dotnet/machinelearning · error · InvalidOperationException

SMAC sweeper localSearch threw exception

Error message

SMAC sweeper localSearch threw exception

What it means

LocalSearch in SmacSweeper evaluates expected improvement over candidate configurations; any exception inside that optimization is rethrown as InvalidOperationException with the original exception as InnerException.

Source

Thrown at src/Microsoft.ML.AutoML/Sweepers/SmacSweeper.cs:249

                for (; ; )
                {
                    ParameterSet[] neighborhood = GetOneMutationNeighborhood(currentBestConfig);
                    double[] eis = EvaluateConfigurationsByEI(forest, bestVal, neighborhood, isMetricMaximizing);
                    int bestIndex = eis.ArgMax();
                    if (eis[bestIndex] - currentBestEI < _args.Epsilon)
                        break;
                    else
                    {
                        currentBestConfig = neighborhood[bestIndex];
                        currentBestEI = eis[bestIndex];
                    }
                }

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

        /// <summary>
        /// Computes a single-mutation neighborhood (one parameter at a time) for a given configuration. For
        /// numeric parameters, samples K mutations (i.e., creates K neighbors based on that parameter).
        /// </summary>
        /// <param name="parent">Starting configuration.</param>
        /// <returns>A set of configurations that each differ from parent in exactly one parameter.</returns>
        private ParameterSet[] GetOneMutationNeighborhood(ParameterSet parent)
        {
            List<ParameterSet> neighbors = new List<ParameterSet>();
            SweeperProbabilityUtils spu = new SweeperProbabilityUtils();

            for (int i = 0; i < _sweepParameters.Length; i++)
            {
                // This allows us to query possible values of this parameter.
                IValueGenerator sweepParam = _sweepParameters[i];

View on GitHub (pinned to 7b76e69cf9)

Solutions

1. Inspect InnerException for the actual cause and fix that underlying failure.
2. Ensure sweeper parameter ranges are valid (min < max, no invalid floats).
3. Increase the number of initial trials/history so the EI model is well-formed.
4. Catch InvalidOperationException around sweeping and fall back to a simpler sweeper (RandomSweeper).

Example fix

// before
var sweeper = new SmacSweeper(mlContext, smacSweeperArgs);
// after
try { var sweeper = new SmacSweeper(mlContext, smacSweeperArgs); }
catch (InvalidOperationException ex) { /* fall back to random sweeper; check ex.InnerException */ }

foreach (var p in searchSpace)
    if (p.MinValue >= p.MaxValue) throw new ArgumentException($"Invalid range for {p.Name}");

try { var best = smacSweeper.Sweep(); }
catch (InvalidOperationException ex) { logger.LogError(ex.InnerException, "SMAC local search failed"); best = randomSweeper.Sweep(); }

When it happens

Trigger: SMAC sweeper's LocalSearch crashes while computing EI over the parameter space — e.g. NaN scores, malformed parameter sets from sweepers, or failures inside EPM (expected improvement model) evaluation.

Common situations: Tuning with SmacSweeper where the surrogate model degenerates (too few trials, constant metric values) or parameter ranges contain invalid 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

• Smart sweeper can only sweep over discrete and numeric param
• {nameof(ChannelMessageKind)}.{e.Kind} is not yet implemented
• Training failed with the exception: {_history.Last().Excepti
• Metric {typeof(TMetrics)} not implemented
• Cannot set parameter {param.Name} for {obj.GetType()}

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/fdb68d2c49f81239. Report an issue: GitHub.