diff --git a/astrbot/core/provider/sources/dify_source.py b/astrbot/core/provider/sources/dify_source.py
index 9af198aa7..e4cc76263 100644
--- a/astrbot/core/provider/sources/dify_source.py
+++ b/astrbot/core/provider/sources/dify_source.py
@@ -99,6 +99,9 @@ class ProviderDify(Provider):
         try:
             match self.api_type:
                 case "chat" | "agent":
+                    if not prompt:
+                        prompt = "请描述这张图片。"
+
                     async for chunk in self.api_client.chat_messages(
                         inputs={
                             **payload_vars,