ShroomPenAdd to Chrome

AI replies in Gmail with your own API key

Write a reply in Gmail's own reply box, with the thread as context, using a model you choose and pay for directly.

Published

The short answer

ShroomPen is a free Chrome extension that drafts Gmail replies in Gmail's own reply box. It reads the conversation you have open, sends it with your instruction to a model you connect (your own OpenRouter, OpenAI, Anthropic or Gemini key, or a local model with Ollama or LM Studio), and inserts the draft for you to edit and send. There is no ShroomPen account, subscription or server.

Set it up and write a reply

  1. Install ShroomPen and connect a model

    Add ShroomPen to Chrome, press Ctrl+Q on any page, accept the data disclosure and connect OpenRouter, OpenAI, Anthropic, Gemini or a local server.

    Get ShroomPen from the Chrome Web Store. OpenRouter is the quickest start: one sign-in, and a free model is preselected. The provider setup guide covers the other options.

  2. Open the email and click Reply

    Open the conversation in Gmail, expand the messages you want the reply to take into account, and click Reply so the cursor is in the reply box.

  3. Open ShroomPen over the reply box

    Press Ctrl+Q. Current page is on by default, so ShroomPen reads the visible thread. You can also use the Reply button that appears next to the field.

    The small helper next to the reply box has Reply, Rewrite, Fix Grammar and Translate buttons. Reply writes an answer straight into an empty box. The Ctrl+Q overlay lets you type an instruction and review the draft first.

  4. Say what the reply should do

    Type a short instruction, for example "Accept the Tuesday slot, ask for the agenda" and press Enter.

  5. Review and insert

    Edit the draft if needed, then choose Insert draft. ShroomPen puts the text into the reply box and never sends the email for you.

What goes into the request

  • Current page (on by default): the text Gmail shows on screen, such as the open thread, up to 12,000 characters. Collapsed messages may be left out, so expand the ones that matter.
  • Current field: whatever you have already typed in the reply box.
  • Selected text: if you highlight a passage before opening ShroomPen, it is treated as the most important part.
  • Tabs: other tabs you pick, for example a PDF with the contract terms or a Google Doc with your notes.

You can switch each source off before you generate. Nothing is sent until you press Enter.

Tips for better replies

  • Say the decision, not the wording: "decline politely, offer next month" works better than "write a reply".
  • Use a Workspace for standing instructions, such as your sign-off, tone or the language you reply in, so you don't repeat them.
  • After a draft appears, type a follow-up like "shorter" or "less formal" to revise it before inserting.

What it costs

ShroomPen is free. A reply with the thread as context costs about $0.0002 to $0.002 on small models such as GPT-4o mini or Gemini 2.5 Flash, and nothing on OpenRouter's free models or a local model. See the cost breakdown.

Limits

  • ShroomPen has no Gmail-specific integration. It works with the reply box like any other text field.
  • It doesn't read attachments inside Gmail. Open a PDF attachment in its own Chrome tab (or download it and open the file) and pick that tab as context instead.
  • It doesn't run automatically on new mail or send anything. Every draft starts with you.

Related pages

Questions

Is there a Gmail AI reply extension that uses my own API key?

Yes. ShroomPen is a free Chrome extension that writes Gmail replies with your own OpenRouter, OpenAI, Anthropic or Gemini key, or with a local model through Ollama or LM Studio. There is no ShroomPen account or subscription.

Does ShroomPen read my whole inbox?

No. It reads the text visible on the page when you ask for a draft, up to 12,000 characters, and only with Current page switched on. It does not open other emails or use the Gmail API.

Can it send the email?

No. ShroomPen only puts text into the reply box. You read it and press Send yourself.

Does my email go to ShroomPen?

No. ShroomPen has no server. Your instruction and the context you switched on go straight from your browser to the AI provider you connected. With a local model, they stay on your computer.

Does it work in Outlook on the web or other webmail?

ShroomPen works with ordinary text fields on most websites, so it is not limited to Gmail. We have described Gmail here because it is the most common case.