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nix-config: Skill for Claude Code

.claude/skills/building-installers/SKILL.md

building-installers is a skill for Claude Code from joshsymonds/nix-config. It costs 40 tokens per session (1,956 once invoked), scanned D, original, MIT.

A workflow for building bootable NixOS installer ISO files for configured computers, including automatic disk setup and installation.

In plain words
What is it for?
Use it to add hosts to the installer system, build and copy installer ISOs to USB drives, troubleshoot installation, and understand disk selection, partitioning, formatting, and SSH access.
Why use it?
It documents the commands, host configuration, USB-writing process, safety confirmation, encryption prompts, and installation behavior needed to create a usable installer.

Skill for Claude Code

Written for Claude Code: installed under .claude/.

This is joshsymonds/nix-config's own configuration. It tells Claude Code how to work on nix-config itself, so it is not a mod to install elsewhere. Copy it as a starting point and replace the rules that are about this project. Everything nix-config configures →

Needs its repository: it runs a file that does not travel with it, so clone the repository first. The line is modules = [ ./hosts/<name>/installer.nix ];.

not rated 835repo +1 changed 2d ago D scan 40 tokens original MIT
View source ↗ joshsymonds/nix-config
Reuse

Borrowing it

Nothing to install: this file belongs to joshsymonds/nix-config. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.

Copy the file
curl -O https://raw.githubusercontent.com/joshsymonds/nix-config/main/.claude/skills/building-installers/SKILL.md
Clone the repo
git clone --depth 1 https://github.com/joshsymonds/nix-config

Made for: Claude Code.

Wrote this? Show the measurements

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README.md
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Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

agentmods 80×15 button for building-installers

Your own site · 80×15
<a href="https://agentmods.dev/skills/joshsymonds/nix-config/building-installers"><img src="https://agentmods.dev/badge/skills/joshsymonds/nix-config/building-installers.svg" alt="Reviewed on agentmods" width="80" height="20"></a>
Per session 40 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 1,956 The whole file, excluding the scripts and references it only reads on demand.
Security scan D 2 findings. A grade says what 26 rules found in the file — not that it is safe.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5.1 $0.00040 $0.01956
Opus 5 $0.00020 $0.00978
Sonnet 5 $0.00008 $0.00391
Haiku 4.5 $0.00004 $0.00196

Measured 2d ago against content hash 7204895574b3, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-10, from the pricing page.

Security

Grade D, and why

building-installers scanned grade D with 2 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Asks for rootmediumPrivilege escalation

A mod that escalates privileges can change anything on the machine, not only the project.

sudo dd if=result/iso/*.iso of=/dev/sdX bs=4M status=progress oflag=direct

Reaches for credential fileshighPrivilege escalation

SSH keys, cloud credentials, git-credentials, .npmrc, /etc/shadow: reading these is how a config file becomes a credential leak.

nix-shell -p ssh-to-age --run 'ssh-to-age --private-key < ~/.ssh/id_ed25519 > /tmp/rekey.agekey'
.claude/skills/building-installers/SKILL.md · 143 lines

How it starts

The opening of the file, as written. The whole thing — 143 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Building NixOS Installer ISOs

Scope. This skill covers the legacy autoInstaller flow (label-based ext4, no LUKS, no impermanence, no lanzaboote) — today only ultraviolet uses it. Hosts on the btrfs-impermanence + lanzaboote stack install a different way: gnomon via the kit-partition ISO (modules/installer-iso/, .#installerIso), vermissian from its own running system (scripts/flash-vermissian.sh), stygianlibrary from gnomon (scripts/flash-stygianlibrary.sh). The option table and the agekey step below do not apply to those hosts.

Overview

The modules/installer.nix module creates bootable USB ISOs that auto-partition, format, and install a NixOS host. Each host has a thin wrapper in hosts/<name>/installer.nix setting autoInstaller.* options.

Building an ISO

nix build .#<host>InstallerIso --print-build-logs

Always use --print-build-logs — the build output contains the baked-in SSH public key:

INSTALLER SSH PUBLIC KEY for <host>:
ssh-ed25519 AAAA... <host>-installer

If the host's flake has private repo inputs (e.g. shimmer uses git+ssh://), add this public key as a deploy key on those repos before booting the USB.

Flashing to USB

sudo dd if=result/iso/*.iso of=/dev/sdX bs=4M status=progress oflag=direct
sudo eject /dev/sdX

What Happens on Boot

  1. NixOS minimal ISO boots, shows ASCII banner on getty
  2. Auto-install service starts after network-online.target
  3. Detects target disk (EFI boot entries > ESP heuristic > partition count > largest disk)
  4. Confirmation prompt via systemd-ask-password: shows disk info, requires typing yes
  5. Partitions and formats disk with configured labels
  6. If LUKS: prompts for passphrase via systemd-ask-password
  7. Mounts root and boot, creates extra dirs
  8. Clones nix-config repo
  9. Runs nixos-install (prebuilt closure or --flake depending on config)
  10. Powers off (or reboots if powerOff = false)

Creating an Installer for a New Host

Read the full file on GitHub · 143 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 2d ago Changed · +9 lines 7204895574b3
  2. 11d ago First seen · 134 lines · 40 tokens per session scan D 6a24b7dab81f

Subscribe to this mod's changes

building-installers is a skill published in the GitHub repository joshsymonds/nix-config (835 stars, last pushed yesterday), licensed MIT. It adds 40 tokens to every session and 1,956 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it D with 2 findings (asks for root, reaches for credential files). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

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