i want to prevent some users from modifying the assignee, where i can do that?
i searched but all i found that i can stop him from assigning or i can let him assigne. but what i want to do is to let him Assign calls to him self "using Assign to me" but i dont want him to change the assignee after the call was assigned to someone else..
You can't do it.
The permission "can assign issue" really does mean what it says. You can either assign an issue or you can't. If you have the right, you can assign issues. If you don't, then you can't, and that includes assigning to yourself.
There are some dynamic/flexible ways to handle it - for example "current assignee can assign the issue" or "members of the group named in field X", but there's no split between assign-to-me and assign-to-anyone.
You can sort of do it with workflows though - set it up so people can't assign, then use "change assignee to %current user% in workflow post-functions" as appropriate.
...be more productive while being fun to use at the same time. For some, getting started can be a bit intimidating. This is especially true if Jira Service Desk is your first exposure to Atlassian...
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