Filekin for Business API

Create XRechnung, ZUGFeRD and Factur-X invoices from your own software, and check any e-invoice against the official rules, with plain-language explanations of every error. Every account can use it: the free Developer plan includes 20 invoices and 100 calls a month, no card needed, and the Business + API and API Plus plans include thousands.

Getting started

1. Fill in your company details: they go on every invoice the API makes.
2. Create an API key on your account page. It's shown once: keep it secret, like a password.
3. Send it with every call: Authorization: Bearer fk_…

Create an invoice

POST https://app.filekin.io/api/v1/invoices with a JSON body. Your company, bank and invoice defaults come from your company details; a saved customer number or product code fills in the rest, just like in a spreadsheet.

curl https://app.filekin.io/api/v1/invoices \
  -H "Authorization: Bearer $FILEKIN_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "number": "RE-2026-0042",
    "issue_date": "2026-11-02",
    "customer": {"name": "Beispiel GmbH", "street": "Marktplatz 3", "postcode": "80331",
                 "city": "München", "country": "DE", "vat_id": "DE987654321"},
    "buyer_reference": "04011000-12345-03",
    "lines": [{"description": "Webdesign", "quantity": 12, "unit": "HUR", "price": 85, "vat_rate": 19}]
  }'

The answer (201) is JSON with the file in base64:

{"number": "RE-2026-0042", "file_name": "Rechnung RE-2026-0042.pdf", "content_type": "application/pdf",
 "file": "JVBERi0xLjcK…", "invoices_left": 1999}

Add ?download=1 to get the file itself instead. Every invoice is checked against the official rules before it's returned.

Fields

Validate an e-invoice

POST https://app.filekin.io/api/v1/validate with the file as the body: an XRechnung or UBL XML, or a ZUGFeRD / Factur-X PDF. Add ?lang=de or ?lang=fr for messages in German or French.

curl https://app.filekin.io/api/v1/validate?lang=de \
  -H "Authorization: Bearer $FILEKIN_KEY" \
  --data-binary @rechnung.xml

The answer is a report: the format found, whether it passes, whether it counts as an e-invoice in Germany and France, and every error and warning with where it is, how to fix it, and a link to a full explanation.

{"format": "XRechnung 3.0 (CII)", "valid": false, "germany": "…", "france": "…",
 "errors": [{"rule": "BR-DE-15", "message": "…", "where": "…", "help": "…",
             "explanation_url": "https://filekin.io/de/e-rechnung-fehler/br-de-15"}], "warnings": []}

Python

import base64, os, requests

key = os.environ["FILEKIN_KEY"]
r = requests.post("https://app.filekin.io/api/v1/invoices", timeout=60,
                  headers={"Authorization": f"Bearer {key}"},
                  json={"customer": {"code": "K-1001"}, "lines": [{"product_code": "WEB", "quantity": 3}]})
r.raise_for_status()
invoice = r.json()
open(invoice["file_name"], "wb").write(base64.b64decode(invoice["file"]))

Usage

GET https://app.filekin.io/api/v1/usage returns your plan, this month's use and your allowance.

Limits

• Each call counts as one API call; creating an invoice also counts as one invoice. Calls refused before any work don't count.
• 5 calls a second, and one call at a time, per account.
• 2 MB per request; 200 lines per new invoice; 1,000 lines per validated invoice.
• When a monthly allowance is used up, calls are refused until the 1st (UTC). You're never charged for more.

Errors

Errors come back as JSON: {"error": "code", "message": "what happened"}.