Format Error
A "Format Error" decline means the transaction message sent to the network was malformed — missing a required field, containing an invalid value, or structured incorrectly. It is almost always caused by a bug or misconfiguration in the merchant's payment gateway, POS, or integration rather than anything related to the cardholder.
Decline code by card network
| Card network | Code | Network's wording |
|---|---|---|
| Visa | 30 | Format Error |
| Mastercard | 30 | Format Error |
Common causes
- A required field, such as expiration date, merchant category code, or currency, is missing from the transaction message.
- A field contains data in the wrong format or length, such as an incorrectly padded account number.
- A recent software update to the POS, gateway, or terminal introduced a bug in how transaction messages are constructed.
What merchants should do
- Escalate to your payment gateway or processor immediately — this is a technical integration issue, not something the customer or merchant can fix at checkout.
- Avoid repeatedly retrying the same transaction, since a malformed message will fail identically every time until corrected.
- Check for recent updates to your POS software, terminal firmware, or gateway integration that coincide with when the errors began.
How to prevent this
- Thoroughly test all required and optional fields against your processor's message specification before deploying integration changes.
- Use a certified payment gateway or SDK that handles message formatting automatically, reducing the risk of manual formatting bugs.
- Set up alerting for a spike in format errors so integration bugs are caught quickly after any software deployment.
Format Error declines are a strong signal to check your integration rather than any individual transaction, since the issue lies in how the message itself was constructed. They often appear suddenly after a software update, new terminal deployment, or processor migration.
Frequently asked questions
Is a "Format Error" decline something the customer can fix?
No. This decline is caused by a malformed transaction message from the merchant's payment system, not anything about the customer's card or account, so the customer can't resolve it by retrying or using a different card.
How urgently should we address a Format Error decline?
Very — since the underlying message format is broken, every transaction using that same code path will likely fail the same way, so it should be escalated to your gateway or processor as soon as it's noticed.