Background From FHIR-46443 document https://docs.google.com/document/d/1WiKR6OD3rmLNovFvxTn7D_5AawdDex391LFtd7cIt0Q/edit                   X12 270 “Beneficiary Eligibility Check” Provider -------------------------------------------> Payer                  x12 271 Response (FHIR Coverage) Provider <------------------------------------------ Payer   x12 271 Response (FHIR Coverage) has a well known URL field to supply the endpoints including davinci_cdex_attachsubmit_endpoint: The ‘base’ for the endpoint used to invoke the $submit-attachment operation Rationale Reviewing the CDex transactions for Data Consumer endpoints Endpoint Discovery Needed For Unsolicited Attachments: The Data Consumer Endpoint is not discoverable in-band.  Therefore, for this transaction, there is a need for Payer-supplied endpoints. Endpoint Discovery Not Needed For Solicited Attachments: The $submit-attachment endpoint is a mandatory Task.input  slice in the [CDex Task Attachment Request Profile] ( http://hl7.org/fhir/us/davinci-cdex/StructureDefinition/cdex-task-attachment-request ) Direct Query: HTTP response so no endpoint discovery is needed Task Based Transactions: The FHIR endpoint for queried data is not needed because the `Task.output` supplies the URLs that are pulled ( or contained in the Task ) by the consumer. URL to canonical* for questionnaires in the conditionally mandatory `Task.input` slice in the [CDex Task Attachment Request Profile] ( http://hl7.org/fhir/us/davinci-cdex/StructureDefinition/cdex-task-attachment-request ) and [CDex Task Data Request Profile] ( https://hl7.org/fhir/us/davinci-cdex/StructureDefinition-cdex-task-data-request.html ) other possible requirement is to know the endpoint to retrieve the Questionnaire (and associated value sets, CQL Libraries, etc.)  However, this function is handled by DTR and there's no reason for CDex to define a mechanism as well. The $submit-attachment endpoint coming from PAS responses will need endpoint discovery, but that will be managed by PAS.   Pre-applied here: https://hl7.org/fhir/us/davinci-cdex/sending-attachments.html#submit-attachment-operation