Preview Tax

Query the tax rules that apply to a checkout session and, once enough billing information is available, get the calculated tax amount. This endpoint is designed for progressive collection: only billingDetail.country is required. The response tells you which additional fields that country needs via rules.requiredFields, so the checkout form can ask for them one step at a time instead of demanding everything upfront.

POST /v1/actions/checkout/preview-tax

Authentication: Store Slug (visitor role)

This endpoint is called by the hosted checkout page as the customer fills in their billing details. If you are building your own checkout UI, call it whenever the customer’s country or business status changes.

Request Body

Field	Type	Required	Description
`checkoutSessionId`	string	Yes	Session ID returned by Create Checkout Session
`billingDetail`	object	Yes	Billing information collected so far (see below)

billingDetail

Field	Type	Required	Description
`country`	string	Yes	ISO 3166-1 alpha-2 country code (e.g. `US`, `DE`, `JP`)
`isBusiness`	boolean	No	Whether the customer is buying as a business
`state`	string	No	State / province code
`postcode`	string	No	Postal code
`businessName`	string	No	Legal business name
`taxId`	string	No	Business tax ID (e.g. EU VAT number)

Only country is required. Which of the remaining fields you need to collect is decided by the response, not by a fixed rule per country — see rules.requiredFields.

Response

Field	Type	Description
`currency`	string	Currency of the checkout session
`rules`	object	Tax rules for the given country
`calculation`	object \| null	Calculated amounts, or `null` when more fields are still needed

rules

Field	Type	Description
`country`	string	Country code the rules apply to
`taxName`	string	Name of the tax (e.g. `JCT`, `VAT`)
`standardRate`	number	Standard rate as a decimal (`0.1` = 10%)
`isTaxFree`	boolean	Whether this country is tax-exempt
`isEuReverseChargeEligible`	boolean	Whether EU reverse charge can apply
`hasReducedRates`	boolean	Whether reduced rates exist for some product categories
`requiredFields`	array	Fields still needed before tax can be calculated (see below)

requiredFields

Each entry describes one field the checkout form should collect next:

Field	Type	Description
`field`	string	One of `isBusiness`, `state`, `postcode`, `businessName`, `taxId`
`label`	string	Human-readable label for the input
`reason`	string	Why this country needs it — safe to show to the customer
`inputType`	string	Suggested input control

When requiredFields is empty, calculation is populated.

calculation

Field	Type	Description
`subtotal`	string	Amount before tax, in display format (e.g. `"8.39"`)
`taxAmount`	string	Tax amount, in display format
`total`	string	Amount the customer will be charged
`taxRate`	number	Rate applied to this order

Amounts are display-format strings, matching the rest of the API. taxRate is a decimal (0.19 = 19%).

Examples

curl -X POST https://api.waffo.ai/v1/actions/checkout/preview-tax \
  -H "X-Store-Slug: my-store-abc123" \
  -H "X-Environment: prod" \
  -H "Content-Type: application/json" \
  -d '{
    "checkoutSessionId": "cs_xxx",
    "billingDetail": { "country": "US" }
  }'

curl -X POST https://api.waffo.ai/v1/actions/checkout/preview-tax \
  -H "X-Store-Slug: my-store-abc123" \
  -H "X-Environment: prod" \
  -H "Content-Type: application/json" \
  -d '{
    "checkoutSessionId": "cs_xxx",
    "billingDetail": { "country": "DE", "isBusiness": false }
  }'

More information needed — the form should now ask for state:

{
  "data": {
    "currency": "USD",
    "rules": {
      "country": "US",
      "taxName": "Sales Tax",
      "standardRate": 0,
      "isTaxFree": false,
      "isEuReverseChargeEligible": false,
      "hasReducedRates": false,
      "requiredFields": [
        {
          "field": "state",
          "label": "State",
          "reason": "Sales tax in the United States is determined at state level",
          "inputType": "select"
        }
      ]
    },
    "calculation": null
  }
}

Ready to calculate:

{
  "data": {
    "currency": "EUR",
    "rules": {
      "country": "DE",
      "taxName": "VAT",
      "standardRate": 0.19,
      "isTaxFree": false,
      "isEuReverseChargeEligible": true,
      "hasReducedRates": true,
      "requiredFields": []
    },
    "calculation": {
      "subtotal": "8.39",
      "taxAmount": "1.60",
      "total": "9.99",
      "taxRate": 0.19
    }
  }
}

Errors

Status	Message	Cause
400	`Missing required field: checkoutSessionId`	`checkoutSessionId` not provided
400	`Missing required field: billingDetail.country`	`billingDetail` missing or has no `country`
400	`Missing or invalid header: x-context-environment`	`X-Environment` header missing or not `test` / `prod`
404	`Checkout session not found or expired`	Session does not exist, has expired, belongs to another store, or was created in a different environment
500	`Internal server error`	Unexpected server-side failure

Overview

Auth

Merchant API (API Key)

Customer Portal API (Session Token)

GraphQL

Request Body

billingDetail

Response

rules

requiredFields

calculation

Examples

Errors

​Request Body

​billingDetail

​Response

​rules

​requiredFields

​calculation

​Examples

​Errors

Request Body

billingDetail

Response

rules

requiredFields

calculation

Examples

Errors