diff --git a/lib/datadog/ai_guard/evaluation.rb b/lib/datadog/ai_guard/evaluation.rb
index 844d51a1b0..435684183f 100644
--- a/lib/datadog/ai_guard/evaluation.rb
+++ b/lib/datadog/ai_guard/evaluation.rb
@@ -74,7 +74,7 @@ def perform_no_op
 
         def truncate_messages(serialized_messages)
           max_length = Datadog.configuration.ai_guard.max_messages_length
-          serialized_messages.first(max_length)
+          serialized_messages.last(max_length)
         end
 
         # Truncates content in serialized messages to stay within the configured byte limit.
diff --git a/spec/datadog/ai_guard/evaluation_spec.rb b/spec/datadog/ai_guard/evaluation_spec.rb
index 58f55c7961..c3c9dfca8b 100644
--- a/spec/datadog/ai_guard/evaluation_spec.rb
+++ b/spec/datadog/ai_guard/evaluation_spec.rb
@@ -213,8 +213,8 @@
         ])
 
         expect(ai_guard_span.get_metastruct_tag("ai_guard").fetch(:messages)).to eq([
-          {content: "System prompt", role: :system},
           {content: "User message", role: :user},
+          {content: "Assistant reply", role: :assistant},
         ])
       end