diff --git a/openfold3/core/utils/chunk_utils.py b/openfold3/core/utils/chunk_utils.py
index 4c9b850c..4037d519 100644
--- a/openfold3/core/utils/chunk_utils.py
+++ b/openfold3/core/utils/chunk_utils.py
@@ -372,17 +372,20 @@ def test_chunk_size(chunk_size):
             except RuntimeError:
                 return False
 
-        min_viable_chunk_size_index = 0
+        # Binary search for largest viable chunk size (min_chunk_size is assumed
+        # to be viable).
+        lo = 0
+        hi = len(candidates)
         i = len(candidates) - 1
-        while i > min_viable_chunk_size_index:
+        while lo < i < hi:
             viable = test_chunk_size(candidates[i])
-            if not viable:
-                i = (min_viable_chunk_size_index + i) // 2
+            if viable:
+                lo = i
             else:
-                min_viable_chunk_size_index = i
-                i = (i + len(candidates) - 1) // 2
+                hi = i
+            i = (lo + hi) // 2
 
-        return candidates[min_viable_chunk_size_index]
+        return candidates[lo]
 
     def _compare_arg_caches(self, ac1, ac2):
         consistent = True
diff --git a/openfold3/tests/utils/test_utils.py b/openfold3/tests/utils/test_utils.py
index 63a33b28..ed3907db 100644
--- a/openfold3/tests/utils/test_utils.py
+++ b/openfold3/tests/utils/test_utils.py
@@ -18,7 +18,7 @@
 import torch
 
 from openfold3.core.model.primitives import Linear
-from openfold3.core.utils.chunk_utils import _chunk_slice, chunk_layer
+from openfold3.core.utils.chunk_utils import ChunkSizeTuner, _chunk_slice, chunk_layer
 from openfold3.core.utils.rigid_utils import (
     Rigid,
     Rotation,
@@ -196,3 +196,51 @@ def test_chunk_slice_dict(self):
                 chunked_flattened = x_flat[i:j]
 
                 self.assertTrue(torch.all(chunked == chunked_flattened))
+
+    def test_chunk_size_tuner_does_not_retest_candidates(self):
+        # Based on previous bug: the binary search forgot which candidates it
+        # had already proven non-viable and re-tested them.
+        for max_viable in (128, 64, 256, 512):
+            with self.subTest(max_viable=max_viable):
+                tested = []
+
+                def fn(arg, chunk_size, _max=max_viable, tested=tested):
+                    tested.append(chunk_size)
+                    if chunk_size > _max:
+                        raise RuntimeError("simulated OOM")
+
+                ChunkSizeTuner._determine_favorable_chunk_size(
+                    fn, args=(None,), min_chunk_size=4, max_chunk_size=1024
+                )
+
+                self.assertEqual(
+                    len(tested),
+                    len(set(tested)),
+                    f"Some candidate was tested more than once: {tested}",
+                )
+
+    def test_chunk_size_tuner_picks_largest_viable(self):
+        # When the cutoff sits between two power-of-2 candidates, the tuner
+        # should pick the largest viable power of 2.
+        cases = [
+            # (max_viable, expected_chunk_size)
+            (1024, 1024),
+            (512, 512),
+            (511, 256),
+            (256, 256),
+            (255, 128),
+            (128, 128),
+            (4, 4),
+            (3, 4),  # nothing viable above min, returns min
+        ]
+        for max_viable, expected in cases:
+            with self.subTest(max_viable=max_viable):
+
+                def fn(arg, chunk_size, _max=max_viable):
+                    if chunk_size > _max:
+                        raise RuntimeError("simulated OOM")
+
+                result = ChunkSizeTuner._determine_favorable_chunk_size(
+                    fn, args=(None,), min_chunk_size=4, max_chunk_size=1024
+                )
+                self.assertEqual(result, expected)