diff --git a/benchmarks/bench.py b/benchmarks/bench.py
index 74f19c5..ae13288 100644
--- a/benchmarks/bench.py
+++ b/benchmarks/bench.py
@@ -71,6 +71,17 @@ def number(value):
     )
 
 
+WIRE_DECIMALS = 4  # workers round probabilities on the wire to four decimal places
+
+
+def sum_allowance(count):
+    """How far `count` wire probabilities may miss a normalized sum: each rounded value
+    carries up to half a decimal step, so the allowance grows with the label count. A
+    flat 1e-4 rejected four-value distributions summing to 0.9999 at the float boundary
+    (PR #40, first GPU run)."""
+    return count * 0.5 * 10**-WIRE_DECIMALS + 1e-9
+
+
 def read_answers(case, payload):
     """Validate wire results and normalize probabilities by label, never by order."""
     answers = payload["answers"]
@@ -97,7 +108,8 @@ def read_answers(case, payload):
             value = answer[kind]
         if any(not number(p) or not 0 <= p <= 1 for p in probabilities.values()):
             raise ValueError("probabilities must be finite and in [0, 1]")
-        if not math.isclose(sum(probabilities.values()), 1, abs_tol=1e-4):
+        total = math.fsum(probabilities.values())
+        if abs(total - 1) > sum_allowance(len(probabilities)):
             raise ValueError("probabilities must sum to one")
         if kind == "choice":
             if value not in probabilities or probabilities[value] != max(
diff --git a/tests/benchmarks/test_bench.py b/tests/benchmarks/test_bench.py
index 99cf8b0..3ca1791 100644
--- a/tests/benchmarks/test_bench.py
+++ b/tests/benchmarks/test_bench.py
@@ -176,6 +176,32 @@ def test_all_primitives_and_invalid_distributions(self):
         with self.assertRaises(ValueError):
             bench.read_answers(CASES[2], payload)
 
+    def test_rounded_probabilities_at_the_sum_boundary(self):
+        """PR #40's first GPU run: the worker rounds probabilities to four decimals, and
+        four rounded values legitimately miss 1 by a full rounding step — a flat 1e-4
+        tolerance rejected both vectors below at the floating-point boundary."""
+        case = {
+            "id": "mmlu-test-10312",
+            "request": {
+                "questions": {"q": {"type": "choice", "criteria": ["A", "B", "C", "D"]}}
+            },
+            "expected": {},
+        }
+        for probabilities in (
+            {"A": 0.1099, "B": 0.1211, "C": 0.5589, "D": 0.2100},
+            {"A": 0.1130, "B": 0.5135, "C": 0.3151, "D": 0.0583},
+        ):
+            answer = {
+                "choice": max(probabilities, key=probabilities.get),
+                "probabilities": probabilities,
+            }
+            bench.read_answers(case, {"answers": {"q": answer}})
+        off = {"A": 0.4, "B": 0.3, "C": 0.2, "D": 0.09}
+        with self.assertRaises(ValueError):
+            bench.read_answers(
+                case, {"answers": {"q": {"choice": "A", "probabilities": off}}}
+            )
+
     def test_duplicate_ids(self):
         with tempfile.TemporaryDirectory() as directory:
             path = Path(directory) / "requests.jsonl"