Cassis
Grasshopper plugin for Rhino 8 that runs an MCP server inside Grasshopper so agentic LLMs and AI coding assistants can read, create and modify Grasshopper definitions directly.
What it does and how it connects
Cassis is a bridge between AI coding assistants and Grasshopper. It is a Grasshopper plugin that runs an MCP server inside Grasshopper, so an AI assistant such as VS Code Copilot, Cursor, or Claude Code can read, create, and modify Grasshopper definitions directly.
Setup has two sides: install the Cassis plugin in Grasshopper, and point the AI assistant at the server. The server auto starts with all tools at http://localhost:3003/mcp/ when Grasshopper opens, and the plugin is installed through the Rhino Package Manager or from the release zip. The current release supports Rhino 8 on Windows and macOS.
The README states Cassis accepts connections only from the local machine and has no application level authentication, so it should be used only on a trusted machine and session. An optional Cassis component gives Start and Stop UI and tool toggles.
Alternatives
Compare side by side- MecAgent
A commercial desktop copilot that acts inside an open SolidWorks session, rather than an open source MCP plugin inside Grasshopper.
- fusion-cad-mcp
A typed MCP tool surface over Autodesk Fusion, rather than an MCP server running inside Grasshopper for Rhino 8.
- AnkusDrive
A CLI and MCP server that drives FreeCAD with FEM and CFD simulation, rather than a Grasshopper plugin for Rhino 8.
- Agent-CAD
An open source AI CAD workbench that turns natural language into build123d solids and 2D drawings, rather than a plugin that lets agents edit Grasshopper definitions.
- build123d-mcp (pzfreo)
A CAD toolbox an AI app uses through MCP to build build123d models, rather than an MCP server inside Grasshopper.
Recurring errors it addresses
Error Index entries this tool is built to reduce or work around.
- a headless Rhino.Inside worker behind an MCP server loaded a placeholder System.Management.dll, got a different device id, and Cloud Zoo licensing then voided every open desktop Rhino (McNeel RH-99363); the fix commits sat unpushed from Oct 22.Error Index
- The MCP tool's four-point surface rebuild kept only a face's corners, so a curved panel with coplanar corners (e.g. a cylinder section) was replaced by a flat surface instead of skipped.Error Index
- Generic Rhino commands showed up as successes in usage reports even when Rhino returned ok: false. Version 0.7.1 fixes the counting, and the author says historical success rates may be inflated and cannot be rebuilt.Error Index
- The Rhino MCP server silently never started on a Mac with no supported AI client installed: slot listing came back empty, commands timed out at 120 seconds, and nothing was logged.Error Index
Sources
Not verified
- Pricing: none stated on the repo.
- No independent user voice found: recent repo activity is contributor pull requests and releases.
Last verified November 6, 2026. Listings are informational. The evidence score measures public, verifiable evidence and is not an endorsement or a compatibility claim.