Ant debugger

Open build file in editor
Set debug breakpoints:
Right click on editor to show context menu and select the "Debug" menu item to launch Ant debugger:
Wait until the Ant debugger stops on breakpoint then use step or resume debugger commands, investigate Ant variables and execution stack:
You can add Ant debug configuration from the Edit configurations dialog:

Then you select Ant file, Java SDK for debugging session:

Also you adjust Ant settings here: folder with Ant tasks, Java memory amount, debug port.

Note. Plugin requires Java SDK to launch Ant: plugin uses build file module, project or any SDK for JDK list (the plugin searches for Java SDK in this order and uses first found).

Supported Idea versions: Idea 8.1.x, Idea 9 (community and ultimate editions).

Ant debugger plugin is free