Rat Defense: Harmonist: what should Black play?

In the Rat Defense: Harmonist (ECO B00), Black faces an aggressive kingside flank thrust with White's pawn on f4. Black's most accurate theoretical response is c5, which strikes immediately at the center and yields a nearly level evaluation of -0.03. Striking out with c5 dismantles White's central ambitions before the first player can solidify control over the d4 square.

The Strategic Value of c5

Although c5 represents the engine's top choice, it appears in only 5.9% of the database (229K recorded games), with White scoring 51.7% from this position. By challenging White's central space early, Black prepares standard Sicilian-style counterplay while discouraging an unchecked advance of the enemy pawns. An illustrative engine continuation demonstrates this dynamic balance: c5 Nf3 Nc6 Nc3 g6 d4, where Black develops harmoniously while contesting central dominance.

The Popularity of Nf6

By far the most common human approach is Nf6, selected in 65.8% of recorded games (2.54M recorded). Carrying an engine evaluation of +0.10, this move directly attacks the undefended e4 pawn while naturally developing the kingside minor piece. A sample continuation follows Nf6 Nc3 c5 Nf3 Nc6 Bb5, leading to a complex setup where White scores 51.6% across games in the database.

Other Paths for Black

Black also frequently challenges the center with e5, played in 20.3% of games (784K recorded) with an evaluation of +0.15 and a 51.0% score for White; play can proceed e5 d3 g6 Nf3 Bg7 fxe5. Less frequent alternatives include Nc6 (eval +0.26, 5.8% of games) and the immediate counter-thrust f5 (eval +0.32, 2.1% of games), which can lead to f5 Nc3 fxe4 d3 exd3 Nf3.