Comparison
Tonecast vs Superhuman AI
Superhuman AI writes replies inside Superhuman. Tonecast writes replies in your voice, in any app, with free BYOK mode.
| Tonecast | Superhuman AI | |
|---|---|---|
| Works in any email client | -- | |
| Works in chat apps | -- | |
| Learns your voice per contact | -- | |
| Generates reply options | ||
| Voice dictation | -- | |
| Emails used to train AI by default | -- | yes (opt-out in settings) |
| Email content stays on your device | yes (BYOK) | -- |
| Email UX (search, shortcuts) | -- | |
| Price | Free BYOK; Cloud optional | $30/mo |
An AI layer vs an email client
Superhuman is an email client with AI built in. Tonecast is an AI layer that sits on top of whatever you already use. These are fundamentally different approaches, and the right choice depends on what problem you're actually solving.
If you're looking for the fastest, most keyboard-driven email experience available, Superhuman is genuinely excellent. The speed, the shortcuts, the split inbox, the read statuses - it's a premium email client that happens to have AI features. The AI is good, but it's one feature among many.
Tonecast does one thing: it helps you communicate in your own voice. It works in Superhuman, Gmail, Apple Mail, Outlook - and also in WhatsApp, Slack, iMessage, and anything else you use to talk to people.
Locked in vs works everywhere
Superhuman AI only works inside Superhuman. Switch to Gmail for a shared inbox, hop into Slack for a thread, open WhatsApp for a quick message - Superhuman's AI can't follow you there. You're back to writing everything yourself.
Tonecast works wherever you are. It reads the conversation on screen (through the browser DOM, accessibility APIs, or screenshot analysis depending on the app), understands the context, and generates replies. Same hotkey, same experience, regardless of the app. Your voice profile travels with you.
Generic replies vs your voice
Superhuman AI generates reply suggestions. They're competent, professional, and generic. They sound like a well-trained language model writing a business email - because that's exactly what they are. Every Superhuman user gets the same voice.
Tonecast learns how you write. It analyzes your sent messages, builds a voice profile specific to your patterns, and generates replies that sound like something you'd actually send. It does this per contact - because the way you write to your manager is different from how you write to your direct reports, and both are different from how you message your friends.
This isn't a style dropdown with "professional" and "casual" options. It's a voice fingerprint built from your real writing, refined every time you send a message.
Three modes, not one
Superhuman AI offers reply suggestions in email. That's one mode in one app.
Tonecast has three modes. Mode 1: select any text and get three tonal rewrites. Mode 2: read a conversation thread and generate three reply options with different strategic angles. Mode 3: push-to-talk voice input with instant transcription and intelligent polish. Each mode works in any app.
Privacy posture
Superhuman was acquired by Grammarly in 2024. Superhuman's privacy page now redirects to Grammarly's, and the policy that governs your Superhuman emails is Grammarly's policy.
Per that policy, Grammarly receives “emails and drafts, text, documents, files, calendars, images, data” from Superhuman customers and uses that content to train Grammarly's AI models — unless you find and toggle the AI training control in your account settings. The opt-out exists; the default is opt-in.
Tonecast in BYOK mode keeps your emails out of any vendor's training pipeline. When you press a hotkey inside Superhuman (Tonecast works there too), Tonecast reads the thread locally on your Mac and sends the prompt directly to your chosen AI provider via your own key. No Tonecast server in the path. No “we train on your content” clause to opt out of.
Tonecast Cloud is account-based and routes prompts through our API — but we don't store your text and we don't train on it. Optional managed tier, different architecture, different threat model.
Sources: Grammarly Privacy Policy (governs Superhuman after the 2024 acquisition; verified 2026-04-29), Tonecast Privacy.
Where Superhuman wins
Email UX. Superhuman is the best email client on the market for power users. The keyboard shortcuts, instant search, scheduled send, read receipts, split inbox - none of this is what Tonecast does. If you want a better email client, that's Superhuman's territory.
Deep integration. Because Superhuman AI lives inside the email client, it has native access to your entire email history, contacts, and calendar. There's no accessibility API layer, no DOM scraping - it reads your data directly. This makes certain features (like auto-summarization of long threads) more reliable.
Mobile support. Superhuman works on iOS and Android. Tonecast is Mac-only.
The real question
If you want a premium email client that also has AI, Superhuman at $30/month is a reasonable investment. You're paying for the email UX as much as the AI.
If you want AI that sounds like you, works across every communication tool, and includes voice dictation - Tonecast does that for free in BYOK mode. And it works inside Superhuman too, so you can use both.
One is a $360/year email client. The other is a free BYOK communication layer with optional managed Cloud.