Brilliant in one inbox. Silent everywhere else.

An email client vs a writing layer

Concede the whole thing up front: Superhuman Mail earned its reputation. The keyboard shortcuts, the speed, the split inbox — the client was the best way to do email before AI was part of the pitch, and the AI it added is real. Auto Drafts promise to “craft a complete email in your voice and tone,” Instant Reply puts a draft on every email, Ask AI draws from “your inbox, calendar, and the web,” and Auto Summarize floats a one-line summary above every conversation. Inside that inbox, it's genuinely superb.

One thing worth reading closely on their pricing page, though: those headliners are not the $30 tier. Starter ($30/mo, $25 annual) includes Superhuman AI; Auto Drafts and Ask AI — what their pricing calls “our most advanced AI” — sit on Business at $40/mo ($33 annual). If the AI drafting is the reason you're buying, the honest price is the Business tier.

And notice the shape of the product: the AI is one feature of one application. A brilliant application — but everything the AI does, it does to the inbox. Tonecast is the other category entirely: a writing layer. It isn't an email client and never will be. It's the hotkey that reads the conversation on your screen — whichever app is showing it — and drafts the reply in your voice.

Where your conversations actually live

Now map an ordinary working day against that inbox. The customer thread is in a Gmail shared inbox that never touches your personal mail. The decision that actually matters is happening in Slack. The contractor answers only on WhatsApp; your family lives in iMessage. Superhuman's AI page is, top to bottom, a list of inbox features — Auto Drafts, Instant Reply, Ask AI, Auto Summarize all operate inside the Mail client. Every time you switch apps, the drafting stays behind.

The Superhuman company knows this, to be fair — its suite includes Superhuman Go, a separate cross-app assistant marketed as working “across 100+ applications.” But Go is its own product with its own pitch. It isn't the email AI, and it isn't what the Mail tiers are selling. The thing this page compares — the drafting you'd buy Superhuman Mail for — lives in Superhuman Mail.

Tonecast's answer is one hotkey with no home app. Press it over the thread you're reading and it resolves the conversation through per-app integrations — Gmail, Apple Mail, Superhuman, WhatsApp, Slack, iMessage — then drafts three replies, each labeled by intent, and pastes the one you pick at your cursor. Yes, including inside Superhuman: Tonecast isn't an alternative to the client, it works in it. The demo above is the whole argument — their AI chip is real, and it stays in its tile.

Your voice vs the house voice

Superhuman markets Voice & Tone Match, and Auto Drafts promise an email “in your voice and tone.” Take that seriously — inside your inbox it has years of your sent mail to learn from, and the drafts come out sounding plausible.

But two limits are structural, and no amount of model quality fixes them. The first is range: a system that can only read your email can only learn your email voice — the most buttoned-up register you own. You don't write your group chat the way you write your inbox, and you don't write your oldest friend the way you write the board. Tonecast learns your voice per channel and per contact, from messages you've actually sent in each. The second is custody: their model of you lives inside their model. Tonecast's lives in plain markdown files at ~/Library/Application Support/Tonecast/voices/ that you can open, edit, or delete. Not training data — notes you can read.

Privacy posture

The policy that governs your inbox deserves a careful read. The Superhuman company's privacy policy — the one Grammarly built — says: “We also use information we collect to train our AI models. You can decide whether Superhuman can use your user content to train our AI models by adjusting the available training control(s) in your account settings.” For individual self-serve accounts that control defaults to on, the same “on for individual users by default” rule that has long governed Grammarly. Only sales-sold Business, 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 the client is today Superhuman Mail, one product inside the Superhuman company, alongside Grammarly and Coda. If the writing tool 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. Press the hotkey inside Superhuman and the thread is read locally, the draft comes back through your key. 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 Superhuman wins

The best email client, full stop. The shortcuts, the instant search, the split inbox, follow-up reminders, scheduled sends — none of that is what Tonecast does, and nobody does it better than Superhuman. If what you want is a faster inbox, that's their territory, and the Starter tier is worth it for the UX alone.

Native access to your data. Because the AI lives inside the client, it reads your mail directly — Ask AI draws from “your inbox, calendar, and the web,” and Auto Summarize sits above every conversation with no integration layer in between. Tonecast reads threads through per-app integrations over the text macOS already exposes — and where an app exposes nothing readable, it says so rather than screenshotting your screen. Native access is simply more reliable on its home turf.

Every platform, today. Superhuman Mail is native on macOS, Windows, iPhone, and Android, and runs on the web in Chrome and Edge. Tonecast is macOS today · iPhone coming soon · Linux & Windows in the works. If your inbox has to follow you to a PC or your pocket this afternoon, they're there and we aren't yet.

Every claim on this page comes from Superhuman's own pricing and AI pages, the Superhuman company privacy policy, and its training-control support article, and the Superhuman acquisition and company-rename announcements, verified July 5, 2026. If they move a feature between tiers, we'll update this page.