diff --git a/src/format_bench/model.py b/src/format_bench/model.py
index ed234ec..123659c 100644
--- a/src/format_bench/model.py
+++ b/src/format_bench/model.py
@@ -225,7 +225,7 @@ def validate(self) -> None:
         if self.kind is WorkloadKind.FILTER:
             if (
                 not self.column
-                or self.operator is None
+                or not is_comparison_operator(self.operator)
                 or self.value is None
             ):
                 raise ValueError(f"filter workload {self.operation} needs a supported predicate")
diff --git a/tests/test_model.py b/tests/test_model.py
index d2dee23..caac697 100644
--- a/tests/test_model.py
+++ b/tests/test_model.py
@@ -19,6 +19,7 @@
     robustness_verdict,
     transition,
 )
+from format_bench.workload_contract import ComparisonOperator
 
 
 def test_lifecycle_accepts_the_public_contract() -> None:
@@ -125,6 +126,20 @@ def test_workload_spec_rejects_incomplete_predicates() -> None:
         )
 
 
+def test_workload_spec_rejects_unsupported_direct_operator() -> None:
+    unsupported = cast(ComparisonOperator, "contains")
+    spec = WorkloadSpec(
+        "filter",
+        WorkloadKind.FILTER,
+        column="value",
+        operator=unsupported,
+        value="needle",
+    )
+
+    with pytest.raises(ValueError, match="supported predicate"):
+        spec.validate()
+
+
 @pytest.mark.parametrize("value", [1.5, "1.5"])
 def test_workload_spec_rejects_fractional_integer_fields(value: object) -> None:
     with pytest.raises(ValueError, match="must be an integer"):