ISO 8583 MTI Decoder
Split a 4-digit message type indicator into version, class, function and origin.
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Common message types
| MTI | Meaning |
|---|---|
| Authorisation request. Reserves funds without clearing them — the classic card-present or card-not-present auth. | |
| Authorisation response. Carries the response code in DE 39 and the auth code in DE 38. | |
| Authorisation advice. Tells the issuer about an authorisation already granted, e.g. a stand-in or offline approval. | |
| Authorisation advice repeat. | |
| Authorisation advice response. | |
| Financial request. Authorises and clears in one message — typical for ATMs and single-message domestic schemes. | |
| Financial response. | |
| Financial advice. Notifies the issuer of a completed financial transaction, often an offline or forced-post sale. | |
| Financial advice repeat. | |
| Financial advice response. | |
| Batch upload / file action advice. | |
| Reversal request. Undoes a prior authorisation, usually after a timeout or a void. | |
| Reversal response. | |
| Reversal advice. Reversal the acquirer has already applied. | |
| Reversal advice repeat. Very common — a repeat means the acquirer never got an 0430. | |
| Reversal advice response. | |
| Reconciliation request (settlement cutover). | |
| Reconciliation response. | |
| Reconciliation advice. | |
| Reconciliation advice response. | |
| Administrative request. | |
| Administrative response. | |
| Administrative advice. | |
| Administrative advice response. | |
| Network management request. Sign-on, sign-off, echo test, key exchange — see DE 70. | |
| Network management response. | |
| Network management advice. |
Four digits, four independent meanings
Unlike most of ISO 8583, the MTI structure is genuinely standard and does not need an acquirer-specific profile to read. Each position means one thing:
Position 1 — version. Almost always 0, meaning the 1987 edition.
The 1993 and 2003 editions exist but adoption was limited; most of the world still runs 1987
with proprietary extensions.
Position 2 — class. 1 is authorisation (reserve funds),
2 is financial (authorise and clear in one message), 4 is reversal or
chargeback, 8 is network management.
Position 3 — function. 0 request, 1 response,
2 advice, 3 advice response. The distinction between a request and an
advice matters: a request asks permission, an advice reports something that already happened.
Position 4 — origin. 0 acquirer, 2 issuer, and the odd
values are repeats. A 1 or 3 here means retransmission.
Dual message versus single message
The 01xx family authorises without clearing — funds are held, and a separate clearing file moves the money later. This is how most card-present and e-commerce card transactions work. The 02xx family does both at once, which is standard for ATMs and for domestic debit schemes that settle in real time.
Getting this wrong causes a specific and confusing bug class: sending an 0200 where the endpoint expects an 0100 will appear to work and then double-settle.
Why you keep seeing 0421
An 0420 is a reversal advice. An 0421 is the same message sent again because the acquirer never received an 0430 acknowledgement. A flood of 0421s usually means responses are being dropped somewhere, not that reversals are genuinely being retried — check whether your 0430s are actually reaching the sender before investigating the reversals themselves.
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