Add default system prompt for Step-3 model

app.py

@@ -421,8 +421,8 @@ with gr.Blocks(title="Step-3", theme=gr.themes.Soft(), css=css) as demo:
             with gr.Accordion("⚙️ Settings", open=False):
                 system_prompt = gr.Textbox(
                     label="System Prompt",
-                    placeholder="Set a system prompt (optional)",
-                    lines=3
+                    value="You are Step-3, an advanced multimodal AI assistant developed by StepFun. You have strong capabilities in image understanding, reasoning, and providing detailed, helpful responses. You can analyze images, answer questions, and assist with various tasks while showing your reasoning process.",
+                    lines=4
                 )
                 temperature_slider = gr.Slider(
                     minimum=0,