Fundamentals (part 10)

Spanton (151) - J Simons (130)
Kings Head (London) Major 1990

Black has just captured on e5 - how should White proceed?

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It might be thought Black is obviously better. After all, he has more space in the centre and he has an undamaged pawn majority. But Komodo10 reckons Black has just a slight edge, while Stockfish10 rates the position as level. However, the engines agree White's next move is critical.
27.b4?
The engines give 27.Kc4. But as soon as the move is played on the board, Stockfish10's assessment switches from dead-equal to Black being the equivalent of almost a pawn ahead. However, given enough time, Stockfish10's assessment changes to about a half-pawn advantage for Black, and eventually to equality. Komodo10 goes through a similar process.
Best play after 27.Kc4, the engines agree, is 27...Ke4 28.g4 c6 29.Kc5 d3 30.cxd3+ Kxd3 31.a4 Kc2 32.b4 Kb3 33.a5 Kb2 34.f3 Kb3 35.f4 Kb2 36.g3 Kb3 37.f5 Kc3 38.Kd6 Kxb4 39.Ke6 c5 40.Kf7 c4 40.Kxg7 c3 41.f6 c2 42.f7 c1=Q 43.f8=Q+ - a long line that is largely forced and seems to lead to equality.
27...c6?
The engines reckon Black should play 27...b5 or 27...Kd5, in both cases preventing 28.Kc4.
28.a4? Kd5
Black seems to be winning in all lines.
The game finished:
29.g4 b6 30.f3 c5 31.b5 c4+ 32.Kd2 Kc5 33.c3 Kd5 34.g3 dxc3+ 35.Kxc3 Kc5 36.f4 Kd5 37.g5 hxg5 38.fxg5 Kc5 39.g6 Kd5 40.Kc2 Kd4 41.Kd2 c3+ 42.Kc2 Kc4 43.Kc1 Kb3 0-1