Most SWIFT message UAT plans test that a message gets generated and that the mandatory fields are populated. That is the easy 80%. The failures that reach production come from the other 20% — field interdependency, correlation across message types, and the ISO 20022 migration most teams are testing as a format change when it is really a data-model change.
MT700: The Fields That Depend on Each Other
An MT700 (Issue of a Documentary Credit) has over forty fields, and most UAT plans test them independently — field 32B has a valid amount, field 44C has a valid date, field 45A has a goods description. What gets missed is that several fields constrain each other, and a system can populate every field correctly in isolation while producing a message that is logically inconsistent.
Field 44C (latest shipment date) and field 31D (date and place of expiry) have a mandatory relationship — expiry must allow enough time after the latest shipment date for document presentation, typically 21 days under UCP 600 Article 14(c) unless the credit states otherwise. A test that checks both fields are populated but never checks the date arithmetic between them will pass a credit that is unworkable in practice.
MT700 field 44C/31D date-arithmetic validation — presentation period sufficiency check. MT707 amendment correlation — confirming amended fields trace back to the original MT700 reference. MT700 partial shipment flag (field 43P) consistency against field 39A tolerance.
MT103 and MT202: Same Payment, Two Messages, One Reference
A correspondent banking payment generates an MT103 (customer credit transfer) and, where an intermediary is involved, an MT202 or MT202COV (financial institution transfer covering it). The two messages must correlate — same UETR, consistent amount net of correspondent charges, and a traceable reference chain. UAT plans frequently test MT103 generation and MT202 generation as separate test cases, which means the correlation between them is never actually tested.
The MT202COV specifically exists to satisfy correspondent banks' regulatory obligation to see the underlying customer details for sanctions screening — a plain MT202 without the COV extension hides the originator and beneficiary from the correspondent. A system that defaults to MT202 instead of MT202COV when cover is required will pass a basic UAT script and fail a real compliance review.
If your test plan does not include a scenario verifying MT202COV is generated (not a plain MT202) whenever an MT103 involves an intermediary correspondent, you have not tested the screening-visibility requirement that regulators actually examine.
MT940 / MT942: Reconciliation Breaks on Timing, Not Format
MT940 (customer statement) and MT942 (interim transaction report) are usually tested for field structure — but the failures that show up post-implementation are timing failures. MT942 is intraday and can report a transaction that later reverses before end of day; a reconciliation engine that treats every MT942 line as final will create phantom balances. The test case that matters is not "does the system generate an MT942" — it is "does the system correctly handle an MT942 entry that is reversed in a later MT942 the same day, before the MT940 is generated."
The ISO 20022 Migration Was a Data Model Change, Not a Format Change
SWIFT's MX messages (pacs.008, pacs.009, camt.053, camt.054) are not MT messages translated into XML — they carry structured party and purpose data that MT messages compress into free-text fields. The MT/MX coexistence period for cross-border payments ran from March 2023 and ended on 22 November 2025, after which MT103 and MT202 were retired for these flows in favour of their ISO 20022 equivalents. A UAT plan written before that cutover and never revisited is testing a format that the network no longer accepts for these message categories. The next deadline is November 2026, when unstructured postal addresses are retired entirely from CBPR+ messages — a fully structured or hybrid address becomes mandatory.
Banks still relying on unstructured address fields have until November 2026 before CBPR+ messages using them are rejected outright. If your UAT plan has not added a scenario testing structured/hybrid address rejection, that gap will surface as a live NAK, not a test failure.
GPI Tracking: The UETR Has to Survive Every Hop
SWIFT GPI tracking depends on the Unique End-to-End Transaction Reference surviving unchanged across every intermediary in the payment chain. A test plan that verifies UETR generation at origination but does not verify it is correctly carried through MT202COV cover messages and into the receiving MT103 has not tested the one thing GPI tracking actually depends on — continuity, not generation.