Skills

Skills

Skills are file-backed capability packs discovered at startup and exposed to the model as:

• lightweight metadata in the system prompt (name + description)
• on-demand content via read skill://...
• optional interactive /skill:<name> commands

This document covers current runtime behavior in src/extensibility/skills.ts, src/discovery/builtin.ts, src/internal-urls/skill-protocol.ts, and src/discovery/agents-md.ts.

What a skill is in this codebase

A discovered skill is represented as:

• name
• description
• filePath (the SKILL.md path)
• baseDir (skill directory)
• source metadata (provider, level, path)

The runtime only requires name and path for validity. In practice, matching quality depends on description being meaningful.

Required layout and SKILL.md expectations

Directory layout

For provider-based discovery (native/Claude/Codex/Agents/plugin providers), skills are discovered as one level under skills/:

• <skills-root>/<skill-name>/SKILL.md

Nested patterns like <skills-root>/group/<skill>/SKILL.md are not discovered by provider loaders.

For skills.customDirectories, scanning uses the same non-recursive layout (*/SKILL.md).

Provider-discovered layout (non-recursive under skills/):

<root>/skills/
  ├─ postgres/
  │   └─ SKILL.md      ✅ discovered
  ├─ pdf/
  │   └─ SKILL.md      ✅ discovered
  └─ team/
      └─ internal/
          └─ SKILL.md  ❌ not discovered by provider loaders

Custom-directory scanning is also non-recursive, so nested paths are ignored unless you point `customDirectories` at that nested parent.

SKILL.md frontmatter

Supported frontmatter fields on the skill type:

• name?: string
• description?: string
• globs?: string[]
• alwaysApply?: boolean
• additional keys are preserved as unknown metadata

Current runtime behavior:

• name defaults to the skill directory name
• description is required for:
  • native .xcsh provider skill discovery (requireDescription: true)
  • skills.customDirectories scans via scanSkillsFromDir in src/discovery/helpers.ts (non-recursive)
• non-native providers can load skills without description

Discovery pipeline

discoverSkills() in src/extensibility/skills.ts does two passes:

1. Capability providers via loadCapability("skills")
2. Custom directories via scanSkillsFromDir(..., { requireDescription: true }) (one-level directory enumeration)

If skills.enabled is false, discovery returns no skills.

Built-in skill providers and precedence

Provider ordering is priority-first (higher wins), then registration order for ties.

Current registered skill providers:

1. native (priority 100) — .xcsh user/project skills via src/discovery/builtin.ts
2. claude (priority 80)
3. priority 70 group (in registration order):
   • claude-plugins
   • agents
   • codex

Dedup key is skill name. First item with a given name wins.

Source toggles and filtering

discoverSkills() applies these controls:

• source toggles: enableCodexUser, enableClaudeUser, enableClaudeProject, enablePiUser, enablePiProject
• glob filters on skill name:
  • ignoredSkills (exclude)
  • includeSkills (include allowlist; empty means include all)

Filter order is:

1. source enabled
2. not ignored
3. included (if include list present)

For providers other than codex/claude/native (for example agents, claude-plugins), enablement currently falls back to: enabled if any built-in source toggle is enabled.

Collision and duplicate handling

• Capability dedup already keeps first skill per name (highest-precedence provider)
• extensibility/skills.ts additionally:
  • de-duplicates identical files by realpath (symlink-safe)
  • emits collision warnings when a later skill name conflicts
  • keeps the convenience discoverSkillsFromDir({ dir, source }) API as a thin adapter over scanSkillsFromDir
• Custom-directory skills are merged after provider skills and follow the same collision behavior

Runtime usage behavior

System prompt exposure

System prompt construction (src/system-prompt.ts) uses discovered skills as follows:

• if read tool is available:
  • include discovered skills list in prompt
• otherwise:
  • omit discovered list

Task tool subagents receive the session’s discovered/provided skills list via normal session creation; there is no per-task skill pinning override.

Interactive /skill:<name> commands

If skills.enableSkillCommands is true, interactive mode registers one slash command per discovered skill.

/skill:<name> [args] behavior:

• reads the skill file directly from filePath
• strips frontmatter
• injects skill body as a follow-up custom message
• appends metadata (Skill: <path>, optional User: <args>)

skill:// URL behavior

src/internal-urls/skill-protocol.ts supports:

• skill://<name> → resolves to that skill’s SKILL.md
• skill://<name>/<relative-path> → resolves inside that skill directory

skill:// URL resolution

skill://pdf
  -> <pdf-base>/SKILL.md

skill://pdf/references/tables.md
  -> <pdf-base>/references/tables.md

Guards:
- reject absolute paths
- reject `..` traversal
- reject any resolved path escaping <pdf-base>

Resolution details:

• skill name must match exactly
• relative paths are URL-decoded
• absolute paths are rejected
• path traversal (..) is rejected
• resolved path must remain within baseDir
• missing files return an explicit File not found error

Content type:

• .md => text/markdown
• everything else => text/plain

No fallback search is performed for missing assets.

Skills vs AGENTS.md, commands, tools, hooks

Skills vs AGENTS.md

• Skills: named, optional capability packs selected by task context or explicitly requested
• AGENTS.md/context files: persistent instruction files loaded as context-file capability and merged by level/depth rules

src/discovery/agents-md.ts specifically walks ancestor directories from cwd to discover standalone AGENTS.md files (up to depth 20), excluding hidden-directory segments.

Skills vs slash commands

• Skills: model-readable knowledge/workflow content
• Slash commands: user-invoked command entry points
• /skill:<name> is a convenience wrapper that injects skill text; it does not change skill discovery semantics

Skills vs custom tools

• Skills: documentation/workflow content loaded through prompt context and read
• Custom tools: executable tool APIs callable by the model with schemas and runtime side effects

Skills vs hooks

• Skills: passive content
• Hooks: event-driven runtime interceptors that can block/modify behavior during execution

Practical authoring guidance tied to discovery logic

• Put each skill in its own directory: <skills-root>/<skill-name>/SKILL.md
• Always include explicit name and description frontmatter
• Keep referenced assets under the same skill directory and access with skill://<name>/...
• For nested taxonomy (team/domain/skill), point skills.customDirectories to the nested parent directory; scanning itself remains non-recursive
• Avoid duplicate skill names across sources; first match wins by provider precedence