dotnet/machinelearning · error · InvalidOperationException

SMAC sweeper localSearch threw exception

Error message

SMAC sweeper localSearch threw exception

What it means

Same pattern as SmacSweeper's LocalSearch: SmacTuner.LocalSearch computes expected improvement over mutation neighborhoods, and any exception inside is wrapped in InvalidOperationException ('SMAC sweeper localSearch threw exception') with the original exception as InnerException.

Source

Thrown at src/Microsoft.ML.AutoML/Tuner/SmacTuner.cs:254

                for (; ; )
                {
                    Parameter[] neighborhood = GetOneMutationNeighborhood(currentBestConfig);
                    var eis = neighborhood.Select(p => EvaluateConfigurationsByEI(forest, bestLoss, p)).ToArray();
                    var maxIndex = eis.ArgMax();
                    if (Math.Abs(eis[maxIndex] - currentBestEI) < _epsilon)
                        break;
                    else
                    {
                        currentBestConfig = neighborhood[maxIndex];
                        currentBestEI = eis[maxIndex];
                    }
                }

                return new Tuple<double, Parameter>(currentBestEI, currentBestConfig);
            }
            catch (Exception e)
            {
                throw new InvalidOperationException("SMAC sweeper localSearch threw exception", e);
            }
        }

        private Parameter[] GetOneMutationNeighborhood(Parameter currentBestConfig)
        {
            var neighborhood = new List<Parameter>();
            var features = _searchSpace.MappingToFeatureSpace(currentBestConfig);
            for (int d = 0; d != _searchSpace.FeatureSpaceDim; ++d)
            {
                var newFeatures = features.Select(x => x).ToArray();
                if (_searchSpace.Step[d] is int step)
                {
                    // if step is not null, it means the parameter on that index is discrete.
                    // in that case, to sample a new value, we need to add the current feature value with 1/step
                    var nextStep = features[d] + (1.0 / step);
                    if (nextStep > 1)
                    {
                        nextStep -= 1;

View on GitHub (pinned to 7b76e69cf9)

Solutions

  1. Inspect InnerException to find the true root cause and fix it.
  2. Validate sweeper parameter ranges and types given to SmacTuner.
  3. Increase fit/local-search sample counts so the model has sufficient history.
  4. Catch the InvalidOperationException and fall back to another tuner.

Example fix

// before
var best = tuner.ProposeSweep(); // may throw wrapped localSearch exception
// after
try { var best = tuner.ProposeSweep(); }
catch (InvalidOperationException ex) { logger.LogError(ex.InnerException, "SMAC local search failed"); }
Defensive patterns

Strategy: try-catch

Validate before calling

if (!double.IsNaN(history.BestScore)) Console.WriteLine("model healthy"); // ensure non-degenerate history before tuning

Try / catch

try { var cfg = smacTuner.ProposeSweep(); }
catch (InvalidOperationException ex) { logger.LogError(ex.InnerException, "SmacTuner local search failed"); cfg = fallbackTuner.ProposeSweep(); }

Prevention

When it happens

Trigger: SmacTuner's bestChildKvp → LocalSearch path throws while evaluating one-mutation neighborhoods of the best configuration — typically due to failures in EPM predictions or invalid generated parameters.

Common situations: Running SMAC-tuned AutoML where the expected-improvement model produces NaN/invalid outputs, or parameter mutation generates out-of-range values.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/b141b8900bc087df. Report an issue: GitHub.