The Multiple Unicast Conjecture is False

The undirected multiple-unicast conjecture [LL04] asserts that network coding offers no throughput advantage over multicommodity flow. We refute this conjecture by constructing a deterministic linear network code over ${\mathbb{F}}_9$ on a 182-vertex bipartite subgraph of the point-line incidence graph of $\mathrm{P}\mathrm{G}(2,9)$. The construction supports $157$ independent unicast sessions at common coding rate at least $1$, while every fractional multicommodity flow has common rate at most $147\mathrm{/}157$. By the amplification theorem of~[BGS17], this yields a family of undirected multiple-unicast instances with coding gap $\mathrm{\Omega }((\log n{)}^{\epsilon })$ for some $\epsilon >0$. We also introduce a nondeterministic model of network coding based on locally verifiable certificates, which guides our construction and may be of independent interest. Building on the high-girth graph and error-correcting code framework of [BH25], we use GPT-6 to find a nondeterministic counterexample based on a new choice of Reed–Solomon local codes, and then convert this example into a causal code using an edge orientation and local search.