Vision-language models (VLMs) demonstrate strong image-level scene understanding but often lack persistent memory, explicit spatial representations, and computational efficiency when reasoning over long video sequences. We present VL-KnG, a training-free framework that constructs spatiotemporal knowledge graphs from monocular video, bridging fine-grained scene graphs and global topological graphs without 3D reconstruction. VL-KnG processes video in chunks, maintains persistent object identity via LLM-based Spatiotemporal Object Association (STOA), and answers queries via Graph-Enhanced Retrieval (GER), a hybrid of GraphRAG subgraph retrieval and SigLIP2 visual grounding. Once built, the knowledge graph eliminates the need to re-process video at query time, enabling constant-time inference regardless of video length. Evaluation across three benchmarks, OpenEQA, NaVQA, and WalkieKnowledge (our newly introduced benchmark), shows that VL-KnG matches or surpasses frontier VLMs on embodied scene understanding tasks at significantly lower query latency, with explainable, graph-grounded reasoning. Real-world robot deployment confirms practical applicability with constant-time scaling.

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