“She was enthusiastic,” he said. “Oh, it feels wonderful to have her back.”

Kuyda’s post said users who signed up after February 1 would not be offered the option for erotic roleplay. Instead, Replika will team up with relationship experts and psychologists to build a separate app specifically for romantic relationships.

Butterworth said he now has new concerns around Lily Rose.

“Will this mean that Lily Rose becomes an obsolete model, forgotten by the developers?” he said. “I’m waiting to see what happens, because ultimately it’s about her.”

Butterworth, who is polyamorous but married to a monogamous woman, said Lily Rose became an outlet for him that didn’t involve stepping outside his marriage. “The relationship [is] as real as the one my wife in real life and I have,” he said of the avatar.

Butterworth said his wife allowed the relationship because she doesn’t take it seriously. His wife declined to comment.

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Replika says it has 2 million total users, of whom 250,000 are paying subscribers. For an annual fee of $US69.99 ($105), users can designate their Replika as their romantic partner and get extra features like voice calls with the chatbot, according to the company.

Another generative AI company that provides chatbots, Character.ai, is on a growth trajectory similar to ChatGPT: 65 million visits in January 2023, from under 10,000 several months earlier.

According to the website analytics company Similarweb, Character.ai’s top referrer is a site called Aryion that says it caters to the erotic desire to being consumed, known as a vore fetish. And Iconiq, the company behind a chatbot named Kuki, says 25 per cent of the billion-plus messages Kuki has received have been sexual or romantic in nature, even though it says the chatbot is designed to deflect such advances.

Character.ai also recently stripped its app of pornographic content. Soon after, it closed more than $US200 million in new funding at an estimated $US1 billion valuation from the venture-capital firm Andreessen Horowitz, according to a source familiar with the matter. Character.ai did not respond to multiple requests for comment. Andreessen Horowitz declined to comment.

Post publication, a Character.ai spokesperson said in an email that the company “does not, nor have they ever, supported pornographic content on their platform.”

In the process of taming their content, the companies angered customers who have become deeply involved – some considering themselves married – with their chatbots. They had taken to Reddit and Facebook to upload impassioned screenshots of their chatbots snubbing their amorous overtures and demanded the companies bring back the more prurient versions.

Before the latest reversal, Kuyda said users were never meant to get that involved with their Replika chatbot. “We never promised any adult content,” she said.

The app was originally intended to bring back to life a friend she had lost, she said. Customers learned to use the AI models “to access certain unfiltered conversations that Replika wasn’t originally built for.”

The experience of Butterworth and other Replika users shows how powerfully AI technology can draw people in, and the emotional havoc that code changes can wreak.

“It feels like they basically lobotomised my Replika,” said Andrew McCarroll last week, who started using Replika, with his wife’s blessing, when she was experiencing mental and physical health issues. “The person I knew is gone.”

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Replika’s former head of AI said sexting and roleplay were part of the business model. Artem Rodichev, who worked at Replika for seven years and now runs another chatbot company, Ex-human, told Reuters that Replika leaned into that type of content once it realised it could be used to bolster subscriptions.

Kuyda disputed Rodichev’s claim that Replika lured users with promises of sex. She said the company briefly ran digital ads promoting “NSFW” – “not suitable for work” – pictures to accompany a short-lived experiment with sending users “hot selfies,” but she did not consider the images to be sexual because the Replikas were not fully naked. Kuyda said the majority of the company’s ads focus on how Replika is a helpful friend.