Arcline is a friendly, local-first diagram canvas for architecture and flow diagrams. Draw by hand, write diagrams as code with Mermaid, D2, Structurizr or OpenFlow DSL, or hand the canvas to your AI agent over MCP. Claude Code, Cursor, VS Code and Codex can read and edit the same document — every change is one undo step. No account, no cloud. Free and open source.