diff --git a/lib/braintrust/contrib/anthropic/instrumentation/beta_messages.rb b/lib/braintrust/contrib/anthropic/instrumentation/beta_messages.rb
index 8b9aca9..1dba91a 100644
--- a/lib/braintrust/contrib/anthropic/instrumentation/beta_messages.rb
+++ b/lib/braintrust/contrib/anthropic/instrumentation/beta_messages.rb
@@ -169,8 +169,8 @@ def set_input(span, params)
               input_messages = []
 
               begin
-                if params[:system]
-                  system_content = params[:system]
+                if params[:system_]
+                  system_content = params[:system_]
                   if system_content.is_a?(Array)
                     system_text = system_content.map { |blk|
                       blk.is_a?(Hash) ? blk[:text] : blk
diff --git a/lib/braintrust/contrib/anthropic/instrumentation/messages.rb b/lib/braintrust/contrib/anthropic/instrumentation/messages.rb
index 5512c07..ab112fc 100644
--- a/lib/braintrust/contrib/anthropic/instrumentation/messages.rb
+++ b/lib/braintrust/contrib/anthropic/instrumentation/messages.rb
@@ -98,8 +98,8 @@ def build_metadata(params, stream: false)
             def set_input(span, params)
               input_messages = []
 
-              if params[:system]
-                system_content = params[:system]
+              if params[:system_]
+                system_content = params[:system_]
                 if system_content.is_a?(Array)
                   system_text = system_content.map { |blk|
                     blk.is_a?(Hash) ? blk[:text] : blk
diff --git a/test/braintrust/contrib/anthropic/instrumentation/messages_test.rb b/test/braintrust/contrib/anthropic/instrumentation/messages_test.rb
index 50e087b..12f09aa 100644
--- a/test/braintrust/contrib/anthropic/instrumentation/messages_test.rb
+++ b/test/braintrust/contrib/anthropic/instrumentation/messages_test.rb
@@ -130,7 +130,7 @@ def test_handles_system_prompt
       message = client.messages.create(
         model: "claude-sonnet-4-20250514",
         max_tokens: 20,
-        system: "You are a helpful assistant that always responds briefly.",
+        system_: "You are a helpful assistant that always responds briefly.",
         messages: [
           {role: "user", content: "Say hello"}
         ]