Claude Mods: What They Are and How to Build One in TypeScript

Claude Mods are Claude Code plugins that use JavaScript or TypeScript function hooks to run inside Claude Code’s process, allowing them to intercept and modify events, register commands, and render interface elements such as banners, panes, counters, or games without using model tokens. Enabled by default since Claude Code v2.1.287, mods differ from external settings hooks because they operate as middleware within the application rather than launching separate processes, trading isolation for deeper UI and event-control capabilities. A basic mod requires a plugin manifest, a hooks configuration file, and a module exporting a registration function, supports hot reloading during development, and can be built manually or generated through Claude Code’s plugin-authoring tools. Security restrictions prevent mods from importing npm packages, React libraries, WebAssembly, or using direct filesystem and network access; instead, they must use the provided API, enabling Claude Code’s validation tools to identify their actions before execution. Examples include a toggleable prompt banner, UI-based games such as DOOM or Tetris, and a Firecrawl credit tracker that monitors command costs, while projects requiring unrestricted Node access, dependencies, or WebAssembly may be better suited to pi extensions.

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