Challenge: Create the longest set of moves where the game is able to be perfectly reproduced from only the current board state and move number The challenge is to create the longest list of half moves whereby an AI could re-create that list of moves perfectly, only given the ending board state and the number of moves.
As example I will first give a short example that does NOT work.
[FEN &quot;&quot;]

1. a3 a6 2. b3 

From that final board state, it is impossible to differentiate it from  1. b3 a6 2. a3.
And a short example that DOES work.
[FEN &quot;&quot;]
1. a3 a6 2. a4 a5 

There is no way to get this board state in exactly two moves other than what was done here.
Scoring will work like this.

+1 point per move.
+5 points if the game is complete, i.e. the last move in the sequence is checkmate.

This is my best solution so far.
[FEN &quot;&quot;]

1. a4 h5 2. a5 h4 3. a6 h3 4. axb7 hxg2 5. Rxa7 Rxh2 6. Rxa8 Rxh1 7. Rxb8 Rxg1 8. Rxc8 Rxf1 

This would score 21 points: 1 point for each of the 16 moves plus 5 points for ending in a checkmate for black.
Note: Opening pawns moving 2 spaces instead of one is necessary in this particular solution.  A 9 turn game where the pawns each move one space per turn would be undifferentiated from an identical game where pawns open with the two space move and then rooks spend an extra turn making their first capture. Also, the game cannot be extended by slipping in Bxg2 at any point in time for White, because it would be unclear, when looking back, when such a move had occurred, since there are several possible turns for such a move to occur.
The current length record is an astounding 57.5 moves (115 half-moves).
[Title &quot;Dmitri W. Pronkin &amp; Andriy Frolkin, Die Schwalbe 06/1989, 1st Prize, PG in 57.5&quot;]
[FEN &quot;&quot;]

1. a4 h5 2. a5 h4 3. a6 h3 4. axb7 hxg2 5. h4 d5 6. h5 d4 7. h6 d3 8. h7 dxc2 9. d4 a5 10. Bh6 c1=R 11. e4 Rc5 12. Ne2 Rh5 13. e5 c5 14. e6 Nc6 15. b8=R a4 16. Rb4 a3 17. Ra4 c4 18. b4 c3 19. b5 c2 20. b6 c1=R 21. b7 Rc4 22. b8=R Qa5+ 23. Rbb4 Bb7 24. Nbc3 O-O-O 25. exf7 e5 26. Rc1 Bc5 27. f8=R a2 28. Rf3 a1=R 29. Na2 g1=R 30. Rfa3 Rg6 31. f4 Re6 32. f5 g5 33. f6 g4 34. f7 g3 35. f8=R g2 36. Rf5 g1=R 37. Bf8 Rg7 38. Ng3 e4 39. Bd3 e3 40. O-O e2 41. Rcc3 e1=R 42. Bc2 R1e3 43. d5 Rdd7 44. d6 Rdf7 45. d7+ Kb8 46. Qd6+ Ka8 47. Qc7 Nge7 48. d8=R+ Nc8 49. Rdd3 Rhg8 50. h8=R Rae1 51. Rh6 R1e2 52. R1f2 Rce4 53. Kf1 Bd4 54. Rfc5 Ne5 55. Nf5 Nc4 56. Nd6 Nb2 57. Rbc4 Nb6 58. Qb8+

This length record was reportedly beaten in 2017 by Pronkin, Frolkin and Keym (to 58.5 moves) but unfortunately it turned that the longer problem had more than one solution. I do not know if the authors managed to make the problem sound.
Although this problem is not computer-checked, it has withstood human testing for 41 years, so it might as well be considered computer checked by the greatest computer of them all-the human mind.