We prove the MDS length bound for nontrivial irreducible cyclic codes of every composite length. The classical trace realization turns the MDS property into independence of elements of a multiplicative subgroup. Every nonidentity subgroup must have the full extension degree. This restriction yields length congruences, and the Frobenius-orbit description of the coefficient field gives an exact descent obstruction. A cyclotomic argument reduces each fixed dimension to finitely many parameters. For dimensions at most thirty, established geometric bounds reduce the alphabet to a small finite range; the remaining twelve parameter triples are excluded by explicit products of at most three determinant factors and a low-degree multiplier argument. An independent argument proves the composite-length theorem in arbitrary dimension. The standardness conclusion is stated separately as a conditional consequence of the classification statement of Feng, Hollmann, Li, and Xiang; the correct extension-field reduction retains a semilinear quotient. The general prime-length problem is not settled here. Neither length proof uses the automorphism classification or the recent odd-characteristic range announced by Fan.

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