Skill Architecture

Skill Architecture

Comprehensive guide for creating effective Claude Code skills following Anthropic's official standards with emphasis on security and progressive disclosure architecture.

Self-Evolving Skill: This skill improves through use. If instructions are wrong, parameters drifted, or a workaround was needed — fix this file immediately, don't defer. Only update for real, reproducible issues.

Scope: Claude Code Agent Skills (~/.claude/skills/), not Claude.ai API skills

Self-Evolution Protocol

This skill — and every skill it creates — must actively evolve through use. This section is placed first because it governs all other sections.

During execution, watch for these signals: friction in instructions, missing edge cases, better patterns discovered, repeated manual steps, drift between documentation and reality.

Before writing any change, pass all three admission gates:

Most executions should produce no evolution. Convergence to stability is success, not stagnation.

When all gates pass: Pause current work → fix SKILL.md or references → log in evolution-log.md with trigger + evidence → resume. Do NOT defer — the next invocation inherits whatever you leave behind.

What never passes the gate: Major structural changes (discuss with user first), speculative improvements without empirical evidence, cosmetic preferences.

When to Use This Skill

Use this skill when:

• Creating new Claude Code skills from scratch
• Learning skill YAML frontmatter and structure requirements
• Validating skill file format and portability
• Understanding progressive disclosure patterns for skills

Task Templates

Select the appropriate template before starting skill work -- templates encode common workflows and prevent missing steps that cause silent failures.

See Task Templates for all templates (A-F) and the quality checklist.

Post-Change Checklist (Self-Maintenance)

After modifying THIS skill (skill-architecture):

1. Templates and 6 Steps tutorial remain aligned
2. Skill Quality Checklist reflects current best practices
3. All referenced files in references/ exist
4. Append changes to evolution-log.md
5. Update user's CLAUDE.md if triggers changed

About Skills

Skills are modular, self-contained packages that extend Claude's capabilities with specialized knowledge, workflows, and tools. Think of them as "onboarding guides" for specific domains -- transforming Claude from general-purpose to specialized agent with procedural knowledge no model fully possesses.

What Skills Provide

1. Specialized workflows - Multi-step procedures for specific domains
2. Tool integrations - Instructions for working with specific file formats or APIs
3. Domain expertise - Company-specific knowledge, schemas, business logic
4. Bundled resources - Scripts, references, assets for complex/repetitive tasks

Skill Discovery and Precedence

Skills are discovered from multiple locations. When names collide, higher-precedence wins:

1. Enterprise (managed settings) -- highest
2. Personal (~/.claude/skills/)
3. Project (.claude/skills/ in repo)
4. Plugin (namespaced: plugin:skill-name)
5. Nested (monorepo .claude/skills/ in subdirectories -- auto-discovered)
6. --add-dir (CLI flag, live change detection) -- lowest

Management commands:

• claude plugin enable <name> / claude plugin disable <name> -- toggle plugins
• claude skill list -- show all discovered skills with source location

Monorepo support: Claude Code automatically discovers .claude/skills/ directories in nested project roots within a monorepo. No configuration needed.

cc-skills Plugin Architecture

This section applies specifically to the cc-skills marketplace plugin structure. Generic standalone skills are unaffected.

Canonical Structure

plugins/<plugin>/
└── skills/
    └── <skill-name>/
        └── SKILL.md   <- single canonical file (context AND user-invocable)

skills/<name>/SKILL.md is the single source of truth. The separate commands/ layer was eliminated -- it required maintaining two identical files per skill and caused Skill() invocations to return "Unknown skill". See migration issue for full context.

How Skills Become Slash Commands

Two install paths, both supported:

Hooks

sync-hooks-to-settings.sh reads hooks/hooks.json directly -> merges into ~/.claude/settings.json. Bypasses path re-expansion bug #18517.

Creating a New Skill in cc-skills

Place the SKILL.md under plugins/<plugin>/skills/<name>/SKILL.md. No commands/ copy needed. The validator (bun scripts/validate-plugins.mjs) checks frontmatter completeness.

Skill Creation Process

See Creation Tutorial for the detailed 6-step walkthrough, or Creation Workflow for the comprehensive guide with examples.

Quick summary: Gather requirements -> Plan resources -> Initialize -> Edit SKILL.md -> Validate -> Register and iterate.

Testing and Iteration

Good skills emerge through testing and feedback, not from getting the first draft perfect. After writing or updating a skill, verify it works by running it against realistic prompts.

Write Test Prompts

Come up with 2-3 realistic test prompts -- the kind of thing a real user would actually say. Not abstract requests, but concrete tasks with enough detail to exercise the skill. Share them with the user for confirmation before running.

Run and Evaluate

For each test prompt, run the skill and examine the output:

• Did the skill trigger? If not, the description may need stronger trigger language.
• Did it follow the workflow? Check whether instructions were followed or ignored.
• Was the output useful? Compare against what you'd expect from a skilled human.

When subagents are available, run with-skill and without-skill versions in parallel to measure the skill's actual value-add. When not available, run test cases yourself as a sanity check.

Iterate Based on Feedback

After evaluating results, improve the skill and retest. Keep iterating until the user is satisfied or feedback is consistently positive. Key principles for each iteration:

1. Generalize from specific feedback. Skills will be used across many different prompts. Avoid overfitting to test cases with fiddly, narrow fixes. If a pattern keeps failing, try a different approach or metaphor rather than adding more constraints.

2. Keep the skill lean. Every section must earn its tokens. Read the execution transcripts -- if the skill causes the model to waste time on unproductive steps, cut those instructions and see what happens.

3. Explain the why, not just the what. LLMs respond better to understanding why a rule exists than to being commanded with rigid directives. Instead of "ALWAYS do X", explain: "Do X because skipping it causes Y, which leads to Z." This produces more robust behavior that generalizes to novel situations.

4. Look for repeated work across test cases. If every test run independently creates the same helper script or takes the same multi-step approach, bundle that script in scripts/ so future invocations don't reinvent the wheel.

5. Bundle common patterns as scripts. When test runs reveal that the model writes similar boilerplate code every time, extract it into a bundled script. This saves tokens and improves reliability.

Skill Writing Principles

These principles (aligned with Anthropic's official guidance) apply to all skill content:

• Imperative form: "Run the script", "Check the output" -- not passive or indirect phrasing.
• Explain reasoning over rigid rules: If you find yourself writing MUST/NEVER/ALWAYS in all caps, that's a signal to reframe. Explain the reasoning so the model internalizes the principle rather than treating it as an arbitrary constraint. The model is smart -- help it understand, don't just command it.
• Pushy descriptions for triggering: Claude tends to undertrigger skills. Descriptions should actively claim territory: "Use this skill whenever the user mentions X, Y, or Z, even if they don't explicitly ask for it." Include negative triggers too: "Do NOT use for A or B."
• Natural language descriptions: Write descriptions as sentences a human could read, not keyword lists. "Use this skill whenever..." is better than "TRIGGERS - keyword1, keyword2".
• Keep execution out of descriptions: Descriptions tell Claude when to trigger. The skill body tells Claude how to execute. Don't mix them.

See Writing Guide for extended guidance with examples.

Skill Anatomy

skill-name/
├── SKILL.md                      # Required: YAML frontmatter + instructions
├── scripts/                      # Optional: Executable code (Python/Bash)
├── references/                   # Optional: Documentation loaded as needed
│   └── evolution-log.md          # Required for self-evolving: Change history
└── assets/                       # Optional: Files used in output

YAML Frontmatter (Required)

See YAML Frontmatter Reference for the complete field reference, invocation control table, permission rules, description guidelines, and YAML pitfalls.

Minimal example:

---
name: my-skill
description: Does X when user mentions Y. Use for Z workflows.
---

Key rules: name is lowercase-hyphen, description is single-line max 1024 chars with trigger keywords, no colons in description text.

Progressive Disclosure (3 Levels)

Skills use progressive loading to manage context efficiently:

1. Metadata (name + description) - Always in context (~100 words)
2. SKILL.md body - When skill triggers (<5k words)
3. Bundled resources - As needed by Claude (unlimited*)

*Scripts can execute without reading into context.

Skill Description Budget

Skills are loaded into the context window based on description relevance. Large skills may be excluded if the budget is exceeded:

• Budget: ~2% of context window (16K character fallback)
• Check: Run /context to see which skills are loaded vs excluded
• Override: Set SLASH_COMMAND_TOOL_CHAR_BUDGET env var to increase budget
• Mitigation: Keep SKILL.md body lean, move detail to references/

Bundled Resources

Skills can include scripts/, references/, and assets/ directories. See Progressive Disclosure for detailed guidance on when to use each.

CLI-Specific Features

CLI skills support allowed-tools for granting tool access without per-use approval. See Security Practices for details.

String Substitutions

Skill bodies support these substitutions (resolved at load time):

Dynamic Context Injection

Use the pattern ! + `command` (exclamation mark followed by a backtick-wrapped command) in skill body to inject command output at load time:

Current branch: <exclamation>`git branch --show-current`
Last commit: <exclamation>`git log -1 --oneline`

(Replace <exclamation> with ! in actual usage.)

The command runs when the skill loads -- output replaces the pattern inline.

Extended Thinking

Include the keyword ultrathink in a skill body to enable extended thinking mode for that skill's execution.

Structural Patterns

See Structural Patterns for detailed guidance on:

1. Workflow Pattern - Sequential multi-step procedures
2. Task Pattern - Specific, bounded tasks
3. Reference Pattern - Knowledge repository
4. Capabilities Pattern - Tool integrations
5. Suite Pattern - Multi-skill lifecycle management (bootstrap, operate, diagnose, configure, upgrade, teardown)

User Conventions Integration

This skill follows common user conventions:

• Absolute paths: Always use full paths (terminal Cmd+click compatible)
• Unix-only: macOS, Linux (no Windows support)
• Python: uv run script.py with PEP 723 inline dependencies
• Planning: OpenAPI 3.1.1 specs when appropriate

Marketplace Scripts

See Scripts Reference for marketplace script usage.

Reference Documentation

For detailed information, see:

• Task Templates - Templates A-F and quality checklist
• Creation Tutorial - 6-step creation process walkthrough
• YAML Frontmatter - Field reference, invocation control, description guidelines
• Structural Patterns - 5 skill architecture patterns (including Suite Pattern)
• Workflow Patterns - Workflow skill implementation patterns
• Progressive Disclosure - Context management patterns
• Creation Workflow - Step-by-step process with examples
• Scripts Reference - Marketplace script usage
• Security Practices - Threats and defenses (CVE references)
• Phased Execution - Preflight/Execute/Verify/Reflect/Rectify patterns and variants
• Post-Execution Reflection - Compulsory self-rectification pattern for stepwise skills
• Theory: Self-Evolving Skills - Research synthesis from Gemini DR #70 (6 threads)
• Invocation Control - Skill invocation modes, permission rules, legacy commands migration
• Interactive Patterns - AskUserQuestion integration patterns
• Token Efficiency - Context optimization
• Advanced Topics - CLI vs API, composition, bugs
• Path Patterns - Safe/unsafe path references (known bugs documented)
• Bash Compatibility - Shell portability patterns
• Validation Reference - Quality checklist
• Writing Guide - Tone, reasoning-based instructions, description optimization
• Script Design - Designing scripts for agentic consumption
• Troubleshooting - Common issues and solutions
• SYNC-TRACKING - Marketplace version tracking
• Evolution Log - This skill's change history

Post-Execution Reflection (Compulsory)

This section is placed last so it is the final thing processed before the skill exits — maximizing recency effect.

Every skill MUST include a Post-Execution Reflection section — workflow skills, task skills, and capability skills alike. This is a structural requirement, not advisory. Without it, errors and drift repeat silently across sessions. Task-pattern skills are just as susceptible: scripts change interfaces, parameters get added, error messages drift from documentation.

After this skill completes, reflect before closing

0. Locate yourself. — Find this SKILL.md's canonical path (Glob for this skill's name) before editing. All corrections target THIS file and its sibling references/ — never other documentation.
1. What failed? — Fix the instruction that caused it. If it could recur, add it as an anti-pattern.
2. What worked better than expected? — Promote it to recommended practice. Document why.
3. What drifted? — Any script, reference, or external dependency that no longer matches reality gets fixed now.
4. Pass the admission gates. — Apply the Self-Evolution Protocol (top of this file). VALUE + REDUNDANCY + FRESHNESS must all pass before writing any change.
5. Log it. — Every change gets an evolution-log entry with trigger, fix, and evidence.

Do NOT defer. The next invocation inherits whatever you leave behind.

Template: Workflow / Stepwise Skills

For skills with multiple phases, evolution-log, and references/:

## Post-Execution Reflection

After this skill completes, reflect before closing the task:

0. **Locate yourself.** — Find this SKILL.md's canonical path before editing.
1. **What failed?** — Fix the instruction that caused it.
2. **What worked better than expected?** — Promote to recommended practice.
3. **What drifted?** — Fix any script, reference, or dependency that no longer matches reality.
4. **Log it.** — Evolution-log entry with trigger, fix, and evidence.

Do NOT defer. The next invocation inherits whatever you leave behind.

Template: Task-Pattern Skills

For single-action skills wrapping a CLI command or script:

## Post-Execution Reflection

After this skill completes, check before closing:

1. **Did the command succeed?** — If not, fix the instruction or error table that caused the failure.
2. **Did parameters or output change?** — If the script's interface drifted, update Usage examples and Parameters table to match.
3. **Was a workaround needed?** — If you had to improvise (different flags, extra steps), update this SKILL.md so the next invocation doesn't need the same workaround.

Only update if the issue is real and reproducible — not speculative.

See Post-Execution Reflection Reference for the full pattern, validation requirements, and examples.