diff --git a/README.md b/README.md
index f0449f5..9697dc6 100644
--- a/README.md
+++ b/README.md
@@ -269,6 +269,20 @@ option. To additionally suppress all but critical log messages, use `--quiet`. F
 via the `--log-level` option. `--debug` is equivalent to `--log-level=DEBUG`, and `--quiet` is equivalent to
 `--log-level=CRITICAL --no-status`.
 
+### Environment Variables
+`GRAPHTAGE_BATCH_BACKEND` pins the backend that computes Levenshtein distances in batch. Graphtage picks one by batch
+size: `python` prices a single pair at a time, and `numpy` advances a whole batch through one array pass per row. Both
+return identical distances, so set this variable only to benchmark one backend against the other, or to work around a
+problem with the automatic choice.
+
+| Value | Effect |
+|-------|--------|
+| `python` | Price every pair one at a time. |
+| `numpy` | Always use the batched scan. |
+| unset | Pick by batch size. This is the default. |
+
+An unrecognized value raises a `ValueError` rather than falling back to a default.
+
 ### Version Information
 `--version` or `-v` writes a line such as `Graphtage version 0.4.0` to STDERR. If you pass it without any input files,
 Graphtage prints the version and exits; if you pass input files as well, it prints the version and then computes the
diff --git a/test/test_batch_distance.py b/test/test_batch_distance.py
index aab786f..37f19cc 100644
--- a/test/test_batch_distance.py
+++ b/test/test_batch_distance.py
@@ -69,6 +69,18 @@ def mixed_corpus() -> list[str]:
     ]
 
 
+def no_backend_pin():
+    """Removes any ambient ``GRAPHTAGE_BATCH_BACKEND`` for the duration of the context.
+
+    A test that watches the *automatic* backend choice can only observe it when nothing pins one. Without this, a
+    developer who exports the variable to exercise the other backend gets spurious failures from tests that never
+    mention it.
+
+    """
+    unpinned = {name: value for name, value in os.environ.items() if name != BACKEND_ENV_VAR}
+    return patch.dict(os.environ, unpinned, clear=True)
+
+
 class TestBackendAgreement(TestCase):
     def test_backends_agree_with_levenshtein_distance(self):
         """Every backend returns exactly what ``levenshtein_distance`` returns, over a varied corpus.
@@ -245,7 +257,8 @@ def test_environment_variable_overrides_the_automatic_choice(self):
             with patch.dict(os.environ, {BACKEND_ENV_VAR: 'python'}):
                 all_pairs(corpus, corpus)
             batched.assert_not_called()
-            all_pairs(corpus, corpus)
+            with no_backend_pin():
+                all_pairs(corpus, corpus)
             self.assertTrue(batched.called, 'this batch should have gone to the numpy backend on its own')
 
     def test_environment_variable_names_are_validated(self):
@@ -493,7 +506,9 @@ def test_one_big_pair_goes_to_a_vectorized_backend(self):
         short_left, short_right = 'kitten', 'sitting'
         self.assertGreaterEqual(len(long_left) * len(long_right), batch_distance.VECTORIZED_MIN_CELLS)
         self.assertLess(len(short_left) * len(short_right), batch_distance.VECTORIZED_MIN_CELLS)
-        with patch.object(NumpyBackend, 'distances', autospec=True, side_effect=NumpyBackend.distances) as batched:
+        with no_backend_pin(), patch.object(
+            NumpyBackend, 'distances', autospec=True, side_effect=NumpyBackend.distances
+        ) as batched:
             self.assertEqual(levenshtein_distance(short_left, short_right), cost(short_left, short_right))
             batched.assert_not_called()
             self.assertEqual(levenshtein_distance(long_left, long_right), cost(long_left, long_right))