# Subcontract loading
WeChat Client 6.6.0, base library {% version ('1.7.3')%} and above are supported.Developer Tools 1.01.1712150 and above can be downloaded here](../devtools/download.md) .
In some cases, developers need to divide Weixin Mini Program into different subpackages, packaged as different subpackages at build time, and loaded as needed by users when they use it.
When you build a Weixin Mini Program subcontract project, the build outputs one or more sub-packages.Each sub-packageing Mini Program must contain a master .The so-called main package, that is, the default startup page / TabBar page, and some of the subpackages all need to use public resources / JS scripts;The subcontract is divided according to the developer's configuration.
When Weixin Mini Program starts, the main package is downloaded by default and the page within the main package starts. When the user enters a page within a subpackage, the client downloads the corresponding subpackage and displays it when the download is complete.
The current Weixin Mini Program subcontract size limits are as follows:
- The size of the entire Weixin Mini Program all sub-packages does not exceed 30 M (Mini programs developed on behalf of the service provider do not exceed 20 M)
- A single subcontract / master package cannot exceed 2 M in size
Subcontracting Weixin Mini Program optimizes the download time for the first time an Mini Program starts, as well as better decoupling collaboration when multiple teams work together.
For more information on how to use it, please refer to: