user25130: I've just played a game on Chess.com, and I'm surprised that the engine considers a promotion that I made a huge blunder. Like, my position was rated -62.8 (basically a won game), and after I promoted my pawn to a queen the rating dropped to -6.40. That's more than 50 points with a single move!
This is the position:
[FEN "8/8/2B5/p7/8/p4p2/P2K1kp1/8 w - - 0 1"]

1. Be4 g1=Q (1... Kg3 2. Kd3 g1=Q 3. Kc4 f2 4. Bd3 f1=Q 5. Bxf1 Qxf1) 2. Bxf3 Kxf3 3. Kc2 Qc5+ 4. Kd3 Qb5+ 5. Kd2 Qb2+ 6. Kd1 Qxa2 7. Kc1 Qb2+ 8. Kd1 a2 9. Ke1 a1=Q# 0-1

I played g1=Q, but the engine says I should have played Kg3 instead, which would have kept the rating at -62.8. I don't understand why. My pawn in f3 was threatened by the bishop, but it was protected by the king. The white king was unable to come closer, because of my king. And the pawns on the left were irrelevant.
The engine's move did lead to another pawn being promoted... Only to be captured by the bishop, which in turn was taken by the first queen. So after a few moves the situation was identical: king and queen vs king (plus the pawns on the side). And even with my "blunder", winning was easy.
Why does the engine find such a huge difference? Should I just ignore this, or am I missing something important?

user15223: This sounds like a bug. Using Lichess's analysis engine, it takes Stockfish 14 less than ten seconds to find a mate in 11 moves. Both 1...Kg3 and 1...g1=Q lead to mate in 11.
So there should be no difference in which move you start with, even objectively (of course as a human I'd go 1...g1=Q without thinking).

user13433: 50 points sounds huge, but there actually is no meaningful difference between -6.4 and -62 in such an endgame.*

6.4 is more or less "Black has a queen for a bishop (but I haven't found a way to make progress yet)"

62 on the other hand is "I have found that way to a position that is 100% won (but I haven't found a forced mate yet)"


Why does the engine rate both moves differently? That might just come down to  random chance: Since Black has a winning position either way, almost all moves along the calculation tree are equal to the engine's eye and it loses its guidance so to speak which branches are best to pursue. At the same time, the tree is still relatively big (White has a lot of ways to shuffle the bishop around, etc.).
So the tree gets deepened in one more or less random direction and the engine will keep expanding from there (downwards and sideways) because all moves it finds are keeping the advantage.
By chance, the trees for 1...Kg3 and 1...g1=Q start growing a bit differently. They will of course converge eventually (to the mate in 11 Allure mentions) if the engine gets enough time to calculate all of the tree. But in for example automatic game analysis, it often doesn't (20-30 seconds for all moves = not a lot of time per single move). In this case, the search for both moves just happened to be cut off at different states of incompleteness (with different temporary evaluation scores).
Which is a bit annoying if it taints your statistics for an otherwise well played game - but you can safely ignore it.
*For this reason, at least Lichess internally caps the scale at +10 and -10 so that a drop from -60 to -10 would not be labelled a "blunder". Don't know how chess.com handles this, maybe the drop from -6.4 to -10 was still enough for a "blunder" though.

25130: <p>I've just played a game on Chess.com, and I'm surprised that the engine considers a promotion that I made a huge blunder. Like, my position was rated -62.8 (basically a won game), and after I promoted my pawn to a queen the rating dropped to -6.40. That's more than 50 points with a single move!<br />
This is the position:</p>
<pre><code>[FEN &quot;8/8/2B5/p7/8/p4p2/P2K1kp1/8 w - - 0 1&quot;]

1. Be4 g1=Q (1... Kg3 2. Kd3 g1=Q 3. Kc4 f2 4. Bd3 f1=Q 5. Bxf1 Qxf1) 2. Bxf3 Kxf3 3. Kc2 Qc5+ 4. Kd3 Qb5+ 5. Kd2 Qb2+ 6. Kd1 Qxa2 7. Kc1 Qb2+ 8. Kd1 a2 9. Ke1 a1=Q# 0-1
</code></pre>
<p>I played g1=Q, but the engine says I should have played Kg3 instead, which would have kept the rating at -62.8. I don't understand why. My pawn in f3 was threatened by the bishop, but it was protected by the king. The white king was unable to come closer, because of my king. And the pawns on the left were irrelevant.</p>
<p>The engine's move did lead to another pawn being promoted... Only to be captured by the bishop, which in turn was taken by the first queen. So after a few moves the situation was identical: king and queen vs king (plus the pawns on the side). And even with my &quot;blunder&quot;, winning was easy.</p>
<p>Why does the engine find such a huge difference? Should I just ignore this, or am I missing something important?</p>
 15223: <p>This sounds like a bug. Using Lichess's analysis engine, it takes Stockfish 14 less than ten seconds to find a mate in 11 moves. Both 1...Kg3 and 1...g1=Q lead to mate in 11.</p>
<p>So there should be no difference in which move you start with, even objectively (of course as a human I'd go 1...g1=Q without thinking).</p>
 13433: <p>50 points sounds huge, but there actually is no meaningful difference between -6.4 and -62 in such an endgame.*</p>
<ul>
<li><p>6.4 is more or less &quot;Black has a queen for a bishop (but I haven't found a way to make progress yet)&quot;</p>
</li>
<li><p>62 on the other hand is &quot;I <em>have</em> found that way to a position that is 100% won (but I haven't found a forced mate yet)&quot;</p>
</li>
</ul>
<p>Why does the engine rate both moves differently? That might just come down to  random chance: Since Black has a winning position either way, almost all moves along the calculation tree are equal to the engine's eye and it loses its guidance so to speak which branches are best to pursue. At the same time, the tree is still relatively big (White has a lot of ways to shuffle the bishop around, etc.).
So the tree gets deepened in one more or less random direction and the engine will keep expanding from there (downwards and sideways) because all moves it finds are keeping the advantage.</p>
<p>By chance, the trees for 1...Kg3 and 1...g1=Q start growing a bit differently. They will of course converge eventually (to the mate in 11 Allure mentions) if the engine gets enough time to calculate all of the tree. But in for example automatic game analysis, it often doesn't (20-30 seconds for all moves = not a lot of time per single move). In this case, the search for both moves just happened to be cut off at different states of incompleteness (with different temporary evaluation scores).</p>
<p>Which is a bit annoying if it taints your statistics for an otherwise well played game - but you can safely ignore it.</p>
<p>*For this reason, at least Lichess internally caps the scale at +10 and -10 so that a drop from -60 to -10 would not be labelled a &quot;blunder&quot;. Don't know how chess.com handles this, maybe the drop from -6.4 to -10 was still enough for a &quot;blunder&quot; though.</p>
