I’m seeing this in both 10.1.5 & 10.1.7 when stepping through code with the GUI debugger. The initial gspy breakpoint is taken OK, but this happens after the first ‘step into’: [INTERNAL ERROR: Diff: '.'(_, roots, A), '.'(A, patterns, _). *** <-> '.'(_, roots, _). Different text depending on what …

[image] alanbur: [INTERNAL ERROR: Diff: '.'(_, roots, A), '.'(A, patterns, _). *** <-> '.'(_, roots, _). Please share the code that triggers that (as well as where to spy and other relevant details).

I can’t provide the code but I’ll see if I can come up with a reproducer. It seems to happen when you are calling a predicate that takes a dict as an argument, and on the initial step into the predicate.

I burned through some tokens and generated the attached reproducer. I’ve provided the :robot:'s analysis of the issue, I make absolutely no claims about its correctness :grinning_face_with_smiling_eyes: but the code reproduces the bug, gspy_dict.pl (1.4 KB)

BUG: internal error in GUI debugger

Help!

alanbur May 15, 2026, 11:29am 2

If it helps, these two strings look like they are dict keys.

Ann: SWI-Prolog 10.1.7

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BUG: internal error in GUI debugger

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