source (dp_size=2)
+        state = mux.state_dict()
+        assert state["idx"] == 2
+
+
+class TestDPMuxFastStep:
+    def test_fast_step_advances_same_as_next(self):
+        items = list(range(12))
+
+        source_slow = SimpleNextable(items)
+        mux_slow = DPMux(source_slow, dp_size=3, dp_rank=0)
+
+        source_fast = SimpleNextable(items)
+        mux_fast = DPMux(source_fast, dp_size=3, dp_rank=0)
+
+        # Advance mux_slow with __next__, mux_fast with fast_step
+        next(mux_slow)
+        mux_fast.fast_step()
+
+        # Internal state should match
+        assert mux_slow.state_dict() == mux_fast.state_dict()
+
+        # Both should produce the same next item
+        assert next(mux_slow) == next(mux_fast)
+
+    def test_fast_step_multiple_times(self):
+        items = list(range(20))
+        dp_size = 4
+
+        source_slow = SimpleNextable(items)
+        mux_slow = DPMux(source_slow, dp_size=dp_size, dp_rank=2)
+
+        source_fast = SimpleNextable(items)
+        mux_fast = DPMux(source_fast, dp_size=dp_size, dp_rank=2)
+
+        for _ in range(3):
+            next(mux_slow)
+            mux_fast.fast_step()
+
+        assert mux_slow.state_dict() == mux_fast.state_dict()
+        assert next(mux_slow) == next(mux_fast)
+
+
+class TestDPMuxLen:
+    def test_len_delegates_to_source(self):
+        items = list(range(10))
+        source = SimpleNextable(items)
+        mux = DPMux(source, dp_size=4, dp_rank=0)
+        assert len(mux) == len(source)
+
+
+class TestDPMuxRequiresSlowFast:
+    def test_rejects_non_slowfast(self):
+        # An object without fast_step should fail the isinstance check
+        class NotSlowFast:
+            pass
+
+        with pytest.raises(AssertionError, match="SlowFastNextable Required"):
+            DPMux(NotSlowFast())
+
+
+class TestDPMuxIterProtocol:
+    def test_works_with_for_loop(self):
+        items = list(range(6))
+        source = SimpleNextable(items)
+        mux = DPMux(source, dp_size=2, dp_rank=0)
+
+        results = []
+        for item in mux:
+            results.append(item)
+            if len(results) == 3:
+                break
+        assert results == [0, 2, 4]