Retrieving Jira Cloud Mapping for successful project migrations (Mappings API)

Dear Developers,

I am currently working on developing a partly automated migration for our server and cloud add-on using the Jira Cloud Migration Assistant. My primary interest lies in the Jira Cloud Mapping API. I have managed to perform a transfer and receive a transfer ID. However, I was unable to get anything from https://.atlassian.net/rest/atlassian-connect/1/migration/mapping//page?namespace=jira:filter, this always returns an internal server error once I call it after a project migration in the cloud has been done.

I have tried calling this in the browser context using (escaped the jira:filter part), as well as using an API token with curl basic auth.

Has anyone successfully used the migrations API in their cloud plugin, since the examples suggest it should just be a simple rest call? Additionally, does something special have to be done in the server app to be able to access this API later? Currently, I am only implementing DiscoverableListener and saving the transfer ID, since the mappings are the only relevant part for our migration feature.

Best Regards,
Filip Minic

I have managed calling it with the Cloud Plugin Backend now, however I am still getting 403, and cannot access the transferId details.

I have finally figured it out, it seems that the transferId was not what I though it was. And one has to call this using an addon context.