fix(evaluators): budget R5 -- isfinite guard on estimated cost

R5 finding: Inf pricing rates produced inf cost, permanently locking
buckets in exceeded state. max(0.0, inf) = inf.

Fix: isfinite + negative check on _estimate_cost return value.
Tests: Inf pricing rate test, strengthened negative pricing assertion.

59 budget tests passing.

Author: amabito

evaluators/builtin/src/agent_control_evaluators/budget/evaluator.py

@@ -145,7 +145,9 @@ def _estimate_cost(
     input_rate = rates.get("input_per_1k", 0.0)
     output_rate = rates.get("output_per_1k", 0.0)
     cost = (input_tokens * input_rate + output_tokens * output_rate) / 1000.0
-    return max(0.0, cost)  # never return negative cost
+    if not math.isfinite(cost) or cost < 0:
+        return 0.0
+    return cost
 
 
 def _extract_metadata(data: Any, metadata_paths: dict[str, str]) -> dict[str, str]:

evaluators/builtin/tests/budget/test_budget.py

@@ -477,4 +477,20 @@ async def test_negative_pricing_does_not_reduce_budget(self) -> None:
         for _ in range(10):
             await ev.evaluate({"model": "model", "usage": {"input_tokens": 1000, "output_tokens": 1000}})
         snap = ev._store.get_snapshot("__global__", "", limit_usd=0.01)
-        assert snap.spent_usd >= 0.0  # must not go negative
+        assert snap.spent_usd == pytest.approx(0.0)  # negative rates clamped to 0
+
+    @pytest.mark.asyncio
+    async def test_inf_pricing_does_not_cause_inf_cost(self) -> None:
+        """Inf pricing rates must not produce inf cost (permanent false positive)."""
+        import math as _math
+        from agent_control_evaluators.budget.evaluator import BudgetEvaluator
+        config = BudgetEvaluatorConfig(
+            limits=[{"limit_usd": 100.0}],
+            pricing={"model": {"input_per_1k": float("inf"), "output_per_1k": 0.01}},
+            model_path="model",
+        )
+        ev = BudgetEvaluator(config)
+        await ev.evaluate({"model": "model", "usage": {"input_tokens": 100, "output_tokens": 100}})
+        snap = ev._store.get_snapshot("__global__", "", limit_usd=100.0)
+        assert _math.isfinite(snap.spent_usd)
+        assert snap.spent_usd == pytest.approx(0.0)  # inf rate -> cost clamped to 0