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I was using Apronus to make an new chess puzzle for Puzzling Stack Exchange, but then it did something odd. Take a look here:

[FEN ""]
1. Nh3 f5 2. e4 fxe4 3. d3 exd3 4. Be2 Kf7 5. Qd2 dxe2 6. O-O

It's letting the white king castle through a square that is attacked by a black pawn; that is an illegal move!

Do you have any idea why this is happening?

EDIT: I’m doing this here since comments can’t include pictures. In regards to what you said Brian Towers, you actually are able to use Apronus to validate games. Take a look at this.

enter image description here

That’s what happens when you put in an illegal move for a position and tap the “Absorb FEN Or PGN Button.”

EDIT 2: I forgot to include this proof link of my claim.

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    Short answer: because the developers are either bad chess players or bad software developers! – David Apr 5 at 7:05
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I happen to be the author of the chessboard editor under discussion. Let me clarify the issue.

This editor is supposed to enforce legal moves but it has a bug which prevents it from being able to make a pawn capture onto the final rank. Therefore it cannot notice that a black chessman controls the f1 square. If you place a black bishop on the a6-f1 diagonal then it won't allow White to castle.

There is a newer chessboard editor that is free from this bug: https://www.apronus.com/chess/puzzle/editor.php?editpuzzle=0, which is meant as a powerful puzzle editor but it can serve as a PGN editor, too.

The "position setup" interface can be used to illustrate an arbitrary sequence of chessboards, not necessarily legal or illegal moves. If you tick the checkbox "Animation sequence" you will see that it keeps a record of all board positions like frames in a movie. It allows you to delete certain frames which effectively allows you to set up an arbitrary sequence of board position and generate an animated GIF diagram. I am the author of this tool.

  • Well I have have found that you can get around that by typing in the moves and clicking the "Absorb" button. Plus, you can always copy and paste games you have with the Apronus computer, where you can do it. – Rewan Demontay Apr 5 at 11:36
  • And, just wondering here, can you prove that you are the author? – Rewan Demontay Apr 5 at 11:38
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    You can write an email message to Apronus.com and ask them to point to this page where you ask about proving me to be the author. I will see the message and reply with a link to this very page. – DrCapablasker Apr 5 at 11:42
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Well, for a definitive answer you need to ask the authors/maintainers of the website. Especially since it has a "legal moves only" option above the board which is selected by default. Note that this very website's replayer also executes the illegal move in your post.

I can imagine that one wants to analyze/record a complete game even if an illegal move was made in it, or show an almost-legal constructed game to reach a certain position. However, the PGN standards forbid this:

8.2: Movetext section

The movetext section is composed of chess moves, move number indications, optional annotations, and a single concluding game termination marker.

Because illegal moves are not real chess moves, they are not permitted in PGN movetext. They may appear in commentary, however. One would hope that illegal moves are relatively rare in games worthy of recording.

  • Well, the position editor is what you use for illegal moves. And I believe that this site’s chess replayer isn’t built to detect illegal moves for the purpose of allowing them for some questions. – Rewan Demontay Apr 4 at 12:54
  • No, the position editor is to setup positions where you don't care about past moves. At least that's what I would use it for ... – Glorfindel Apr 4 at 12:56
  • Well, even so, it stil is in a WAY I would say. – Rewan Demontay Apr 4 at 12:58
  • The "position setup" interface can be used to illustrate an arbitrary sequence of chessboards, not necessarily legal or illegal moves. If you tick the checkbox "Animation sequence" you will see that it keeps a record of all board positions like frames in a movie. It allows you to delete certain frames which effectively allows you to set up an arbitrary sequence of board position and generate an animated GIF diagram.I am the author of this tool. – DrCapablasker Apr 5 at 11:38
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Do you have any idea why this is happening?

Because it is a chessboard editor, not an engine.

Here is what the Apronus site says:

PGN Viewer and Editor with Chess Puzzle Editor

  • online interactive chessboard with PGN viewer and editor, diagram editor and puzzle editor
  • set up any position to play against computer or analyze with a chess engine
  • make diagrams with arrows and selected squares or animations indicating moves with arrows
  • create links to your chess positions or embed chessboards on your own pages
  • design chess puzzles:

Its main function is to help you design diagrams and puzzles not validate your games.

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    It should be able to validate games but it has a bug as I explained in my answer chess.stackexchange.com/a/24121/3456 – DrCapablasker Apr 5 at 11:33
  • Did you literally just link something that you you can get by simply scrolling above? – Rewan Demontay Apr 5 at 11:40
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    At the time of writing my answer was below not above. You can never tell because these things shift position according to the votes they get. – DrCapablasker Apr 5 at 11:48

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