Writing A Mosaicast Plugin

Use when creating or modifying a Mosaicast plugin (any mosaicast-plugin-* repo or the plugin-sample). Covers the plugin.json manifest (slots and placements, the page slot behind /p/<id>/*, data access floors, backendOwned keys, consent services, doc vs schema storage, the blobs file-storage block, the tags vocabulary block, the external-services block, the identity block (ctx.users, resolving user UUIDs to a name/avatar), the notifications block (ctx.notify/ctx.notifier, putting a message in a user's inbox), nav entrances into the host menu, config fields with a localized label/description and a closed options set, the license/author/homepage/attribution credit fields), the backend PluginBackend/PluginContext contract (including onSchedule's Supplier<Duration> overload for a schedule that actually follows config) and its optional extension points (ShareMetadataProvider and its per-URL OgMeta.locale, SitemapProvider and its hreflang-alternates SitemapUrl.alternates, PageRouteProvider, SearchProvider, UserDataHandler), per-user data in the USER scope, the frontend Web Component via the SDK ctx (ctx.docs, ctx.feeds, ctx.tags, ctx.users, ctx.notify, ctx.schema reads, ctx.blobs uploads, ctx.links, ctx.locale.available/content, ctx.translation, ctx.route.navigate, typed ctx.api errors) plus defineMosaicastElement's MosaicastHandle (a render surviving a reassigned ctx instead of being torn down) and theme tokens including the --mc-icon-* icon set, testing against the SDK test kit, and installing a plugin by spec (owner/repo@tag#sha256). Trigger whenever writing the manifest, adding a slot, wiring ctx, storing per-user data, uploading or serving files, linking to core pages, querying schema tables from the frontend, reading or writing the shared tag vocabulary, contributing to site search, handling account deletion/export, real 404s for a page plugin's unknown subpaths, declaring external-service (translation) use, declaring what language a plugin page or sitemap translation group is written in, resolving user ids to a display name/avatar, sending an in-app notification to a user, adding a nav entrance, labeling or restricting a config field, making a scheduled task follow a configurable interval, keeping a component's state across a re-rendered ctx, sanitizing HTML a plugin did not write (ctx.sanitize), aggregating over every user's data (data.readsAllUsers / ctx.allUsers), bounding a config value, labelling a consent category, navigating inside a page plugin, declaring backend-owned or schema storage, styling a plugin tile's icons, crediting a plugin's license or data source, bumping platformApi, installing or releasing a plugin, or building a plugin's backend or frontend.

mkdir -p ~/.claude/skills/writing-a-mosaicast-plugin
curl -L https://claudskills.com/skills/writing-a-mosaicast-plugin/SKILL.md \
  -o ~/.claude/skills/writing-a-mosaicast-plugin/SKILL.md

@misc{writing-a-mosaicast-plugin-2026,
  author    = {Mosaicast},
  title     = {Writing A Mosaicast Plugin [Claude Code skill]},
  year      = {2026},
  publisher = {ClaudSkills},
  url       = {https://claudskills.com/skills/writing-a-mosaicast-plugin/}
}