To view a previously requested calculation, send an HTTP GET to https://api.fonoa.com/tax/v2/calculations/<uuid> replacing <uuid> with the identifier received in the calculation response (data.id)

If the uuid is valid, the API will respond with an HTTP 200 OK and a response body containing the details of the calculation:

{
    "data": {
        "uuid": "1010b4d8-9361-4a7e-98cc-9419dbba695c",
        "status": "calculated",
        "input": {
            "supplier": {
                "taxable": true,
                "country": "gb"
            },
            "customer": {
                "taxable": false,
                "country": "nl"
            },
            "transaction": {
                "price_includes_tax": false,
                "date": "2026-01-14T20:48:59.000Z",
                "items": [
                    {
                        "id": "34db79d1-24bf-4c22-863e-1f7832f24be3",
                        "unit_price": 100,
                        "tax_category_code": "fs_0503010000",
                        "quantity": 1
                    }
                ]
            }
        },
        "output": {
            "id": "1010b4d8-9361-4a7e-98cc-9419dbba695c",
            "result": {
                "total_gross": 121,
                "total_net": 100,
                "total_indirect_tax_amount": 21,
                "items": [
                    {
                        "id": "34db79d1-24bf-4c22-863e-1f7832f24be3",
                        "gross_amount": 121,
                        "net_amount": 100,
                        "indirect_tax_amount": 21,
                        "effective_indirect_tax_rate": 0.21,
                        "tax_breakdown": [
                            {
                                "country": "nl",
                                "country_name": "Netherlands",
                                "rate_class": "s",
                                "tax_level": "federal",
                                "name": "BTW",
                                "rate": 0.21,
                                "amount": 21
                            }
                        ]
                    }
                ]
            }
        }
    }
}

🚧
Eventual Consistency
Requested calculations are saved in the background. Expect a minor delay between an HTTP POST of a calculation and it appearing in a subsequent GET
Given the calculation result is returned immediately on the POST request, we don’t see a use case to call the GET endpoint immediately and thus deem a minor delay to be acceptable. The result is expected be available within 1 minute.
In the interim, the HTTP GET to https://api.fonoa.com/tax/v2/calculations/<uuid> will respond with an HTTP 202 Accepted and a Retry-After: <time in s> header suggesting when you can call the GET endpoint again, and the following response body:
{
  "data": {
    "id": "<uuid>",
    "status": "pending",
    "message": "The calculation is in the process of being stored. Please retry later"
  }
}
Feedback is appreciated, Fonoa will monitor this behaviour and adjust if required.

⚠️ Calculation storage

Please do not rely on Fonoa’s calculation resource urls to be long lived.

Fonoa provides both hot (immediate access) and cold (long term, no immediate access) storage. Data will be stored in cold storage for the duration specified by law. Data in hot storage is subject to contractual agreements (we default to 3 months)