Skill Creator

Skill Creator

When to Use This Skill

Activate this skill when:

• The user asks to create, add, or write a new Agent Skill
• The ai-engineer promotes an issue and the routing table says to create

a new skill

• An existing skill needs its frontmatter fixed, its description improved, or

its body restructured

• The user asks "how do I write a good skill" or similar

The Agent Skills Standard

prompt-forge skills follow the agentskills.io specification, an open format supported by Claude Code, GitHub Copilot, OpenAI Codex, Cursor, Gemini CLI, and many others.

Directory Structure

Each skill is a directory under .github/skills/ containing a SKILL.md file. The directory name must match the name field in the frontmatter.

.github/skills//
├── SKILL.md          # Required: YAML frontmatter + Markdown body
├── scripts/          # Optional: executable code the agent can run
├── references/       # Optional: additional docs loaded on demand
└── assets/           # Optional: templates, images, data files

Frontmatter Specification

The SKILL.md file begins with YAML frontmatter between --- delimiters. The agentskills.io spec defines these fields:

| Field | Required | Constraints | |-------|----------|-------------| | name | Yes | Max 64 chars. Lowercase letters, numbers, hyphens only. Must match directory name. | | description | Yes | Max 1024 chars. Describes what the skill does AND when to use it. | | license | No | SPDX identifier or reference to bundled license file | | compatibility | No | Max 500 chars. Environment requirements (OS, packages, network) | | metadata | No | Arbitrary key-value map. Use for extended attributes. | | allowed-tools | No | Experimental. Space-separated list of pre-approved tools. | | disable-model-invocation | No | If true, skill loads only on explicit user request, not automatic trigger |

prompt-forge Extended Metadata

prompt-forge adds these fields inside metadata for every skill:

| Field | Purpose | |-------|---------| | metadata.author | GitHub username of the skill author | | metadata.url | Canonical URL for the skill source | | metadata.version | Semantic version string | | metadata.tokens | Estimated token count of the Markdown body (4 chars per token) | | metadata.sources | Comma-separated list of reference URLs used to build the skill |

These fields are optional per the spec but required by prompt-forge convention. Always include them when creating a new skill.

Creating a New Skill

Step 1: Choose the Name

The name must match the directory that will contain the skill.

Rules:

• Lowercase letters, numbers, and hyphens only
• Must not start or end with a hyphen
• Must not contain consecutive hyphens (--)
• Max 64 characters

Good names: pdf-processing, data-analysis, code-review Bad names: PDF-Processing, -pdf, pdf--processing

Step 2: Write the Description

The description is the most critical field. It is the ONLY information the agent has when deciding whether to activate the skill. A poorly written description means the skill never triggers, or triggers on irrelevant tasks.

A good description:

• States what the skill does in concrete terms
• Includes specific keywords the agent should match against
• Mentions the context or user intent that should trigger it
• Is between 50 and 300 characters

Example of a strong description:

description: |
  Create, update, review, fix, or debug Agent Skills following the
  agentskills.io specification and prompt-forge conventions. Use when
  the user wants to add a new skill, improve an existing skill, or when
  ai-engineer promotes an issue to a new skill. Covers frontmatter
  format, description writing, body structure, token estimation, and
  progressive disclosure patterns.

Example of a weak description:

description: Helps with skills.

Step 3: Structure the Body

The body after the frontmatter contains the instructions the agent follows. Follow these patterns from the agentskills.io best practices:

Add what the agent lacks, omit what it knows. Do not explain what a PDF is or how HTTP works. The agent already knows. Focus on project-specific conventions, non-obvious edge cases, and particular tool choices.

Include a gotchas section. The highest-value content is environment-specific facts that defy reasonable assumptions. If the agent would get something wrong without being told, put it in a gotchas section.

Favor procedures over declarations. Teach the agent how to approach a class of problems, not what to produce for a specific instance. "Read the schema file, join tables using the _id foreign key convention, and format results as a Markdown table" is reusable. "Join orders to customers on customer_id" works only for one query.

Provide defaults, not menus. Pick a recommended approach and mention alternatives only when necessary.

Use templates for output format. When the agent needs to produce output in a specific format, provide a concrete Markdown template. Agents pattern-match well against structures.

Use checklists for multi-step workflows. An explicit checklist helps the agent track progress and avoid skipping steps.

Keep the body under 500 lines and 5,000 tokens. Move detailed reference material to references/ files. The agent loads the full body on activation; every token competes for attention.

Step 4: Estimate Token Count

After writing the body, count its characters and divide by 4 to estimate tokens. This is the standard ratio for English text. Set the result in metadata.tokens, rounded to the nearest 0.1k.

PowerShell verification command:

$body = ($content -split '---', 3)[2]
$tokens = [Math]::Round($body.Length / 4)
$k = [Math]::Round($tokens / 100, 1)
Write-Host "$k k"

Step 5: Create the Files

Use the agent create_file tool to write SKILL.md to the new directory under .github/skills//. The directory must not already exist.

After creating the skill, verify:

• The directory name matches the name field exactly
• The description includes both WHAT and WHEN
• The metadata block has all five prompt-forge fields
• The token count is measured, not guessed
• The body uses the structural patterns described above

Updating an Existing Skill

When updating a skill, use replace_string_in_file to make targeted edits. Always include 3-5 lines of surrounding context in the match string. After editing, re-measure the token count and update metadata.tokens if it changed by more than 0.1k.

Fixing Common Frontmatter Issues

Problem: The frontmatter --- delimiter is missing or malformed. Fix: Ensure exactly three dashes on the first and last line of the frontmatter block. No spaces before or after.

Problem: The name field does not match the directory name. Fix: Rename either the directory or the field. They must match exactly.

Problem: The description does not include trigger conditions. Fix: Add "Use when..." followed by specific user intents or task descriptions.

Problem: The description uses > instead of | for multi-line text. Fix: Use | (literal block scalar) for multi-line descriptions. The > folded scalar may collapse newlines, making the description harder to read.

Debugging Skill Activation

If a skill is not triggering when it should:

1. Check the description field. Does it contain keywords the user's request

would match? If the user says "I need to extract text from this PDF" but the description says "Document processing utilities," the match may fail.

1. Check disable-model-invocation. If true, the skill only activates on

explicit user request, never automatically.

1. Consider whether the description is too narrow or too broad. Too narrow:

misses valid triggers. Too broad: activates on irrelevant tasks, crowding out other skills.

1. Read the description aloud as if you were the agent scanning skills. Would

you know when to use it?

Skill Creation Checklist

Before finalizing a new skill, verify:

• [ ] Directory name matches name field (lowercase, hyphens, max 64 chars)
• [ ] description states what the skill does AND when to use it (50-300 chars)
• [ ] license is set (use MIT for prompt-forge skills)
• [ ] metadata.author is set to a GitHub username
• [ ] metadata.url points to the canonical source
• [ ] metadata.version uses semantic versioning
• [ ] metadata.tokens is measured, not estimated by eye
• [ ] Body is under 500 lines
• [ ] Body includes a gotchas section if there are non-obvious pitfalls
• [ ] Body favors procedures over specific one-off instructions
• [ ] No content explains things the agent already knows

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: pablodiazjorge
• Source: pablodiazjorge/prompt-forge
• License: MIT

Install and usage instructions live in the source repository linked above.