highs: wrong optimization result with presolve since highs v1.14.0

[maurerle]

Version Checks (indicate both or one)

• I have confirmed this bug exists on the lastest release of Linopy.

• I have confirmed this bug exists on the current master branch of Linopy.

Issue Description

optimizing:

minimize 2x + y
s. t. 2*x0 + y0 >= 10
with x binary
and 9 <= y

should result in y=10 and x=0 but results (as with HiGHS 1.11) - but gives the wrong result with HiGHS 1.14 and linopy.

Besides downgrading, it helps to disable presolve (m.solve(solver_name='highs', presolve= "off")).

Running HiGHS 1.14.0 (git hash: 7df0786): Copyright (c) 2026 under MIT licence terms
MIP linopy-problem-hu0mxzpo has 1 row; 2 cols; 2 nonzeros; 1 integer variables (1 binary)
Coefficient ranges:
  Matrix  [1e+00, 2e+00]
  Cost    [1e+00, 2e+00]
  Bound   [1e+00, 9e+00]
  RHS     [1e+01, 1e+01]
Presolving model
1 rows, 1 cols, 1 nonzeros 0s
0 rows, 0 cols, 0 nonzeros 0s
Presolve reductions: rows 0(-1); columns 0(-2); nonzeros 0(-2) - Reduced to empty
Presolve: Optimal

Src: B => Branching; C => Central rounding; F => Feasibility pump; H => Heuristic;
     I => Shifting; J => Feasibility jump; L => Sub-MIP; P => Empty MIP; R => Randomized rounding;
     S => Solve LP; T => Evaluate node; U => Unbounded; X => User solution; Y => HiGHS solution;
     Z => ZI Round; l => Trivial lower; p => Trivial point; u => Trivial upper; z => Trivial zero

        Nodes      |    B&B Tree     |            Objective Bounds              |  Dynamic Constraints |       Work      
Src  Proc. InQueue |  Leaves   Expl. | BestBound       BestSol              Gap |   Cuts   InLp Confl. | LpIters     Time

         0       0         0   0.00%   11              11                 0.00%        0      0      0         0     0.0s

Solving report
  Model             linopy-problem-hu0mxzpo
  Status            Optimal
  Primal bound      11
  Dual bound        11
  Gap               0% (tolerance: 0.01%)
  P-D integral      0
  Solution status   feasible
                    11 (objective)
                    0 (bound viol.)
                    0 (int. viol.)
                    0 (row viol.)
  Timing            0.00
  Max sub-MIP depth 0
  Nodes             0
  Repair LPs        0
  LP iterations     0
Status: ok
Objective: 11.0
x0: 1.0
y0: 9.0

Reproducible Example

import linopy

# 1. Initialize the model
m = linopy.Model()

# 2. Define Variables
# x0 is binary, y0 has a lower bound of 9
x0 = m.add_variables(binary=True, name='x0')
y0 = m.add_variables(lower=9, name='y0')

# 3. Add Constraints
m.add_constraints(2*x0 + y0 >= 10, name='c0')

# 4. Set Objective (Minimize 2*x0 + y0)
m.add_objective(2*x0 + y0)

# 5. Solve using HiGHS
m.solve(solver_name='highs')

# 6. Display Results
print(f"Status: {m.status}")
print(f"Objective: {m.objective.value}")
print(f"x0: {x0.solution.item()}")
print(f"y0: {y0.solution.item()}")

Expected Behavior

Running HiGHS 1.11.0 (git hash: 364c83a): Copyright (c) 2025 HiGHS under MIT licence terms
MIP  linopy-problem-5rmiip7y has 1 rows; 2 cols; 2 nonzeros; 1 integer variables (1 binary)
Coefficient ranges:
  Matrix [1e+00, 2e+00]
  Cost   [1e+00, 2e+00]
  Bound  [1e+00, 9e+00]
  RHS    [1e+01, 1e+01]
Presolving model
0 rows, 1 cols, 0 nonzeros  0s
0 rows, 0 cols, 0 nonzeros  0s
Presolve: Optimal

Src: B => Branching; C => Central rounding; F => Feasibility pump; J => Feasibility jump;
     H => Heuristic; L => Sub-MIP; P => Empty MIP; R => Randomized rounding; Z => ZI Round;
     I => Shifting; S => Solve LP; T => Evaluate node; U => Unbounded; X => User solution;
     z => Trivial zero; l => Trivial lower; u => Trivial upper; p => Trivial point

        Nodes      |    B&B Tree     |            Objective Bounds              |  Dynamic Constraints |       Work      
Src  Proc. InQueue |  Leaves   Expl. | BestBound       BestSol              Gap |   Cuts   InLp Confl. | LpIters     Time

         0       0         0   0.00%   10              10                 0.00%        0      0      0         0     0.0s

Solving report
  Model             linopy-problem-5rmiip7y
  Status            Optimal
...
Status: ok
Objective: 10.0
x0: 0.0
y0: 10.0

Installed Versions

Details

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