Caden Redmond, a college student in West Palm Beach, Fla., was on TikTok in April flirting with a woman living in South America. While writing to her via direct message, he told her he had never been to her home country but was planning a trip soon.

The conversation was going smoothly. He asked if she would show him around when he arrived; she said that would be cool. He called her cute, and she called him cute back. At one point, she said she “can’t wait” for him to get there.

Moments later, he took screenshots of their conversation, blocked the woman’s account and sent the images to her boyfriend.

“I just texted him and was like, ‘Hey, she said she wants to go out,’” Mr. Redmond said in a phone interview. “I sent him screenshots and he said, ‘OK, that’s enough, thank you.’”

Mr. Redmond, 19, was hired by the man to test his girlfriend’s loyalty, and according to him, she failed, leading her boyfriend to dump her. All the arrangements to lay the trap were made through Loyalty-Test, a service that allows people to hire “testers” to flirt with their significant others online to see whether they respond to the romantic advances or remain faithful.

Mr. Redmond charges $100 a test and has conducted five since joining the site this spring. Sometimes it takes just one DM exchange; other times it’s two to three days of online conversation: He determines what is included in his flat fee on a case-by-case basis. He only tests women, he said, and he doesn’t share any sexually explicit messages or private information of his customers and wouldn’t conduct tests on behalf of anyone he knows personally.

“I don’t aim to make people cheat,” said Mr. Redmond, who is a running back for Keiser University’s football team and a TikTok and Instagram creator. “I just do it because I’ve been cheated on, and I feel like if someone wants to know, they should know from someone who is actually not going to take their girlfriend.”

“That’s part of the job: Never follow through,” he added.

Since beginning in January, Loyalty-Test has brought aboard 30 testers — like ride-share drivers, they are free to take on as many or as few clients as they wish — and has been used by roughly 1,000 anxious customers who are unsure about their partners’ loyalty, according to Brandan Balasingham, 27, the website’s founder.

In a chaotic dating ecosystem made all the more unpredictable by how easily accessible we are to thousands online, it might not be surprising that those with trust issues would feel justified in resorting to schemes predicated on dishonesty and misrepresentation, all in the name of protecting their hearts.

“Our dating market has become so much bigger than it was before,” Mr. Balasingham said, “and I just noticed around me how that made people a little bit more unfaithful.”

To get people to sign up as testers and customers, Mr. Balasingham posted listings on job sites like Indeed, Handshake and Backstage. He also searched for microinfluencers to promote the service. He offered a $20 signing bonus to anyone who signed up as a tester, he said, and ran ads on Google with terms like “how can I test my husband?”

It doesn’t take much to be approved to be a tester: just an active Instagram account (it doesn’t have to use a real name) and an agreement to abide by Loyalty-Test’s terms.

“They just have to agree to confidentiality when they’re signing up,” he said, adding that testers are expected not to disclose the personal information of clients, including names and exact locations.

It’s mostly women who sign up to use the service, but there’s also “a good amount of guys,” Mr. Balasingham said. Testers can set their own pricing, and he keeps 10 percent of each transaction.

“We have a huge variety of testers,” he said. “And on the website, you can filter it down to whatever your partner would be interested in.”

Another tester, a 23-year-old woman living in Jersey City, originally encountered the service while searching online for ways to test her own boyfriend’s loyalty. She didn’t end up hiring a tester, but after she broke up with him, she became one herself in April.

“I wanted to see what it was about because I know what it feels like to be cheated on,” she said. “So if I could help test other people’s boyfriends to see if they were also cheating, I wouldn’t be mad about it.”

The woman, who spoke on the condition of anonymity (“It doesn’t feel like something I want to publicize, but I want to do it”), was a tester on the website for about two months before deactivating her profile. She doesn’t want her real identity involved with the site, using a “finsta,” or fake Instagram account, to work.

One woman asked her to send her boyfriend a message on Instagram to see if he would respond. She wrote “heyyy cutie,” and he replied with five red-faced hot-and-bothered emojis and asked for her Snapchat. Just like Mr. Redmond, this woman took screenshots of the conversation, blocked the boyfriend and sent her findings to his girlfriend.

The woman estimated that about half the men she tests respond with interest. All together, she has conducted about 40 “loyalty tests,” charging $50 to $60 to start. While she was doing tests regularly, she made a few hundred dollars a week on average, she said, earning close to $500 in one week alone.

Another woman pressed her to test her boyfriend in person, but she declined out of safety concerns. One woman messaged her asking her to write something “really specific” and explicit to her boyfriend.

“So I did it,” she said. “It was just something very sexual.”

He messaged back. So she sent it to the girlfriend, who replied, “I knew it.”

She said she would do tests again in the future but admitted that she did feel bad sometimes. “Anytime you lie to someone, anyone, you feel some sort of guilt about it, even though I feel like I’m lying for the right intentions,” she said.

“I don’t think I regret it, though, just because I’ve been able to help so many people,” she said. “There’s always going to be pros and cons, but I feel like the pros are outweighing the cons for me.”

Send your thoughts, stories and tips to [email protected].