docs(budget): document model_path requirement

Author: lan17

evaluators/contrib/budget/README.md

@@ -99,7 +99,7 @@ ModelPricing(input_per_1k=0.04, output_per_1k=0.16)
 
 `input_per_1k` is applied to input tokens. `output_per_1k` is applied to output tokens.
 
-Pricing is required when any rule uses `limit_unit="usd_cents"`. Token-only rules can omit pricing. If an event uses a model that is not in the pricing table and a cost rule exists, `unknown_model_behavior="block"` fails closed. Use `"warn"` to log a warning and treat the cost as 0.
+Pricing and `model_path` are required when any rule uses `limit_unit="usd_cents"`. Token-only rules can omit both. If an event uses a model that is not in the pricing table and a cost rule exists, `unknown_model_behavior="block"` fails closed. Use `"warn"` to log a warning and treat the cost as 0.
 
 ## Dual Ceiling Pattern
 

evaluators/contrib/budget/src/agent_control_evaluator_budget/budget/config.py

@@ -79,11 +79,13 @@ class BudgetEvaluatorConfig(EvaluatorConfig):
             pricing table and a cost-based rule exists. block=fail closed,
             warn=log warning and treat cost as 0.
         pricing: Optional model pricing table. Maps model name to ModelPricing.
-            Used to derive cost in USD from token counts and model name.
+            Required when any rule uses limit_unit="usd_cents". Used to
+            derive cost in USD from token counts and model name.
         token_path: Dot-notation path to extract token usage from step
             data (e.g. "usage.total_tokens"). If None, looks for standard
             fields (input_tokens, output_tokens, total_tokens, usage).
         model_path: Dot-notation path to extract model name (for pricing lookup).
+            Required when any rule uses limit_unit="usd_cents".
         metadata_paths: Mapping of metadata field name to dot-notation path
             in step data. Used to extract scope dimensions (channel, user_id, etc).
     """