How can I identify if a callback URL is for first-time app installation or reopening?

At this point I only really know as much as you do. I take it the ‘state’ param isn’t returning anything promising.

Again, if you had a list of shop names stored in your database, you could just check to see if the shop in shop param had an entry yet.

Here’s a link to the Rails Shopify callback controller , even if you’re not a Ruby developer, it’s really nicely written and easy to understand. Maybe there’s a clue in there.

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