Unless you have a lot (more than one year) of time, you will probably want to develop the agent as a group. This means that you have to coordinate concurrent development between more several developers. A source code management system is very helpful here.
There are several non-commercial source code management systems available. During the development of AT Humboldt 98 the system CVS was used. It is much less restrictive than the system RCS it is based on. For example, it allows several developers to edit the same project file and merges the changes. For more information, see the CVS manual. Another such system is PRCS. It is more tailored towards smaller projects and is simpler to use. One the other hand it is newer and less thoroughly tested than CVS.
The CVS manual can be found at http://www.loria.fr/E molli/cvs/doc/cvs_toc.html.
The PRCS home page is located at http://www.XCF.Berkeley.EDU/E jmacd/prcs.html.