OpenCastle is an open-source agent harness and developer tool orchestration framework maintained by monkilabs. Distributed as an npm package, it is designed to manage development stack context, skills, and Model Context Protocol (MCP) integrations across developer environments. In day-to-day operation, OpenCastle functions via a CLI interface. During repository initialization, it automatically detects supported application frameworks such as Next.js and Astro, and provides configuration reconciliation for MCP tool endpoints. It manages a JSON-based skill matrix alongside dedicated plugin configurations to inject relevant domain tools and contexts into agent workflows. What distinguishes OpenCastle is its modular plugin and skills architecture designed to reconcile MCP server configurations dynamically. Rather than manually configuring disparate agent tools, developers can define and update project stacks using post-initialization commands and structured skill matrices spanning testing, hosting, and backend providers. OpenCastle is open source under active development on GitHub, published through npm, and funded via community sponsorships.
Tags: agent harness, ai agent, cli, coding agent, developer tools, OpenSource