> For the complete documentation index, see [llms.txt](https://docs.nexusglobalpayments.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.nexusglobalpayments.org/payment-processing/investigation-and-enquiry.md).

# Investigation & Enquiry

This section sets out the Nexus failure scenarios, the Nexus resolution and the expected IPSO handling.

### Common principles

* An HTTP response is not a payment status
  \
  An HTTP response only confirms the outcome of the API request handling. It does not by itself represent the final business status of the payment.
* No unilateral cancellation after uncertain submission
  \
  If a sender does not receive a clear response, it must not assume the payment was not received. It must retry using the Nexus defined retry mechanism. If uncertainty remains, it must use the investigation process.
* Duplicate Detection

  Nexus validates the UETR and the Message ID. Duplicates will be responded with a pacs.002 duplicate error code.
* A-synchronous Technical Rejection

  Nexus will use the ISO 20022 admi.002 (MessageRejectV01) as the Nexus standard asynchronous technical rejection message. It is emitted when an inbound ISO 20022 message has been accepted at the transport layer (HTTP 202) but subsequently fails Nexus validation in the domain pipeline, and no ISO-compliant business rejection counterpart applies.