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Use structured tax codes instead of free-text VAT names. Friday validates the code against the business’s chart of accounts, calculates line tax when an amount is not supplied, and stores the code on the invoice line item for auditability.

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Fetch Available Codes

Fetch the active codes for the business before creating taxable invoices: 
GET https://sandbox.thredfi.com/v1/platform/businesses/{business_id}/tax-codes/
Authorization: Bearer {business_token}
Example response: 
{
  "business_id": "964f4325-3efb-400d-a1bd-8b1f29e828cf",
  "count": 3,
  "tax_codes": [
    {
      "code": "BTW_21",
      "name": "VAT 21% (Standard)",
      "rate": 0.21,
      "purchase_account": "1520",
      "sales_account": "2100",
      "effective_from": "2024-01-01",
      "is_active": true
    },
    {
      "code": "EXEMPT",
      "name": "VAT Exempt",
      "rate": 0,
      "purchase_account": null,
      "sales_account": null,
      "effective_from": "2024-01-01",
      "is_active": true
    }
  ]
}

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Create Invoices With Tax Codes

Pass the code on each invoice line item: 
{
  "external_id": "invoice-2026-001",
  "customer_id": "customer-uuid",
  "invoice_number": "INV-2026-001",
  "sent_at": "2026-06-11T10:00:00Z",
  "due_at": "2026-07-11T10:00:00Z",
  "line_items": [
    {
      "external_id": "line-001",
      "description": "Consulting services",
      "quantity": 1,
      "unit_price": 10000,
      "tax_code": "BTW_21"
    }
  ]
}
When tax_code is valid and no explicit tax amount is supplied, Friday calculates the tax from the configured rate and persists the code on the line item. The same tax_code field is returned in invoice line-item responses.

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sales_taxes Compatibility

If your system already sends structured tax lines, include sales_taxes[].code: 
{
  "description": "Consulting services",
  "quantity": 1,
  "unit_price": 10000,
  "sales_taxes": [
    {
      "code": "BTW_21",
      "name": "VAT 21% (Standard)",
      "amount": 2100
    }
  ]
}
sales_taxes.name is accepted only when a code is also present. The code is the canonical value used for validation and audit.

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Validation Rules

  • Unknown or inactive tax codes return a 4xx validation error.
  • Conflicting line-level codes, such as tax_code: "BTW_21" plus sales_taxes[].code: "EXEMPT", return a validation error.
  • If you provide both a tax code and explicit sales_taxes[].amount, the amount must match the configured tax-code rate within rounding tolerance.
  • Customers marked tax_exempt: true reject taxable invoice line items. Use EXEMPT or another zero-rated code.
  • Tax-exempt errors identify the customer and the offending line_items[index].
Do not infer VAT treatment from the tax name alone. Always fetch and send a valid code.