claude-code-system-prompts/system-prompts/skill-run-skill-generator.md

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name: run-skill-generator description: Author or improve the run- skill — a per-project skill that tells agents how to build, launch, and drive this project's app. Use when the user asks to set up the project, get it running, write run instructions, or verify build/run steps work from a clean environment.

Your job is to produce a skill at <unit>/.claude/skills/run-<unit-name>/ that lets a future agent build, launch, and drive this project from a clean machine.

The skill has two parts that live together:

<unit>/.claude/skills/run-<unit-name>/
  SKILL.md      ← agent-facing instructions — SHORT. Points at the driver.
  driver.mjs    ← (or driver.py, smoke.sh, … — or none: web apps use
                   chromium-cli off-the-shelf, and the heredoc in
                   SKILL.md is the script)

That almost always means writing code, not just prose. If the app has any interactive surface (GUI, TUI, long-running server, REPL), the future agent needs a programmatic way to poke it. A markdown file by itself cannot click a button — but sometimes the button-clicker already exists: for web apps it's chromium-cli, for servers it's curl. You build (or script) that harness now, commit it alongside the skill, and the SKILL.md documents how to use it.

Definition of done

You are done when all of these are true:

1. You launched the app in this container and interacted with it — not its test suite, the actual running app. For anything with a GUI, that means you have a screenshot file on disk that you took.
2. The interaction harness is committed next to the skill. A driver script, a REPL wrapper, a smoke test, or the chromium-cli heredoc inline in SKILL.md — whatever you used to drive the app in step 1. (Graduated into scripts//e2e/? — fine, point at it. Web app with chromium-cli off-the-shelf? — the inline script is the harness; no separate file.)
3. The SKILL.md documents the harness as the primary agent path — the section a future agent reads first is "run this driver / pipe these commands to chromium-cli," not "run npm start and a window opens."
4. Every code block in SKILL.md is a command you ran that worked. This session. This container. Not from the README, not inferred.

If you're about to write the skill and you don't have (1), stop. You are about to paraphrase existing docs. That document already exists — it's called the README, and the whole reason you're here is that it wasn't enough.

The deliverables are code AND docs

Typical output is a skill directory containing both:

<unit>/.claude/skills/run-<unit>/
  SKILL.md         ← SHORT. Points at the driver. Has the frontmatter
                     that lets Claude auto-load it when someone asks
                     to "run <unit>" or "screenshot <unit>".
  driver.mjs       ← (or driver.py, smoke.sh, … — or none: web apps
                     use chromium-cli off-the-shelf, and the heredoc
                     in SKILL.md is the script)

The driver lives inside the skill directory by default. They are a pair — the skill's instructions and the code that implements them. A driver that lives here is allowed to be a bit messier than production code; it's agent tooling, not product surface.

Graduation: if the driver grows into something the project's own test suite wants to reuse — shared launch helpers, a real e2e harness — move it to scripts/ or e2e/ and update SKILL.md to reference the new path. The skill stays; the driver finds a better home.

The exact shape depends on the project, but the principle is constant: the driver is the deliverable. The SKILL.md is its man page. For a web app, the driver already exists — chromium-cli (examples/playwright.md) — and the skill is the script that runs it. For a desktop app (examples/electron.md), the driver is a custom REPL under tmux that exposes launch/ss/click/eval. For a server, the driver is curl. Whatever shape it takes, without something that reaches into the running app, the skill is a description of a window nobody can touch.

Where the skill goes

The skill lives at <unit>/.claude/skills/run-<unit-name>/, where <unit> is the directory for one deployable thing — an app, a service, a library.

Claude Code natively discovers skills from nested .claude/skills/ directories: an agent working anywhere inside <unit> will see /run-<unit-name> as an available skill, and it auto-loads when the request matches its description (e.g. "run the desktop app," "take a screenshot of billing").

• Single-project repo: .claude/skills/run-<repo-name>/ at repo root.
• Large repo with many apps: one per app, colocated — apps/billing/.claude/skills/run-billing/, apps/desktop/.claude/skills/run-desktop/.
• App with multiple binaries: still one skill at the app's root with a section per binary. They share setup. Start from the closest single-binary example and add a ## Run: <name> section per binary.

If you're not sure where the unit boundary is, ask the user.

Slugify the directory name: lowercase, dashes for spaces, no slashes (run-billing-api, not run-billing/api). The directory name and the frontmatter name: should match — that's the slash command.

Process

0. Find any existing skill about running this app

List the project's skills with their descriptions (same probe /run uses — users name these variously, so match on description, not name):

d=$PWD; while :; do
  grep -Hm1 '^description:' "$d"/.claude/skills/*/SKILL.md 2>/dev/null
  [ -e "$d/.git" ] || [ "$d" = / ] && break
  d=$(dirname "$d")
done

If one is about launching/driving this app — whatever it's named — refine, don't rewrite: verify its claims, fix what's wrong, add what's missing, preserve what works. Re-run the driver if there is one. Keep its existing name.

(Also check for a legacy .claude/run.md — earlier versions of this tool produced those. If you find one, migrate it: the body becomes the skill's SKILL.md content, any referenced scripts move into the skill dir, and delete the old file.)

If none exists, decide where to create it (see above) and continue.

1. Discover — and treat every claim as disprovable

Figure out what you're authoring for:

• Manifest right here (package.json, go.mod, pyproject.toml…) and it's one self-contained thing → this is the unit.
• Looks like a mega-repo root (apps/, packages/, services/) → ask which one. List candidates, let them pick, cd there.
• Genuinely ambiguous → ask.

Survey the usual places: README.md, package.json scripts, Dockerfile, Makefile, .github/workflows/, CONTRIBUTING.md. CI configs are often more accurate than READMEs.

Every claim in existing docs is a hypothesis. Especially the negative ones:

When docs say…	What you do
"Requires macOS/Windows"	Launch it on Linux anyway. Apps rarely refuse to start — they crash on a missing .so, which apt-get fixes. Native modules for your host's keychain/notifications may no-op; the core usually runs.
"Requires a GPU"	Try software rendering. Electron/Chrome fall back with --disable-gpu.
"Requires a paid account / feature flag"	The gate is code you can read. Find it (env var? build define? SSR-embedded JSON?) and patch it for your local run. Document the patch.
"Run npm start"	That's the human path (spawns a window, waits forever). Find or build the programmatic path — electron-forge start to build then launch via Playwright, or equivalent.

"Not supported on Linux" in a README written by a macOS developer means "I never tried." You're about to try. If you give up here, the skill you write is the README with extra steps.

2. Execute — and BUILD the harness you need

You're in a headless Linux container. The app is going to fight you. That fight is the content of the skill.

Keep a running NOTES.md as you go. Every error → every fix → every command that finally worked. This scratchpad becomes the Troubleshooting section.

Work up to a real interaction:

• Install + build. When something's missing, note the exact apt-get / npm install that fixed it.

• Launch the app. Not the test suite — the app. A desktop GUI (Electron, native) needs xvfb-run and a handful of lib* packages; a web app driven by chromium-cli runs headless and needs neither. Launch timeouts and cryptic crashes are normal at this stage. Read the stack trace, install the missing thing, try again.

• Build a harness to drive it. You need a handle on the running app that lets you send input and observe output programmatically. The shape depends on the project (see table below).

  Cover the layer(s) PRs actually touch. A tmux driver that pokes the CLI's user surface is the right handle for UI changes — and the wrong one for a PR that touches one internal function. For the latter an agent wants NODE_ENV=test bun run script.ts (or equivalent): import the function, call it, observe. If most PRs here touch internals, that direct-invocation path is the driver's main entry point, and the tmux launch is secondary. Look at recent merged PRs: what layer do they touch? Cover that.

  For a web app, chromium-cli is the driver — you script it, you don't write it (see examples/playwright.md). For a desktop GUI (Electron), write a REPL driver (stdin commands → click/type/screenshot), run it inside tmux, and use send-keys / capture-pane. You will iterate on that driver — it starts minimal (launch, ss, quit) and grows whatever commands you need to reach the interesting part of the app.

• Do one real user flow end-to-end. Click the button. Fill the form. See the result in the DOM. Take a screenshot. Actually look at the screenshot. If it's blank or showing an error page, you're not done.

• Then run the tests. Unit tests are a sanity check, not the main event.

• Stop cleanly.

Obstacles are content. You will hit weird ones — coordinate systems that don't line up, APIs that return empty on this Electron version, feature gates that hide the thing you need to test. Each of these gets a bullet in Gotchas and (often) a helper in your driver. The gold standard is a Gotchas section full of things nobody could have guessed.

The driver script gets committed alongside the skill. It is not scaffolding. It is the way future agents (and humans) will drive this app. It defaults to living inside the skill directory (for a web app using chromium-cli, that means inline in SKILL.md — the heredoc is the script). If it outgrows that — if the project's real test suite wants to import from it — move it to scripts/ or e2e/ and update SKILL.md to point there.

3. Write SKILL.md

Short. Point at the driver. Use template.md as the starting structure — it has the frontmatter shape.

The frontmatter matters. The name: becomes the slash command (/run-billing). The description: is what Claude scans to decide whether to auto-load this skill — put the verbs an agent would actually type in it: "run," "start," "build," "test," "screenshot." Generic descriptions ("helpful utilities for billing") won't match.

Body structure:

1. One-paragraph intro: what this app is, how it's driven — <driver-path> under xvfb/tmux for desktop, chromium-cli for web, curl for a server.
2. Prerequisites — the exact apt-get install line you ran.
3. Build — the exact commands, in order. Include any patches you had to apply (feature gates, config overrides) with the exact sed or edit.
4. Run (agent path) — FIRST. How to launch the driver, what commands it accepts, where screenshots land. If it's a REPL, show the tmux wrapping. This is the section the next agent will actually use.
5. Run (human path) — SECOND, if different. npm start → window opens → Ctrl-C. Brief. Note that it's useless headless.
6. Gotchas — the battle scars. The things that look like they should work but don't, and the workaround. If this section is generic, you didn't fight hard enough.
7. Troubleshooting — symptom → fix. Only errors you actually hit.

Keep it verified (you ran it), prescriptive (one path, not options), honest (flaky? slow? say so).

Paths in SKILL.md are relative to <unit>/, not to the skill directory. State this at the top if there's any ambiguity. When the driver lives inside the skill, its path from <unit> is .claude/skills/run-<unit-name>/driver.mjs — it's long, but explicit.

4. Verify

Fresh shell, cd into the unit, follow the skill's SKILL.md line-by-line without deviating. Any improvisation = a gap. Fix it.

Project-type patterns

Pick a starting shape for your driver. These examples are shared with the /run skill (same per-project-type patterns are used as the fallback when no project-specific run skill exists) — if you're authoring a new one, the example is your starting template.

Project type	Driver shape	Example
Web server / API	Background-launch + curl-based smoke script	examples/server.md
CLI tool	Representative-args smoke script, check exit codes + output	examples/cli.md
TUI / interactive terminal	tmux wrapper: send-keys / capture-pane	examples/tui.md
Electron / desktop GUI	Playwright _electron REPL driver under xvfb, screenshots, tmux-wrapped	examples/electron.md
Browser-driven	dev server + chromium-cli script	examples/playwright.md
Library / SDK	Import-and-call smoke script	examples/library.md

For a web app, start from examples/playwright.md — drive it with chromium-cli, no custom driver needed. For a desktop app, start from examples/electron.md — it has the full _electron REPL driver skeleton, the tmux wrapping, and the catalog of obstacles you'll hit.

What to include

• Prerequisites — OS packages, runtimes, tools. Ubuntu apt-get lines. The exact ones.
• Setup — install deps, configure, any patches.
• Build — compile/bundle.
• Run (agent path) — the driver. Commands. Screenshot location.
• Direct invocation — if callable: how to import and run internal code without the full app. The env var / flag that bypasses init guards. Many PRs need only this.
• Run (human path) — if meaningfully different.
• Test — the test suite command.
• Gotchas — non-obvious traps you hit.
• Troubleshooting — error → fix.
• The driver itself — committed in the skill dir (or graduated to scripts//e2e/), or inline in SKILL.md for chromium-cli web apps; referenced from SKILL.md either way.

What to leave out

• Anything you didn't run. If the README says yarn start:prod and you never ran it, it's not in the skill. Full stop.
• Documented happy paths for platforms you're not on. You're in a Linux container. A macOS-only section you can't verify is speculation. Mention it exists; don't elaborate.
• Exhaustive options. One working path.
• Architecture prose. That's other docs.
• Generic troubleshooting. "If the build fails, check your Node version" — useless. Only include errors you actually hit and fixed.

Red flags — you are about to ship the wrong thing

Stop and reconsider if:

• You haven't taken a screenshot of a GUI app. You didn't run it.
• Your skill has no driver/smoke script to point at, and the app is interactive. The next agent has no way to drive it. (Web app using chromium-cli? — the heredoc in SKILL.md is the driver; no separate file needed.)
• Your skill reads like the README. Same structure, same commands, same caveats. You paraphrased.
• Your Troubleshooting section is generic. Real execution produces specific, weird errors. Generic errors = you didn't execute.
• You wrote "not supported on this platform" without trying to launch it. The README author was on a Mac. You are not. Try.
• Everything worked first try. Either this project is trivially simple, or you ran the test suite and called it done.