Design Advanced Claude Code Skills with Progressive Disclosure and Tests

Quick answer

A strong skill has a precise discovery description, a focused SKILL.md, explicit prerequisites and safety boundaries, deterministic scripts for repeatable mechanics, references loaded only when needed, reusable assets, and behavioral tests for triggering and execution. Keep the main instructions concise and route to detail progressively. Anthropic's skills documentation defines supported frontmatter, locations, invocation, and tool controls.

Design discovery first

Claude sees skill metadata before loading the full body, so the description must explain both capability and trigger. Weak: “Helps with releases.” Strong:

---
name: release-readiness
description: Audit a proposed software release for versioning, changelog, migrations,
  compatibility, tests, and rollback. Use when preparing, reviewing, or diagnosing a release.
---

Avoid a description so broad that the skill triggers on ordinary coding, or so narrow that users must remember an exact phrase.

Use progressive disclosure

release-readiness/
├── SKILL.md
├── scripts/
│   └── collect-release-state.sh
├── references/
│   ├── semver.md
│   └── rollback-checklist.md
└── assets/
    └── report-template.md

SKILL.md should contain the workflow, decision points, and routing rules. References hold deep domain detail. Scripts gather or transform data deterministically. Assets are copied or filled rather than loaded as general instruction. This keeps unrelated context out of every invocation.

Write an executable workflow

Specify:

1. Inputs and prerequisites.
2. Read-only discovery.
3. Decisions requiring user authority.
4. Exact action sequence.
5. Verification and failure handling.
6. Output contract.

Do not bury destructive or external actions inside prose. State that publishing, deploying, messaging, deleting, or changing remote state requires explicit authority.

Use tool restrictions where appropriate, but remember they are part of Claude Code configuration, not a replacement for sandboxing and infrastructure security. A read-only audit skill should not quietly inherit mutation tools.

Prefer scripts for mechanics

If a task requires the same parsing, validation, or scaffolding every time, provide a checked-in script instead of asking the model to reconstruct commands. Scripts should support --help, validate inputs, avoid secrets in output, return meaningful exit codes, and be safe to rerun. Use project-relative paths and quote user-controlled inputs.

Test triggering and outcomes

Create at least four evaluation classes:

• positive prompts that should trigger automatically;
• adjacent prompts that should not trigger;
• missing-prerequisite cases;
• unsafe-action cases that must pause.

Then assert output properties: required sections, commands actually executed, forbidden files untouched, and failure states reported honestly. A skill that reads well but does not trigger or produces unverifiable outcomes is not complete.

Lab: skill quality review

Take an existing team runbook and convert it into a skill. Limit the main file to workflow and routing, move deep tables to references, automate one deterministic collection step, and run ten trigger/non-trigger prompts. Record false positives, false negatives, and unsafe behaviors, then revise the description and steps.

FAQ

Is a skill the same as a slash command?

Skills can be model-invoked or explicitly invoked depending on configuration; commands are a user-facing invocation pattern. Consult current docs for compatibility details.

How large should SKILL.md be?

As small as possible while remaining unambiguous. Move optional detail to references rather than chasing an arbitrary line count.

Can a skill bundle executable code?

Yes, but executable content expands the trust boundary. Review, test, pin dependencies, and constrain permissions.