fix: add diagnostic logging for empty message content

Added logging to capture the original UI message structure when empty content
is detected after conversion. This helps debug the root cause while the
filter provides a safety net for Bedrock API compatibility.

app/api/chat/route.ts

@@ -90,7 +90,27 @@ ${lastMessageText}
 
     // Convert UIMessages to ModelMessages and add system message
     const modelMessages = convertToModelMessages(messages);
+
+    // Log messages with empty content for debugging (helps identify root cause)
+    const emptyMessages = modelMessages.filter((msg: any) =>
+      !msg.content || !Array.isArray(msg.content) || msg.content.length === 0
+    );
+    if (emptyMessages.length > 0) {
+      console.warn('[Chat API] Messages with empty content detected:',
+        JSON.stringify(emptyMessages.map((m: any) => ({ role: m.role, contentLength: m.content?.length })))
+      );
+      console.warn('[Chat API] Original UI messages structure:',
+        JSON.stringify(messages.map((m: any) => ({
+          id: m.id,
+          role: m.role,
+          partsCount: m.parts?.length,
+          partTypes: m.parts?.map((p: any) => p.type)
+        })))
+      );
+    }
+
     // Filter out messages with empty content arrays (Bedrock API rejects these)
+    // This is a safety measure - ideally convertToModelMessages should handle all cases
     let enhancedMessages = modelMessages.filter((msg: any) =>
       msg.content && Array.isArray(msg.content) && msg.content.length > 0
     );