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SKILL.md
writing-skills.SKILL.md
---name: writing-skills
description: Use when creating new skills, editing existing skills, or verifying skills work before deployment
---# Writing Skills
## Overview
**Writing skills IS Test-Driven Development applied to process documentation.**
**Personal skills live in your runtime's skills directory**
You write test cases (pressure scenarios with subagents), watch them fail (baseline behavior), write the skill (documentation), watch tests pass (agents comply), and refactor (close loopholes).
**Core principle:** If you didn't watch an agent fail without the skill, you don't know if the skill teaches the right thing.
**REQUIRED BACKGROUND:** You MUST understand superpowers:test-driven-development before using this skill. That skill defines the fundamental RED-GREEN-REFACTOR cycle. This skill adapts TDD to documentation.
**Official guidance:** For Anthropic's official skill authoring best practices, see anthropic-best-practices.md. This document provides additional patterns and guidelines that complement the TDD-focused approach in this skill.
## What is a Skill?
A **skill** is a reference guide for proven techniques, patterns, or tools. Skills help future agents find and apply effective approaches.
**Skills are:** Reusable techniques, patterns, tools, reference guides
**Skills are NOT:** Narratives about how you solved a problem once
## TDD Mapping for Skills
- Two required fields: `name` and `description` (see [agentskills.io/specification](https://agentskills.io/specification) for all supported fields)
- Max 1024 characters total
- `name`: Use letters, numbers, and hyphens only (no parentheses, special chars)
- `description`: Third-person, describes ONLY when to use (NOT what it does)
- Start with "Use when..." to focus on triggering conditions
- Include specific symptoms, situations, and contexts
- **NEVER summarize the skill's process or workflow** (see SDO section for why)
- Keep under 500 characters if possible
```markdown
---
name: Skill-Name-With-Hyphens
description: Use when [specific triggering conditions and symptoms]
---
# Skill Name
## Overview
What is this? Core principle in 1-2 sentences.
## When to Use
[Small inline flowchart IF decision non-obvious]
Bullet list with SYMPTOMS and use cases
When NOT to use
## Core Pattern (for techniques/patterns)
Before/after code comparison
## Quick Reference
Table or bullets for scanning common operations
## Implementation
Inline code for simple patterns
Link to file for heavy reference or reusable tools
## Common Mistakes
What goes wrong + fixes
## Real-World Impact (optional)
Concrete results
```
## Skill Discovery Optimization (SDO)
**Critical for discovery:** Future agents need to FIND your skill
### 1. Rich Description Field
**Purpose:** Your agent reads the description to decide which skills to load for a given task. Make it answer: "Should I read this skill right now?"
**Format:** Start with "Use when..." to focus on triggering conditions
**CRITICAL: Description = When to Use, NOT What the Skill Does**
The description should ONLY describe triggering conditions. Do NOT summarize the skill's process or workflow in the description.
**Why this matters:** Testing revealed that when a description summarizes the skill's workflow, an agent may follow the description instead of reading the full skill content. A description saying "code review between tasks" caused an agent to do ONE review, even though the skill's flowchart clearly showed TWO reviews (spec compliance then code quality).
When the description was changed to just "Use when executing implementation plans with independent tasks" (no workflow summary), the agent correctly read the flowchart and followed the two-stage review process.
**The trap:** Descriptions that summarize workflow create a shortcut agents will take. The skill body becomes documentation agents skip.
```yaml
# ❌ BAD: Summarizes workflow - agents may follow this instead of reading skill
description: Use when executing plans - dispatches subagent per task with code review between tasks
# ❌ BAD: Too much process detail
description: Use for TDD - write test first, watch it fail, write minimal code, refactor
# ✅ GOOD: Just triggering conditions, no workflow summary
description: Use when executing implementation plans with independent tasks in the current session
# ✅ GOOD: Triggering conditions only
description: Use when implementing any feature or bugfix, before writing implementation code
```
**Content:**
- Use concrete triggers, symptoms, and situations that signal this skill applies
- Describe the *problem* (race conditions, inconsistent behavior) not *language-specific symptoms* (setTimeout, sleep)
- Keep triggers technology-agnostic unless the skill itself is technology-specific
- If skill is technology-specific, make that explicit in the trigger
- Write in third person (injected into system prompt)
- **NEVER summarize the skill's process or workflow**
```yaml
# ❌ BAD: Too abstract, vague, doesn't include when to use
description: For async testing
# ❌ BAD: First person
description: I can help you with async tests when they're flaky
# ❌ BAD: Mentions technology but skill isn't specific to it
description: Use when tests use setTimeout/sleep and are flaky
# ✅ GOOD: Starts with "Use when", describes problem, no workflow
description: Use when tests have race conditions, timing dependencies, or pass/fail inconsistently
# ✅ GOOD: Technology-specific skill with explicit trigger
description: Use when using React Router and handling authentication redirects
| "Academic review is enough" | Reading ≠ using. Test application scenarios. |
| "No time to test" | Deploying untested skill wastes more time fixing it later. |
**All of these mean: Test before deploying. No exceptions.**
## Match the Form to the Failure
Before writing guidance, classify the baseline failure. The form that bulletproofs one failure type measurably backfires on another.
| Baseline failure | Right form | Wrong form |
|---|---|---|
| Skips/violates a rule under pressure (knows better, does it anyway) | Prohibition + rationalization table + red flags (see Bulletproofing below) | Soft guidance ("prefer...", "consider...") |
| Complies, but output has the wrong shape (bloated prompt, buried verdict, restated spec) | Positive recipe or contract: state what the output IS — its parts, in order | Prohibition list ("don't restate", "never narrate") |
| Omits a required element from something they already produce | Structural: REQUIRED field or slot in the template they fill in | Prose reminders near the template |
| Behavior should depend on a condition | Conditional keyed to an observable predicate ("if the brief exists, reference it") | Unconditional rule + exemption clauses |
**Why prohibitions backfire on shaping problems:** under a competing incentive ("make the prompt self-contained"), agents negotiate with "don't X". In head-to-head wording tests on dispatch-prompt guidance, the prohibition arm produced clearly more of the unwanted content than the recipe arm (fully separated distributions), and trended worse than even the no-guidance control — micro-test your own case rather than assuming, but never reach for the prohibition by default. A recipe leaves nothing to negotiate: the output matches the stated shape or it doesn't.
**Rules for whichever form you pick:**
- **No nuance clauses.** "Don't X unless it matters" reopens the negotiation — appending a single nuance clause to a winning recipe degraded it from consistent to noisy in the same wording tests. Express a real exception as its own conditional on an observable predicate.
- **Exemption clauses don't scope.** "This limit doesn't apply to code blocks" still suppresses code blocks. If part of the output must be exempt, restructure so the rule can't reach it.
## Bulletproofing Skills Against Rationalization
Skills that enforce discipline (like TDD) need to resist rationalization. Agents are smart and will find loopholes when under pressure.
**Scope:** this toolkit is for discipline failures — an agent that knows the rule and skips it under pressure. For wrong-shaped output or omitted elements, prohibition-based bulletproofing backfires; use the forms in Match the Form to the Failure instead.
**Psychology note:** Understanding WHY persuasion techniques work helps you apply them systematically. See persuasion-principles.md for research foundation (Cialdini, 2021; Meincke et al., 2025) on authority, commitment, scarcity, social proof, and unity principles.
### Close Every Loophole Explicitly
Don't just state the rule - forbid specific workarounds:
<Bad>
```markdown
Write code before test? Delete it.
```
</Bad>
<Good>
```markdown
Write code before test? Delete it. Start over.
**No exceptions:**
- Don't keep it as "reference"
- Don't "adapt" it while writing tests
- Don't look at it
- Delete means delete
```
</Good>
### Address "Spirit vs Letter" Arguments
Add foundational principle early:
```markdown
**Violating the letter of the rules is violating the spirit of the rules.**
```
This cuts off entire class of "I'm following the spirit" rationalizations.
### Build Rationalization Table
Capture rationalizations from baseline testing (see Testing section below). Every excuse agents make goes in the table:
```markdown
| Excuse | Reality |
|--------|---------|
| "Too simple to test" | Simple code breaks. Test takes 30 seconds. |
| "Tests after achieve same goals" | Tests-after = "what does this do?" Tests-first = "what should this do?" |
```
### Create Red Flags List
Make it easy for agents to self-check when rationalizing:
```markdown
## Red Flags - STOP and Start Over
- Code before test
- "I already manually tested it"
- "Tests after achieve the same purpose"
- "It's about spirit not ritual"
- "This is different because..."
**All of these mean: Delete code. Start over with TDD.**
```
### Update SDO for Violation Symptoms
Add to description: symptoms of when you're ABOUT to violate the rule:
```yaml
description: use when implementing any feature or bugfix, before writing implementation code
```
## RED-GREEN-REFACTOR for Skills
Follow the TDD cycle:
### RED: Write Failing Test (Baseline)
Run pressure scenario with subagent WITHOUT the skill. Document exact behavior:
- What choices did they make?
- What rationalizations did they use (verbatim)?
- Which pressures triggered violations?
This is "watch the test fail" - you must see what agents naturally do before writing the skill.
### GREEN: Write Minimal Skill
Write skill that addresses those specific rationalizations. Don't add extra content for hypothetical cases.
Run same scenarios WITH skill. Agent should now comply.
### REFACTOR: Close Loopholes
Agent found new rationalization? Add explicit counter. Re-test until bulletproof.
### Micro-Test Wording Before Full Scenarios
Full pressure-scenario runs are the final gate, but they are slow and expensive per iteration. Verify the wording itself first with micro-tests:
1. **One fresh-context sample per call** — a raw API call, or a single-shot subagent if you don't have API access. System prompt = the realistic context the guidance will live in (the full skill or prompt template, not the guidance in isolation); user message = a task that tempts the failure.
2. **Always include a no-guidance control.** If the control doesn't exhibit the failure, there is nothing to fix — stop, don't author the guidance.
3. **5+ reps per variant.** Single samples lie.
4. **Manually read every flagged match.** Score programmatically if you like, but template echoes and quoted counter-examples masquerade as hits; automated counts alone overstate both failure and success.
5. **Variance is a metric.** When guidance lands, reps converge on the same shape. Five different interpretations across five reps means the wording isn't binding — tighten the form before adding words.
Micro-tests verify wording; they do not replace pressure scenarios for discipline skills.
**Testing methodology:** See [testing-skills-with-subagents.md](testing-skills-with-subagents.md) for the complete testing methodology:
- [ ] Run scenarios WITHOUT skill - document baseline behavior verbatim
- [ ] Identify patterns in rationalizations/failures
**GREEN Phase - Write Minimal Skill:**
- [ ] Name uses only letters, numbers, hyphens (no parentheses/special chars)
- [ ] YAML frontmatter with required `name` and `description` fields (max 1024 chars; see [spec](https://agentskills.io/specification))
- [ ] Description starts with "Use when..." and includes specific triggers/symptoms
- [ ] Description written in third person
- [ ] Keywords throughout for search (errors, symptoms, tools)
- [ ] Clear overview with core principle
- [ ] Address specific baseline failures identified in RED
- [ ] Guidance form matches the failure type (see Match the Form to the Failure)
- [ ] For behavior-shaping guidance: wording micro-tested against a no-guidance control (5+ reps, every flagged match read manually) — N/A for pure reference skills
- [ ] Code inline OR link to separate file
- [ ] One excellent example (not multi-language)
- [ ] Run scenarios WITH skill - verify agents now comply
**REFACTOR Phase - Close Loopholes:**
- [ ] Identify NEW rationalizations from testing
- [ ] Add explicit counters (if discipline skill)
- [ ] Build rationalization table from all test iterations
- [ ] Create red flags list
- [ ] Re-test until bulletproof
**Quality Checks:**
- [ ] Small flowchart only if decision non-obvious
- [ ] Quick reference table
- [ ] Common mistakes section
- [ ] No narrative storytelling
- [ ] Supporting files only for tools or heavy reference
**Deployment:**
- [ ] Commit skill to git and push to your fork (if configured)
- [ ] Consider contributing back via PR (if broadly useful)
