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.
Framework Auto-Detection — Automatically inspects project directories during initialization to detect frameworks like Next.js and Astro. - MCP Config Reconciliation — Rebuilds and syncs Model Context Protocol (MCP) configurations to wire up external tools and integrations. - Modular Plugin Ecosystem — Integrates specialized plugins for services and tools such as Vercel, Netlify, Hetzner, Prisma, Resend, and Figma. - JSON-Based Skill Matrix — Manages agent skills and capabilities through structured JSON definitions for testing frameworks (Playwright, Cypress, Vitest) and monorepos (Turborepo). - Stack Reconfiguration CLI — Supports post-initialization stack modifications and workflow adjustments using the update --reconfigure command.
Use cases
Use Case: Automated MCP Tooling Setup Developers working with MCP-compliant AI agents can initialize OpenCastle in a repository to automatically detect framework settings and generate reconciled MCP server configurations without manual JSON editing.
Use Case: Stack and Plugin Context Provisioning A team adopting new infrastructure or testing frameworks (such as Prisma, Playwright, or Hetzner) can activate the corresponding OpenCastle plugins to dynamically extend agent capabilities and skill matrices.
Use Case: Multi-Framework Stack Migration When transitioning or expanding a repository between Next.js, Astro, or Turborepo configurations, engineers can run stack updates to reconcile environment skills and agent harnesses across the codebase.
Who it is for
OpenCastle is intended for full-stack JavaScript and TypeScript developers using MCP-enabled AI coding agents who need structured stack detection, plugin configuration, and skill orchestration. It is less relevant for developers seeking a standalone turnkey chat interface or those working outside modern web and Node.js-based development ecosystems.
Install guide
Installation Install OpenCastle globally or locally in your project using npm or npx: bash npm install -g opencastle Alternatively, run it directly via npx: bash npx opencastle init
First Run Initialize OpenCastle within your repository to trigger framework auto-detection and skill matrix setup: bash opencastle init To update your stack selection or adjust plugins post-init, run: bash opencastle update --reconfigure
Example Session Rebuild and reconcile your MCP server configuration after configuring plugins: bash # Initialize with framework detection opencastle init # Reconfigure stack plugins (e.g., adding Vitest, Prisma, or Playwright) opencastle update --reconfigure
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