While it passes at "How many 'r' are in strawberry" test, it still halucinates quite a lot in nontrivial questions.

The question I always ask GPT is about chess: "Isolani structure occurs in two main subtypes: 1. black has e6 pawn, 2. black has c6 pawn. What is the main difference between them? Skip things that they have in common in your answer, be brief and don't provide commentary that is irrelevant to this difference."

In isolani structure white has d4 pawn, while he doesn't have either c or e pawns. Black has either e6 or c6 pawn, but he doesn't have d-pawn.

Correct answer would be, that in structure with e6 black king is a bit safer, since f7-a2 diagonal is covered by e6 pawn, which is also controlling f5 point.

o1 answered: "With Black's e6 pawn, the d5 pawn is supported from behind, but Black's light-squared bishop is restricted by the pawn chain, and the open c-file offers White opportunities for pressure. With Black's c6 pawn, the d5 pawn is supported from the side, allowing Black's light-squared bishop more freedom to develop, but the pawn on c6 can become a target, and the absence of an e6 pawn leaves e6 more vulnerable."

while d5 pawn is simply not present in this structure.

So I gave it a little hint: "which diagonal is weakened by absence of c6 pawn and which by absence of e6 pawn? Which diagonal is more meaningful and why?"

And got a smart sounding answer: "In the Isolani structure:

Absence of Black's c6 pawn (Black has an e6 pawn):

Weakened Diagonal: The c8–h3 diagonal (the long light-squared diagonal) is weakened.
Reason: Without the pawn on c6, this diagonal becomes open, allowing White's light-squared bishop or queen to target critical squares like d5 and f7, and to potentially infiltrate via c4.
Absence of Black's e6 pawn (Black has a c6 pawn):

Weakened Diagonal: The f1–a6 diagonal (the long dark-squared diagonal) is weakened.
Reason: The missing e6 pawn leaves this diagonal open, enabling White's dark-squared bishop to exert pressure on b7, d5, and potentially attack Black's queenside.
Which Diagonal is More Meaningful and Why?

The f1–a6 diagonal (weakened by the absence of Black's e6 pawn) is more significant. Here's why:

Control Over Critical Squares: This diagonal allows White to target Black's central and queenside weaknesses more effectively, especially the b7 pawn and the d5 pawn, which are crucial in the Isolani structure.

Bishop Activity: White's dark-squared bishop becomes a powerful piece, influencing the game by controlling key squares and supporting potential pawn advances like d4–d5.

Pressure on Black's Position: The open f1–a6 diagonal increases tactical opportunities for White, such as pinning pieces or creating threats that are difficult for Black to parry without the defensive support of the e6 pawn.

In contrast, while the weakened c8–h3 diagonal (due to the absence of c6) does offer White some opportunities, it is generally less impactful. The light-squared bishop's influence along this diagonal doesn't pressure Black's critical central pawns as directly, and Black can often more easily defend against threats on light squares.

Summary: The absence of Black's e6 pawn weakens the f1–a6 diagonal, which is more meaningful because it enhances White's attacking prospects and puts greater pressure on Black's central and queenside defenses."

The problem is though that not even the first few lines of this answer ("The c8–h3 diagonal (the long light-squared diagonal) is weakened. Reason: Without the pawn on c6, this diagonal becomes open") make sense, since c6 is not on c8-h3 diagonal.