Checkpoint Engine is a production-grade Python library that serves as a lightweight middleware for updating model weights in large language model (LLM) inference engines such as vLLM and SGLang, a critical step in reinforcement learning fine-tuning. The system is designed to efficiently broadcast or peer-to-peer transfer sharded checkpoint data (safetensors format) across a cluster of GPUs, drastically reducing weight update time—for example, updating a 1-trillion-parameter model across thousands of GPUs takes approximately 20 seconds. The architecture is a distributed parameter server colocated with inference workers, using ZeroMQ for inter-process communication (IPC) and optionally Mooncake Transfer Engine for RDMA-based P2P transfers. It supports multiple accelerator backends (CUDA, NPU, XPU) through a unified device abstraction layer. The codebase is well-factored, with clear separation of concerns: IPC handling, memory pinning, distributed communication, and worker integration are each encapsulated in their own modules. The project includes an extensive test suite covering hardware-gated and CPU-only scenarios, CI/CD pipelines, and pre-commit linting. While tightly integrated with vLLM via a worker extension plugin, the core components (IPC handler, device manager, checkpoint loading) are designed for reuse and could be extracted for other inference frameworks.