SAN FRANCISCO — On Wednesday, the newsletter service Substack announced that it had built a Twitter competitor. On Thursday, Twitter prevented Substack writers from sharing tweets in their newsletters. And on Friday, Twitter took steps to block Substack newsletters from circulating on the platform.
Twitter’s move to swat an upstart was an abrupt deviation from normal behavior among internet companies and publishers. It also provided more grist for critics who say that while Elon Musk, Twitter’s new owner, has often hailed the importance of free speech, he has not shied from restricting competitors and content that he doesn’t like.
The new fight with a young company is the latest controversy in Mr. Musk’s chaotic ownership of Twitter, which he acquired about six months ago. He has laid off more than 75 percent of its employees, has been sued by commercial landlords for failing to pay office rent and has lost advertisers.
While Mr. Musk has long clashed with mainstream news outlets, targeting Substack largely affects independent writers, some of whom depend on Twitter to drive readers to their work.
“This is a huge inconvenience,” said Hunter Harris, a writer who distributes her newsletter, Hung Up, on Substack. “It’s incredibly petty.”
Substack’s founders, Chris Best, Hamish McKenzie and Jairaj Sethi, said in a statement that they were “disappointed” by Twitter’s decision to stifle engagement with any tweets that featured a Substack link.
“Writers deserve the freedom to share links to Substack or anywhere else,” they said. “This abrupt change is a reminder of why writers deserve a model that puts them in charge, that rewards great work with money and that protects the free press and free speech.”
Mr. Musk did not respond to a request for comment.
Twitter and Substack share a major investor, the venture capital firm Andreessen Horowitz, which could be called to play referee in the spat. A spokeswoman for Andreessen Horowitz, which led a $65 million investment round in Substack in 2021 and invested $400 million in Mr. Musk’s Twitter last year, did not respond to a request for comment.
Substack’s new feature, called Notes, mimics Twitter in several ways. It allows users to post brief updates and lets other people like, repost or reply to them. The changes by Twitter on Friday meant that Twitter users could still share links to Substack newsletters, but blocked other users from liking or resharing those links.
Inside Elon Musk’s Twitter
Before Mr. Musk acquired it, Twitter sometimes restricted likes and retweets as a way to prevent content that violated its policies from spreading widely on the platform. The measure was used to limit the reach of former President Donald J. Trump’s tweets in which he made false assertions about how votes would be counted in the 2020 election.
Twitter has been an important distribution outlet for Substack authors, many of whom are independent of traditional media outlets and rely on subscriptions to make money.
Some of Substack’s most popular writers have been vocal supporters of Mr. Musk, who gave them special access to comb through and release the “Twitter Files,” internal communications that the billionaire said showed the biases of Twitter’s previous management.
The changes outraged writers who use both Twitter and Substack to distribute their work. “I cannot explain how absurd and silly this is,” Rohit Krishnan, who writes the newsletter Strange Loop Canon, wrote in a tweet. “Not to mention petty and vindictive.”
Ms. Harris said that, while Twitter did not drive significant traffic to her newsletter, the restrictions penalized her readers who might want to share links to her work or discuss it on Twitter. Preventing Substack writers from including tweets in their newsletters also stripped proper credit from Twitter users, she added.
“Any Twitter alternative would be great,” Ms. Harris said of Substack’s move to build a competitor. “I want another place just like Twitter that’s not Twitter.”
Other writers said Mr. Musk’s latest move contradicted his statements about enabling free speech at Twitter.
“In cutting off access to Substack, Elon is banning access to free and highly informative content on the internet,” said Simon Rosenberg, who writes a newsletter on politics. “It is censorship of the very worst kind.”
The Substack kerfuffle is not the first time that Mr. Musk has blocked competing services from being shared on Twitter. In December, he suspended Twitter users, including several journalists, for linking to Mastodon after a Mastodon user shared public information about the location of Mr. Musk’s private jet.
He went on to bar users from sharing links to Facebook, Instagram and several other social media companies, but reversed course after a backlash. Under pressure from supporters who saw the move as an abandonment of his free speech principles, Mr. Musk said in December that he would step down as Twitter’s chief executive once he found a replacement. Mr. Musk has not yet done so.
Mr. Musk has also escalated his long-running feud with mainstream media. Last Saturday, he revoked The New York Times’s verification check mark, which had distinguished its Twitter account from impersonators. Mr. Musk had said Twitter would start removing verification badges starting on April 1, but most verified accounts have kept their check marks.
On Wednesday, Twitter added a label to the Twitter account of NPR, calling it “state-sponsored media.” The label, which the platform used in the past to identify propaganda outlets, sparked backlash from press freedom organizations. In emails to an NPR reporter on Wednesday, Mr. Musk acknowledged that the label “might not be accurate” and added, “We should fix it.”
This week, Mr. Musk has made a number of unexplained changes to Twitter, replacing the company’s bird logo for a few days with the image of a Shiba Inu dog that’s associated with the cryptocurrency dogecoin.
On Tuesday, the company also covered the letter “w” in the name “Twitter” on a large public sign outside its San Francisco headquarters, in what some observers assumed was a vulgar joke.
