The combination of artificial intelligence and extended reality has the potential to unlock a new paradigm of immersive intelligent computing, but a significant gap exists between the ecosystems of these two fields. To bridge this gap, the XR Blocks framework was introduced, a cross-platform framework designed to accelerate human-centered AI and XR innovation. XR Blocks provides a modular architecture with plug-and-play components for core abstraction in AI and XR, including user, world, interface, AI, and agents. The framework is designed with the mission of accelerating rapid prototyping of perceptive AI and XR apps, and it is built upon accessible technologies such as WebXR, threejs, LiteRT, and Gemini. The architectural and API design choices of XR Blocks are guided by three principles: simplicity and readability, prioritizing the creator experience, and pragmatism over completeness. The XR Blocks framework accelerates the prototyping of real-time AI and XR applications across desktop simulators and Android XR devices, and it provides a high-level, human-centered abstraction layer that separates the what of an interaction from the how of its low-level implementation. The framework proposes a new Reality Model composed of high-level abstractions to guide the implementation of XR Blocks, which consists of replaceable modules for XR interaction. The Reality Model is realized by XR Blocks's modular Core engine, which provides high-level APIs that enable developers to harness subsystems such as perception and input pipeline, AI as a core utility, and experience and visualization toolkit. The goal of XR Blocks is to allow creators to move from high-level, human-centric ideas to interactive prototypes much more quickly, and to enable a future where any declarative prompt could be directly translated to high-level instructions in XR Blocks. Overall, XR Blocks is a foundational step toward a future where the boundaries between programming, design, and conversation disappear, enabling us to script realities as fluidly as we script stories.