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A mod is a Claude Code plugin whose behaviour lives in a function-hooks module: TypeScript that registers middleware on engine events ($.turn.step, $.tool.call, $.ui.render…) to add a pane, a status line, a slash command, or to change a request before it is sent. Anthropic’s reference is anthropics/claude-code/mods and the mods documentation.
Writes the complete plugin to .claude/skills/stack-viewer/, which Claude Code loads as stack-viewer@skills-dir. Then run /stack in Claude Code.

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Requirements

The $ API can change between Claude Code releases. Every mod in the catalog is type-checked against the current API in CI, and its README states the minimum version it was tested on.
A mod in a project’s .claude/skills/ loads only after you accept the trust prompt for that folder. claude -p never shows the prompt, so a headless run in a fresh folder will not load it. Open claude once in the folder, or install to ~/.claude/skills/, which always loads.

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Options

Options are declared in the mod’s .claude-plugin/plugin.json (userConfig) and read from your user or managed settings, never from project settings:
~/.claude/settings.json
The key is the plugin’s full id: <name>@skills-dir when it is auto-loaded from .claude/skills/, plain <name> when you load it with --plugin-dir.

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A mod that seems silent

Check these before suspecting the mod’s code:
  1. Untrusted project. See the warning above.
  2. Wrong place to look. $.ui.log writes a dim transcript line; in claude -p there is no transcript, so look in ~/.claude/debug/<session-id>.txt. A mod that rewrites the model or effort changes each request, not the header or status line.
  3. Options under the wrong key. The debug log says no pluginConfigs["<name>@skills-dir"].options when it finds none.
  4. Function hooks off on Claude Code below 2.1.287.
claude --debug shows hooks module <name>@skills-dir loaded … events: … when the mod is running. Browse all mods at aitmpl.com/mods. To write one, see Building a mod.