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# An AI email agent: inbound email → Claude drafts a reply → MailKite sends it, threaded.

# Give your product an inbox that answers itself.

#

# Flow: MailKite POSTs the inbound `email.received` event → verify it → Claude composes a concise

# reply → send it back with `inReplyTo` so it threads to the sender.

#

# Run: MAILKITE_API_KEY=mk_live_… MAILKITE_WEBHOOK_SECRET=whsec_… ANTHROPIC_API_KEY=sk-ant-… \

# ruby 03_agent_email_reply.rb # serves on :3000

# Deps: gem install mailkite sinatra anthropic

require "json"

require "sinatra"

require "mailkite"

require "anthropic"

mk = Mailkite::Client.new(ENV.fetch("MAILKITE_API_KEY"))

claude = Anthropic::Client.new # reads ANTHROPIC_API_KEY

SECRET = ENV.fetch("MAILKITE_WEBHOOK_SECRET")

SYSTEM = "You are the support agent for Acme. Read the customer's email and write a short, friendly " \

"reply that directly answers them. Plain text. If you can't help, say a human will follow up."

set :port, 3000

post "/hooks/mailkite" do

raw = request.body.read # verify against the RAW body — re-serialized JSON breaks the HMAC

signature = request.env["HTTP_X_MAILKITE_SIGNATURE"]

halt 401, "bad signature" unless mk.verifyWebhook(signature, raw, SECRET)

event = JSON.parse(raw)

halt 200, Mailkite.reply_ok unless event["type"] == "email.received"

m = event["message"] || event # { from, to, subject, text, html, messageId, … }

# 1. Claude drafts the reply.

msg = claude.messages.create(

model: :"claude-opus-4-8", # swap to claude-sonnet-4-6 / claude-haiku-4-5 for lower cost

max_tokens: 1024,

system_: SYSTEM,

messages: [{ role: "user", content: "From: #{m['from']}\nSubject: #{m['subject']}\n\n#{m['text'] || m['html']}" }]

)

reply = msg.content.find { |b| b.type == :text }&.text || "Thanks — a human will follow up."

# 2. Send it back, threaded to the original.

subject = m["subject"].to_s.start_with?("Re:") ? m["subject"] : "Re: #{m['subject']}"

mk.send(

"from" => m["to"], # reply from the address that received the mail

"to" => m["from"],

"subject" => subject,

"text" => reply,

"inReplyTo" => m["messageId"]

)

puts "🤖 replied to #{m['from']}"

content_type :json

Mailkite.reply_ok

end