diff --git a/modules/streaming-generation.js b/modules/streaming-generation.js
index 96e3fa6..95b63be 100644
--- a/modules/streaming-generation.js
+++ b/modules/streaming-generation.js
@@ -139,8 +139,8 @@ class StreamingGeneration {
             const isClaudeModel = modelLower.includes('claude');
 
             if (isClaudeModel && Array.isArray(messages) && messages.length > 0) {
-                if (this._mergeAssistantPrefillIntoPrevious(messages)) {
-                    console.log('[xbgen] Claude model: merged trailing assistant prefill into previous message');
+                if (this._convertTrailingAssistantToSystem(messages)) {
+                    console.log('[xbgen] Claude model: converted trailing assistant prefill to system message');
                 }
             }
 
@@ -800,29 +800,15 @@ class StreamingGeneration {
         return '';
     }
 
-    _mergeAssistantPrefillIntoPrevious(messages) {
-        if (!Array.isArray(messages) || messages.length < 2) return false;
+    _convertTrailingAssistantToSystem(messages) {
+        if (!Array.isArray(messages) || messages.length < 1) return false;
         const lastMsg = messages[messages.length - 1];
         if (lastMsg?.role !== 'assistant') return false;
 
         const assistantText = this._extractTextFromMessage(lastMsg).trim();
-        const previousMsg = messages[messages.length - 2];
-        if (!previousMsg) return false;
-
-        if (assistantText) {
-            const mergedText = `assistant:\n${assistantText}`;
-            if (typeof previousMsg.content === 'string') {
-                previousMsg.content = previousMsg.content
-                    ? `${previousMsg.content}\n\n${mergedText}`
-                    : mergedText;
-            } else if (Array.isArray(previousMsg.content)) {
-                previousMsg.content.push({ type: 'text', text: `\n\n${mergedText}` });
-            } else {
-                previousMsg.content = mergedText;
-            }
-        }
-
-        messages.pop();
+        const systemText = assistantText ? `Assistant:\n${assistantText}` : 'Assistant:';
+        lastMsg.role = 'system';
+        lastMsg.content = systemText;
         return true;
     }
 
@@ -1515,4 +1501,3 @@ if (typeof window !== 'undefined') {
         eventSource: (window)?.eventSource || eventSource
     });
 }
-