AI skill

Yulin has an AI skill that teaches a coding agent how to test AWS code with the simulator well. It is yulin-aws-simulation, a SKILL.md written to the Agent Skills specification and installable into Claude Code, Codex CLI, Cursor, VS Code and anything else that reads one. It covers the part that lives outside the API.

These docs and the skill answer different questions. A page here says what a simulated service does and what its commands take. The skill says what to do with that. An AI agent reaching for Yulin without it tends to build a harness around the simulator, leave hand-rolled stubs in place beside it, or write an instanceof check against an SDK exception class that passes in production and fails against the simulation.

Install

npx @kensio/skills add yulin-aws-simulation

That copies the skill into .agents/skills/, the directory Codex CLI, Cursor, VS Code and Gemini CLI read. --agent claude puts it in .claude/skills/, and --user installs it under your home directory for every project.

As a Claude Code plugin:

claude plugin marketplace add KensioSoftware/kensio.ai
claude plugin install yulin-aws-simulation@kensio

It is also on npm as @kensio/yulin-aws-simulation, and every kensio.ai release carries it as a zip for a machine with no registry reach.

What it covers

• Using SimAws and SimSdk directly, and spotting the helper class or setupSimulatedAws() wrapper that starts to grow around them.
• One synthesized CDK template behind the tests, the dev server and production, deployed with deployTemplateFile or deployCdkOut.
• When to register a resource at a chosen id, and when to deploy the stack that creates it.
• Interception over hand-rolled stubs, down to the requests a fake accepts and the simulation refuses.
• Freezing the clock, then advancing it on purpose.
• Assertions that read the simulation back.
• Why service errors match by name and not by instanceof.
• Deploying an expensive stack once per test file.
• Running a handler as a real simulated Lambda, under its execution role, its declared environment and its own log group.
• Refusals as a feature, and gaps raised upstream.

Reading the API alongside it

The skill sends the AI agent to these docs for anything API-shaped, and llms.txt is the index it uses. Every page here is available as plain markdown by appending llms.txt to its URL, one file per guide and one per simulated service. That index works with or without the skill installed.

The skill lives at kensio.ai/skills/yulin-aws-simulation, versioned separately from Yulin and licensed Apache-2.0.