The [skill capability family](../../packages/skill) is split across three packages: the registry ([dsh-skill](../../packages/skill/skill), `ctx.skills`) merges provider catalogs; the local provider ([dsh-skill-local](../../packages/skill/skill-local)) scans and watches project/custom/user directories; the consumer ([dsh-tool-skill](../../packages/skill/tool-skill)) owns the initial and replacement catalogs plus the model-facing `skill` tool. Skills are optional instructions, not session events, so their vocabulary lives here rather than in [core.md](core.md).
Source: [`packages/skill/skill/src/index.ts`](../../packages/skill/skill/src/index.ts), [`packages/skill/skill-local/src/index.ts`](../../packages/skill/skill-local/src/index.ts), and [`packages/skill/tool-skill/src/index.ts`](../../packages/skill/tool-skill/src/index.ts).
`ctx.skills` combines local, embedded, remote, or other providers. Registration is synchronous; remote initialization and discovery belong in awaited `list()`. Provider objects, options, and candidates are borrowed readonly, while semantic fields are validated.
Duplicate names resolve by rank, provider order, then local order; summaries sort by name. A rejected `list()` is logged and omitted from an incomplete observation, while an explicit incomplete observation contributes usable candidates without making the result cacheable; malformed candidates fail fast. Each provider factory receives a registration-scoped control whose `invalidate()` clears completed catalogs only while that exact registration remains active and whose signal aborts on failed registration or disposal. An in-flight discovery retries once when its provider generation changes; a second change returns the latest candidates incomplete and uncached. Provider and runtime mutations emit the unfiltered `skills/change` invalidation event; it carries no diff, so consumers refetch `snapshot()` with their own lookup options.
An array returned by `SkillProvider.list()` is complete-discovery shorthand. `SkillProviderObservation` lets a provider expose candidates that remain directly loadable while reporting that the observation is not authoritative.
```ts type-equiv
/** Provider candidates plus whether the current discovery is authoritative. */
interface SkillProviderObservation {
/** Candidates available from the current provider discovery. */
readonly candidates: readonly SkillCandidate[]
/** Whether discovery completed and these candidates may be cached. */
The project root is the nearest ancestor containing `.git`; without one, the current cwd is used. When `ctx.fs` is available, the git-root walk probes `.git` through the filesystem service so remote or sandboxed workspaces do not fall back to the host filesystem boundary. The user DSH root skips its `.system` child. The local provider does not ship built-in system skills; deployments supply built-ins through another provider.
Chokidar watches existing roots for direct bundle/flat-entry additions and removals plus direct skill-entry changes. A missing root is followed one absent path segment at a time from its nearest existing ancestor until Chokidar can attach. Resource files below a bundle are not catalog changes. Model-facing `write` and `edit` observations synchronously invalidate the provider when their target is catalog-relevant, while the host watcher covers IDE, Git, shell, and external-process mutations. Watcher failures make the current observation incomplete without hiding readable candidates from direct loads; project-scoped watchers use a configured bounded LRU.
Skill names are kebab-case (`^[a-z0-9]+(?:-[a-z0-9]+)*$`). The local provider accepts directory bundles (`<name>/SKILL.md`) and flat Markdown files (`<name>.md`). Nested recursive `**/SKILL.md` discovery is intentionally outside v1.
`SkillSummary` is the registry's invocation-neutral summary shape. Consumers choose which entries and fields to render; the model session catalog uses only model-invocable `name` and `description`, never the body or absolute file path. `SkillInvocationPolicy` normalizes the two independent invocation controls into positive booleans, and every resolved summary, candidate, and definition carries it without turning arbitrary frontmatter into the domain model.
`ctx.skills.list()` preserves all four policy combinations. `isModelInvocable(skill)` and `isUserInvocable(skill)` read the corresponding required field. A model-only skill sets `{ modelInvocable: true, userInvocable: false }`, a user-only skill sets `{ modelInvocable: false, userInvocable: true }`, and setting both fields to `false` keeps the skill available only through trusted `ctx.skills.get()` callers. The local provider reads the exact kebab-case frontmatter keys `disable-model-invocation` and `user-invocable`, defaults omitted fields to `true`, and projects every parsed skill into this normalized policy.
`SkillCatalogSnapshot` distinguishes authoritative absence from transient provider failure or a catalog that kept changing during discovery. `skills` contains the sorted invocation-neutral summaries collected in that observation; `complete` is true only when every registered provider completed without a concurrent catalog revision. Incomplete snapshots are not cached, allowing each consumer to retain its last-good filtered catalog and retry.
`SkillCandidate` is the provider-to-registry shape. `locator` is opaque provider state; the registry only stores it and gives it back to the winning provider's `get()`.
`SkillDefinition` is the complete parsed result returned by `ctx.skills.get()` and used by the `skill` tool. `resourceBase` tells the tool how to render relative-resource guidance for local, URL, or provider-managed skills.
Runtime skill inputs may omit invocation controls and the provider label. The registry resolves both defaults once, then uses the same complete definition shape and first-wins collection order as providers. The returned disposer removes the contribution and invalidates discovery caches.
Skill lookup is cwd-sensitive because providers may expose workspace-local skills, and its optional signal cancels provider work for the caller. Providers receive the same readonly options object used for cache identity and loading. Cancellation is checked before and after catalog selection, including cache hits, and races both discovery and full-definition loading. If no git root is found, the local provider treats the supplied cwd itself as the project root.
Full definitions are not cached by the registry. Each `get()` calls the winning provider with the selected candidate, so the local provider rereads the current body. A definition whose name no longer matches that candidate is rejected and invalidates the exact provider for rediscovery.
The registry owns only its discovery-cache bound. The local provider owns filesystem roots (`dshHome`, `agentsHome`, `customSkillDirs`, and optional `bundledSkillDir`/`DSH_BUNDLED_SKILL_DIR`) plus watcher enablement, polling, stability, symlink, and project-capacity controls. The consumer owns its catalog description bound. Exact defaults and validation are in the generated [config catalog](../config-catalog.md).
`dsh-tool-skill` injects the initial durable user-role `<system-reminder>` at the first `agent/pre-step` of a live session that observes a non-empty complete view. The catalog contains sorted skill `name` and normalized, XML-escaped `description` only; it omits bodies, paths, sources, providers, and routing hints. Discovery forwards the step's abort signal through `SkillLookupOptions`. `catalogDescriptionMaxLength` is the consumer config for the description bound, with default `500` and integer minimum `3`.
Before each later model step, the consumer applies exact tool visibility and digests the exact rendered entries between the `<available_skills>` tags from a complete snapshot. It derives the comparison baseline from the same entries in the newest recognizable visible catalog message sourced by the plugin. A changed digest appends a durable full replacement through `agent.inject()`; deleting every skill appends an explicit empty replacement. Incomplete snapshots preserve the last-good model view. If compaction hides every historical catalog message, the next complete snapshot re-establishes the current catalog; an empty view with no prior catalog emits nothing. These catalog messages are session history, not World State.
The model-facing `skill({ name })` tool validates the kebab-case name, finds the summary in the invocation-neutral catalog, rejects it before loading unless `isModelInvocable` permits access, then rereads the complete definition for the calling agent cwd and rechecks the policy before returning content. It reports an unresolved skill as unknown or no longer available and returns a tool result containing `<skill_content name="...">`, `<skill_resources>`, and `<skill_instructions>`. `resourceBase` resolves explicitly referenced scripts, references, and assets only as needed; the loaded result does not enumerate a skill directory. Body-only edits therefore change later tool calls without producing catalog messages or rewriting earlier tool results.
