We give a deterministic $O(m)$-time algorithm for exact modular subset sum over every modulus $m$. The input gives distinct residues and their multiplicities. The algorithm reports all reachable residues and answers one target query. It returns a witness when the target is reachable. The algorithm uses $O(m)$ auxiliary words on an arithmetic word-RAM. This improves Potępa's deterministic $O(m\log m\,α(m))$-time bound under the same input convention. The running time matches the cost of explicitly reporting all $m$ reachability bits. We represent reachable residues as runs along cycles of repeated addition. Newly reached residues pay for scans of partial runs. We process prime factors in increasing order so that cycle rebuilding takes linear time. A theorem on subset sums of distinct units limits the number of adaptive boundary batches to $O(m^{3/4})$. Radix sorting their $O(m)$ total keys also takes $O(m)$ time.

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