<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=27472) </details> <!--IssueSummary end--> ### Summary The API endpoint use to update an issue accept some multi-valued property updates (eg: labels, assignees). Serializing values using a comma works well for labels, but seems to be badly implemented for assignees (API returns `{"error":"assignee_ids is invalid"}`). ### Steps to reproduce #### Working example `curl -XPUT https://gitlab.com/api/v4/projects/12345/issues/12345?labels=FIRST%20LABEL%2CSECOND%20LABEL -H "Private-Token: XXXX"` => Issue has now labels `FIRST LABEL` and `SECOND LABEL` #### Failing example `curl -XPUT https://gitlab.com/api/v4/projects/12345/issues/12345?assignee_ids=1%2C2 -H "Private-Token: XXXX"` => Issue is not updated with users `1` and `2` as assignees, API returns message `{"error":"assignee_ids is invalid"}` with status line `400 Bad Request`. ### What is the current *bug* behavior? API can't be use to update an issue assigning many users. ### What is the expected *correct* behavior? API should be usable to update an issue, choosing many assignees. ### Output of checks This bug happens on GitLab.com