diff --git a/lib/ruby_llm/attachment.rb b/lib/ruby_llm/attachment.rb
index f5098365f..36251e136 100644
--- a/lib/ruby_llm/attachment.rb
+++ b/lib/ruby_llm/attachment.rb
@@ -37,7 +37,7 @@ def active_storage?
         @source.is_a?(ActiveStorage::Attached::Many)
     end
 
-    def content
+    def content # rubocop:disable Metrics/PerceivedComplexity
       return @content if defined?(@content) && !@content.nil?
 
       if url?
@@ -53,6 +53,8 @@ def content
         nil
       end
 
+      @content&.force_encoding(Encoding::UTF_8) if text?
+
       @content
     end
 
diff --git a/spec/fixtures/multilingual.txt b/spec/fixtures/multilingual.txt
new file mode 100644
index 000000000..a6042ec87
--- /dev/null
+++ b/spec/fixtures/multilingual.txt
@@ -0,0 +1 @@
+Hei på deg
diff --git a/spec/ruby_llm/attachment_spec.rb b/spec/ruby_llm/attachment_spec.rb
index 2bcd8cd92..2096200f3 100644
--- a/spec/ruby_llm/attachment_spec.rb
+++ b/spec/ruby_llm/attachment_spec.rb
@@ -22,4 +22,10 @@
     expect(status.success?).to be(true), stderr
     expect(stdout.strip).to eq('ruby.txt,text/plain')
   end
+
+  it 'serializes non-ASCII text/* attachment content as JSON' do
+    attachment = described_class.new('spec/fixtures/multilingual.txt')
+
+    expect { JSON.generate(text: attachment.content) }.not_to output.to_stderr
+  end
 end