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We study the canonical string-based Assembly Index (ASI), defined as the minimum number of binary concatenations needed to construct a target word under full reuse. NP-completeness of ASI-DEC over general finite alphabets and an equivalence between ASI plans and straight-line programs (SLPs) under the same size convention has been established. We emphasize that all transfers between decision variants are effected by explicit polynomial-time mappings and (where needed) an explicit reparameterization of the threshold by an absolute constant or a simple affine function. The remaining technical obstacle for the binary alphabet is that a naive encoding reduction may allow an optimizer to exploit “cross-boundary” substrings created by overlaps of codewords. We give a fully self-contained binary-alphabet proof: we construct an explicit self-synchronizing (comma-free) codebook of 17 fixed-length binary codewords and prove a boundary-normalization lemma showing that optimal plans can be assumed aligned to codeword boundaries. This yields a polynomial reduction from fixed-alphabet ASI-DEC to binary ASI-DEC, proving NP-completeness over {0, 1}. Using the recalled ASI–SLP equivalence (with a short proof for completeness), we obtain NP-completeness of binary SLP-DEC. We additionally provide an explicit, fully formal translation between our binary-rule counting convention and the standard SGP size measure (sum of right-hand side lengths), showing that the NP-completeness classification transfers to common one-string SGP/SLP decision variants over {0, 1}.