Representation, Not Search Strategy: Typed DSL Operators Transfer to Unseen Routing Variants Where Free-Form Code Does Not

Large language models can evolve local-search operators for vehicle routing, and the published systems that do so — EoH, FunSearch, ReEvo, HiFo-Prompt — all evolve them as free-form source code. We ask whether that choice of representa- tion is what decides whether an evolved operator works on a routing variant the designer never had in mind. We study operator-set evolution: the search starts from an empty operator set, which is pure construction, and each round a newly proposed operator is tried in every position of the current set — replacing one of the k incumbents, or appended to make k + 1 — with the incumbent set replaced only on strict improvement. Because there is no hand-written floor, a failed round cannot be blamed on the floor having already absorbed the available gains. On three unseen variants, a typed DSL front-end with a semantics-preserving com- piler reaches 20.6% mean improvement over its empty-set start, the best of four LLM baselines that evolve free-form code reaches 19.2%, and a free-form arm under the identical 100,000-token budget reaches 4.8%. The gap is not a tuning artefact: it comes with a 11.8×higher cost per proposal for free-form code, a 3/5 static-rejection rate, and candidates that compile, run, and accept zero improving moves. All trajectories, operator sets and the harness are released.