PromptMason is a private prompt library application for building and managing prompts used with AI tools. The application allows users to construct prompts from reusable components called blocks, which can represent a role, background information, a task, a format, or a rule. These blocks are stored locally on the user's computer in encrypted form, with no account required and no data kept on the developer's servers. Users can share their library with selected team members and decide what level of access each connected AI application receives, including full access, read-only, or off. The core workflow involves saving a block once and reusing it across multiple prompts. Prompts are assembled by dragging blocks onto a canvas and arranging them in order, with a live preview of the finished prompt displayed as it is built. Because prompts are dynamic, editing a block automatically updates every prompt that uses it. The application ships with 103 pre-written blocks and 28 prompts organized into seven starter packs covering starter, finance, legal, marketing, sales, human resources, and customer support use cases, with variables already configured. PromptMason includes a built-in Model Context Protocol (MCP) connection for integrating the library with desktop AI applications such as Claude Desktop, Cursor, and Claude Code. It also offers a built-in private AI model that runs entirely on the user's computer, requiring no internet connection, API key, or account, and ensuring that text never leaves the machine. Users can run saved prompts in a built-in chat interface using the local model or their own API keys from OpenAI, Anthropic, Google Gemini, or xAI. A privacy check feature scans prompts for personal, financial, or confidential details before they are pasted into public AI services. Additional features include a Chrome extension for accessing the library from any website, the ability to open external AI websites inside the application for direct prompt insertion, and support for storing AI Skills and Markdown documents alongside prompts, with formatted preview and download as a zip folder.