HiGHS

What it is

HiGHS is the high-performance, open-source linear programming, mixed-integer programming, and quadratic programming solver developed at the University of Edinburgh. It is the default solver in FlexTool and the only one that ships with the FlexTool installation — no separate setup, no licence, no environment variables. Every FlexTool scenario that does not explicitly set the solver parameter runs on HiGHS.

Internally FlexTool runs HiGHS via one of two paths:

• Warm-active path — the default for cascade runs. Keeps a single live highspy.Highs instance across structurally-compatible sub-solves so each iteration's right-hand-side updates flow into the same in-memory model.
• Subprocess path — for cold solves, --save-memory, and single-solve invocations. FlexTool writes the LP to a temporary MPS via the polar-high polars→MPS writer, then runs python -m flextool.cli.cmd_solve_mps in a clean child process so HiGHS' simplex/IPM working set lives outside the FlexTool address space. The retired in-process cold-HiGHS path is no longer reachable from the orchestrator — set FLEXTOOL_SAVE_MEMORY=1 explicitly to silence the soft-promote warning on the cold path.

Official site: https://highs.dev/

License options

• Free and open source under the MIT licence. No commercial vs academic split, no node lock, no annual renewal — install and use without paperwork.

Installation

Already installed. HiGHS is pulled in automatically when you install FlexTool because highspy is a hard dependency. If for any reason it is missing from your environment, install it directly:

pip install highspy

There are no licence files to place, no environment variables to set, and no binaries to fetch separately from the vendor.

Verification

Run:

python -c "from polar_high.solvers import available_solvers; print(available_solvers)"

Expected output (the list ordering may vary depending on which other wrappers are installed, but 'highs' must appear):

['gurobi', 'cplex', 'xpress', 'copt', 'highs']

If 'highs' is missing, your FlexTool install is broken — reinstall FlexTool or run pip install highspy to repair.

The --matrix-file-format choices for HiGHS

The on-disk format used when a solver is dispatched via a matrix file is selected by the --matrix-file-format CLI flag (it replaced the old solver_io_api DB parameter):

Value	What it does	When to use it
mps	Default. On-disk MPS when a matrix file is written.	The default; required for commercial-solver dispatch.
lp	On-disk LP format instead of MPS.	When you prefer human-readable LP files.

For HiGHS without --save-memory, FlexTool dispatches in-process (the fastest path) and the flag has no effect. With --save-memory, HiGHS round-trips through MPS internally; the flag again has no effect on HiGHS. The flag matters for commercial solvers, which are always dispatched via a matrix file.

HiGHS-specific options worth knowing

HiGHS exposes a long list of options; FlexTool surfaces them through the generic solver_arguments Map plus the convenience knobs. The most commonly useful HiGHS option names are:

HiGHS option name	Type	Notes
presolve	string	"on" (default), "off", or "choose".
parallel	string	"on", "off", or "choose" (default).
time_limit	float	Wall-clock seconds. Equivalent to the --solver-time-limit CLI flag.
mip_rel_gap	float	Relative MIP optimality gap. Equivalent to FlexTool's solver_mip_gap.
threads	int	Thread cap. Equivalent to the --highs-threads CLI flag.
solver	string	"simplex", "ipm", or "choose". (Note: this is HiGHS's own solver option — not FlexTool's.)

The former highs_* parameters

Three older FlexTool parameters (highs_presolve, highs_method, highs_parallel) predated the generic mechanism and have been removed. Author the equivalent HiGHS option directly inside the solver_arguments Map instead:

Removed FlexTool parameter	HiGHS option key in solver_arguments	Type
highs_presolve	presolve	string
highs_method	solver	string
highs_parallel	parallel	string

For example, to turn presolve off:

solver_arguments:
  presolve = "off"

Common errors

HiGHS is unusual in that license, install, and environment errors are essentially absent. The two failure modes you can realistically see are:

• Model infeasibility. FlexTool reports INFEASIBLE status from HiGHS; see the FlexTool log for the path to the matrix file (when a file-based dispatch was used) or use keep_solver=True inside the engine for in-process debugging.
• Numerical issues. The LP-scaling guard already in FlexTool catches the most common cases; if you see HiGHS warn about ill-conditioning, the scenario probably has cost or capacity coefficients spanning more than ~12 orders of magnitude.

If you suspect a HiGHS internal issue, pass --solver-log-level verbose to capture HiGHS's own diagnostics in the FlexTool log.

How to set it in FlexTool

HiGHS is the default. No parameter is needed. To make the choice explicit (or to set HiGHS-specific options), author on the relevant solve entity:

solve_advanced.solver = "highs"
solve_advanced.solver_arguments:
  presolve = "on"
  parallel = "on"

Run-wide options such as the wall-clock cap are passed as CLI flags:

flextool <input_db_url> --solver-time-limit 600