Deep Reinforcement Learning (DRL) systems are widely used across various domains. However, testing these systems presents significant challenges. The DRL agent, which serves as the core decision-maker, generates continuous value estimates rather than discrete labels and operates under nonstationary policies within complex and stochastic environments. Consequently, there is no definitive “correct answer” for each state-action pair, complicating automated test generation due to the known oracle problem. To address this challenge, we propose a Metamorphic Testing (MT) framework (MDPMORPH) specifically designed for validating DRL agents. Our framework is based on Markov Decision Processes (MDP) and focuses on the core reasoning properties of agents to automatically uncover potential faults. To support MDPMORPH, we introduce a Metamorphic Relation (MR) design methodology tailored for DRL agents, based on the temporal characteristics of MDP. Using this method, we define nine generic MRs that encapsulate common and expected properties of an agent’s reasoning process. Furthermore, based on established assumptions and definitions within the context of MDPs, we theoretically demonstrate the soundness of these MRs. Finally, we specialize these generic MRs into environment-specific MRs by determining appropriate thresholds through training on three classic DRL environments. Our experimental results demonstrate that MDPMORPH and the proposed MRs are highly effective in automatically detecting mutants within these studied environments, with a 0.84 average mutation detection rate.

We present MDPMorph, a tool for metamorphic testing of Deep Reinforcement Learning (DRL) agents. MDPMorph is based on the Markov Decision Process (MDP) and targets the core reasoning properties of DRL agents to automatically uncover potential faults. It can generate metamorphic test suites and corresponding mutants directly from the DRL system under test. MDPMorph uses a subset of the metamorphic test suite and models to train the thresholds of the nine proposed Metamorphic Relations (MRs) using stochastic gradient descent. These MRs are based on the temporal characteristics of the MDP, and the training aims to determine the optimal threshold for each MR. After obtaining the optimal threshold, MDPMorph leverages the MRs to compare the execution results of different metamorphic test suites on the model under test and reports whether each test passes or fails. Finally, by collecting the execution results, MDPMorph calculates the mutant detection rate of MR to validate its effectiveness. Experimental results show that MDPMorph and the proposed MRs are highly effective in automatically detecting seeded faults (mutants).