Pfp comes with a built-in debugger. You can drop into the interactive debugger by calling the Int3() <pfp.native.dbg.int3> function within a template.
All commands are documented below in the debug reference documentation. Command methods begin with do_.
While the pfp interpreter is handling AST nodes, it decides if a node can be "breaked" on using the _node_is_breakable method. If the interpreter is in a debug state, and the current node can be breaked on, the user will be dropped into the interactive debugger.
pfp.dbg
pfp.native.dbg