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Multi-agent pathfinding

Multi-agent pathfinding (MAPF) is the combinatorial problem of computing collision-free paths for a team of cooperative agents, each moving from a start location to a designated goal in a shared environment, such as a grid or graph, while avoiding collisions with other agents and obstacles.^[1]^[2] MAPF has emerged as a foundational challenge in artificial intelligence and robotics, enabling coordinated motion planning for multi-robot systems in dynamic settings.^[2] Its importance stems from real-world applications, including warehouse automation for order fulfillment robots, urban air mobility for drone traffic management, train scheduling on rail networks, and semiconductor manufacturing where robots handle wafer transport.^[1]^[2] These scenarios often involve dozens to thousands of agents navigating complex, time-varying spaces, where efficient solutions can significantly reduce operational costs and improve throughput.^[2] Classical MAPF solvers dominate the field, employing techniques like conflict-based search (CBS) for optimal solutions on grids up to 200×200 with over 1,000 agents, or compilation-based methods such as satisfiability (SAT) formulations for bounded-suboptimal paths.^[2] In contrast, learning-based approaches, including reinforcement learning and neural networks, offer scalability for larger or continuous environments but typically handle 10–100 agents, often hybridizing with classical methods for robustness.^[2] Ongoing research addresses variants like lifelong MAPF (for repeated tasks), kino-dynamic constraints (for realistic motion), and decentralized settings with limited communication.^[1]

Introduction

Definition and Scope

Multi-agent pathfinding (MAPF) is the combinatorial optimization problem of computing collision-free paths for a set of multiple agents, each moving from a designated start location to a specified goal location within a shared environment, such that all agents can execute their paths simultaneously without conflicts.^[3] The environment is typically modeled as an undirected graph G = (V, E), where V represents the set of vertices (locations) and E the set of edges (connections between locations allowing movement).^[3] Agents are treated as point masses that occupy vertices, and paths are sequences of actions—either moving along an edge to an adjacent vertex or waiting at the current vertex—ensuring no two agents violate collision constraints at any point.^[3] Key assumptions in classical MAPF include discrete time steps, where all agents act synchronously at each timestep t = 0, 1, 2, \dots, and unit speed for agents, meaning each can traverse at most one edge per step.^[3] The graph is static, with no changes to V or E during execution, and agents have perfect knowledge of the environment and each other's goals in centralized formulations.^[3] Basic terminology encompasses agents (the moving entities), vertices (discrete positions), edges (traversable links), and the time-expanded graph representation, which extends G by incorporating time dimensions to model temporal occupancy and avoid conflicts over sequences of steps.^[3] Unlike single-agent pathfinding, which focuses on finding an optimal path for one agent—such as using the A* algorithm to minimize distance or cost from start to goal—MAPF introduces inter-agent conflicts as the primary challenge, requiring coordination to resolve potential overlaps in space and time.^[3] The scope of MAPF is delimited to scenarios emphasizing centralized planning (where a single solver computes all paths jointly) versus decentralized approaches (where agents plan independently but communicate or negotiate to ensure feasibility), and static environments over dynamic ones with moving obstacles.^[3] This distinction highlights MAPF's emphasis on scalability and coordination in multi-entity systems, rather than isolated navigation.^[3]

Historical Development

The roots of multi-agent pathfinding (MAPF) trace back to the 1970s and 1980s in robotics and AI planning, where initial efforts focused on cooperative path planning in distributed systems and multi-agent coordination. Early work in distributed artificial intelligence (DAI) explored coordination among multiple intelligent agents, laying foundational concepts for handling interactions in shared environments, as seen in seminal collections on DAI that emphasized problem decomposition and cooperation among software agents. By the late 1980s, robotics research advanced these ideas with decentralized approaches to motion planning for multiple mobile robots, such as Yeung and Bekey's 1987 framework for avoiding collisions in dynamic settings.^[4] These developments, often simulation-based with small numbers of agents (≤3), set the stage for physical implementations and highlighted challenges like communication and conflict resolution in distributed systems. The field emerged as a distinct area in the 1990s and 2000s, shifting toward systematic algorithms for discrete grid-based problems in cooperative robotics. Key early contributions included Silver's 2005 proposal of cooperative pathfinding methods like Windowed Hierarchical Cooperative A* (WHCA*), which addressed coordination in grid worlds with multiple agents.^[5] Optimal solutions gained traction with Standley's 2010 work on finding collision-free paths under sum-of-costs objectives, marking a pivotal advancement in theoretical foundations.^[6] The introduction of Conflict-Based Search (CBS) by Sharon et al. in 2012 further revolutionized the field, providing an optimal, two-level search algorithm that branches on conflicts while planning individual paths with A*, and became widely adopted for its completeness and efficiency.^[7] Refinements to prioritized planning, such as Felner et al.'s 2012 Enhanced Push and Swap (EPEA*) algorithm, improved scalability by incorporating swapping and pushing operators to resolve deadlocks in priority-based approaches.^[8] The 2010s saw rapid growth, driven by community efforts and a focus on large-scale instances. The first workshop on MAPF (WoMP) at AAAI in 2012 fostered collaboration across communities, followed by annual AAAI and ICAPS workshops from 2014 onward, which standardized discussions on scalability and real-world applications.^[9] Benchmarks proliferated, starting with the Moving AI Lab's grid-based datasets in 2012 for evaluating pathfinding on diverse maps like games and urban layouts, and Stern et al.'s 2019 comprehensive benchmark suite, which introduced varied grid maps and scenarios to test solvers on up to 1000 agents.^[3] This period emphasized shifts to massive instances, with enhancements like Barer et al.'s 2014 Explicit Estimation CBS (ECBS) for bounded suboptimal solutions on 200×200 grids.^[10] Recent developments from 2020 to 2025 have integrated deep reinforcement learning (DRL) and learnable solvers, addressing dynamic and lifelong MAPF scenarios. Post-2020, DRL methods like PRIMAL (Sartoretti et al., 2019) evolved into team-based navigation frameworks, with a 2024 review by Alkazzi and Okumura highlighting DRL's role in adaptive coordination for 10-100 agents in interactive environments.^[11] Benchmarks expanded in 2024 with dynamic maps like POGEMA for hardness analysis, enabling evaluations up to 512 agents.^[12] In 2025, learnable approaches advanced significantly, including MAPF-GPT (Andreychuk et al., 2025), an imitation learning model using transformers trained on expert trajectories for zero-shot solving of unseen problems at scale, and MAPF-World (Yang et al., 2025), an action world model that predicts conflict-free paths via generative modeling for efficient planning in complex grids.^[13] An AAMAS 2025 paper by Ren et al. further analyzed empirical hardness, identifying challenges like algorithm selection and instance diversity in benchmarks to guide future scalability research.^[14]

Problem Formalization

Environment and Agents

Multi-agent pathfinding (MAPF) is formalized on an undirected graph G = ([V](/page/V.), [E](/page/E!)), where [V](/page/V.) represents the set of locations (such as grid cells in a 2D map) and [E](/page/E!) denotes the traversable connections between them.^[15] Obstacles are modeled as locations excluded from [V](/page/V.) or without incident edges in [E](/page/E!).^[15] To account for discrete time steps in the planning process, the environment is often represented using a time-expanded graph, where each node is a pair (v, t) with v \in [V](/page/V.) and t denoting the time step, and edges connect compatible positions across consecutive times (e.g., staying at v or moving along an edge in [E](/page/E!)). The environment is assumed to be static, with a fully known map available to the planner, and all edges have uniform cost of length 1 (corresponding to one time unit per move or wait action).^[15] Agents operate in this shared space, aiming to navigate from individual starts to goals while avoiding collisions with each other.^[15] There are k agents, each labeled a_i for i = 1, \dots, k, with a unique start position s_i \in V and goal position g_i \in V.^[15] At each discrete time step t \geq 0, agent a_i can either move along an edge in E to an adjacent vertex or wait in place.^[15] A path \pi_i for agent a_i is a sequence of vertex-time pairs \langle (v_{i,0}, 0), (v_{i,1}, 1), \dots, (v_{i,l_i}, l_i) \rangle, where v_{i,0} = s_i, v_{i,l_i} = g_i, and consecutive pairs are either the same vertex (wait) or adjacent in G (move). After time l_i, the position of agent a_i remains at g_i for all subsequent time steps t > l_i.^[15] For collision checking in a solution, all paths are considered up to a common horizon equal to the makespan, the maximum arrival time across agents. Understanding MAPF requires familiarity with single-agent pathfinding prerequisites, such as using breadth-first search (BFS) to compute exact reachability and shortest paths in unweighted graphs like G.^[15] For heuristic-guided search, the A* algorithm employs an admissible heuristic such as the Manhattan distance h(n) = |x_n - x_g| + |y_n - y_g| in grid-based environments, ensuring optimal paths by h(n) \leq true cost to the goal.^[15]

Collision Types

In multi-agent pathfinding (MAPF), collisions represent invalid interactions between agents' paths in both space and time, forming the core constraints that solvers must avoid to produce feasible solutions. These collisions are typically categorized into vertex and edge types, with edge collisions often specified as swap conflicts to prevent head-on traversals. Less common variants, such as vertex-wait collisions, arise in scenarios involving agent idling. The definitions of these collisions rely on the paths of individual agents, denoted as sequences of vertices over discrete time steps, and their detection is fundamental to conflict identification in MAPF algorithms.^[16]^[17] A vertex collision occurs when two or more agents occupy the same vertex v at the same time step t. Formally, for the paths \pi_i and \pi_j of agents i and j, a vertex collision exists if there is some time step x such that \pi_i = \pi_j = v. This type of collision is the most straightforward to detect and is central to standard MAPF formulations on graphs, where agents cannot share vertices simultaneously.^[16]^[17] An edge collision, commonly known as a swap conflict, takes place when two agents traverse the same undirected edge (u, v) in opposite directions between consecutive time steps t and t+1. This is defined formally as \pi_i = u, \pi_i[t+1] = v, \pi_j = v, and \pi_j[t+1] = u for some t, preventing agents from exchanging positions in a single step. In grid worlds, which are prevalent in MAPF benchmarks, swap conflicts are particularly emphasized to model realistic movement restrictions without allowing direct passes.^[16]^[17] A vertex-wait collision is a specialized variant where one agent remains stationary (waiting) at a vertex v at time t, while another agent moves into v at the same time step, resulting in overlap. This is less commonly distinguished as a separate type, as it subsumes under the general vertex collision definition when wait actions are permitted, but it highlights issues in lifelong or extended MAPF settings where agents may idle post-goal.^[17] More broadly, a collision between any two paths \pi_i and \pi_j can be formalized as the existence of a time t where the positions coincide (\position(\pi_i, t) = \position(\pi_j, t)) for vertex cases, or where the positions at t and t+1 are adjacent vertices forming the same edge but in reverse order for edge cases. Extensions to these definitions include swapping conflicts tailored to grid structures and higher-order collisions involving more than two agents, such as cycles where agents rotate through a loop of positions; however, the latter are rare in classical MAPF, which primarily focuses on pairwise interactions.^[16]^[17] The incorporation of these collision types renders MAPF NP-hard in its general form (with variable number of agents), with the problem remaining computationally intractable even on directed graphs, due to the combinatorial explosion with the number of agents.^[18]

Objective Functions

In multi-agent pathfinding (MAPF), objective functions quantify the quality of collision-free path sets for multiple agents, guiding the optimization process while ensuring paths from start to goal locations avoid conflicts. These functions typically measure aspects such as total effort, completion time, or resource usage, with solutions evaluated under constraints like vertex or edge collisions. Common objectives balance individual agent efficiency against collective performance, often leading to trade-offs in solution characteristics.^[17] The sum-of-costs (SOC) objective minimizes the total effort across all agents, defined as the sum over agents i of the cost c(\pi_i) for each agent's path \pi_i, where c(\pi_i) includes the path length and any waiting times. Formally, for a set of paths \Pi = \{\pi_1, \dots, \pi_k\} on a graph with time-expanded representation, SOC is given by

\text{SOC}(\Pi) = \sum_{i=1}^k (|\pi_i| - 1),

where |\pi_i| is the number of positions in \pi_i (including the start), so |\pi_i| - 1 counts actions (moves or waits) to reach the goal. This metric favors balanced workloads and lower overall energy consumption, as it penalizes unnecessary waits or detours for any agent.^[17]^[19] In contrast, the makespan objective prioritizes rapid collective completion by minimizing the time until the last agent reaches its goal, defined as the maximum cost over all agents: \text{makespan}(\Pi) = \max_i c(\pi_i). More precisely, in a time-expanded graph where paths consist of vertex-time pairs (v, t), it is

\text{makespan}(\Pi) = \max_i \max \{ t \mid (v, t) \in \pi_i \}.

This suits time-critical applications, such as warehouse logistics, by reducing idle system time but may increase total waits for faster agents. Empirical studies show makespan optimization often yields solutions with higher SOC values, as agents may wait excessively to synchronize.^[17]^[20] A variant of SOC is the fuel (or sum-of-fuel) objective, which focuses on physical movement costs by counting only non-waiting actions, excluding waits from the path cost. This is particularly relevant in energy-constrained settings, such as battery-limited robots, where waiting incurs no fuel penalty but movement does; it can incorporate non-uniform edge costs for heterogeneous terrains. Like SOC, fuel minimization is NP-hard, but it encourages more direct paths with strategic waits to avoid detours.^[21]^[22] Trade-offs between these objectives are inherent: SOC promotes equitable individual paths and total efficiency, ideal for resource-limited multi-agent teams, while makespan emphasizes synchronization for deadline-driven tasks, potentially at the expense of overall effort. Multi-objective approaches, such as lexicographic ordering, resolve conflicts by prioritizing one objective (e.g., makespan first, then SOC), producing a single Pareto-optimal solution without enumerating frontiers; this is useful in hierarchical scenarios like mixed-criticality agents. For example, in grid-based MAPF, lexicographic methods can reduce computation by focusing on primary goals before secondary refinements.^[20] Optimizing MAPF under SOC or makespan is NP-hard, even on grids or planar graphs, due to the combinatorial explosion in path interactions. For makespan, certain variants (e.g., connected MAPF requiring agent communication graphs) are PSPACE-complete, highlighting increased complexity in constrained settings. These hardness results underscore the need for heuristic or bounded-suboptimal solvers in practical deployments.^[23]^[24]^[25]

Algorithms

Rule-Based Methods

Rule-based methods in multi-agent pathfinding (MAPF) represent early heuristic-driven approaches that rely on simple prioritization and local repair rules to generate fast, suboptimal collision-free paths for multiple agents in shared environments. These methods decouple the joint planning problem into sequential or local decisions, making them suitable for real-time applications such as robotics and logistics where speed is critical. Unlike optimal solvers, they sacrifice guarantees on solution quality for efficiency, often using single-agent pathfinders like A* as subroutines.^[26] Prioritized planning is a core rule-based technique where agents are ordered by priority, and paths are computed sequentially from highest to lowest priority. Higher-priority agents plan their paths ignoring lower-priority ones, while lower-priority agents treat the paths of higher-priority agents as dynamic obstacles to avoid collisions. Conflicts may arise, requiring replanning for affected agents. This approach was first proposed by Erdmann and Lozano-Pérez in 1987 for coordinating multiple moving objects in robotic manipulation, establishing the foundational decoupled strategy for MAPF. An influential extension appeared in Silver's 2005 work on cooperative pathfinding, which applied prioritized planning to video game scenarios and demonstrated its practicality through empirical evaluations on grid maps.^[27] The method's computational complexity is typically O(k \cdot t), where k is the number of agents and t is the time for single-agent planning, enabling scalability to dozens of agents.^[28] Push and swap methods build on prioritized planning by incorporating local repair mechanisms to resolve conflicts without full replanning. In the "push" operation, a conflicting agent is temporarily moved aside to allow the primary agent to proceed, while the "swap" operation exchanges positions between two adjacent agents to resolve edge conflicts. These rules ensure progress toward goals while maintaining collision avoidance. Luna and Bekris introduced the push and swap algorithm in 2011 as a complete yet suboptimal solver for MAPF on graphs, proving its completeness under certain conditions like sufficient free space and evaluating it on benchmarks where it solved instances with up to 100 agents faster than exhaustive methods.^[29] Extensions, such as push and rotate, further refine these rules to handle rotational deadlocks by incorporating vertex swaps in cycles.^[30] Rule-based heuristics often include deadlock avoidance strategies, such as parity-based rules in grid environments, where agents move only on odd or even time steps along certain directions to prevent head-on collisions and cyclic dependencies. These heuristics, integrated into prioritized frameworks, enhance robustness in structured spaces like warehouses. Early implementations around 2012, such as those in Sturtevant and Buro's work on real-time strategy games, showed these methods achieving success rates over 90% on small maps (up to 50 agents) but with higher failure rates in dense scenarios due to unresolvable conflicts. The primary advantages of rule-based methods lie in their low computational overhead and real-time suitability, allowing deployment in dynamic settings with frequent replanning. However, their greedy nature results in order sensitivity—poor priority assignments can lead to failures or highly suboptimal solutions, with sum-of-costs (SOC) often several times the optimal value, as demonstrated in benchmarks where bad orderings increased SOC by factors of 2–5 compared to optimal solvers.^[31] Additionally, they lack optimality bounds and completeness guarantees in general graphs, limiting their use in safety-critical applications requiring verifiable performance.^[28]

Optimal Solvers

Optimal solvers for multi-agent pathfinding (MAPF) aim to compute solutions that are provably optimal with respect to a specified objective, such as the sum-of-costs (SOC), where the total cost is the sum of individual agent path lengths, or the makespan, defined as the maximum completion time among all agents. These solvers typically employ search-based techniques or mathematical programming formulations to explore the vast solution space exhaustively until an optimal conflict-free plan is found. Unlike heuristic or rule-based approaches, optimal solvers provide formal guarantees of optimality, though they often face exponential time complexity due to the combinatorial nature of coordinating multiple agents.^[32] A prominent search-based optimal solver is Conflict-Based Search (CBS), introduced in 2015, which operates on a two-level hierarchy to decouple high-level conflict resolution from low-level path planning. At the high level, CBS performs a best-first search over a tree where each node represents a set of constraints on agent movements, expanding nodes by detecting and resolving conflicts between pairs of agents. The low level computes individual paths for each agent using a single-agent planner like A* with the accumulated constraints, ensuring no vertex or edge collisions occur. Optimality is achieved through the best-first expansion of the high-level tree, which guarantees the first conflict-free solution found minimizes the SOC.^[16] Formally, a constraint in CBS is defined as c = (i, l, t), prohibiting agent i from being at location l at time t. Upon detecting a conflict between two agents' paths, the high-level search branches into two child nodes, each adding one of the corresponding constraints, and re-plans the affected agent's path. The time complexity is exponential in the number of conflicts, as the branching factor grows with pairwise interactions, but CBS remains empirically efficient for grid-based environments with up to 100 agents, solving many instances optimally in seconds on standard benchmarks like random or room layouts.^[16] Another optimal approach formulates MAPF as a mixed-integer linear program (MILP), which encodes the problem as an optimization over discrete variables subject to linear constraints, solvable by off-the-shelf solvers like CPLEX or Gurobi. A key MILP model uses binary variables x_{i,v,t} = 1 if agent i occupies vertex v at time t, with the objective minimizing \sum_{i,v,t} x_{i,v,t} for SOC optimization. Constraints include flow conservation, ensuring each agent moves from its start to goal without revisiting unless necessary: \sum_{v' \in N(v)} x_{i,v',t-1} = x_{i,v,t} for all i, v, t, where N(v) are neighbors of v; non-collision rules like \sum_i x_{i,v,t} \leq 1 for vertex conflicts and similar for edges; and boundary conditions fixing starts and goals. This formulation guarantees optimality for SOC or makespan but scales poorly beyond dozens of agents due to the integer programming's NP-hardness.^[33]^[34] Other notable optimal methods include the Increasing Cost Tree Search (ICTS), proposed in 2013, which builds a search tree where each level corresponds to a monotonically increasing cost value k, planning joint moves for all agents at cost exactly k while allowing waits or lower-cost moves below. ICTS expands the tree level-by-level until a conflict-free solution is found at the minimal k, ensuring SOC optimality, and performs well on instances with balanced agent densities compared to CBS.^[35] These solvers provide optimality guarantees with respect to SOC by default, though adaptations like CBS with makespan heuristics or multi-objective extensions optimize hybrid criteria, such as lexicographic ordering of makespan then SOC. Recent advances, such as 2023 work on exact algorithms exploiting treelike graph topologies, improve solvability for centralized networks by reducing the search space through structural properties, achieving fixed-parameter tractability in treewidth.^[16]^[36]

Bounded Suboptimal Solvers

Bounded suboptimal solvers for multi-agent pathfinding (MAPF) compute collision-free paths for multiple agents that are guaranteed to have a total cost (typically the sum-of-costs, or SOC) no worse than a user-specified factor w \geq 1 times the optimal solution cost, enabling significant speedups over optimal methods for larger-scale problems while providing formal quality guarantees. These solvers are essential for practical deployments where computational time is limited, but predictability in solution quality is required, such as in warehouse automation with dozens to hundreds of agents. Unlike purely optimal solvers, they explicitly trade a bounded amount of suboptimality for scalability, often solving instances with 100-1000 agents that are intractable for exact methods.^[37] A foundational class of bounded suboptimal solvers builds on Conflict-Based Search (CBS) by introducing controlled approximations at the high or low levels of its two-layer architecture. Enhanced CBS (ECBS), introduced in 2014, employs focal search on the high-level CBS tree to prioritize nodes likely to yield good solutions quickly; specifically, it maintains an open list for all nodes and a focal list containing only nodes whose f-values (estimated total cost) lie between the current best solution cost C_{\min} and w \cdot C_{\min}, where w is the suboptimality bound. This restricts exploration to a bounded suboptimal subspace, ensuring the final solution cost is at most w times optimal while reducing the high-level search space by focusing on promising branches. Experiments with w = 5 to $10 often yield effective suboptimality of 10-50% in practice on grid maps, balancing speed and quality for dense scenarios.^[38] To accelerate the low-level single-agent pathfinding within CBS variants, weighted A* (wA*) is commonly integrated, inflating the admissible heuristic to achieve bounded suboptimality per agent. In wA*, the evaluation function becomes f(n) = g(n) + w \cdot h(n), where g(n) is the path cost from start to node n, h(n) is an admissible heuristic (e.g., Manhattan distance), and w \geq 1 is the weight; this guarantees that each agent's path cost is at most w times its individual optimal, propagating an explicit overall MAPF bound of \leq w times optimal when combined with high-level bounding. The inflated heuristic satisfies h'(n) \leq w \cdot h(n), maintaining consistency for the bound while allowing faster expansion toward goals under constraints from conflicts.^[38] Meta-agent CBS (MA-CBS) extends this framework by introducing implicit agent prioritization through meta-agents—groups of agents treated as single entities at the high level—resolving intra-group conflicts separately to reduce branching in dense environments; when integrated into bounded suboptimal variants like ECBS, it enhances scalability by implicitly favoring coordinated paths without explicit ordering.^[39] Anytime variants of CBS further support bounded suboptimal solving by producing an initial feasible solution quickly and iteratively refining it toward optimality within the bound. For instance, Explicit Estimation CBS (EECBS), a 2021 advancement, combines focal search with online-learned cost estimates based on conflict counts, reoptimizing from a suboptimal incumbent to improve quality over time; it guarantees solutions within w = 1.02 to $1.20 of optimal and can solve MAPF instances with up to 500 agents on 32×32 grids, achieving less than 20% SOC excess relative to optimal lower bounds in under a minute on standard hardware. Suboptimality bounds in these methods are explicit via the w-factor or implicit through heuristic lower bounds on joint costs, ensuring no violation of the guarantee.^[37] Recent developments emphasize scalability for real-world applications. The Adaptive Delay-based anytime MAPF solver (ADDRESS), proposed at AAAI 2025, uses a destroy-and-repair framework with adaptive delays and self-learning to generate bounded suboptimal neighborhoods, enabling optimization for hundreds of agents in dynamic settings by iteratively improving initial rule-based plans within user-defined bounds. Similarly, local guidance for configuration-based MAPF, introduced in 2025, augments bounded search with agent-specific local hints in the configuration space (joint positions of all agents), reducing search depth while maintaining suboptimality guarantees through prioritized expansions.^[40]^[41] Benchmark evaluations demonstrate that bounded suboptimal solvers like EECBS and ECBS outperform rule-based methods (e.g., PUSH-and-SWAP) on large maps with 200+ agents, yielding 20-50% lower SOC while remaining complete within the bound, as shown in standard MAPF benchmarks. Integrations with mechanism design, such as strategyproof allocation in IJCAI 2024, further leverage these solvers for fair, scalable path assignment in competitive multi-agent systems, solving 100-agent instances 10x faster than optimal baselines with bounded cost overhead.^[37]^[42]

Learning-Based Approaches

Learning-based approaches to multi-agent pathfinding (MAPF) leverage machine learning techniques, particularly deep reinforcement learning (DRL) and imitation learning, to generate scalable, decentralized policies that enable agents to navigate complex environments without explicit centralized coordination. These methods train neural networks on simulated MAPF scenarios to predict collision-free paths, prioritizing generalization over optimality guarantees to handle large-scale instances with hundreds or thousands of agents. Unlike traditional search-based solvers, learning-based techniques emphasize end-to-end learning from data, allowing agents to reactively adapt to dynamic obstacles and agent interactions. Deep reinforcement learning (DRL) has emerged as a prominent paradigm within multi-agent DRL (MADRL), often employing centralized training with decentralized execution (CTDE) frameworks to coordinate agents during training while enabling independent action selection at runtime. In these setups, agents receive rewards shaped for collision avoidance—such as penalties for vertex or edge conflicts—and positive reinforcement for reaching goals within minimal steps, fostering cooperative behaviors in shared environments. A 2024 review highlights how CTDE-based MADRL, including algorithms like QMIX, addresses the non-stationarity challenges in MAPF by mixing individual agent Q-values into a centralized critic during training, improving scalability and success rates on benchmarks with up to 64 agents. For instance, QMIX-integrated approaches have demonstrated effective path planning in grid-world scenarios by enforcing monotonic value factorization, ensuring stable learning across agent interactions. Neural solvers represent another key advancement, combining imitation learning with neural architectures to approximate optimal paths from expert demonstrations. PRIMAL, introduced in 2019 and extended in subsequent works, uses reinforcement and imitation multi-agent learning to train decentralized policies where agents reactively plan paths based on local observations, achieving high success rates on standard MAPF benchmarks. More recent extensions incorporate graph neural networks (GNNs) for enhanced path prediction, modeling agent interactions as graph structures to propagate spatial dependencies and predict conflict-free trajectories efficiently. Complementing this, MAPF-GPT (2025) employs a transformer-based architecture for supervised imitation learning, trained on vast datasets of expert MAPF solutions to generate paths autoregressively, demonstrating zero-shot generalization to unseen maps and agent counts up to 100 agents. World models further enhance decentralized planning by learning predictive representations of the environment and agent dynamics. MAPF-World (2025), an autoregressive action world model, integrates situation encoding with action generation to simulate future states, enabling agents to plan without full global visibility and outperforming prior learnable solvers in success rate and path quality on diverse benchmarks. Active learning techniques, such as those in MAPF-GPT-DDG (2025), refine these models through targeted fine-tuning on challenging instances, using delta-data generation to focus on failure cases and accelerate convergence while boosting performance on hard scenarios. Training for these approaches typically involves supervised learning on large MAPF datasets generated by optimal solvers like CBS, or reinforcement learning via MADRL algorithms such as QMIX, where agents explore joint action spaces in simulated environments. These methods scale to over 1000 agents in warehouse-like settings by leveraging parallel training and lightweight neural policies, achieving runtime efficiencies that surpass classical methods on dense maps. For example, transformer-based models have shown coordination improvements on the POGEMA benchmark, handling 1000+ agents with reduced collisions compared to earlier RL baselines. Learning-based approaches excel in dynamic environments, where agents must adapt to moving obstacles or replanning needs, and demonstrate empirical success on empirically hard MAPF instances characterized by high agent density and bottlenecks, as explored in AAMAS 2025 analyses. However, they lack formal optimality guarantees, often producing longer paths than search-based solvers, and exhibit strong dependency on high-quality training data, limiting transfer to novel map topologies without retraining.

Variations

Anonymous and Lifelong Variants

In anonymous multi-agent pathfinding (AMAPF), agents are treated as indistinguishable, meaning specific identities or pre-assigned targets are not required; instead, the objective is to find collision-free paths from start locations to a set of target locations, optimally assigning agents to targets to minimize metrics such as the sum of costs (SOC) across all paths.^[43] This variant arises in scenarios where agent interchangeability simplifies planning, such as in symmetric robotic fleets. Unlike classical MAPF, where vertex or edge collisions must be avoided as defined in the problem formalization, AMAPF focuses on collective coverage of targets without individual labeling.^[44] The assignment subproblem in AMAPF, which involves matching agents to targets while accounting for path conflicts, can be solved efficiently by reducing the problem to a maximum flow computation on an auxiliary time-expanded graph, enabling polynomial-time optimality for objectives like makespan or SOC.^[45] Algorithms such as the Ford-Fulkerson method compute these flows directly, while more advanced solvers incorporate bulk search techniques to compress and explore search states in batches, reducing runtime and memory usage on benchmarks.^[43] Adaptations of conflict-based search (CBS) integrated with bipartite matching further handle conflicts by iteratively resolving them and reassigning paths, achieving completeness and optimality on standard grid maps.^[46] Anonymity significantly reduces the state space compared to non-anonymous MAPF, as permutations of agent identities are irrelevant, allowing scalability to hundreds of agents.^[44] Lifelong multi-agent pathfinding (LLMAPF) extends MAPF to ongoing operations where agents continuously receive new goal assignments upon task completion, modeling real-world systems like automated warehouses with recurring logistics.^[47] The primary objective shifts to maximizing throughput—the average number of tasks completed per timestep—over an indefinite horizon, rather than optimizing a single batch of paths.^[48] This requires real-time replanning to adapt to dynamic goal releases, emphasizing amortized efficiency and low-latency decisions to sustain high agent utilization. Algorithms for LLMAPF often build on prioritized planning, where agents are sequenced by a fixed or dynamic priority order, and each plans a conflict-free path using low-level searches like A* while respecting higher-priority reservations.^[49] State-of-the-art methods, such as MAPF-LNS2 (a large neighborhood search variant) and priority inheritance with backtracking (PIBT), are adapted with windowed execution—replanning over short horizons (e.g., 20-100 timesteps)—to balance solution quality and speed, achieving near-optimal throughput in dense environments.^[48] Relative to standard MAPF, LLMAPF prioritizes scalability and robustness to interruptions, reducing per-task overhead through techniques like guidance graphs that precompute low-congestion routes.^[49] Benchmarks from 2024, including 3D warehouse maps from dedicated datasets such as the ICAPS 2024 benchmark, evaluate these algorithms on recurring scenarios with 100-10,000 agents, reporting throughput improvements via optimized guidance and agent disabling strategies (up to over 50% in some 2D scenarios).^[50]^[51]

Task-Oriented Problems

Task-oriented problems in multi-agent pathfinding (MAPF) extend the standard formulation by incorporating complex logistics such as fetching and delivering items, which introduce temporal constraints, dependencies between actions, and dynamic task assignments. In these settings, agents must not only navigate collision-free paths but also coordinate to fulfill sequences of operations, often under deadlines or resource limitations, making the problem more computationally demanding than basic MAPF.^[52]^[53] The multi-agent pickup and delivery (MAPD) problem is a prominent example, where a set of agents operates in a known environment represented as an undirected graph G = (V, E) to execute a batch or stream of tasks. Each task \tau is formalized as a tuple consisting of a pickup location (source s_\tau \in V), a delivery location (sink g_\tau \in V), and potentially an assigned agent, with paths for the assigned agent divided into a fetch segment from its current position to s_\tau and a deliver segment from s_\tau to g_\tau. Tasks may include release times or deadlines, requiring agents to arrive at the pickup before the delivery can proceed, while ensuring no vertex or edge collisions among all agents. Well-formed MAPD instances assume finite tasks, sufficient non-task endpoints for agent parking (at least as many as agents), and connectivity of endpoints without unnecessary traversals. The objective is typically to minimize the makespan (completion time of the last task) or average service time, though variants prioritize success rates or fairness.^[52]^[53] Key challenges in MAPD include efficient task allocation to agents, often addressed via auction-based mechanisms where agents bid on tasks based on estimated costs like travel time or load capacity, and synchronization to avoid deadlocks during handoffs or shared resource access. These issues arise because assigning tasks without considering future paths can lead to conflicts, and the problem is PSPACE-hard due to the inherent scheduling and path coordination complexities that generalize the PSPACE-completeness of standard MAPF. Scalability is a further concern, as solving for dozens of agents and hundreds of tasks requires balancing optimality with real-time performance.^[53]^[54]^[55] Solvers for MAPD often build on MAPF techniques, such as the CBS-MAPD approach from 2019, which adapts Conflict-Based Search (CBS) via prioritized planning and hybrid methods like ICBS for task agents combined with min-cost flow for idle agents, achieving scalability for up to 100 agents and 500 tasks in warehouse-like grids. Prioritized methods, including token-passing algorithms, decouple planning by ordering agents and reserving paths to prevent deadlocks, offering efficient approximations at the cost of potential suboptimality. Optimal solvers like branch-and-cut-and-price (BCP-MAPD) exist but are limited to smaller instances due to exponential complexity.^[53]^[52]^[56] Variants of MAPD include lifelong settings with continuous task streams, where agents handle an online sequence of deliveries without resting, as studied in early work using decoupled token-passing for real-time execution in dynamic environments.^[52]

Continuous and Kinematic Extensions

In continuous-time multi-agent pathfinding (CT-MAPF), agents navigate through environments where time is modeled as a continuous variable rather than discrete timesteps, allowing paths to be represented as smooth functions over real time. This extension addresses limitations of discrete MAPF by accommodating real-world dynamics, where agents move at varying speeds and trajectories may intersect at non-integer times. Formally, each agent's trajectory is defined as \tau_i(t) = (x_i(t), y_i(t)) for t \in [0, T], with a collision occurring if there exists some t such that the Euclidean distance \|\tau_i(t) - \tau_j(t)\| < 2r, where r is the agents' radius.^[57] CT-MAPF solvers must ensure completeness and optimality in this infinite state space, often approximating solutions through discretization or hybrid methods.^[57] Kinematic constraints further extend CT-MAPF to model realistic agent dynamics, such as velocity limits, acceleration bounds, and non-holonomic movement (e.g., minimum turning radii via Dubins paths for differential-drive robots). These constraints prevent instantaneous direction changes, requiring paths that respect maximum translational and rotational velocities while maintaining safety margins. For instance, post-processing algorithms like MAPF-POST take discrete MAPF outputs and refine them into executable schedules using temporal networks, ensuring compliance with kinematics in polynomial time and absorbing execution uncertainties.^[58] Velocity obstacle (VO) methods, including reciprocal variants (RVO), provide local collision avoidance by computing velocity sets that steer clear of predicted obstacle trajectories, scaling to dense environments with hundreds of agents and supporting kinematic limits like maximum speeds.^[59] Hybrid approaches combine discrete and continuous planning for efficiency; for example, Continuous-time Conflict-Based Search (CCBS) adapts CBS by using Safe Interval Path Planning (SIPP) for single-agent continuous paths and resolves multi-agent conflicts without grid assumptions, achieving optimality on benchmarks with up to 50 agents.^[57] Recent extensions, such as robust CT-MAPF variants, incorporate delay tolerance for imperfect execution in dynamic settings.^[60] In robotics applications, CT-MAPF with kinematics enables coordinated drone swarms for tasks like search-and-rescue, where scalable testbeds simulate physics-based interactions for thousands of agents, advancing real-world deployment through integrated execution monitoring.^[61] Key challenges in CT-MAPF include the infinite state space, which complicates exact solving and often necessitates approximations like time discretization or sampling-based methods, alongside computational demands for large-scale kinematic planning.^[57] Despite these, advances in 2025 emphasize decentralized solvers for drone swarms, improving scalability in uncertain environments.^[61]

Applications

Industrial Automation and Logistics

Multi-agent pathfinding (MAPF) plays a pivotal role in industrial automation and logistics, particularly in automated warehouses where fleets of mobile robots coordinate to transport inventory pods or goods without collisions. One seminal example is Amazon's Kiva system, introduced in the mid-2000s and acquired by Amazon in 2012, which deploys over 1 million robots across fulfillment centers to move shelves to picking stations, relying on MAPF to ensure collision-free navigation in dense environments.^[62]^[63]^[64] This approach has enabled scalable operations for thousands of agents, transforming order picking from manual walking to robot-assisted delivery.^[65] The primary benefit of MAPF in these settings is enhanced operational efficiency, with studies demonstrating throughput improvements through optimized routing that minimizes idle time and maximizes pod movements per hour.^[66] In large-scale warehouses, MAPF ensures robots continually engage with tasks, avoiding bottlenecks and supporting high-volume e-commerce demands.^[67] These gains are particularly evident in 24/7 operations, where lifelong MAPF variants sustain continuous replanning to handle perpetual task streams without downtime.^[47] Key challenges in deploying MAPF for industrial logistics include managing dynamic obstacles, such as human workers or fallen items, which necessitate real-time replanning to maintain safety and flow.^[68] Warehouses often feature unpredictable elements like temporary blockages or varying robot speeds, complicating collision avoidance in shared spaces.^[69] Lifelong and task-oriented MAPF extensions help mitigate these by enabling adaptive, ongoing coordination.^[70] Real-world case studies illustrate MAPF's impact, such as Alibaba's Cainiao Network warehouses, which as of 2018 integrated multi-robot systems handling over 700 units for Singles' Day peaks, using prioritized and conflict-based hybrids for efficient pod routing.^[71]^[72] Similarly, Ocado's automated fulfillment centers employ grid-based shuttles in grocery warehouses, achieving seamless scaling for dynamic order volumes.^[73] Integration of MAPF with multi-agent pickup and delivery (MAPD) further optimizes order fulfillment, reducing cycle times from order receipt to dispatch by streamlining task assignment and path execution.^[74] In practice, this has led to measurable reductions in fulfillment cycle times, often by 30-40% in robot-dense environments, enhancing overall logistics responsiveness.^[75]

Transportation Systems

Multi-agent pathfinding (MAPF) plays a critical role in transportation systems by enabling coordinated, collision-free trajectories for vehicles in dynamic urban and aerial environments, such as airports, roadways, and airspace. In airport taxiing operations, MAPF algorithms facilitate aircraft path planning on tarmacs, ensuring safe ground movements amid high traffic densities. For instance, multi-agent systems have been developed to compute conflict-free trajectories for dozens of aircraft using tailored motion planning techniques, addressing the need for real-time routing and priority-based search in complex airport layouts. These approaches support collision avoidance for over 100 aircraft during peak operations at major hubs, with integrations into advanced surface movement guidance systems explored by aviation authorities in the 2020s, including FAA advisory circulars on ground operations automation.^[76]^[77] In autonomous vehicle fleets within smart cities, MAPF supports decentralized routing through vehicle-to-vehicle (V2V) communication, allowing agents to negotiate paths at intersections and avoid gridlock in real-time. Algorithms adapted from classical MAPF, such as priority-based and conflict-based search variants, enable cooperative driving where vehicles share intentions to resolve potential collisions dynamically. This V2V-enabled coordination optimizes fleet-wide trajectories, reducing the computational burden on centralized systems and enhancing scalability for urban traffic management.^[78] Drone swarms for delivery and surveillance leverage continuous kinematic MAPF extensions to navigate 3D spaces, accounting for velocity constraints and time-continuous movements in urban airspace. Recent advances in 2025 include hybrid frameworks that integrate task allocation with trajectory optimization for swarms of up to hundreds of drones, ensuring collision-free paths during package routing or area monitoring. These methods build on continuous-time MAPF solvers to handle 3D kinematic models, enabling efficient operations in cluttered environments like cityscapes.^[79]^[80]^[81] The application of MAPF in these systems yields significant benefits, including reduced operational delays; simulations of lifelong MAPF in traffic scenarios demonstrate up to 30% decreases in average travel times by mitigating congestion through optimized flows. Real-world pilots, such as Uber Elevate's urban air mobility initiatives from 2019 to 2021, explored concepts applicable to MAPF for eVTOL coordination at vertiports, paving the way for scalable aerial fleets integrated with ground transport. However, challenges persist, including handling uncertainty from unpredictable obstacles or weather in dynamic environments, which requires robust probabilistic MAPF variants. Scalability to thousands of agents, such as 10,000 vehicles or drones, remains computationally intensive, demanding efficient decentralized solvers to maintain real-time performance.^[82]^[83]^[84]^[85]

Simulation and Entertainment

Multi-agent pathfinding (MAPF) plays a crucial role in virtual environments, enabling realistic crowd behaviors and non-player character (NPC) navigation in video games and simulations. In open-world titles such as The Sims and Grand Theft Auto (GTA), MAPF algorithms manage the movement of hundreds of autonomous agents, ensuring collision-free paths while simulating emergent social interactions and pedestrian dynamics. For instance, in The Sims, agents navigate household and community spaces using rule-based pathfinding to perform daily routines without overlapping, drawing on multi-agent systems for crowd simulation that prioritize local avoidance and global goal achievement. Similarly, GTA employs real-time MAPF variants for NPC traffic and pedestrian flows, handling dynamic obstacles like vehicles and player actions to create immersive urban environments. These applications rely on efficient heuristics, such as hierarchical navigation meshes (HNA*), which accelerate path computation for large agent counts by abstracting environments into layered graphs, achieving up to 15x speedups over standard A* for over 500,000 agents in real-time scenarios.^[86]^[87] In training simulators, MAPF facilitates scenario-based learning for military and disaster response teams by modeling coordinated agent movements in virtual crises. For example, virtual reality (VR) modules simulate active shooter events on campuses, where crowds of agents use behavior trees and utility-based path selection to flee threats, dynamically adjusting goals like "run away" or "seek cover" while avoiding collisions. This approach, implemented in Unity, supports intention-driven navigation, with agents weighting mandatory evasion over routine behaviors to mimic human panic responses, as validated in user studies showing responsive crowd dynamics. Another system, Metis, integrates MAPF for crisis management in dynamic environments, allowing reinforcement learning agents to plan paths amid evolving hazards like fires or structural collapses, aiding first responders in strategy testing. These simulators leverage MAPF to generate scalable, repeatable scenarios without real-world risks, incorporating learning-based methods for adaptive NPC behaviors in team coordination tasks.^[88]^[89] Specialized tools enhance MAPF integration in game engines, with plugins and platforms enabling developers to prototype crowd systems efficiently. The Unreal-MAP framework, built on Unreal Engine, supports multi-agent reinforcement learning tasks like flag capture and navigation games, where heterogeneous agents (e.g., ground and aerial) compute collision-free paths using algorithms such as MAPPO, achieving high win rates (up to 0.97) in benchmarks with 5-10 agents per team over 50,000-150,000 episodes. While Unity-specific MAPF plugins are less formalized, its navigation components often extend to multi-agent extensions via custom scripts, as seen in VR crowd avoidance prototypes that use reciprocal velocity obstacles for real-time steering among 100+ agents. These tools allow for 2024-2025 benchmarks demonstrating scalability, such as handling 100,000 AI agents at over 100 FPS in Unreal Engine 5 with local partitioning for collision detection.^[90]^[91]^[92] The primary benefits of MAPF in simulation and entertainment include cost-effective realism and rigorous scalability testing, enabling developers to iterate AI behaviors in virtual spaces before deployment. By avoiding physical hardware expenses, these systems facilitate emergent crowd phenomena, such as flocking or evasion, which enhance immersion without compromising performance. Demonstrations at the 2025 WoMAPF workshop highlighted MAPF's role in virtual fleet coordination, with tools like swarm platforms testing thousands of agents in simulated environments to inform entertainment applications. Notable examples include StarCraft II bots via the StarCraft Multi-Agent Challenge (SMAC), where decentralized RL agents perform MAPF-like micromanagement on combat maps, coordinating unit paths against opponents using partial observability. In VR crowd avoidance, multi-agent systems ensure safe user interactions by predicting and resolving path conflicts in real-time, as explored in studies on proximity effects in immersive crowds.^[93]^[94]^[95]^[96]

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Introduction. Path planning for multi-agents in large virtual environments is a central problem in the fields of robotics, video games, and crowd simulation.
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[PDF] Pathfinding Algorithms in Multi-Agent Systems
Multi agent systems can be used to simulation traffic and pedestrian activity. The main purpose of such a simulation could be for research.
[77]
[PDF] Multi-agent Crowd Simulation in an Active Shooter Environment
This paper presents a VR training module for active shooter events for a campus building emergency response in an institute of higher education (IHE). The VR ...
[78]
[PDF] Metis: Multi-Agent Based Crisis Simulation System - CEUR-WS
Sep 4, 2020 · Simply put, the system's features focus on dynamic environment design and crisis management, interconnection with popular Reinforcement Learn-.
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Unreal-MAP: Unreal-Engine-Based General Platform for Multi-Agent ...
Mar 20, 2025 · Unreal-MAP allows users to freely create multi-agent tasks using the vast visual and physical resources available in the UE community, and ...Unreal-Map... · 4 Unreal-Map · 7 Experiments
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100000 AI Agents in UE5 with Collision & Pathfinding at 100+ FPS
Mar 19, 2025 · Recent progress on my interactive crowd simulation project. 10K -> 100k AI. - Local partitioning for static & dynamic collision. - Multi ...
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How to Implement Continuous-Time Multi-Agent Crowd Simulation
Jan 24, 2020 · In this article, we discuss the mathematics on how to implement a continuous-time multi-agent crowd simulation.
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The 6th International Workshop on Multi-Agent Path Finding
The 6th International Workshop on Multi-Agent Path Finding. part of the 39th AAAI Conference on Artificial Intelligence (AAAI) 2025. March 3, 2025
[83]
oxwhirl/smac - The StarCraft Multi-Agent Challenge - GitHub
SMAC is WhiRL's environment for research in the field of cooperative multi-agent reinforcement learning (MARL) based on Blizzard's StarCraft II RTS game.
[84]
https://cdn.aaai.org/ojs/13866/13866-40-17384-1-2-20201228.pdf
[85]
Human‐virtual crowd interaction: Towards understanding the effects ...
May 11, 2023 · This paper focuses on understanding how study participants interact and perceive a virtual crowd in an immersive virtual environment.<|separator|>