A command for creating or changing a sysext, an extension image that adds files or software to an operating-system image. It guides the agent through package settings, file paths, update metadata, and immutable-filesystem rules.

Use it to build or update sysext images, define their shipped paths, configure updates, and adapt packages for an immutable Linux filesystem.

It turns a detailed set of operating-system image requirements into a focused implementation task, reducing missed files or incompatible paths.

How frostyard/snosi configures GitHub Copilot

Reuse

Borrowing it

Nothing to install: this file belongs to frostyard/snosi. 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/frostyard/snosi/main/.github/prompts/add-sysext.md

Clone the repo

git clone --depth 1 https://github.com/frostyard/snosi

Made for: GitHub Copilot.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/commands/frostyard/snosi/add-sysext/github.svg)](https://agentmods.dev/commands/frostyard/snosi/add-sysext)

Your own site

<a href="https://agentmods.dev/commands/frostyard/snosi/add-sysext"><img src="https://agentmods.dev/badge/commands/frostyard/snosi/add-sysext/github.svg" alt="Measured on agentmods" height="20"></a>

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.

Your own site · 80×15

<a href="https://agentmods.dev/commands/frostyard/snosi/add-sysext"><img src="https://agentmods.dev/badge/commands/frostyard/snosi/add-sysext.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 0 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.

When invoked 753 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 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.

Model	Per session	Once invoked
Fable 5.1	$0.00000	$0.00753
Opus 5.5	$0.00000	$0.00301
Sonnet 5.5	$0.00000	$0.00151
Haiku 4.5	$0.00000	$0.00075

Measured 6d ago against content hash 620570e64a21, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

add-sysext scanned grade A with 0 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 6d 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.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

.github/prompts/add-sysext.md · 51 lines

What it actually says

Prompt: add or change a sysext

Goal: <what the sysext must provide>

Read first: docs/design/sysexts.md, plus the "Sysext Constraints" section of CLAUDE.md.

Work to do:

Create or edit mkosi.images/<name>/ using an existing sysext as the template. Set KEYPACKAGE= in its mkosi.conf; bump SYSEXT_REVISION= when republishing a tree-only fix under an unchanged package version.
Ship matching <name>.transfer and <name>.feature files in mkosi.images/base/mkosi.extra/usr/lib/sysupdate.<name>.d/. Never add them to the shared usr/lib/sysupdate.d/ directory, which is reserved for the native A/B OS transfers.
List every path the sysext itself ships in mkosi.images/<name>/required-paths.txt. For Overlay=yes images the finalize $BUILDROOT is the delta, so paths already present in the base image must not be listed.
Respect the immutable-filesystem rules: /usr only. Relocate anything a package installs into /opt to /usr/lib/<package> with symlinks in /usr/bin. Configs needed in /etc are captured into /usr/share/factory/etc (only the exact paths the tmpfiles rules reference) and injected via tmpfiles.
For services, ship a usr/lib/systemd/system/multi-user.target.d/10-<name>.conf drop-in with Upholds=<name>.service in addition to the preset — a plain .wants symlink is dropped at boot because the sysext is not merged yet. Also ship mkosi.extra/usr/lib/extension-release.d/extension-release.<name> with the line EXTENSION_RELOAD_MANAGER=1 (model it on mkosi.images/incus/), list it in required-paths.txt, and wire shared/sysext/finalize/sysext-reload-manager-guard.sh in FinalizeScripts=: a sysext with any system unit entry fails the build without the key, and without it PID 1 loads the units only after a reboot (snosi#1189).
For desktop applications, include shared/sysext/finalize/sysext-strip-icon-cache.sh in FinalizeScripts= so no hicolor icon cache leaks into the delta.
Any direct download must go through verified_download() with a pinned URL and an entry in shared/download/sysext-checksums.json, plus a check in .github/workflows/check-dependencies.yml (or check-packages.yml for APT sentinels). A first-party release .deb that its workflow attests also gets an attestation object in that entry (ADR-0024); build.yml and check-dependencies.yml then verify it with shared/download/verify-attestations.sh.

Validate: just sysexts (or a targeted build of the one image), then the relevant scripts under test/ and the checks in .github/workflows/validate.yml.

Finish by updating CLAUDE.md and docs/design/sysexts.md if any constraint or contract changed.

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.

6d ago Changed · +6 lines 620570e64a21
8d ago Changed · +4 lines a313b35ddb30
2mo ago First seen · 41 lines · 0 tokens per session scan A 817be3a8b884

Subscribe to this mod's changes

add-sysext is a command published in the GitHub repository frostyard/snosi (32 stars, last pushed today), licensed MIT. It costs nothing until one of its globs matches a file; then it loads 753 tokens. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-06.

DevOps and cloud

add-sysext next to 6 commands from other repositories
Mod	Type	Stars	Last push	Licence	Scan	Tokens	Works with
add-sysextfrostyard/snosi	Command	32	today	MIT	A	0	GitHub Copilot
checklistgithub/spec-kit	Command	148k	yesterday	MIT	A	13	Claude Code
clarifygithub/spec-kit	Command	148k	yesterday	MIT	A	27	Claude Code
specifygithub/spec-kit	Command	148k	yesterday	MIT	A	13	Claude Code
analyzegithub/spec-kit	Command	148k	yesterday	MIT	A	26	Claude Code
convergegithub/spec-kit	Command	148k	yesterday	MIT	A	35	Claude Code
implementgithub/spec-kit	Command	148k	yesterday	MIT	A	14	Claude Code

snosi: Command for GitHub Copilot