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Custral brings AI into the inbox the way Superhuman does: a short summary of every substantial thread, a one-click draft reply, and one-tap rewrites of the draft you’re writing: translate, change tone, fix grammar, or adjust length. Everything the AI produces is returned for you to edit. Nothing is sent until you send it yourself.

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How it works

Each feature sends only the plain text it needs (the thread, or your draft) to an AI model and shows you the result:
  • Summarize reads the whole thread and returns a one- or two-sentence gist.
  • Draft with AI reads the thread plus an optional instruction and writes a reply in your voice.
  • Rewrite with AI takes the draft you’ve written and applies a single transform: translate, tone, grammar, or length.
None of these write, send, or save anything on their own.

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Using it

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Summarize a thread

Open an email. For any thread with real substance, a short Summary appears at the top of the conversation (right under the subject, above the messages) as a few bullet points. Click Summary to collapse or expand it. Trivial one-line emails are left alone.

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Draft a reply

  1. Open a reply (or a new email).
  2. In the composer toolbar, click the wand (Draft with AI).
  3. Optionally type what the reply should say, e.g. “politely decline and suggest next week”. Leave it blank to let Custral write the most appropriate reply to the latest message.
  4. Click Generate. The draft fills the composer, ready to edit and send.

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Rewrite your draft

With a draft in the composer, click the pen (Rewrite with AI) in the toolbar and choose:
  • Translate → pick a language (Spanish, French, German, Italian, Portuguese, Dutch, Japanese, Chinese).
  • Make formal / Make friendly. Change the tone.
  • Fix grammar & spelling: clean up mistakes without changing your meaning.
  • Make shorter / Make longer, adjust the length.
The rewritten text replaces your draft in place. Rewrite is disabled until there’s something in the composer to rewrite.

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Troubleshooting

Summaries are shown only for threads with enough content. A single short message (like “ok, thanks”) is left alone on purpose. Very short threads, or a thread whose summary couldn’t be generated, simply show nothing rather than an empty box.
These call an AI model, so an occasional failure can happen. The error notification includes a request ID, use Report this to send it to us. Try again in a moment; your existing draft is left untouched on failure.
Rewrite works on the draft you’ve written, so it’s disabled while the composer is empty. Write (or draft) something first, then rewrite it.
AI writing is an aid, not a source of truth. Always read what it produced before sending, and correct anything wrong. Never send a date, price, name, or commitment you haven’t verified.
If your workspace’s AI usage is temporarily exhausted (for example in a sandbox), the request is declined. Try again later.