Hook Development for Claude Code Plugins
anthropics/claude-plugins-official
Explains how to write Claude Code plugin hooks, both prompt-based checks and bash commands, for events such as PreToolUse, Stop and SessionStart.
Initialize a COSA workdir, build or fetch Fedora CoreOS images, and launch local QEMU VMs using coreos-assembler
Apache-2.0Auto-check passed
Install Cosa Basics
$ npx skills add coreos/coreos-assembler --skill cosa-basics -a claude-codeProject install by default; add -g for ~/.claude/skills/.
$ gh skill install coreos/coreos-assembler cosa-basics --agent claude-codeProject scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).
$ git clone --depth 1 https://github.com/coreos/coreos-assembler.git skills-src && mkdir -p .claude/skills && cp -r skills-src/.agents/skills/cosa-basics .claude/skills/cosa-basics && rm -rf skills-srcUse ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.
Claude Code skills documentation · loads skills from .claude/skills/
Install the "cosa-basics" agent skill from https://github.com/coreos/coreos-assembler/tree/main/.agents/skills/cosa-basics into .claude/skills/cosa-basics/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "cosa-basics", then confirm the skill loads.Claude Code copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$skill-installer install https://github.com/coreos/coreos-assembler/tree/main/.agents/skills/cosa-basicsType this inside Codex. $skill-installer <name> installs a curated skill from openai/skills. The installer writes to $CODEX_HOME/skills (default ~/.codex/skills). Restart Codex if the skill does not show up.
$ npx skills add coreos/coreos-assembler --skill cosa-basics -a codexProject install goes to .agents/skills/; add -g for ~/.codex/skills/.
$ gh skill install coreos/coreos-assembler cosa-basics --agent codexProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/coreos/coreos-assembler.git skills-src && mkdir -p .agents/skills && cp -r skills-src/.agents/skills/cosa-basics .agents/skills/cosa-basics && rm -rf skills-srcUse ~/.agents/skills/ instead of .agents/skills for a personal install.
Codex skills documentation · loads skills from .agents/skills/
Install the "cosa-basics" agent skill from https://github.com/coreos/coreos-assembler/tree/main/.agents/skills/cosa-basics into .agents/skills/cosa-basics/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "cosa-basics", then confirm the skill loads.Codex copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ npx skills add coreos/coreos-assembler --skill cosa-basics -a cursorProject install goes to .agents/skills/; add -g for ~/.cursor/skills/.
$ gh skill install coreos/coreos-assembler cosa-basics --agent cursorProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/coreos/coreos-assembler.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/.agents/skills/cosa-basics .cursor/skills/cosa-basics && rm -rf skills-srcUse ~/.cursor/skills/ instead of .cursor/skills for a personal install.
Cursor skills documentation · loads skills from .cursor/skills/, .agents/skills/, .claude/skills/, .codex/skills/
Install the "cosa-basics" agent skill from https://github.com/coreos/coreos-assembler/tree/main/.agents/skills/cosa-basics into .cursor/skills/cosa-basics/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "cosa-basics", then confirm the skill loads.Cursor copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ gemini skills install https://github.com/coreos/coreos-assembler.git --path .agents/skills/cosa-basics--scope user (default) or --scope workspace; --path is the subfolder of the repo that holds the skill; --consent skips the security confirmation prompt.
$ npx skills add coreos/coreos-assembler --skill cosa-basics -a gemini-cliProject install goes to .agents/skills/; add -g for ~/.gemini/skills/.
$ gh skill install coreos/coreos-assembler cosa-basics --agent gemini-cliProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/coreos/coreos-assembler.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/.agents/skills/cosa-basics .gemini/skills/cosa-basics && rm -rf skills-srcUse ~/.gemini/skills/ instead of .gemini/skills for a personal install, then run /skills reload.
Gemini CLI skills documentation · loads skills from .gemini/skills/, .agents/skills/
Install the "cosa-basics" agent skill from https://github.com/coreos/coreos-assembler/tree/main/.agents/skills/cosa-basics into .gemini/skills/cosa-basics/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "cosa-basics", then confirm the skill loads.Gemini CLI copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ gh skill install coreos/coreos-assembler cosa-basicsInstalls for Copilot at project scope by default; add --scope user for a personal install. Preview a skill first with gh skill preview. Needs GitHub CLI 2.90.0 or later (public preview).
$ npx skills add coreos/coreos-assembler --skill cosa-basics -a github-copilotProject install goes to .agents/skills/; add -g for ~/.copilot/skills/.
$ git clone --depth 1 https://github.com/coreos/coreos-assembler.git skills-src && mkdir -p .github/skills && cp -r skills-src/.agents/skills/cosa-basics .github/skills/cosa-basics && rm -rf skills-srcUse ~/.copilot/skills/ instead of .github/skills for a personal install. Commit .github/skills so cloud agent and code review can use it.
GitHub Copilot skills documentation · loads skills from .github/skills/, .claude/skills/, .agents/skills/
Install the "cosa-basics" agent skill from https://github.com/coreos/coreos-assembler/tree/main/.agents/skills/cosa-basics into .github/skills/cosa-basics/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "cosa-basics", then confirm the skill loads.GitHub Copilot copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ npx skills add coreos/coreos-assembler --skill cosa-basics -a opencodeOpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.
$ gh skill install coreos/coreos-assembler cosa-basics --agent opencodeProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/coreos/coreos-assembler.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/.agents/skills/cosa-basics .opencode/skills/cosa-basics && rm -rf skills-srcUse ~/.config/opencode/skills/ instead of .opencode/skills for a personal install.
OpenCode skills documentation · loads skills from .opencode/skills/, .claude/skills/, .agents/skills/
Install the "cosa-basics" agent skill from https://github.com/coreos/coreos-assembler/tree/main/.agents/skills/cosa-basics into .opencode/skills/cosa-basics/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "cosa-basics", then confirm the skill loads.OpenCode copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
Facts
- Skill name
cosa-basics- GitHub stars
- 395
- Token cost
- ~1.8k tokens
- SKILL.md length
- 945 words
- Files
- 1
- Skills in repo
- 4
- Repo updated
- First seen
- Licence
- Apache-2.0
At a glance
Initialize a COSA workdir, build or fetch Fedora CoreOS images, and launch local QEMU VMs using coreos-assembler
- Works in 3 steps: Set up a COSA workdir → Obtain a build → Launch the VM
- SKILL.md covers Out of scope, Key documentation references, Running COSA and Workflow, plus 1 more section
- Calls podman; reaches github.com
What it does
Cosa Basics is an agent skill from coreos/coreos-assembler. Initialize a COSA workdir, build or fetch Fedora CoreOS images, and launch local QEMU VMs using coreos-assembler
Its SKILL.md is about 1.8k tokens, which your agent loads only when the skill is triggered. It is a single SKILL.md file with no bundled scripts.
It works with Bash. The repository describes itself as: Tooling container to assemble CoreOS-like systems. The licence is Apache-2.0.
Example prompts
- “/cosa-basics”
Workflow steps
3 steps, taken from the step headings in SKILL.md.
What it can do on your machine
Read from SKILL.md and the folder at commit 4ab8b08. It shows what the files ask for, not the result of running them.
Tool permissions
Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.
From allowed-tools in the SKILL.md frontmatter.
Runs code
Shell commands in SKILL.md call:
podman
From the folder's file list and the shell code blocks in SKILL.md.
Network
Hosts in commands or code, which the agent is likely to contact:
github.com
From URLs in SKILL.md, links to its own repository left out.
Credentials
Names no API keys, tokens, secrets or passwords.
From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.
Context cost
Cosa Basics loads about 1.8k tokens when it runs. Until then it costs about 31 tokens; SKILL.md has 945 words of instructions outside code blocks.
Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.
Safety
Auto-check passed
The automated check found no risky patterns in SKILL.md.
Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.
SKILL.md
The full file from coreos/coreos-assembler at commit 4ab8b08, republished under its Apache-2.0 licence (© coreos). 945 words, ~1,757 tokens.
Download SKILL.mdSave it as
.claude/skills/cosa-basics/SKILL.md (or your agent's skills folder).- name
- cosa-basics
- description
- Initialize a COSA workdir, build or fetch Fedora CoreOS images, and launch local QEMU VMs using coreos-assembler
COSA Basics
This skill guides you through the core coreos-assembler (COSA) workflow: initializing a build directory, obtaining a CoreOS build (either from source or by fetching a published build), and launching a local QEMU VM.
Out of scope
Cloud platform image building and uploading (AWS, GCP, Azure, etc.) are not covered here. Load the cosa-platforms skill for those workflows.
Key documentation references
Do NOT duplicate these docs. Read them at runtime for command details:
| File | What it covers |
|---|---|
docs/building-fcos.md | Full build walkthrough, prerequisites, the cosa() bash alias |
docs/cosa.md | Command reference overview (all cosa subcommands) |
docs/cosa/run.md | Detailed cosa run and QEMU VM options |
docs/working.md | Overrides, customization, advanced usage |
docs/devel.md | Running cosa locally outside a container |
src/cmd-init | cosa init source -- all supported flags |
src/cmd-buildfetch | cosa buildfetch source -- all supported flags |
Running COSA
Container mode (default)
COSA is designed to run inside a container. The container image is:
quay.io/coreos-assembler/coreos-assemblerBefore running any cosa commands, read docs/building-fcos.md and extract the
cosa() bash alias function defined there. This alias wraps podman run with
the necessary privileges, device mounts (/dev/kvm, /dev/fuse), user
namespace mapping, and volume mounts. The current working directory is mounted
as /srv/ inside the container.
Set up the alias in the user's shell before running any cosa commands.
Local mode (for cosa developers)
Developers who have cosa installed locally can run commands directly without
the container wrapper. Read docs/devel.md for details on this workflow. The
commands themselves are the same; only the execution environment differs.
Workflow
Step 1: Set up a COSA workdir
- Ask the user for a working directory path. Default to
./fcosif they don't specify one. - If the directory does not exist, create it.
cdinto the directory.- Set up the
cosa()bash alias (read it fromdocs/building-fcos.md). - Initialize the workdir:
cosa init https://github.com/coreos/fedora-coreos-configThe user can override the config repo URL. Additional useful flags (read
src/cmd-init for the full list):
--branch BRANCH-- use a specific branch of the config repo--variant VARIANT-- select a build variant--force-- allow init in a non-empty directory
After init, the directory structure will contain src/config/ (the cloned
config repo), plus builds/, cache/, tmp/, and overrides/ directories
that are populated during builds.
Step 2: Obtain a build
Choose one of the following paths based on what the user wants to do.
Option A: Build from source
This builds a fresh CoreOS image from the config repo and RPM sources:
cosa build
cosa osbuild qemucosa build creates the bootable OCI container (ostree commit). cosa osbuild qemu derives a QEMU disk image from that container. Both are required before
cosa run will work.
Note: cosa fetch is a no-op in the current build flow. Package fetching
happens automatically inside cosa build via buildah.
Show full SKILL.md (501 more words)Show less
Option B: Fetch an existing published build
This downloads a previously published build from the Fedora CoreOS build servers. Use this when you want to test or inspect an existing release without building from source.
Default flags: Include --artifact qemu --decompress when the user
needs a local QEMU disk image (e.g. for cosa run or QEMU-based kola
tests). The published artifacts are stored compressed on the server;
--decompress expands them so the resulting qcow2 can be used directly.
If the user only needs to run tests on a cloud platform where images already
exist (e.g. AMIs in AWS), --artifact and --decompress are not needed.
A plain cosa buildfetch -b <build-id> fetches only a few small metadata
files (including meta.json), which contain cloud image references that
kola picks up automatically. See the cosa-platforms skill for details.
Fetch the latest build for a stream:
cosa buildfetch --stream <stream> --artifact qemu --decompressAvailable streams: stable, testing, testing-devel, next, rawhide
If the user does not specify a stream, default to stable.
Fetch a specific build by ID:
cosa buildfetch -b <build-id> --artifact qemu --decompressWhen -b/--build is given, --stream is not required -- the stream is
auto-detected from the build ID's version number (the third dotted component
maps to a stream, e.g. .2. = testing, .3. = stable). Read
src/cmd-buildfetch for the full version-to-stream mapping. If --stream
is also provided it must match or the command will error.
If the user wants a non-qemu artifact: replace --artifact qemu with the
appropriate artifact name (e.g., metal, aws, gcp). But note that cloud
platform workflows are better served by the cosa-platforms skill.
Step 3: Launch the VM
cosa runThis launches a QEMU VM using the latest build image with -snapshot (changes
are discarded on exit). By default it establishes an SSH connection with
auto-login as the core user.
Common options (read docs/cosa/run.md for the full list):
| Flag | Purpose |
|---|---|
-c / --devshell-console | Use serial console instead of SSH (see full boot, GRUB menu) |
--qemu-image PATH | Use a specific qcow2 image instead of the latest build |
-B / --butane PATH | Pass a Butane config for the VM |
-i / --ignition PATH | Pass an Ignition config for the VM |
--add-disk SIZE[:OPTS] | Attach additional disks |
--kargs 'ARGS' | Append kernel arguments |
-m SIZE | Set VM memory in MB |
Exiting the VM:
- SSH mode:
exitorCtrl-D - Console mode:
Ctrl-a x
Important notes
- Each
cosacommand (via the alias) creates a transient container. Usecosa shellfor a persistent interactive session inside the container. /dev/kvmis required for both building disk images and running VMs. The host must be bare metal or have nested virtualization enabled.- The build directory at
$PWDis mounted at/srv/inside the container. Build artifacts persist across container invocations. - The workdir's
builds/latestsymlink always points to the most recent build. - To run kola tests against a build, load the cosa-kola skill. It covers
test discovery,
cosa kola run, upgrade tests, and result inspection. - Fedora infrastructure services (like Bodhi or Koji) are behind Anubis to prevent abuse. When querying APIs or web pages for information set a User-Agent and Accept headers to increase the chance of making it through unchallenged.
© coreos, Apache-2.0. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file
Files
Just SKILL.md in .agents/skills/cosa-basics of coreos/coreos-assembler.
Open the folder on GitHubat commit 4ab8b08
Compare with similar skills
Cosa Basics next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.
| Skill | Stars | Used in | Tokens | Auto-check | Licence | Repo updated |
|---|---|---|---|---|---|---|
| Cosa Basics this skillcoreos/coreos-assembler | 395 | — | ~1.8k | Automated safety check: Pass | Apache-2.0 | |
| Hook Development for Claude Code Pluginsanthropics/claude-plugins-official | 38k | 11 repos | ~4.1k | Automated safety check: Notes | Apache-2.0 | |
| Plugin Settings Patternanthropics/claude-plugins-official | 38k | 7 repos | ~3k | Automated safety check: Pass | Apache-2.0 | |
| Mole Bug Patternstw93/Mole | 70k | — | ~2k | Automated safety check: Pass | GPL-3.0 | |
| Neat-Freak Knowledge CloseoutKKKKhazix/khazix-skills | 21k | — | ~1.9k | Automated safety check: Pass | MIT | |
| E2Ecallstack/react-native-pager-view | 3.4k | 1 repos | ~2.1k | Automated safety check: Pass | MIT |
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Cosa Dev
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Develop and test changes to coreos-assembler itself -- build mantle Go binaries (kola, ore, plume), iterate on Python/shell scripts, or do full container rebuilds
Cosa Kola
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Run kola tests against CoreOS builds using coreos-assembler, including native and external tests
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Related
Works with
Questions about Cosa Basics
What does Cosa Basics do?
Initialize a COSA workdir, build or fetch Fedora CoreOS images, and launch local QEMU VMs using coreos-assembler. Cosa Basics is an agent skill from coreos/coreos-assembler.
How do I install Cosa Basics in Claude Code?
Run `npx skills add coreos/coreos-assembler --skill cosa-basics -a claude-code`. Or copy the skill folder (.agents/skills/cosa-basics in coreos/coreos-assembler) into .claude/skills/cosa-basics in your project. Claude Code loads it when a task matches its description.
How do I install Cosa Basics in Codex?
Run `npx skills add coreos/coreos-assembler --skill cosa-basics -a codex`. Or copy the skill folder (.agents/skills/cosa-basics in coreos/coreos-assembler) into .agents/skills/cosa-basics in your project. Codex loads it when a task matches its description.
Can I use Cosa Basics in Cursor, Gemini CLI or GitHub Copilot?
Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add coreos/coreos-assembler --skill cosa-basics -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/cosa-basics, .gemini/skills/cosa-basics, .github/skills/cosa-basics and .opencode/skills/cosa-basics in your project.
What does Cosa Basics need to run?
Going by SKILL.md and its folder, Cosa Basics needs the command-line tools its instructions call (podman).
Does Cosa Basics access the network?
SKILL.md names 1 domain. In commands or code: github.com; the agent is likely to contact it when it follows the instructions. This is read from the text; nothing was executed.
Is Cosa Basics safe to install?
Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.
What licence does Cosa Basics use?
Cosa Basics is published under the Apache-2.0 licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.
How many tokens does Cosa Basics use?
About 1.8k tokens (SKILL.md is roughly 7k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full.
What are the alternatives to Cosa Basics?
Skills that share tags, products or a category with Cosa Basics: Hook Development for Claude Code Plugins (anthropics/claude-plugins-official, 38k stars), Plugin Settings Pattern (anthropics/claude-plugins-official, 38k stars), Mole Bug Patterns (tw93/Mole, 70k stars) and Neat-Freak Knowledge Closeout (KKKKhazix/khazix-skills, 21k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.
Who maintains Cosa Basics?
coreos (a GitHub organization) maintains it in coreos/coreos-assembler, which has 395 GitHub stars. The repository holds 4 skills in this directory. The repository was last updated on October 7, 2026.
Source: coreos/coreos-assembler on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.