NAV Online Invoice integration — a practical guide

What is NAV Online Invoice?

NAV Online Invoice (in Hungarian: Online Számla, OSA) is a real-time data reporting system: every domestic B2B invoice must be reported to the Hungarian Tax Authority (NAV) server within 5 calendar days of issuance (thresholds and exact deadlines have evolved — check nava.gov.hu for the current rules).

In practice: if your invoicing or accounting software is not connected to the NAV API, invoices must be uploaded manually — time-consuming and error-prone.

How does the API work?

The NAV Online Invoice API (currently version 3.0) is an XML-based REST interface. The reporting flow:

1. XML generation — invoice data is serialised into an XML file conforming to NAV's XSD schema.
2. Signing — the XML is signed with a SHA3-512 hash using the technical user's password.
3. Token request — the NAV server issues a one-time token.
4. Submission — the invoice package (1–100 invoices at a time) is sent using the token.
5. Result polling — asynchronous feedback: the system queries the processing result a few seconds later.

Implementing the integration takes an experienced developer 2–5 days, but allow extra time for testing in the NAV sandbox, handling edge cases, and retry logic.

What you need to know about the reporting obligation

• Threshold: all domestic B2B invoices must be reported (the exact VAT-content threshold has been adjusted over time — verify current rules).
• Amendments and cancellations: if an invoice is amended or cancelled, that must also be submitted.
• Buyer query: the buyer can query the NAV system for what their supplier has reported — which increases transparency in business relationships.

How to integrate with accounting software

Three approaches:

1. Built-in integration

Modern accounting systems (such as the DevTools accounting software) have NAV Online Invoice integration built in — the report is sent automatically when an invoice is posted or approved, with confirmation of successful processing shown immediately.

2. Middleware

If the accounting software has no NAV integration, a middleware layer is needed to process exported invoice files (typically XML or CSV) and submit them.

3. Custom API integration

If invoices originate from a custom ERP, web shop, or other system, a development team needs to implement the NAV API. It is not complex, but requires precise XML schema handling and robust error recovery.

Common mistakes and how to avoid them

• Schema validation error — the XML does not conform to NAV's XSD. Fix: automated validation before submission.
• Duplicate submission — the same invoice number submitted twice. Fix: idempotent submission logic, dedup by invoice number.
• Token expiry — the token is only valid for a few minutes; generation and submission must happen in quick succession.
• Data mismatch — a field value does not match the actual invoice content. Fix: thorough testing in the sandbox environment.

Summary

NAV Online Invoice integration is now a baseline requirement for any enterprise-grade accounting system operating in Hungary. If your current system doesn't handle it automatically, it's worth either upgrading or commissioning a custom integration.

Get in touch — we'll assess the simplest way to connect your existing system to the NAV API.

Related pages:

• Accounting software details
• Custom software development