# Skill Design Principles

> Principles for writing effective agent skills. A skill is a set of instructions injected into an agent's context window — every line competes for space with the user's actual task.

- Skill: `tools-only/skill-design-principles-2` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add tools-only/skill-design-principles-2`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tools-only/skill-design-principles-2/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: tools-only (https://skillmd.com/u/tools-only)
- Updated: 2026-09-29
- Page: https://skillmd.com/skills/tools-only/skill-design-principles-2

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# Skill Design Principles

Principles for writing effective agent skills. A skill is a set of instructions injected into an agent's context window — every line competes for space with the user's actual task.

## Conciseness

The context window is shared between the skill instructions and the agent's working memory. Only include what the agent doesn't already know.

**Include:**
- Domain knowledge specific to this task
- Decision logic the agent can't infer
- Output format requirements
- Concrete examples of correct behavior

**Omit:**
- General programming knowledge
- How to use standard tools (Read, Grep, Bash)
- Obvious instructions ("be thorough", "check for errors")
- Lengthy explanations when a table or example suffices

**Rule of thumb:** If a senior engineer would skip reading it, the agent doesn't need it either.

## Degrees of Freedom

Match the specificity of your instructions to the fragility of the task.

| Fragility | Instruction Style | Example |
|-----------|------------------|---------|
| **High** — wrong output is costly | Prescriptive steps, exact formats | Commit message format, API output schema |
| **Medium** — multiple valid approaches | Guidelines with examples | Code review priorities, refactoring strategy |
| **Low** — many correct answers | Goals and constraints only | "Explain this code", "Summarize these changes" |

Over-constraining low-fragility tasks wastes context and limits the agent. Under-constraining high-fragility tasks leads to inconsistent results.

## Progressive Disclosure

Structure skills so agents load only what they need, when they need it.

**Three-tier loading:**

1. **Metadata** (always loaded) — frontmatter `name` and `description` determine whether the skill activates
2. **Instructions** (loaded on activation) — the SKILL.md body with the core workflow
3. **Resources** (loaded on demand) — reference files, loaded conditionally based on the task context

```markdown
## Step 3: Load Language Guide

| File Extension | Read This Reference |
|---------------|-------------------|
| `.py`         | `${CLAUDE_SKILL_ROOT}/references/python.md` |
| `.js`, `.ts`  | `${CLAUDE_SKILL_ROOT}/references/javascript.md` |
```

This keeps the base context small while making deep knowledge available when needed.

## Description as Trigger

The `description` field determines when agents activate the skill. It must contain the phrases users actually say.

**Effective descriptions:**
```yaml
# Good — includes natural trigger phrases
description: Create commit messages following Sentry conventions. Use when committing code changes, writing commit messages, or formatting git history.

# Good — includes action verbs and domain terms
description: Security code review for vulnerabilities. Use when asked to "security review", "find vulnerabilities", "check for security issues", "audit security", "OWASP review".
```

**Ineffective descriptions:**
```yaml
# Bad — too vague, no trigger phrases
description: A helpful skill for code quality.

# Bad — describes internals, not when to use it
description: Runs a Python script that parses AST and generates reports.

# Bad — too short, won't match varied user phrasing
description: Code review.
```

**Pattern:** `<What it does>. Use when <trigger phrases>. <Key capabilities>.`

## Imperative Voice

Skills are instructions to an agent, not documentation for humans. Write in imperative voice throughout.

| Imperative (correct) | Descriptive (avoid) |
|---------------------|-------------------|
| Read the diff and identify changes | This skill reads the diff and identifies changes |
| Report findings in the table format below | Findings should be reported in the table format below |
| Ask the user before making destructive changes | The agent may want to ask the user before making destructive changes |
| Skip test files unless explicitly requested | Test files are generally skipped unless explicitly requested |

The agent interprets imperative instructions as direct commands. Descriptive language introduces ambiguity about whether an action is required or optional.