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Remove the "Skill file format" and "Creating skills" sections which duplicated content from the Agent Skills specification. Link to agentskills.io for the format spec and introduction instead. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
73 lines
3.5 KiB
Text
73 lines
3.5 KiB
Text
---
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title: "Skills"
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description: "Reusable prompt templates that extend agent capabilities"
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---
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Skills are reusable prompt templates that give agents structured instructions for common tasks -- code review, committing, test writing, refactoring, and more. They are discovered automatically from convention directories, listed in the agent's system prompt, and can be invoked either by the user (with `/skill-name` syntax) or by the agent itself (via the `use_skill` tool).
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Arc implements the open [Agent Skills](https://agentskills.io/home) format. Each skill is a directory containing a `SKILL.md` file with YAML frontmatter and Markdown instructions. See the [Agent Skills specification](https://agentskills.io/specification) for the full file format, and [What are skills?](https://agentskills.io/what-are-skills) for an introduction.
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## Discovery
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Arc discovers skills from three convention directories, searched in order:
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| Directory | Scope |
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|---|---|
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| `~/.arc/skills/` | Global -- applies to all projects |
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| `{git_root}/.arc/skills/` | Project -- checked into the repo |
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| `{git_root}/skills/` | Project (alternate location) |
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Within each directory, Arc looks for `*/SKILL.md` files (one level of nesting).
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When the same skill name appears in multiple directories, **later directories override earlier ones**. This means project-level skills take precedence over global skills, letting you customize behavior per-repo.
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<Note>
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You can override the default directories by setting `skill_dirs` in the session configuration. When set, only the specified directories are searched.
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</Note>
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## Invocation
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Skills can be invoked in two ways.
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### Slash syntax (user input)
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Users can reference a skill with a `/` prefix in their input:
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```
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/commit fix the off-by-one error in pagination
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```
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Arc detects the `/commit` token, finds the matching skill, and expands the input by replacing the skill reference with the full template. The remaining text (`fix the off-by-one error in pagination`) is substituted into the `{{user_input}}` placeholder.
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Rules for slash syntax:
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- The `/` must be preceded by whitespace or be at the start of the input
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- The skill name must be followed by whitespace or end of input
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- Only one skill reference per input is allowed
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- File paths like `/usr/bin/bash` are not matched (the character after `/` must be a lowercase letter, and the name cannot contain `/`)
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### use_skill tool (agent-initiated)
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When skills are available, Arc adds a `use_skill` tool to the agent's toolset and lists the available skills in the system prompt:
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```
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# Available Skills
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When the user's request matches a skill below, call the `use_skill` tool
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to load its instructions, then follow them.
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- `commit`: Create a git commit following best practices
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- `review`: Review code changes for issues and improvements
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```
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The agent can call `use_skill` with a skill name to load the template, then follow the instructions:
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| Parameter | Type | Required | Description |
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|---|---|---|---|
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| `skill_name` | string | yes | Name of the skill to load (without the `/` prefix) |
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Returns the skill's template text.
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## Lifecycle
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1. **Discovery** -- On session startup, Arc scans the skill directories and parses all valid `SKILL.md` files
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2. **Registration** -- If any skills are found, the `use_skill` tool is registered and the skill list is added to the system prompt
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3. **Expansion** -- When user input contains a `/skill-name` reference, Arc expands it before sending to the LLM
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4. **Tool use** -- The agent can also call `use_skill` at any time to load a skill's template mid-conversation
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