An error arrives as a code
In the official documentation, error codes carry no explanation — and the person invoicing is not the one who reads them.
SLAVKO · e-invoicing helper
In development
An obligation that changes every month. Help that changes with it.
A deadline tracker and error interpreter for the national e-invoicing system (SEF), in Serbian: it follows changes, explains errors against the official guidance and raises a flag before a problem occurs.
It also runs on a local model, so financial data never leaves the company. An application of the same core that runs the institutions portal and the school assistant in production.
An operational guide with cited sources, not legal or tax advice. An independent product, not an official SEF service.
Why it is hard
The system works — but it does not explain.
The national e-invoicing system has been mandatory for VAT payers since 2023. The obligation is clear; the path to meeting it is not.
The deadline does not announce itself
Acceptance, rejection, tax recording — every action has its own deadline, and a miss shows only when it is too late.
The rules keep moving
A new release of the system went live on 31 July 2026 after two postponements, with officially announced slowdowns; the next update followed on 2 August. What worked yesterday may return an error tomorrow.
When the system stops, so does the work
An outage is noticed only when someone tries to send an invoice. No taxpayer has hired a person to read the technical manual.
What it does
It watches the obligations instead of waiting for a question.
The conversation is only the entrance. The helper's job is deadlines, changes, errors and system availability.
Deadlines
It flags before the deadline, not after. It follows acceptance, rejection and tax recording.
System changes
When a rule changes, the answer changes with it. The knowledge base remembers what applied and since when, so an old answer never lies.
Error interpreter
A code goes in, a sentence comes out: what is wrong, where it is fixed and what comes next — according to the official guidance.
E-delivery notes
The same approach for the e-delivery note system: guides, the law and the rulebook in the same knowledge base. The obligation is phased in; for the whole private sector it applies from 1 October 2027.
Knowledge from official sources
It answers from the official text and says where every answer comes from. If the guidance has no answer, it says so.
How it works
The path of a single question.
From an error code to an answer with a source, in six steps — on the same core as every SLAVKO application.
Question
It arrives through a channel already in use — a web app or Telegram — as text or voice. Speech is transcribed on the organization's own infrastructure.
Who is asking
The job is opened under that person's name, in their own separate space. The system knows how much may be spent and what may be run.
Context
From the knowledge base: official guides, the law, rulebooks and error codes, marked with what applies on which day — plus the organization's own rules and history.
Candidate
The model of the organization's choice — local or cloud — proposes an answer through a single point of access that alone holds the keys. The proposal is only a candidate.
Check
Structure, rules and an independent review; for e-invoicing, also a check that every cited source exists. An answer that fails goes back for repair or is discarded.
Answer
In plain language, with the document and the place in it cited, and with the date of validity. An action that cannot be undone waits for a human to confirm.
Data stays in the company
Read-only. The key stays with the taxpayer.
In local mode the model runs on the organization's infrastructure and financial data never leaves the company. In cloud-model mode a request goes to the model vendor, with personal data removed when the organization requires it; the choice of mode rests with the organization.
Read-only access
Checking statuses and deadlines needs read-only access. The helper does not send invoices on anyone's behalf.
The key stays with the taxpayer
Access goes through the taxpayer's own key, generated in the official system; there is no intermediary accreditation.
An independent product
A product of Metatron Production; not an official service of the e-invoicing system or of the Ministry of Finance.
A guide, not advice
An operational guide with cited sources, not legal or tax advice. Borderline cases are referred to the source or to the accountant.
Maturity
What exists, what is being built.
Product label: in development. No understatement and no overstatement — every item carries its own state.
Knowledge base from official sources
The technical manual, the API documentation, the law and rulebooks on e-invoicing and e-delivery notes, VAT recording guides, the list of error codes, frequently asked questions — every document with its address, fingerprint and date.
existsTime dimension
Every record carries “valid from” and “valid to”; an old rule is not deleted but stays marked. Search returns only current law by default, in Cyrillic and Latin.
existsTables of hard facts
Deadlines, statuses, system versions, error codes and tax categories — every row with its source.
existsAnswering layer with citation check
An answer in plain language, with a check that every cited source exists and that numbers match the table values. Without a source, no answer goes out.
being builtGlossary and links between concepts
An article per concept with citations, and links: which article amends which, error → fix → procedure.
being builtChannels and monitoring of announcements
Connection to the web app and Telegram, and automatic monitoring of announcements on the official site, so the base is refreshed for a new version of the manual.
being builtFrequently asked questions
Direct answers.
Answers to the questions business owners and accountants ask most often.
Does this replace SEF or the accountant?
No. The helper works alongside the official system: it only reads and explains, and invoices are sent by a person. Decisions in borderline cases stay with the accountant.
Where do the answers come from?
From official texts: the guides, the law and the rulebooks from the system's site and the Ministry of Finance. Every answer carries where it comes from and since when it applies.
What if the guidance has no answer?
It says so and refers to the source or to the accountant. It does not guess — an answer without a source is not delivered.
Does financial data leave the company?
In local mode, no: the model runs on the organization's infrastructure. In cloud-model mode a request goes to the model vendor with personal data removed when the organization requires it; the choice rests with the organization.
What happens when the rules change?
The knowledge base keeps the version and validity date of every record. A new rule gets its own record, the old one stays marked with when it ceased to apply, so the answer follows the change.
Does it cover e-delivery notes too?
Yes. The same approach and the same knowledge base cover the e-delivery note system: guides, the law and the rulebook.
What stage is the product at, and how to start?
In development: the knowledge base exists, the answering layer and channels are being built. Collaboration starts with a conversation by e-mail; a demonstration follows by arrangement.
Contact
A conversation is the first step.
For questions and to arrange a demonstration: e-mail below. The company behind the product: Metatron Production.
ognjen.vucurevic@gmail.com