AtCoder Parallel Alert is a free Chrome add-on developed by Tomii9273. This add-on is designed to display a warning message in the submission field of non-AHC contests when the user has both AHC and non-AHC contest problem statements open simultaneously on AtCoder.

The main purpose of AtCoder Parallel Alert is to prevent violations of the contest rules by mistakenly submitting AHC contest code to non-AHC contests, such as past problems. It works by detecting if a window tab contains a website with both "atcoder" and "ahc" in the URL, and if the URL contains "atcoder" but not "ahc". If these conditions are met and the tab has the same HTML class as the regular submission field, the add-on displays the warning message "【警告】これは AHC ではありません！" (This is not AHC!) in bold red text.

Please note that there may be cases where the warning message is not displayed even if there is a possibility of a mis-submission that violates the rules. For example, if only non-AHC contests that do not contain "ahc" in the URL are open, the warning message will not be shown in the submission field. Conversely, there may be instances where the warning message is displayed even if there is no possibility of a mis-submission that violates the rules. For instance, if both AHC and non-AHC past problems are open simultaneously, the warning message will be displayed in the non-AHC submission field.

It is important to keep in mind that future changes in AtCoder's specifications may affect the functionality of this add-on, potentially causing it to stop working or behave unexpectedly. The developer of AtCoder Parallel Alert does not assume any responsibility for any disadvantages or violations of contest rules, such as decreased scores or rankings, resulting from the use of this add-on.