Twitter has started marking links to Substack as unsafe. If you click on a link on Twitter with substack.com in the URL, Twitter will show a separate notice warning you that “the link you are trying to access has been identified by Twitter or our partners as being potentially spammy or unsafe.”

Don’t be alarmed — the links we’ve checked appear to be perfectly safe. This notice seems instead to be Twitter’s latest hostile move toward Substack after the email newsletter platform announced its own Twitter-like “Notes” feature on Wednesday.

On Thursday, Twitter blocked people who use Substack from embedding tweets into their stories. Then, late Thursday or early Friday, Twitter started blocking engagement on tweets containing links to Substack; users weren’t able to like or retweet them, but could quote retweet them. Then, on Friday morning, Twitter applied those same restrictions to tweets from the official Substack account.

While those moves are clearly unfriendly, I’d classify the unsafe warning as downright aggressive. Twitter’s URL policy hasn’t changed since 2020, according to the last updated note on the page, and Substack’s site appears to be working as it always has, so it’s hard to say what justification Twitter’s using for the warning. CEO Elon Musk hasn’t tweeted anything about Substack recently and, when reached for comment, Twitter’s press email auto-replied with a poop emoji, which it has been doing since mid-March.

The elephant in the room is Substack’s Notes feature, which adds very Twitter-like elements to the newsletter platform. Matt Taibbi, a journalist who has history reporting stories for Musk, said Friday that he was told by an unspecified party that “Twitter is upset about the new Substack Notes feature, which they see as a hostile rival.” He also notes that he was “given the option of posting my articles on Twitter instead of Substack.”

In December, Musk said he considered “relentless advertising of competitors” to be a policy violation, and blocked sharing links to Instagram, Mastodon, Facebook, and others. However, those restrictions were later removed. It’s possible that he’s decided to bring it back now that he feels like Substack is trying to compete with Twitter, though it is worth noting that, for now at least, you still are able to tweet a Substack link. Anyone trying to follow it will just have to click through the warning to get to the content.

Substack didn’t immediately respond to The Verge’s request for comment on the warning. However, its founders have recently spoken out about how Twitter is treating their platform.