# Skills

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 project/custom/user directories; the consumer ([dsh-tool-skill](../../packages/skill/tool-skill)) owns the session-prefix catalog and 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).

## Provider registry

`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 skipped without caching the degraded catalog, while malformed candidates fail fast.

```ts type-equiv
/** Provider interface for one source of skills, such as local directories or a remote registry. */
interface SkillProvider {
  /** Unique provider name in the `ctx.skills` registry. */
  readonly name: string
  /**
   * List available skill candidates for the current lookup context. Provider
   * plugins register synchronously during `apply()`; remote initialization,
   * authentication, and discovery are awaited inside this method. Implementations
   * should settle promptly when `options.signal` aborts.
   * @param options - lookup options; `cwd` selects workspace-sensitive skills and `signal` cancels work.
   * @returns provider candidates with precedence ranks and opaque locators.
   */
  readonly list: (options: SkillLookupOptions) => Promise<readonly SkillCandidate[]>
  /**
   * Load a complete skill body for a previously listed candidate.
   * @param candidate - the winning candidate originally returned by this provider.
   * @param options - lookup options; `cwd` selects workspace-sensitive skills and `signal` cancels work.
   * @returns the full skill body, or `undefined` if it is no longer loadable.
   */
  readonly get: (candidate: SkillCandidate, options: SkillLookupOptions) => Promise<SkillDefinition | undefined>
}
```

## Local discovery priority

The shipped local provider scans roots in rank order:

| Rank | Source | Root |
|---|---|---|
| 100 | `project-dsh` | `<projectRoot>/.dsh/skills` |
| 200 | `project-agents` | `<projectRoot>/.agents/skills` |
| 300 | `custom` | `Config.customSkillDirs` |
| 400 | `user-dsh` | `<dshHome>/skills` |
| 500 | `user-agents` | `<agentsHome>/skills` |

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.

## Skill identity

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.

```ts type-equiv
/** Origin bucket for a skill contribution. The value is prompt-visible metadata, not precedence by itself. */
type SkillSource = 'project-dsh' | 'project-agents' | 'runtime' | 'user-dsh' | 'user-agents' | 'custom' | (string & {})
```

## Summaries, candidates, and complete definitions

`SkillSummary` is the registry's model-invocable summary shape. Consumers choose which fields to render; the session catalog uses only `name` and `description`, never the body or absolute file path. `disableModelInvocation` hides a skill from model listings while allowing trusted code to load it by name.

```ts type-equiv
/** Model-visible skill metadata returned by `ctx.skills.list()` and rendered into request guidance. */
interface SkillSummary {
  /** Kebab-case identifier used with the `skill` tool. */
  readonly name: string
  /** Short routing description shown to the model. */
  readonly description: string
  /** Optional extra routing guidance shown to the model. */
  readonly whenToUse?: string
  /** Whether the skill is hidden from model listings while remaining loadable by trusted callers. */
  readonly disableModelInvocation?: boolean
  /** Discovery source that produced this winning skill. */
  readonly source: SkillSource
  /** Provider that owns this skill body. */
  readonly provider: string
  /** Provider-specific base for relative resources. */
  readonly resourceBase?: SkillResourceBase
}
```

`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()`.

```ts type-equiv
/** Provider catalog entry used by the registry to merge and later load skills. */
interface SkillCandidate extends SkillSummary {
  /** Lower ranks win duplicate skill names before provider registration order is considered. */
  readonly rank: number
  /** Opaque provider-owned handle passed back to `provider.get()`. */
  readonly locator: unknown
  /** Absolute file path when the provider has one. */
  readonly path?: string
  /** Parsed optional metadata object from provider-specific skill frontmatter. */
  readonly metadata?: Readonly<Record<string, unknown>>
}
```

`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.

```ts type-equiv
/** Optional provider-specific base used by loaded skill bodies to resolve relative resources. */
type SkillResourceBase =
  | { readonly kind: 'directory'; readonly path: string }
  | { readonly kind: 'url'; readonly url: string }
  | { readonly kind: 'opaque'; readonly description: string }
```

```ts type-equiv
/** Complete parsed skill definition, including the body loaded by `ctx.skills.get()`. */
interface SkillDefinition extends SkillSummary {
  /** Markdown instruction body after any provider-specific metadata removal. */
  readonly content: string
  /** Absolute file path when the skill came from disk. */
  readonly path?: string
  /** Parsed optional metadata object from frontmatter. */
  readonly metadata?: Readonly<Record<string, unknown>>
}
```

Runtime skills use the same complete shape and participate in the same first-wins collection order. The returned disposer removes the contribution and invalidates discovery caches.

```ts type-equiv
/** Runtime skill contribution accepted by `ctx.skills.register()`. */
type SkillRegistration = Omit<SkillDefinition, 'provider'> & { readonly provider?: string }
```

## Lookup and configuration

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.

```ts type-equiv
/** Caller context used for cwd-sensitive and abortable provider work. */
interface SkillLookupOptions {
  /** Workspace selector for the current lookup. */
  readonly cwd?: string | undefined
  /** Abort discovery or loading work for the current caller. */
  readonly signal?: AbortSignal | undefined
}
```

The registry owns only its discovery-cache bound. The local provider owns filesystem roots (`dshHome`, `agentsHome`, and `customSkillDirs`). The consumer owns its catalog description bound.

```ts type-equiv
/** Skill registry configuration. */
interface Config {
  /** Maximum number of completed cwd/provider catalogs kept in memory. */
  readonly collectCacheMaxEntries?: number
}
```

## Session catalog and tool contract

`dsh-tool-skill` contributes a user-role `<system-reminder>` through `agent/session-prefix`. The catalog contains sorted skill `name` and normalized, XML-escaped `description` only; it omits bodies, paths, sources, providers, and routing hints. Prefix discovery forwards the caller's abort signal through `SkillLookupOptions`. `catalogDescriptionMaxLength` is the consumer config for the description bound, with default `500` and integer minimum `3`. Its request-only, header-logged lifecycle is defined by the [session-prefix RFC](../rfc/implemented/feature/2026-07-07-session-prefix.md).

The model-facing `skill({ name })` tool validates the kebab-case name, loads the complete definition for the calling agent cwd, reports an unresolved skill as unknown or no longer available, rejects `disableModelInvocation` skills, 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. The tool result is the model-visible path for complete instructions.
