Claude Code plugin overview

The Sextant Claude Code plugin bundles the MCP server, three skills the agent auto-loads, three slash commands, and a sample git pre-commit hook that blocks commits on a dirty grade.

Install

You need the sextant and sextant-mcp binaries on PATH first. See Installation.

Then, from a Claude Code session:

/plugin marketplace add kylebastien/sextant-mcp
/plugin install sextant@kylebastien/sextant-mcp

The plugin lives at plugin/ in the sextant-mcp repo, so the marketplace is the repo itself.

What’s in the box

Piece	What it does
MCP server	Registers `sextant-mcp` so the agent has `grade_diff`, `grade_files`, `list_rules`, `explain_rule`, `get_config`.
Skills	Three auto-loaded skills the agent uses to know when and how to grade.
Slash commands	Three `/sextant-*` commands you can invoke explicitly.
Pre-commit hook	Sample git pre-commit script that blocks commits on a dirty grade.

Why a plugin and not just an MCP server?

The MCP server alone gives the agent the ability to grade. The plugin adds the behaviour: skills tell the agent when to call which tool, slash commands let you invoke grading explicitly, and the sample git pre-commit hook is the gate that blocks commits until the diff is clean.

If you just want the tools, add the MCP server by hand. If you want the skills, commands, and pre-commit gate, install the plugin.

Why no Claude hooks?

Earlier versions of the plugin shipped SessionStart, PostToolUse, and Stop hooks. They’re gone:

The Stop hook (with SEXTANT_ENFORCE_ON_STOP=1) produced dead-end loops when an LLM rule kept flagging the same line and the agent couldn’t make it happy.
The PostToolUse hook surfaced findings the agent could just talk past, and burned tokens every keystroke.
git commit is the natural integration point for “this diff is not allowed to land” — the gate runs once per commit and aborts the commit outright when it fires.

The replacement is the pre-commit hook — same grade_diff check, ran at the right moment, strict by design with no env-var escape hatch.

Authoring

Skills live at plugin/skills/<name>/SKILL.md.
Commands live at plugin/commands/<name>.md.
Pre-commit script lives at plugin/hooks/pre-commit.sh.

Each is plain markdown or bash — no compile step. After editing skills or commands, reload the plugin (/plugin reload sextant) or restart the session.

Troubleshooting

“sextant: command not found” when committing. Your shell PATH doesn’t include the install dir. Install Sextant somewhere already on PATH (~/.local/bin, /usr/local/bin) or add the install directory explicitly to your shell rc so commits launched from git inherit it.

HEAD~1 errors on a fresh repo. The diff grade needs a base commit. The first commit of a repo has no HEAD~1; Sextant returns a friendly “no default base” error and the hook exits silently.

Pre-commit hook blocked the commit. Fix the findings. The gate is strict by design — there is no env-var escape hatch. If a rule fires repeatedly on something that isn’t a real problem, calibrate the rule rather than trying to skip the grade.