As admin, enable "Mobile services": Site administration ► Mobile app ► Mobile settings Ensure that "Messaging" is enabled in Site administration ► Advanced features Create a Token in the mobile app service for any user in the site (not an admin account) Click on Site administration ► Plugins ► Web services ► Manage tokens Open the console an execute this new curl request, replacing wstoken with the token you just created and the site url with yours. You have to replace also the userid with the user id you created the token for: curl 'http://localhost/m/stable_master/webservice/rest/server.php?moodlewsrestformat=json' --data 'wsfunction=core_message_get_conversation&userid=19&includecontactrequests=1&conversationid=100&includeprivacyinfo=1&wstoken=3af67232a7596ceb658df4db329e5ad6' Confirm that: You see a moodle exception with error code set to: conversationdoesntexist Execute now the following CURL request (doing the same replacements than for the previous one) curl 'http://localhost/m/stable_master/webservice/rest/server.php?moodlewsrestformat=json' --data 'wsfunction=core_message_get_conversation_between_users&userid=19&otheruserid=2&includecontactrequests=1&includeprivacyinfo=1&wstoken=3af67232a7596ceb658df4db329e5ad6' Confirm that: You see a moodle exception with error code set to: conversationdoesntexist