REVIEW 4 major objections 8 minor 25 references
NS-Gym: Open-Source Simulation Environments and Benchmarks for Non-Stationary Markov Decision Processes
T0 review · 4 major / 8 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read The paper presents NS-Gym, the first simulation toolkit built explicitly for non-stationary Markov decision processes, along with the first comparative benchmark of six solution algorithms on standardized non-stationary environments.
desk verdict NS-Gym is a genuinely useful toolkit, but the benchmark section needs a revision before the 'first standardized benchmark' claim holds. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing mechanism is the NS-Gym wrapper: it sits between the agent and an underlying Gymnasium environment and holds a dictionary of tunable parameters, each mapped to a scheduler (continuous, stepwise, random, periodic) and an update function (e.g., increment or random walk). The wrapper checks the scheduler at every decision epoch, applies the update, computes the magnitude of change (Wasserstein distance for distributions), and packages the result into a custom observation with env_change and delta_change fields, while an optional get_planning_env() returns a snapshot of the base environment at the appropriate notification level.
What would settle it
Define an NS-MDP where, after some decision epoch, a new action becomes available (the action set changes size) and try to express it with NS-Gym's parameter-update wrapper; the wrapper has no slot for structural changes, so it would fail to simulate the problem, contradicting the claim of a general-purpose NS-MDP toolkit.
Extended reading notes
Core claim
The central discovery is that a broad class of non-stationary decision problems can be captured by a thin wrapper around standard Gymnasium environments: treat the environment's exposed parameters (gravity, pole mass, transition probabilities) as a semi-Markov process that evolves independently of the agent, and let a scheduler plus update function drive that evolution. The wrapper returns custom observations containing the state, a change flag, the magnitude of change, and relative time, and can optionally hand the agent a stationary snapshot of the environment for planning at one of three notification levels. On the benchmark side, the paper reports that detailed notification generally improves most methods, that model-based online search methods (MCTS, PA-MCTS, AlphaZero with notifications) handle continuous change well, and that risk-averse tree searches (ADA-MCTS, RATS) fare better in stochastic grid worlds like FrozenLake.
Load-bearing premise
The toolkit only models non-stationarity as changes to observable parameters that an existing stationary Gymnasium environment already exposes, so shifts that alter the state space, action space, or the very structure of the dynamics cannot be represented.
Editorial extensions
If this is right
- Researchers can now report results on shared NS-MDP environments with a common interface, making head-to-head algorithm comparison meaningful and reproducible.
- New decision-making algorithms can plug into NS-Gym with minimal code by wrapping an existing Gymnasium environment and declaring parameter mappings.
- The three notification levels give a principled way to test how much information an algorithm needs: a boolean flag, exact parameter magnitudes, or a full planning model.
- The benchmark baseline suggests that notification information matters more for online search methods than for pure RL like DDQN, and that risk-averse strategies pay off in stochastic non-stationary domains.
Reading between the lines
- The wrapper architecture implicitly bounds the toolkit: any NS-MDP whose non-stationarity is not a parameter change of a base environment (e.g., changing state or action spaces) cannot be represented; a natural extension would be a wrapper that can also modify observation and action spaces.
- The paper's separation of change detection (notification) from model update suggests a taxonomy where detection is assumed external; an extension could benchmark agents that must detect changes themselves from raw observations, which NS-Gym currently does not simulate.
- The benchmark metric is mean cumulative episode reward; extending the harness to regret or dynamic regret would make results more informative for the bandit-flavored prior work the paper surveys.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces NS-Gym, an open-source Python library built on Gymnasium for constructing non-stationary Markov decision processes (NS-MDPs). The design wraps existing Gymnasium environments and lets users specify which environment parameters change, when they change (via schedulers), and how they change (via update functions), with three notification levels. The paper also presents a taxonomy of prior work, a set of environment instances (CartPole, Mountain Car, Acrobot, Pendulum, FrozenLake, CliffWalking, Bridge), and a benchmark comparing MCTS, AlphaZero, DDQN, PA-MCTS, ADA-MCTS, and RATS under single-change and continuous-change protocols with and without notification. The central claim is that NS-Gym is the first standardized, reproducible NS-MDP toolkit and benchmark suite.
Significance. NS-Gym addresses a real gap: prior NS-MDP papers use ad hoc, mutually incompatible environments, making cross-paper comparison difficult. The library is open-source, integrates directly with the Gymnasium API, and cleanly separates the parameter-evolution process from the agent, with configurable schedulers and update functions. The benchmark breadth—six algorithms, four base environments, two change regimes, and three notification levels—is a useful starting point if the numbers are made reproducible. I credit the authors for releasing the code and for providing a tutorial-style pipeline (Section 3.6) that lowers the adoption barrier. There is no circularity concern: benchmarking the authors' own algorithms is standard practice. However, the benchmark results as currently reported cannot yet serve as a standardized reference because of missing run counts, incomplete hyperparameters, asymmetric information across algorithms, and corrupted table entries.
major comments (4)
- [§4.1, §3.3, Tables 4 and 7] The 'with notification' condition is not the same intervention for all algorithms. Model-based methods (MCTS, AlphaZero, PA-MCTS) receive the current environment model through get_planning_env() at the configured notification level (§3.3), whereas DDQN is given only a fixed wall-clock budget ('time 0.4' in Table 7) to collect data and perform gradient updates (§4.1, item 3). The 'without notification' condition is also asymmetric: model-based planners are handed a stationary snapshot of the environment while DDQN learns only from raw interactions. Consequently, column-wise differences in Tables 3 and 4 confound the notification modality with the amount of privileged model information and with training budget; they do not measure a common 'notification' treatment. The authors should specify exactly what information each algorithm receives in each condition and, if the comparison is meant to reflect each algorithm's intended interface, should say so explicitly and avoid the implication that the columns differ only in notification level.
- [§4.2, Tables 3, 4, and 11] The benchmark tables report only mean ± standard error, with no statement of the number of independent runs, the number of random seeds, or the number of evaluation episodes per condition. Without these counts, the bold 'best' entries cannot be judged against the reported error bars, and the results cannot be independently reproduced. Several table entries are additionally corrupted or unreadable: Table 3's CliffWalking 0.6 row reads '-1216.72±63.686.97±8.2' (two numbers fused), Table 11 contains '2 740.84±43.23' and '0.60.31' with missing separators, and Table 4 uses '+-', '+/', and '+/-' for the same quantity. These data-integrity problems are central to the paper's benchmark contribution and must be fixed, not merely reformatted.
- [§3.2, Abstract, and Contribution 1] The architecture is restricted to non-stationarity that can be expressed as modifications to parameters that an existing stationary Gymnasium environment exposes ('each wrapper introduces non-stationarity by modifying some parameters that the base environment exposes'). Structural changes to the state or action space, changes in observation modality, and non-parametric changes in the transition kernel that cannot be captured by a scalar or distributional parameter of the base environment cannot be represented. This is a real limitation of the toolkit's current design, and it is not stated in the abstract or in Contribution 1, which claim a toolkit for NS-MDPs without qualification. Please state this limitation prominently and adjust the scope claims accordingly.
- [Appendix C, Tables 6–8, and §4.1] The algorithm configuration is incomplete for reproducibility. For DDQN and for the PA-MCTS value/policy network, Tables 7 and 8 list only network sizes and, for DDQN, a notification-time budget; missing are the learning rate, optimizer, batch size, replay-buffer size, target-network update period, exploration schedule, and number of training episodes. For AlphaZero, Table 6 does not state how long the stationary policy network was trained, the number of self-play games, or the training data distribution. Since all of these choices materially change the benchmark numbers, they need to be reported in the paper or supplied as a versioned configuration in the released repository.
minor comments (8)
- [Section 1] The sentence 'We make the following contributions. We make the following contributions:' is duplicated before the contribution list.
- [Section 2, Figure 1 caption] 'textitsojourn' is a LaTeX artifact; the intended text is 'sojourn time'.
- [Section 4.1, item 1] 'UTC' should be 'UCT' (Upper Confidence bounds for Trees).
- [Table 4 caption] The caption says 'WO and WON' but the rows are labeled 'WN' and 'WON'; it also says 'for with agent' instead of 'for an agent'.
- [Section 3.3, paragraph 2] In the second problem setting, 'the user is aware of the exact environmental change' should refer to the agent, not the programmer; the footnote defines 'user' as the programmer.
- [Abstract and Contribution 5] The 'first' claims are not substantiated by a systematic comparison with existing non-stationary benchmark or wrapper efforts; please provide evidence for the priority claim or soften it.
- [Tables 3, 4, and 11] Notation for uncertainty is inconsistent across tables ('±', '+-', '+/-'); unify it, and ensure that all decimal separators and spaces are unambiguous.
- [Appendix A.2.3 and A.2.5] Appendix A.2.3 says 'jail cell' where 'goal cell' is presumably meant, and Appendix A.2.5 has 'is moves' instead of 'it moves'.
Circularity Check
No circularity: NS-Gym is a software and benchmark contribution whose claims rest on an implementation and on measured evaluations, not on a derivation that reduces to its own inputs.
full rationale
This paper makes no formal derivation that could be circular. Its central claims are (1) the presentation of a simulation toolkit and (2) benchmark results comparing six algorithms on non-stationary MDP instances. The toolkit design is described operationally: wrappers introduce non-stationarity by modifying parameters exposed by base Gymnasium environments, schedulers determine when changes occur, and update functions determine how parameters change. The benchmark section reports measured episodic rewards from executing the algorithms in those environments. No parameter is fitted to a subset of data and then presented as a prediction; no quantity is defined in terms of the quantity it is supposed to establish. The authors do benchmark algorithms from their own prior work (PA-MCTS, ADA-MCTS), but the benchmark numbers are produced by running the algorithms in this paper rather than imported by citation, and the central toolkit claim does not rest on a self-citation. The review of prior work and the 'first toolkit' novelty assertion are historical and narrative claims rather than derivations; even if contested, a novelty dispute is not a circularity. No uniqueness theorem, ansatz, or renaming of a known result is invoked to make the argument load-bearing. Accordingly, the analysis finds no significant circularity.
Assumptions & free parameters
free parameters (4)
- Environment change magnitudes =
varied: pole mass 1.0/1.5, transition probabilities 0.4/0.6/0.8, decrements 0.1/0.2/0.02
- PAMCTS alpha interpolation weights =
0.25, 0.5, 0.75
- DDQN notification update time =
0.4 seconds
- Episode truncation lengths =
2500 (CartPole), 100 (FrozenLake), 200 (CliffWalking, Bridge)
assumptions (4)
- domain assumption The dynamics of an NS-MDP are fully determined by observable parameters θ that the agent or wrapper can read and update.
- domain assumption Non-stationarity can be layered onto existing Gymnasium environments by changing exposed scalar parameters.
- domain assumption The four questions (what changes, how, detect, know) are sufficient to categorize the space of NS-MDPs.
- standard math Standard MDP definitions and the semi-Markov formalism from Campo et al. [1991] apply.
invented entities (2)
-
Runtime monitor
independent evidence
-
Model updater
independent evidence
Cite this review
Pith. "Pith review of NS-Gym: Open-Source Simulation Environments and Benchmarks for Non-Stationary Markov Decision Processes." pith.science (2026). https://pith.science/paper/7ZC4ZZ3E
@misc{pith2026250109646,
author = {Pith},
title = {Pith review of: NS-Gym: Open-Source Simulation Environments and Benchmarks for Non-Stationary Markov Decision Processes},
year = {2026},
howpublished = {\url{https://pith.science/paper/7ZC4ZZ3E}},
note = {Machine review of arXiv:2501.09646}
}
read the original abstract
In many real-world applications, agents must make sequential decisions in environments where conditions are subject to change due to various exogenous factors. These non-stationary environments pose significant challenges to traditional decision-making models, which typically assume stationary dynamics. Non-stationary Markov decision processes (NS-MDPs) offer a framework to model and solve decision problems under such changing conditions. However, the lack of standardized benchmarks and simulation tools has hindered systematic evaluation and advance in this field. We present NS-Gym, the first simulation toolkit designed explicitly for NS-MDPs, integrated within the popular Gymnasium framework. In NS-Gym, we segregate the evolution of the environmental parameters that characterize non-stationarity from the agent's decision-making module, allowing for modular and flexible adaptations to dynamic environments. We review prior work in this domain and present a toolkit encapsulating key problem characteristics and types in NS-MDPs. This toolkit is the first effort to develop a set of standardized interfaces and benchmark problems to enable consistent and reproducible evaluation of algorithms under non-stationary conditions. We also benchmark six algorithmic approaches from prior work on NS-MDPs using NS-Gym. Our vision is that NS-Gym will enable researchers to assess the adaptability and robustness of their decision-making algorithms to non-stationary conditions.
Figures
Figures from the paper (14 more)
Reference graph
Works this paper leans on
-
[1]
On state estimation in switching environments
Guy Ackerson and K Fu. On state estimation in switching environments. IEEE transactions on automatic control, 15 0 (1): 0 10--17, 1970
work page 1970
-
[2]
Near-optimal regret bounds for reinforcement learning
Peter Auer, Thomas Jaksch, and Ronald Ortner. Near-optimal regret bounds for reinforcement learning. Advances in neural information processing systems, 21, 2008
2008
-
[3]
Stochastic multi-armed-bandit problem with non-stationary rewards
Omar Besbes, Yonatan Gur, and Assaf Zeevi. Stochastic multi-armed-bandit problem with non-stationary rewards. Advances in neural information processing systems, 27, 2014
2014
-
[4]
State estimation for systems with sojourn-time-dependent markov model switching
L Campo, P Mookerjee, and Y Bar-Shalom. State estimation for systems with sojourn-time-dependent markov model switching. IEEE Transactions on Automatic Control, 36 0 (2): 0 238--243, 1991
work page 1991
-
[5]
Non stationary bandits with periodic variation
Titas Chakraborty and Parth Shettiwar. Non stationary bandits with periodic variation. In Proceedings of the 23rd International Conference on Autonomous Agents and Multiagent Systems, pages 2177--2179, 2024
work page 2024
-
[6]
Towards safe policy improvement for non-stationary mdps
Yash Chandak, Scott Jordan, Georgios Theocharous, Martha White, and Philip S Thomas. Towards safe policy improvement for non-stationary mdps. Advances in Neural Information Processing Systems, 33: 0 9156--9168, 2020 a
work page 2020
-
[7]
Optimizing for the future in non-stationary mdps
Yash Chandak, Georgios Theocharous, Shiv Shankar, Sridhar Mahadevan, Martha White, and Philip S Thomas. Optimizing for the future in non-stationary mdps. Thirty-seventh International Conference on Machine Learning (ICML), 2020 b
work page 2020
-
[8]
Anomaly detection: A survey
Varun Chandola, Arindam Banerjee, and Vipin Kumar. Anomaly detection: A survey. ACM computing surveys (CSUR), 41 0 (3): 0 1--58, 2009
2009
Show all 25 references
-
[9]
Reinforcement learning for non-stationary markov decision processes: The blessing of (more) optimism
Wang Chi Cheung, David Simchi-Levi, and Ruihao Zhu. Reinforcement learning for non-stationary markov decision processes: The blessing of (more) optimism. In International Conference on Machine Learning, pages 1843--1854. PMLR, 2020
2020
-
[10]
On upper-confidence bound policies for switching bandit problems
Aur \'e lien Garivier and Eric Moulines. On upper-confidence bound policies for switching bandit problems. In International Conference on Algorithmic Learning Theory, pages 174--188. Springer, 2011
2011
-
[11]
Markov decision processes with their applications, volume 14
Qiying Hu and Wuyi Yue. Markov decision processes with their applications, volume 14. Springer Science & Business Media, 2007
2007
-
[12]
Deep reinforcement learning for autonomous driving: A survey
B Ravi Kiran, Ibrahim Sobh, Victor Talpaert, Patrick Mannion, Ahmad A Al Sallab, Senthil Yogamani, and Patrick P \'e rez. Deep reinforcement learning for autonomous driving: A survey. IEEE Transactions on Intelligent Transportation Systems, 23 0 (6): 0 4909--4926, 2021
2021
-
[13]
Algorithms for decision making
Mykel J Kochenderfer, Tim A Wheeler, and Kyle H Wray. Algorithms for decision making. MIT press, 2022
2022
-
[14]
Bandit Based Monte - Carlo Planning
Levente Kocsis and Csaba Szepesvári. Bandit Based Monte - Carlo Planning . In Johannes Fürnkranz, Tobias Scheffer, and Myra Spiliopoulou, editors, Machine Learning : ECML 2006 , pages 282--293, Berlin, Heidelberg, 2006. Springer Berlin Heidelberg. ISBN 978-3-540-46056-5
2006
-
[15]
Non-stationary markov decision processes, a worst-case approach using model-based reinforcement learning
Erwan Lecarpentier and Emmanuel Rachelson. Non-stationary markov decision processes, a worst-case approach using model-based reinforcement learning. Advances in neural information processing systems, 32, 2019
2019
-
[16]
Learning to delegate for large-scale vehicle routing
Sirui Li, Zhongxia Yan, and Cathy Wu. Learning to delegate for large-scale vehicle routing. Advances in Neural Information Processing Systems, 34: 0 26198--26211, 2021
2021
-
[17]
Act as you learn: Adaptive decision-making in non-stationary markov decision processes
Baiting Luo, Yunuo Zhang, Abhishek Dubey, and Ayan Mukhopadhyay. Act as you learn: Adaptive decision-making in non-stationary markov decision processes. arXiv preprint arXiv:2401.01841, 2024
2024 arXiv
-
[18]
A review of incident prediction, resource allocation, and dispatch models for emergency management
Ayan Mukhopadhyay, Geoffrey Pettet, Sayyed Mohsen Vazirizade, Di Lu, Alejandro Jaimes, Said El Said, Hiba Baroud, Yevgeniy Vorobeychik, Mykel Kochenderfer, and Abhishek Dubey. A review of incident prediction, resource allocation, and dispatch models for emergency management. A...
2022
-
[19]
Trading financial indices with reinforcement learning agents
Parag C Pendharkar and Patrick Cusatis. Trading financial indices with reinforcement learning agents. Expert Systems with Applications, 103: 0 1--13, 2018
2018
-
[20]
Decision making in non-stationary environments with policy-augmented search
Ava Pettet, Yunuo Zhang, Baiting Luo, Kyle Wray, Hendrik Baier, Aron Laszka, Abhishek Dubey, and Ayan Mukhopadhyay. Decision making in non-stationary environments with policy-augmented search. arXiv preprint arXiv:2401.03197, 2024
2024 arXiv
-
[21]
Markov decision processes: discrete stochastic dynamic programming
Martin L Puterman. Markov decision processes: discrete stochastic dynamic programming. John Wiley & Sons, 2014
2014
-
[22]
Mastering Chess and Shogi by Self - Play with a General Reinforcement Learning Algorithm , December 2017
David Silver, Thomas Hubert, Julian Schrittwieser, Ioannis Antonoglou, Matthew Lai, Arthur Guez, Marc Lanctot, Laurent Sifre, Dharshan Kumaran, Thore Graepel, Timothy Lillicrap, Karen Simonyan, and Demis Hassabis. Mastering Chess and Shogi by Self - Play with a General Reinfor...
2017 arXiv
-
[23]
Terry, Ariel Kwiatkowski, John U
Mark Towers, Jordan K. Terry, Ariel Kwiatkowski, John U. Balis, Gianluca de Cola, Tristan Deleu, Manuel Goulão, Andreas Kallinteris, Arjun KG, Markus Krimmel, Rodrigo Perez-Vicente, Andrea Pierré, Sander Schulhoff, Jun Jet Tai, Andrew Tan Jin Shen, and Omar G. Younis. Gymnasiu...
2023
-
[24]
Deep reinforcement learning with double q-learning
Hado van Hasselt, Arthur Guez, and David Silver. Deep reinforcement learning with double q-learning. CoRR, abs/1509.06461, 2015. URL http://arxiv.org/abs/1509.06461
2015 arXiv
-
[25]
Reinforcement learning in healthcare: A survey
Chao Yu, Jiming Liu, Shamim Nemati, and Guosheng Yin. Reinforcement learning in healthcare: A survey. ACM Computing Surveys (CSUR), 55 0 (1): 0 1--36, 2021
2021
Reviewed August 10, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.