NeurIPS 2026 Papers

Estimating the number of clusters, $k$, is a fundamental prerequisite for most clustering algorithms. Traditionally, this estimation has relied on computationally expensive grid searches over internal cluster validity indices (CVIs) or adaptive clustering algorithms, both constrained by specific statistical assumptions. We introduce TabK, a zero-shot, permutation-invariant transformer architecture that frames the estimation of $k$ as an amortized Bayesian inference problem for tabular data. TabK leverages a novel Quantile Feature Encoder (QFE) to extract position-free, content-derived representations from empirical marginal distributions. By learning a generalized structural bias over a diverse synthetic prior, TabK estimates the number of clusters in a single forward pass. Evaluated across 50 diverse real-world benchmarks, TabK outperforms its closest competitor, TabClustPFN, reducing the Mean Absolute Error (MAE) of the number of clusters estimation by 1.2 (from 2.24 to 1.04). Furthermore, TabK empirically achieves the best position on the Pareto frontier among 12 evaluated methods, providing predictions of $k$ in an average time of less than one second, which is $2\times$ faster than TabClustPFN and $30\times$ faster than traditional CVIs such as Silhouette and Calinski-Harabasz. *Code and data are available at:* https://anonymous.4open.science/r/TabK.

Prior-data Fitted Networks (PFNs) have reframed supervised tabular learning as single-pass in-context inference without per-dataset optimization. Extending this paradigm to unsupervised clustering is appealing yet fundamentally more challenging, due to absent supervision, unknown cluster cardinality, and label switching inherent to partition outputs. Existing PFN-based clustering methods address these challenges only partially, either requiring known cardinality as input or relying on unstable label-ordering conventions and overly restrictive synthetic priors. We introduce TabClustPFN, a clustering PFN that resolves all challenges jointly through co-designed prior, objective, and architecture. Our hybrid pretraining prior captures heterogeneous real-tabular geometry; our decoupled partition inference network-cardinality inference network architecture jointly infers cluster assignments and cardinality in a single pass; and our SoftARI training objective is permutation-invariant by construction, eliminating the need for any label-ordering convention. On a 44 curated real-world tabular benchmark, TabClustPFN achieves state-of-the-art clustering performance against classical, deep, and amortized baselines, with runtime comparable to efficient baselines.

Recent advances in machine learning have accelerated biomedical prediction, yet evaluation on structured biomedical data remains fragmented, especially for datasets with high dimensionality, missingness, class imbalance, and small-sample, many-feature regimes. We introduce TabBioMed, a large-scale benchmark of 95 curated public biomedical tabular datasets spanning electronic health records \cite{data2016secondary}, drug response \cite{wu2018moleculenet}, genomics, transcriptomics, proteomics, metabolomics \cite{yang2025mlomics}, single-cell omics \cite{heumos2023best}, and systems biology. TabBioMed unifies these datasets through a standardized, dataset-aware preprocessing and evaluation framework, enabling reproducible comparison across classification, regression, and multi-target tasks. We benchmark 23 models across classical baselines, gradient-boosted trees, neural tabular architectures, and emerging tabular foundation models. Foundation models deliver the strongest family-level predictive performance. However, their gains are regime-dependent: tree-based models and tuned MLPs remain highly competitive, often dominating under inference-time or training-cost constraints. We further show that foundation-model advantages are most meaningful in low-noise biomedical tasks, where small absolute gains translate into large reductions in residual error. Overall, TabBioMed reveals that biomedical tabular learning is strongly context-dependent, with optimal model choice shaped by data regime, task type, and computational budget. By releasing curated datasets, preprocessing code, baselines, and results, TabBioMed provides a reproducible foundation for practical model selection and future methods development in biomedical machine learning, available at https://anonymous.4open.science/r/tabbiomed.

Tabular prediction has long been dominated by gradient-boosted decision trees and specialized deep tabular models, while large language models (LLMs) remain difficult to make competitive despite their cross-task adaptability and transparent reasoning traces. Existing LLM-for-tabular methods mainly adapt tables into textual inputs, but often fail to capture key structural properties of tabular data. In this work, we propose Permutation Relative Policy Optimization (PRPO), a reinforcement learning post-training method that strengthens LLMs for tabular prediction by injecting column-permutation invariance as a structural prior. By constructing label-preserving column permutations and estimating advantages both within and across them, PRPO converts sparse outcome rewards into denser and more stable optimization signals. Extensive experiments on 139 OpenML datasets show that our 8B model reaches a genuinely competitive regime against strong specialized tabular baselines. It achieves strong fully supervised performance, dominates cross-dataset zero-shot settings, and performs on par with 32-shot strong baselines. Moreover, it substantially outperforms much larger general-purpose and reasoning LLMs, including up to a 53.17% improvement over DeepSeek-R1 (685B). These results show that structural-prior RL post-training is an effective route for making LLMs competitive in tabular prediction.

Benchmarking tabular learning has revealed the benefit of dedicated architectures, pushing the state of the art. But real-world tables often contain string entries, beyond numbers, and these settings have been understudied due to a lack of a solid benchmarking suite. They lead to new research questions: Are dedicated learners needed, with end-to-end modeling of strings and numbers? Or does it suffice to encode strings as numbers, as with a categorical encoding? And if so, do the resulting tables resemble numerical tabular data, calling for the same learners? To enable these studies, we contribute STRABLE, a benchmarking corpus of 108 tables, all real-world learning problems with strings and numbers across diverse application fields. We run the first large-scale empirical study of tabular learning with strings, evaluating 445 pipelines. These pipelines span end-to-end architectures and modular pipelines, where strings are first encoded, then post-processed, and finally passed to a tabular learner. We find that, because most tables in the wild are categorical-dominant, advanced tabular learners paired with simple string embeddings achieve good predictions at low computational cost. On free-text-dominant tables, large LLM encoders become competitive. Their performance also appears sensitive to post-processing, with differences across LLM families. Finally, we show that STRABLE is a good set of tables to study “string tabular” learning as it leads to generalizable pipeline rankings that are close to the oracle rankings. We thus establish STRABLE as a foundation for research on tabular learning with strings, an important yet understudied area.

Tabular data is widely applied in critical domains, where tree-based models remain dominant, yet their performance is often constrained by manual heuristic splitting criteria and limited ensemble diversity. While hyperbolic geometry naturally fits the hierarchical topology of tabular data, existing hyperbolic algorithms are hindered by complex Riemannian optimization and the difficulty of defining subtree priors in non-Euclidean spaces. To address these challenges, we propose HOT, a hyperbolic random forest framework based on data decoupling. First, we leverage intrinsic geometric structures and LLM reasoning to guide subtree construction, replacing traditional manual heuristics with geometric-semantic priors. Second, we introduce an orthogonal hyperbolic subspace projection mechanism via tangent space isometry, effectively decoupling feature dependencies and maximizing ensemble diversity. Finally, we establish an end-to-end collaborative training paradigm where gradient residuals from a graph network directly guide the growth of new hyperbolic trees, bypassing expensive iterative optimization. Experiments on 13 benchmark datasets demonstrate that HOT significantly outperforms state-of-the-art baselines in both computational efficiency and predictive performance.

We study the global convergence of policy gradient for infinite-horizon entropy-regularized Markov decision processes (MDPs) with continuous state and action spaces. We consider log-linear softmax policies with linear function approximation, which extend the tabular softmax parameterization while retaining a tractable policy class. Under $Q^\pi_\tau$-realizability for the regularized state-action value function, we first establish a non-uniform Polyak--Łojasiewicz (PŁ) inequality. The non-uniformity arises through degeneracy of constants associated with the policy geometry, namely the Fisher information matrix or an uncentered feature covariance matrix. We then identify two feature regimes under which this non-uniform constant can be bounded along the gradient flow. For full-affine-span features, we prove radial unboundedness of the KL regularizer and show that the smallest eigenvalue of the Fisher information matrix remains bounded below by an initialization-dependent positive constant. For simplex-valued features, we prove an analogous radial unboundedness result in the subspace orthogonal to the all-ones vector and obtain a uniform lower bound for the smallest eigenvalue of the uncentered covariance matrix. These results imply global linear convergence of the regularized objective along the gradient flow, i.e. suboptimality decaying as $\mathcal{O}(e^{-Ct})$ for some $C>0$. Our analysis extends the global convergence theory of entropy-regularized softmax policy gradient beyond the tabular setting of Agarwal et al. [2020], Bhandari and Russo [2024], Mei et al. [2020].

We introduce Phase-Interference Decision Trees (PIDT), a differentiable routing model for tabular prediction in which each node propagates a bounded complex state rather than only a scalar routing probability. The main model uses two-state norm-preserving branch maps: a gate controls how much amplitude is sent to the left and right children, while learned unitary maps rotate the latent state before subsequent splits. This formulation contains ordinary soft decision trees as the phase-free scalar case, and two-state routing creates explicit score-level cross terms among latent-state trajectories. We give a self-contained PIDT definition with exact mass conservation, prove scalar phase cancellation, establish a scoped containment relation with soft trees, derive an algebraic interference decomposition, give a companion construction for shared two-state parity routing, and derive a fixed-architecture empirical Rademacher upper bound for bounded two-state PIDT scores. We evaluate PIDT on a ten-dataset OpenML suite of low-class-count tabular tasks under matched differentiable-tree controls. Across ten seeds per dataset, the main summaries track accuracy as context and report small descriptive NLL and Brier deltas relative to the scalar PIDT-1S, phase-frozen PFB, and SDT controls. The empirical study is framed as a mechanism-level study for compact differentiable trees, not as a broad tabular-performance claim.