[GitLab API] Cannot update issue using multiple assignees

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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

Edited Aug 18, 2025 by 🤖 GitLab Bot 🤖
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