Missing Mods Checker is a simple utility mod for modpack devs that hooks into the pre-loading process for Forge & Fabric to check if any dependencies are missing before the game loads. If any are found to be missing, a window will pop up prompting the user to download them before restarting.

This utility mod can serve multiple purposes:

Prevent important mods from being accidentally or deliberately removed
Enforce exact versions of mods, to prevent accidental end-user updates
Prompt to download mods that are only available on certain platforms
Allow users to download optional mods the first time the game starts up

Whatever the reason mods are found to be missing, the mod will open up a simple Java Swing window with a list of all missing mods. The user can click "download," which will open the link specified in the config file in their browser. To make things easier, there is a button that opens the mods folder directly.

You can configure the mod by editing missing_mods_checker.json in your modpack's config folder, and adding in entries for each mod. The dependency scanning supports regex for filenames, so you can specify exact versions, or a regex match for anything you would like.