diff --git a/api/core/tools/custom_tool/tool.py b/api/core/tools/custom_tool/tool.py
index f8467c54c5..8a0163760e 100644
--- a/api/core/tools/custom_tool/tool.py
+++ b/api/core/tools/custom_tool/tool.py
@@ -211,6 +211,12 @@ class ApiTool(Tool):
                 body = urlencode(body)
             else:
                 body = body
+                
+        # if there is a file upload, remove the Content-Type header so that httpx can automatically generate the boundary header required for multipart/form-data.
+        # issue: https://github.com/langgenius/dify/issues/13684
+        # reference: https://stackoverflow.com/questions/39280438/fetch-missing-boundary-in-multipart-form-data-post
+        if files:
+            headers.pop("Content-Type", None)
 
         if method in {
             "get",