From d61714b8c708a902b665457a776592a31d5e9896 Mon Sep 17 00:00:00 2001
From: Alex Castillo <alex.castillog33@gmail.com>
Date: Tue, 8 Sep 2026 18:58:46 -0400
Subject: [PATCH] Substitute the layout content token literally, not through
 String#%
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`Options#wrap` inserted the body with `layout % {content: ...}`. `String#%`
reads every `%` in the layout as a format directive, so any HTML layout that
carries a bare `%` — `width: 100%`, a percent-encoded URL, an inline `<style>`
rule — raised `ArgumentError: malformed format string` and the email never
went out.

Replace it with `layout.gsub("%{content}") { wrapped }`. The block form also
keeps the wrapped body verbatim when it contains `\1`, `\\` or `\&`, which the
string form of `gsub` would consume.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
---
 lib/courrier/email/options.rb       |  5 +++-
 test/courrier/email/options_test.rb | 44 +++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+), 1 deletion(-)
 create mode 100644 test/courrier/email/options_test.rb

diff --git a/lib/courrier/email/options.rb b/lib/courrier/email/options.rb
index 1506b6b..b5fb50f 100644
--- a/lib/courrier/email/options.rb
+++ b/lib/courrier/email/options.rb
@@ -56,7 +56,10 @@ def wrap(content, with_layout:)
 
           next wrapped if !layout
 
-          layout % {content: wrapped}
+          # A plain substitution, not `String#%`: an HTML layout routinely carries a
+          # bare `%` (`width: 100%`, an encoded URL), and `format` raises on those.
+          # The block form also keeps the content verbatim when it contains `\1`, `\\`, etc.
+          layout.gsub("%{content}") { wrapped }
         end
       end
 
diff --git a/test/courrier/email/options_test.rb b/test/courrier/email/options_test.rb
new file mode 100644
index 0000000..b8dbbcd
--- /dev/null
+++ b/test/courrier/email/options_test.rb
@@ -0,0 +1,44 @@
+require "test_helper"
+
+class Courrier::Email::OptionsTest < Minitest::Test
+  def test_layout_wraps_the_content_at_the_content_token
+    options = build_options(
+      html: "<p>Hi</p>",
+      text: "Hi",
+      layouts: [{html: "<div>%{content}</div>", text: "%{content}\n\nThanks!"}]
+    )
+
+    assert_equal "<div><p>Hi</p></div>", options.html
+    assert_equal "Hi\n\nThanks!", options.text
+  end
+
+  # An HTML email layout almost always carries a bare `%` (`width: 100%`, an
+  # encoded URL). `String#%` treats it as a format directive and raises.
+  def test_layout_keeps_a_literal_percent_sign
+    options = build_options(
+      html: "<p>Hi</p>",
+      layouts: [{html: "<td style='width:100%'>%{content}</td>"}]
+    )
+
+    assert_equal "<td style='width:100%'><p>Hi</p></td>", options.html
+  end
+
+  # gsub's string replacement would eat `\1`, `\\`, `\&` in the content; the
+  # wrapped body has to come through byte for byte.
+  def test_layout_keeps_backslash_sequences_in_the_content
+    options = build_options(
+      text: 'refund code \1 (\\ and \& too)',
+      layouts: [{text: "%{content}\n--"}]
+    )
+
+    assert_equal "refund code \\1 (\\ and \\& too)\n--", options.text
+  end
+
+  private
+
+  def build_options(**overrides)
+    Courrier::Email::Options.new(
+      {from: "devs@railsdesigner.com", to: "recipient@railsdesigner.com"}.merge(overrides)
+    )
+  end
+end