vcs-detect - SKILL.md Agent Skill

name: vcs-detect description: Detect whether the current project uses jj (Jujutsu), arc (Arcadia), or git for version control. Run this BEFORE any VCS command to use the correct tool.

VCS Detection Skill

Detect the version control system in use before running any VCS commands.

Why This Matters

jj (Jujutsu), arc, and git have different CLIs and workflows
Running git commands in a jj or arc repo (or vice versa) causes errors
Some repos use jj with git colocated (both .jj/ and .git/ exist)
Arcadia uses arc, Yandex's VCS for the Arcadia monorepo

Detection Logic

jj root, arc root, and git rev-parse --show-toplevel walk up the filesystem to find repo root.

Priority order:

jj root succeeds → jj (handles colocated too)
arc root succeeds → arc (Arcadia)
git rev-parse succeeds → git
All fail → no VCS

Detection Command

if jj root &>/dev/null; then echo "jj"
elif arc root &>/dev/null; then echo "arc"
elif git rev-parse --show-toplevel &>/dev/null; then echo "git"
else echo "none"
fi

Command Mappings

Operation	git	jj	arc
Status	`git status`	`jj status`	`arc status` / `arc st`
Log	`git log`	`jj log`	`arc log`
Diff	`git diff`	`jj diff`	`arc diff`
Commit	`git commit`	`jj commit` / `jj describe`	`arc commit` / `arc ci`
Branch list	`git branch`	`jj branch list`	`arc branch` / `arc br`
New branch	`git checkout -b <name>`	`jj branch create <name>`	`arc checkout -b <name>` / `arc co -b <name>`
Push	`git push`	`jj git push`	`arc push`
Pull/Fetch	`git pull` / `git fetch`	`jj git fetch`	`arc pull` / `arc fetch`
Rebase	`git rebase`	`jj rebase`	`arc rebase`

Usage

Before any VCS operation:

Run detection command
Use appropriate CLI based on result
If none, warn user directory is not version controlled

Example Integration

User: Show me the git log
Agent: [Runs detection] -> Result: jj
Agent: [Runs `jj log` instead of `git log`]

Colocated Repos

When both .jj/ and .git/ exist, the repo is "colocated":

jj manages the working copy
git is available for compatibility (GitHub, etc.)
Always prefer jj commands in colocated repos

Arcadia Repos

When arc root succeeds, the repo is managed by arc:

arc is the VCS for Yandex's Arcadia monorepo
arc's CLI is intentionally similar to git for common operations
Use arc --help or arc <mode> -h for command-specific help
Prefer arc commands over git commands inside Arcadia