SolveSession owns a HiGHS instance across calls and supports numerical LP/MILP reoptimization. Backend::Auto within a session selects HiGHS; active globals are unsupported there. The one-shot solve uses the same numerical pipeline for linear models, and routes active native globals to the native compiler.

#include <gecode/optimize.hh>
namespace O = Gecode::Optimize;
O::Model model;
auto x = model.add_continuous(0, 100);
auto demand = model.add_row({{x, 1}}, 4, 100);
model.minimize({{x, 2}}, -3);
O::SolveSession session;
auto before = session.solve(model);  // objective 5
model.set_bounds(demand, 8, 100);
auto after = session.solve(model);   // objective 13, compatible LP basis retained
auto statistics = session.statistics();

Every call validates the original snapshot, compiles it, and compares actual content. A caller cannot bypass invalidation by editing a public snapshot without incrementing its revision. Model identity, active variable/row slots, original variable types and the sparse matrix determine compatibility. Changes to any of those reload the backend. Compatible column bounds, row sides, objective costs, sense and offset changes use HiGHS update methods. Pure naming changes have no mathematical effect and leave the backend names internal.

LP updates retain the available basis; HiGHS decides how to use it. MIP search trees and cuts are discarded on every call. A previous MIP incumbent is a hint only after the independent validator checks it against the new original model, including its logical indicators. An explicit user start takes precedence. Changing model identity or structure discards the old hint. Limits never make an invalid old assignment a feasible incumbent.

Each call receives a fresh monotonic budget. Backend timers, optional node limits, gaps, tolerances and seed are set for that call. Callbacks are detached on normal completion and exception paths, before their local data is destroyed. Invalid conversion or backend/numerical failure discards cached backend state; invalid options rejected before conversion leave it unused. Budget expiry before conversion likewise does not run the cached model. Deadline/cancellation checks remain cooperative and may overrun inside backend calls.

Statistics distinguish successful model loads, compatible updates, unchanged models, runs entered with a valid retained LP basis, and submitted revalidated MIP hints. A retained basis or submitted hint is not a claim of a speedup. reset() discards state and counters. Moving transfers ownership; a moved-from session rejects solves until reset. Historical SolveResult values remain valid after either operation. Concurrent use of one session is not supported; independent sessions have separate model state and use one HiGHS worker each.

Persistent native Gecode sessions, basis import/export, LP sensitivity/dual records, action callbacks, interrupted MIP-tree reuse and mixed worker counts are still unsupported. Set SolveOptions::backend = Backend::Native for the explicit native one-shot bridge described in NATIVE.md.

test/optimize/session.cpp compares edited sessions with cold solves, checks seeded bounded-integer edits against exhaustive enumeration, and covers type, matrix and identity changes, tombstones, historical results, infeasible and unbounded revisions, malformed input, start precedence, moving and reset. The complete sanitizer build instruments the adapter and HiGHS together.