letter_opener for the Resend gem.

Intercepts Resend API calls in development, stores emails as JSON on disk, and provides a web UI to browse, preview, and simulate replies. Production code runs unchanged — same pipeline in dev and prod.

Add to your Gemfile:

gem "resend_robot", group: :development

Run bundle install, then run the install generator:

bin/rails generate resend_robot:install

This creates:

• config/initializers/resend_robot.rb — configuration (reply domain, mount path, etc.)
• .claude/skills/resend-robot-read/skill.md — Claude skill for reading dev emails
• .claude/skills/resend-robot-send/skill.md — Claude skill for simulating inbound emails

The generator is safe to re-run — it will ask before overwriting existing files.

Resend Robot automatically:

• Installs shims that intercept Resend::Emails.send, Resend::Batch.send, etc.
• Sets a dummy API key so Resend::Mailer doesn't raise
• Injects routes at /dev/email for the web UI
• Provides rake tasks for CLI interaction

Visit http://localhost:3000/dev/email to browse your dev mailbox:

• Outbox — all intercepted emails, newest first, with search/filter
• Preview — inline HTML rendering with sanitization (no iframes)
• Reply — simulate inbound email through your real webhook pipeline

# config/initializers/resend_robot.rb
ResendRobot.configure do |config|
  config.reply_domain = "reply.example.com"  # required for inbound simulation
  config.webhook_path = "/webhooks/resend"   # default
  config.dev_port = 3000                     # auto-detected from action_mailer
  config.open_in_browser = true              # auto-open emails in browser
  config.mount_path = "/dev/email"           # change to mount UI elsewhere
end

To send real emails via the Resend API in development:

RESEND_IN_DEV=1 bin/dev

OPEN_EMAILS=0 bin/dev

Resend Robot includes Minitest assertions for verifying emails in your test suite:

class UserSignupTest < ActiveSupport::TestCase
  include ResendRobot::TestHelper

  test "sends welcome email on signup" do
    clear_emails
    User.create!(email: "alice@example.com")

    assert_email_sent_to "alice@example.com"
    assert_email_sent_to "alice@example.com", subject: /Welcome/
    assert_equal 1, emails_sent.count
    assert_equal "Welcome", last_email[:subject]
  end

  test "does not send email without signup" do
    clear_emails
    assert_no_emails_sent
  end
end

For E2E tests (Playwright, Capybara), the web UI also serves JSON:

GET /dev/email.json          → array of all emails
GET /dev/email/:id.json      → single email object

The install generator adds two Claude Code skills:

These let Claude interact with your dev mailbox conversationally:

• "Show me the last email sent" → runs /resend-robot-read
• "Simulate a reply to the welcome email" → runs /resend-robot-send

Skills are plain markdown files in .claude/skills/ — customize them for your app (e.g., add app-specific models or webhook details).

bin/rails resend_robot:outbox              # List recent emails
bin/rails resend_robot:show[rl_xxx]        # Show specific email
bin/rails resend_robot:receive[from,to,subject,body]  # Simulate inbound
bin/rails resend_robot:reply[index,body]   # Reply to Nth most recent
bin/rails resend_robot:clear               # Clear all emails

Resend Robot monkey-patches 5 Resend gem methods in development:

Your production code (mailers, webhook controllers, inbound email handlers) runs unchanged. The shim operates at the Resend gem boundary — everything downstream is real.

• Ruby >= 3.1
• Rails >= 7.0
• resend gem >= 1.0

• ARCHITECTURE.md — System design, component overview, data flow diagrams
• CONTRIBUTING.md — Dev setup, running tests, project structure
• CHANGELOG.md — Release history
• TODOS.md — Planned future work

MIT