Grammarly fixes what you wrote. Tonecast writes it like you.

Editing is not authoring

Credit first, because it's earned at planetary scale: Grammarly catches real mistakes — the agreement error, the comma splice, the sentence that should have been two — in more text fields than any tool ever shipped. And its tone work is described precisely on its own pages: the tone checker “analyzes your word choice, phrasing, punctuation” and suggests refinements. Text you produced goes in; a better version of the same text comes out. That's the category, and Grammarly is the best at it.

But look at what that workflow assumes: by the time Grammarly has anything to underline, you've already done the expensive part. You read the thread, decided what it needed, and typed the stiff first version — the blank compose box was yours alone. Correction polishes minute ten of the job; the minutes that hurt are the first nine. Tonecast starts there: press the hotkey over the thread and it reads who wrote and what they asked, drafts three replies, each labeled by intent, and pastes the one you pick at your cursor.

To be fair, Grammarly has an answer here, and we won't pretend otherwise. GrammarlyGo on the Pro plan “detects an email's context from the sender, subject line, and email content to instantly suggest options for a thoughtful reply” — real generation, no pre-draft required. In email, the blank-box argument is now a fair fight. What survives it is narrower and more interesting: whose voice the draft comes out in, and where the drafting can follow you. GrammarlyGo's replies live in email; Tonecast's Reply mode reads Gmail, Apple Mail, Superhuman, WhatsApp, Slack, and iMessage threads through per-app integrations.

“Improved” toward the average

Every correction engine has a center of gravity. When millions of people accept suggestions from the same model, their prose converges on the same register — clear, competent, and interchangeable. That isn't a flaw in Grammarly; it's what “improve” has to mean when one system improves everyone. Your hedge gets the same trim as everyone else's hedge. The output is better writing, in the sense that a style guide is better writing — and it sounds like the style guide, not like you.

A voice profile points the other way: it diverges. Tonecast learns how you actually write — the vocabulary, the sentence rhythm, how you soften a no — from messages you've already sent, per channel and per contact, because the way you write your CEO and the way you write your oldest friend differ by a lot more than correctness. And the profiles aren't hidden in a model somewhere: they're plain markdown files at ~/Library/Application Support/Tonecast/voices/ that you can open, edit, or delete. Not training data — notes you can read.

The demo above is the whole argument. Both panes answer the same message. The left one ends where editing ends: your stiff draft, corrected, still yours to rewrite. The right one ends with a reply you'd actually send.

Privacy posture

Grammarly's support pages say it plainly: “Product Improvement and Training is on for individual users by default,” and that data is used for “training and validating our AI and machine learning models.” The off switch exists, but you go find it — Free and Premium accounts under account.grammarly.com/security/privacy, single-user Pro under account.grammarly.com/admin/data_settings. Only Enterprise and education accounts are opted out automatically. The opt-out is real; the default is opt-in.

It's also worth knowing who you're dealing with, because the corporate ground moved recently. Grammarly announced it was acquiring Superhuman, the email client, on June 30, 2025 — and then, on October 29, 2025, renamed the parent company itself to Superhuman: “The Grammarly product will still exist, but we're changing our company name to Superhuman.” So Grammarly is today one product inside the Superhuman company, alongside Coda and Superhuman Mail. If the email client is the thing you're actually weighing, we compare it directly.

Tonecast's default is the opposite shape. In BYOK mode your prompt goes from your Mac straight to your own provider on your own key — Anthropic, OpenAI, Google, or Ollama running on-device — with no Tonecast account and no Tonecast server in the path. Keys, voice profiles, vocabulary, and logs all live locally. There's no training toggle to hunt down because there's nothing collecting.

One honest caveat on our side: Tonecast Cloud, the optional managed tier for people who'd rather not handle provider keys, does route prompts through our API. We don't store your text and we don't train on it — but if “no vendor in the path” is the bar, BYOK is the mode that clears it, and it's the default.

Where Grammarly wins

Everywhere you type, today. Grammarly's whole premise is meeting you in the text field you're already in, and it delivers on more surfaces than anything else in this comparison. Tonecast is macOS today · iPhone coming soon · Linux & Windows in the works. If you need your commas caught on every device this afternoon, that's their territory.

A real free tier. Free Grammarly covers spelling and grammar, tone detection, and 100 AI prompts a month. The generative features — tone rewriting, reply options — sit behind Pro at 2,000 prompts a month, but if catching mistakes is the whole job, you may never pay them anything.

Teams, and an organization around it. Pro absorbed the old Premium and Business tiers, Enterprise sits above it with training opted out automatically, and since the rebrand Grammarly ships beside Coda and Superhuman Mail in the Superhuman suite. If you're buying writing tooling for an organization rather than getting your own replies written, they've built for that and we haven't.

Every claim on this page comes from Grammarly's own plans, tone-checker, and training-control pages, its GrammarlyGo announcement, and the Superhuman acquisition and company-rename announcements, verified July 5, 2026. If they change a default, we'll update this page.