Saving Variables in Tavern 1.0.0

I am testing my REST endpoints with Tavern. Basically, all I am doing is doing a GET request, trying to save the response, and then use a part of that response for a subsequent UPDATE test.

# test_.tavern.yaml file

# GET
test_name: Get Rules by ServiceCode
stages:
  - name: Get Rules by a ServiceCode
    request:
      method: GET
      url: http://localhost:9000/record/v1/admin/rules
      params:
        service_code: "ZEL302" #hardcoded
    response:
      status_code: 200
      save:
        $ext:
          function: rule:save_rule_no # saving the results into a variable called "saved_rule_no"
---

# UPDATE
test_name: Update Rule
stages:
  - name: Update a Rule
    request:
      method: PUT
      url: "http://localhost:9000/record/v1/admin/rules/69"
      json:
        ruleNo: !int "{saved_rule_no}" # use the saved variable here
        category: "tavernc"
        identifier: "taverni"
        operator: "taverno"
        value: "tavernv"
        productId: "tavernpid"
        serviceCode: "tavernsc"
        templateNo: 1 #hardcoded
    response:
      status_code: 200

# rule.py file

def save_rule_no(response):
    """
    After creating a Rule, get the Rule's ruleNo field to use for testing update/delete
    """
    ruleNo = response.json()[0]["ruleNo"]
    return Box({"saved_rule_no": ruleNo})

I get the error below saying the variable is not recognized. This approach works on Tavern 0.34.0 but not for 1.0.0. How would you do this for 1.0.0?

scenario/test_.tavern.yaml::Insert Rules PASSED                                                                                                      [ 25%]
scenario/test_.tavern.yaml::Get Rules by ProductId PASSED                                                                                            [ 50%]
scenario/test_.tavern.yaml::Get Rules by ServiceCode PASSED                                                                                          [ 75%]
scenario/test_.tavern.yaml::Update Rule FAILED                                                                                                       [100%]

========================================================================= FAILURES =========================================================================
Format variables:
  saved_rule_no = '???'