How to build string theory in almost 26 dimensions

Zwiebach's formulation of closed string field theory works best in exactly 26 dimensions for bosonic strings, but real physics might happen nearby. This work constructs the precise mathematical machinery—mixed moduli spaces and descent operators—needed to write down the theory when you deviate even slightly from criticality, then tests it on strings in 26-ε dimensions and linear dilaton backgrounds. The result bridges formal consistency and potential physical applications.