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Twitter users on Friday began noticing that they could not retweet or reply to tweets containing links to the Substack.com domain. This behavior seems to have started less than 48 hours after the popular newsletter platform announced a new product called Notes that will compete directly with Twitter.

For example, this tweet references my newsletter, understandingai.substack.com. When I try to retweet it, it gives me an error message saying, "Some actions on this tweet have been disabled by Twitter." If I try to reply to the same tweet, I get an error message saying, "Something went wrong, but don't fret—let's give it another shot." Even liking isn't allowed.

This tweet is identical, except that I linked to my custom domain, understandingai.org. I have no trouble liking, retweeting, or replying to it.