diff --git a/test/test_api_v3_non_finite_numbers.py b/test/test_api_v3_non_finite_numbers.py
index c0392912..4e6c217b 100644
--- a/test/test_api_v3_non_finite_numbers.py
+++ b/test/test_api_v3_non_finite_numbers.py
@@ -23,24 +23,34 @@ sys.path.insert(0, str(Path(__file__).parent.parent))
 from test._api_v3_test_helpers import api_v3_client, api_v3_module  # noqa: F401,E402
 
 
-#: (route, body) pairs that returned 500 before OverflowError was caught.
+#: (route, field) that returned 500 before OverflowError was caught. Both
+#: infinity signs are exercised: int() raises OverflowError for either, but
+#: only one of them was in the original report, and a guard that special-cased
+#: the sign would pass a one-sided test.
+NON_FINITE_ROUTES = [
+    ('/api/v3/config/dim-schedule', 'dim_brightness'),
+    ('/api/v3/errors/clear', 'max_age_hours'),
+    ('/api/v3/config/main', 'multiplexing'),
+    ('/api/v3/config/main', 'row_address_type'),
+]
 NON_FINITE_CASES = [
-    ('/api/v3/config/dim-schedule', '{"dim_brightness": Infinity}'),
-    ('/api/v3/config/dim-schedule', '{"dim_brightness": -Infinity}'),
-    ('/api/v3/errors/clear', '{"max_age_hours": Infinity}'),
-    ('/api/v3/config/main', '{"multiplexing": Infinity}'),
-    ('/api/v3/config/main', '{"row_address_type": Infinity}'),
+    (route, '{"%s": %s}' % (field, literal))
+    for route, field in NON_FINITE_ROUTES
+    for literal in ('Infinity', '-Infinity')
 ]
 
 
 @pytest.mark.parametrize("route,body", NON_FINITE_CASES)
 def test_infinity_is_a_client_error_not_a_server_error(api_v3_client, route, body):
+    """Exactly 400, not merely "some 4xx".
+
+    Accepting any 4xx would let a 404 pass, so renaming one of these routes
+    would leave the test green while testing nothing -- the failure mode this
+    whole file exists to catch.
+    """
     response = api_v3_client.post(route, data=body, content_type='application/json')
-    assert response.status_code != 500, (
-        f"{route} with {body} raised instead of validating"
-    )
-    assert 400 <= response.status_code < 500, (
-        f"{route} answered {response.status_code}; expected a 4xx"
+    assert response.status_code == 400, (
+        f"{route} with {body} answered {response.status_code}; expected 400"
     )
 
 
@@ -52,22 +62,24 @@ def test_nan_is_also_a_client_error(api_v3_client, route, body):
     """int(nan) raises ValueError so this path already worked -- pinned so a
     refactor that narrows the except tuple cannot quietly break it."""
     response = api_v3_client.post(route, data=body, content_type='application/json')
-    assert 400 <= response.status_code < 500
+    assert response.status_code == 400
 
 
-def test_a_valid_number_is_not_rejected_by_the_guard(api_v3_client):
-    """The widened except must not start swallowing ordinary input.
+def test_a_valid_number_is_accepted(api_v3_client, api_v3_module, monkeypatch):
+    """Prove the widened except did not start swallowing ordinary input.
 
-    Asserting on 2xx is not possible here: every manager is a MagicMock, so
-    the save path fails downstream whatever is posted. What this can show is
-    that a valid number gets past *validation* -- it is not answered with a
-    400, and nothing in the response mentions the coercion failing.
+    Asserting "not a 400" would not show that: the mocked save path fails for
+    any input, so the assertion would hold even if validation had rejected the
+    value. Give load_config a real dict and stub the atomic save, and the
+    endpoint reaches its success response -- which only happens if 30 passed
+    validation.
     """
+    api_v3_module.api_v3.config_manager.load_config.return_value = {}
+    monkeypatch.setattr(api_v3_module, '_save_config_atomic',
+                        lambda *a, **k: (True, ''))
     response = api_v3_client.post(
         '/api/v3/config/dim-schedule',
         data='{"dim_brightness": 30}',
         content_type='application/json',
     )
-    assert response.status_code != 400, "a valid brightness was rejected"
-    assert b'must be an integer' not in response.get_data()
-    assert b'OverflowError' not in response.get_data()
+    assert response.status_code == 200, response.get_data(as_text=True)[:200]