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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). Fabro implements the open Agent Skills format. Each skill is a directory containing a SKILL.md file with YAML frontmatter and Markdown instructions. See the Agent Skills specification for the full file format, and What are skills? for an introduction.

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Discovery

Fabro discovers skills from three convention directories, searched in order: Within each directory, Fabro looks for */SKILL.md files (one level of nesting). 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.
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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Invocation

Skills can be invoked in two ways.

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Slash syntax (user input)

Users can reference a skill with a / prefix in their input:
Fabro 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. Rules for slash syntax:
  • The / must be preceded by whitespace or be at the start of the input
  • The skill name must be followed by whitespace or end of input
  • Only one skill reference per input is allowed
  • 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)

When skills are available, Fabro adds a use_skill tool to the agent’s toolset and lists the available skills in the system prompt:
The agent can call use_skill with a skill name to load the template, then follow the instructions: Returns the skill’s template text.

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Lifecycle

  1. Discovery — On session startup, Fabro scans the skill directories and parses all valid SKILL.md files
  2. Registration — If any skills are found, the use_skill tool is registered and the skill list is added to the system prompt
  3. Expansion — When user input contains a /skill-name reference, Fabro expands it before sending to the LLM
  4. Tool use — The agent can also call use_skill at any time to load a skill’s template mid-conversation