What Is Sinceerly: The Anti-Grammarly Tool That Adds Typos to Your Emails

Sinceerly is a Chrome extension that uses AI to make AI-generated emails look like they were written by a human. It does this by deliberately adding typos, stripping formal language, and shortening sentences. The name is misspelled on purpose. That is the whole point.

Ben Horwitz, a graduating student at Harvard Business School and investment partner at Dorm Room Fund, built it in April 2026 using Anthropic's Claude. He announced it on X with a single line: "I made the anti-Grammarly. Mess up your emails with AI."

Why It Exists

AI writing tools like ChatGPT and Claude have made polished emails effortless. The side effect is that inboxes are now full of messages that are technically flawless but feel robotic. Perfect grammar, uniform tone, and identical structure have become markers of AI-generated text rather than signs of professionalism.

Horwitz noticed this and built Sinceerly to reverse it. "I got sick of everyone in my inbox sounding like AI," he said. The tool targets the exact quality that Grammarly produces and removes it.

How It Works

Sinceerly works directly inside your browser. Select the text you want to humanise, choose a mode, and the extension rewrites it using Claude to introduce calculated imperfections. The tool claims it does not store or log email content.

There are three modes:

Subtle removes filler words, simplifies sentence structure, and adds minor imperfections like contractions and occasional typos. The email still reads cleanly but loses the over-engineered quality of AI output.

Human adds a conversational tone and typically inserts a typo early in the message. The logic is that real people make mistakes near the beginning of an email, which signals authenticity in a way a perfectly structured message cannot.

CEO is the most extreme mode. It uses lowercase text, keeps responses extremely brief, and sometimes adds a signature like "sent from my iPhone." It mimics the terse, casual replies busy executives are known for sending from their phones.

After a limited number of free uses, the extension costs $4.99.

Whether It Actually Works

Horwitz tested it before launching. He used Sinceerly to cold email five Fortune 500 CEOs. Four replied. Each response was under ten words, two contained typos, and one CEO addressed him as Larry instead of Ben. His conclusion: "Maybe they were using Sinceerly on CEO mode."

The result is anecdotal and impossible to attribute cleanly to the tool itself. But the viral attention it generated suggests the underlying observation is widely recognised. A growing number of professionals feel that perfectly polished emails now work against the sender rather than for them.

The Irony It Is Built On

Sinceerly is built on a recursive joke. It uses an AI model to hide the fact that an AI model wrote the email. The misspelled name is deliberate. The premise is intentionally absurd. Horwitz describes it as satire: "If we are using AI to write, can we use AI to un-AI our own writing? That'd be funny."

The tool works as satire because the problem it exaggerates is real. Human texture in writing, the slightly off word choice, the abbreviated reply, the early typo, has become a credibility signal precisely because AI consistently fails to replicate it naturally. Sinceerly automates that texture, which is either a practical solution to a real problem or proof that the problem has already gone too far. Probably both.

Frequently Asked Questions

Is Sinceerly free?

Sinceerly offers a limited number of free uses. After the free trial, continued use costs $4.99. It is available as a Chrome extension from the Chrome Web Store.

Does Sinceerly store my emails?

According to its creator, Sinceerly does not store or log email content. The text is processed securely without being retained. As with any browser extension that processes your writing, avoid using it for sensitive or confidential correspondence.

What AI model does Sinceerly use?

Sinceerly was built using Anthropic's Claude. Claude analyses the text and determines where and how to introduce imperfections based on the selected mode.

Is it ethical to use Sinceerly?

That depends on what you use it for. Making your own AI-drafted emails feel more natural and personal is a legitimate use. Using it to deceive recipients about who wrote a message raises different questions. Horwitz built it as satire and a genuine response to inbox uniformity, not as a deception tool, but the line between the two can be thin depending on the context.

Who built Sinceerly?

Ben Horwitz, a graduating student at Harvard Business School and investment partner at Dorm Room Fund, built Sinceerly in his spare time between classes. He described himself as a terrible typist with mild dyslexia who was thrilled when Grammarly arrived and frustrated when AI made his inbox feel impersonal.