Large language model-driven Multi-Agent Systems (MAS) face two key challenges in long-horizon tasks: collaboration workflows struggle to adapt to dynamic task states, and accumulated inter-agent interactions lead to lengthy, noisy contexts that degrade model performance and efficiency. To address these challenges, we propose MAS-OS, a collaborative Operating System (OS) for MAS. MAS-OS consists of three core modules: Memory Manager, Agent Scheduler, and Context Allocator. The Memory Manager maintains a structured memory pool and a global summary of agents’ interaction history. Based on the evolving memory states, the Agent Scheduler selects the next active agent, while the Context Allocator provides task-relevant memories to support effective context utilization. Experiments on four benchmark datasets demonstrate the effectiveness of MAS-OS. Compared with SOTA baselines, MAS-OS achieves an average improvement of 17.42% under the Llama-3.1-8B backbone. The code is available at https://anonymous.4open.science/r/MAS-OS-AD34.

Long video understanding requires reasoning over hours of content that far exceed the context window of multimodal large language models (MLLMs). Existing approaches compress video into fixed-length clips, yet events are fragmented and unrelated content is merged, yielding noisy retrieval. To overcome this challenge, we propose EMMA, an Event-driven Multimodal Memory system with Multi-Agent retrieval, which organizes memory around semantic events in three stages: (1) Event segmentation: a dual-stream detector adaptively locates event boundaries by fusing visual semantic drift with spoken transitions, rather than fixed windows. (2) Memory construction: built on the detected events, a three-level hierarchy of raw video, clip-level memory, and event-level memory stores content at progressively higher semantic abstraction. (3) Memory retrieval: a multi-agent framework collaboratively queries this hierarchy through query decomposition, tiered search, evidence validation, and answer generation, narrowing evidence from coarse events to precise clips. Extensive experiments on three challenging benchmarks demonstrate substantial improvements, showing the effectiveness of our event-aligned memory system. Code, prompt templates, and reproduction scripts are available at https://github.com/BAI-LAB/EMMA.