Referring to googleappengine project in Google Code, issue 9012: 

https://code.google.com/p/googleappengine/issues/detail?id=9012&q=Type%3DDefect&colspec=ID Type Component Status Stars Summary Language Priority Owner Log

Currently, PyDev debugger is broken in new dev_appserver.
Issue is marked as 'MoreInfoRequired', with the question being asked is: Do you know if PyDev used pdb in the backend?

I'd like to get this issue some traction, but I've failed to figure out in the PyDev code how the debugger is implemented. Can you please describe or get me some pointers where I can start reading?