diff --git a/lib/mail_mcp/imap_client.rb b/lib/mail_mcp/imap_client.rb
index aa7a29b..9b12886 100644
--- a/lib/mail_mcp/imap_client.rb
+++ b/lib/mail_mcp/imap_client.rb
@@ -190,12 +190,15 @@ def format_addresses(addrs)
 
     def extract_attachments(mail)
       mail.attachments.map do |att|
+        # Mail#decoded does not memoize — it base64-decodes into a new String on
+        # every call, so decode once and reuse for both the upload and the size.
+        content = att.decoded
         url = AttachmentStore.upload(
-          content: att.decoded,
+          content: content,
           filename: att.filename || "attachment",
           content_type: att.content_type
         )
-        { filename: att.filename, content_type: att.content_type, size: att.decoded.bytesize, url: url }
+        { filename: att.filename, content_type: att.content_type, size: content.bytesize, url: url }
       end
     end
   end
diff --git a/spec/mail_mcp/imap_client_spec.rb b/spec/mail_mcp/imap_client_spec.rb
index 0b962ec..d2b3985 100644
--- a/spec/mail_mcp/imap_client_spec.rb
+++ b/spec/mail_mcp/imap_client_spec.rb
@@ -50,6 +50,63 @@
     end
   end
 
+  describe "#get_message" do
+    let(:attachment_body) { "PDF-CONTENT" * 10 }
+    let(:raw_message) do
+      mail = Mail.new
+      mail.from = "alice@example.com"
+      mail.to = "bob@example.com"
+      mail.subject = "Invoice"
+      mail.text_part = Mail::Part.new { body "see attached" }
+      mail.add_file(filename: "invoice.pdf", content: attachment_body)
+      mail.to_s
+    end
+
+    before do
+      fetch_data = instance_double(
+        Net::IMAP::FetchData,
+        attr: { "RFC822" => raw_message, "FLAGS" => [:Seen] }
+      )
+      allow(imap).to receive(:uid_fetch).and_return([fetch_data])
+      allow(MailMCP::AttachmentStore).to receive(:upload).and_return("https://s3.example.com/invoice.pdf")
+    end
+
+    it "returns the message with its attachment metadata" do
+      result = described_class.new(imap).get_message(folder: "INBOX", uid: 42)
+
+      expect(result[:subject]).to eq("Invoice")
+      expect(result[:attachments]).to contain_exactly(
+        {
+          filename: "invoice.pdf",
+          content_type: a_string_including("application/pdf"),
+          size: attachment_body.bytesize,
+          url: "https://s3.example.com/invoice.pdf"
+        }
+      )
+    end
+
+    # Mail#decoded re-decodes on every call, so decoding twice doubled the peak
+    # memory of the largest allocation in the request path.
+    it "decodes each attachment only once" do
+      decode_count = 0
+      allow_any_instance_of(Mail::Part).to receive(:decoded).and_wrap_original do |original| # rubocop:disable RSpec/AnyInstance
+        # Count only the attachment: format_message legitimately decodes
+        # text_part/html_part too, and those are Mail::Part instances as well.
+        decode_count += 1 if original.receiver.attachment?
+        original.call
+      end
+
+      described_class.new(imap).get_message(folder: "INBOX", uid: 42)
+
+      expect(decode_count).to eq(1)
+    end
+
+    it "returns nil when the uid is not found" do
+      allow(imap).to receive(:uid_fetch).and_return([])
+      expect(described_class.new(imap).get_message(folder: "INBOX", uid: 99)).to be_nil
+    end
+  end
+
   describe "#search_messages" do
     it "passes raw query string to IMAP SEARCH" do
       allow(imap).to receive(:search).with(["UNSEEN"]).and_return([1, 2, 3])