Sema Core: An Open, Embeddable AI Coding Agent Runtime for the IDE and Beyond

AI coding agents are increasingly used in software development, but most are delivered as integrated applications whose reasoning logic is tied to a specific interface, model provider, and deployment setting. This coupling makes it difficult for researchers and tool builders to reuse agent capabilities across different developer tools, workflows, and deployment environments. We present Sema Core, an open, embeddable AI coding agent runtime that is decoupled from any particular client and distributed as a standalone npm package. Through a compact API and typed event stream, Sema Core allows different clients to reuse the same reasoning loop, tool system, permissions, background tasks, Skills/MCP integration, and model adapters. We demonstrate the Sema Core runtime through Sema Code, a flagship VSCode extension built as a thin UI client, and additional clients including a multi-channel assistant and an industrial programming assistant. We provide source code, documentation, a demo video, and an archived release.