Update optuna requirement from <5 to <6

[dependabot]

Updates the requirements on optuna to permit the latest version.

Release notes

Sourced from optuna's releases.

v5.0.0

This is the release note for v5.0.0.

Highlights

Major Enhancements to the Default Sampler

Optuna v5.0 introduces the first major update to its default sampler configuration since v1.5. For single-objective optimization, TPESampler now enables multivariate TPE (#6746) and the constant liar strategy by default (#6738), together with enhanced bandwidth computation (Watanabe 2023). For multi-objective optimization, TPESampler replaces NSGAIISampler as the default sampler (#6766). These settings were selected through comprehensive benchmarking to improve optimization performance.

New Core APIs for Constrained Optimization

Constrained optimization is now represented directly in Optuna’s core Trial API. Constraint values can be set inside the objective function using trial.set_constraint() and retrieved through trial.constraints, providing a consistent interface across supported samplers. Accordingly, the sampler-specific constraints_func arguments are now deprecated. (#6736, #6754, #6773)

PED-ANOVA Becomes the Default Parameter Importance Evaluator

Optuna v5.0 makes PedAnovaImportanceEvaluator the default evaluator used by optuna.importance.get_param_importances(), replacing f-ANOVA. PED-ANOVA computes parameter importances substantially faster, works with Optuna’s standard dependencies, and now supports conditional search spaces and multi-objective studies. The conditional extension, Conditional PED-ANOVA: Hyperparameter Importance in Hierarchical & Dynamic Search Spaces, was accepted at KDD 2026. (#6682, #6728, #6748)

Improved Parallel Optimization with GPSampler

GPSampler, now a stable API in Optuna v5.0, introduces Monte Carlo-based q-batch acquisition functions that account for trials currently under evaluation. Rather than assigning each running trial a single heuristic pseudo-value, these acquisition functions integrate over possible outcomes sampled from the Gaussian process posterior, allowing predictive uncertainty to inform subsequent suggestions.

The new acquisition functions cover all four major problem settings: qLogEI for unconstrained single-objective optimization, qLogCEI for constrained single-objective optimization, qLogEHVI for unconstrained multi-objective optimization, and qLogCEHVI for constrained multi-objective optimization. This provides a more principled approach to parallel Bayesian optimization across the problem settings supported by GPSampler. (#6715, #6640, #6744, #6792, #6804)

Breaking Changes

• Remove optuna.multi_objective module (#6686)
• Remove deprecated integration wrappers for AllenNLP, Chainer, and MXNet (#6693)
• Stabilize GPSampler (#6715)
• Enable constant_liar by default (#6738)
• Support categorical parameters in QMCSampler (#6742, thanks @​saivedant169!)
• Enable multivariate by default in TPESampler (#6746)
• Remove deprecated positional-arg compatibility from study and trial APIs (#6747, thanks @​yen-0!)
• Make PedAnovaImportanceEvaluator the default importance evaluator (#6748)
• Make TPESampler the default sampler for multi-objective optimization (#6766)
• Deprecate constraints_func (#6773)
• Change to treat cases without constraints as feasible (#6774)
• Normalize trial timestamps to UTC in RDBStorage and JournalStorage (#6776)
• Remove axis_order argument from plot_pareto_front (#6781)
• Remove system_attrs from StudySummary (#6782)
• Remove categorical_distance_func from TPESampler (#6783)
• Remove deprecated modules (optuna/optuna-integration#293)
• Remove MXNetPruningCallback (optuna/optuna-integration#294)
• Use constraints property in BoTorchSampler (optuna/optuna-integration#306)
• Remove set_system_attr and system_attrs from TorchDistributedTrial (optuna/optuna-integration#309)

... (truncated)

Commits

• 01ddd17 Merge pull request #6839 from c-bata/ignore-sphinx-warning
• 1339d2c Merge pull request #6840 from y0z/feature/bump-up-version-v5
• 4c975c0 Bump up version to 5.0.0
• 86a1d43 Temporarily disable -W option on Sphinx build
• 6ac05f5 Merge pull request #6799 from yen-0/fix/cliplow
• ab320b6 Merge pull request #6804 from sawa3030/add-constrained-qlogehvi
• 3dbb1d5 Merge branch 'master' of https://github.com/optuna/optuna into add-constraine...
• 027f3b1 Merge pull request #6834 from not522/remove-system-attr
• 0576faf Update document
• 8f98dd9 Update optuna/_gp/acqf.py
• Additional commits viewable in compare view

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