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Installation

fencepost ships as a Claude Code plugin: a manifest that registers the hooks, a small committed JS bundle, the tree-sitter grammars, and a set of presets. There's no build step for users and no 100 MB binary to download.

Requirements​

  • Claude Code
  • A JavaScript runtime on your PATH — Node (which you already have if you run Claude Code via npm) or Bun. The hook wrapper uses whichever it finds.

fencepost's repository doubles as a single-plugin marketplace, so you can add it and install in two commands:

/plugin marketplace add ch4nn0n/fencepost
/plugin install fencepost@fencepost

That's it — the plugin is fetched and cached, the PreToolUse and SessionStart hooks register, and the gate is live on the next tool call. Update later with /plugin marketplace update fencepost.

fencepost is meant to be your permission layer, so run Claude Code in bypassPermissions mode:

claude --permission-mode bypassPermissions

This stands down Claude's own permission prompts and lets fencepost's allow/ask/deny decide every call — no double-prompting, and your fencepost.yaml is the single source of truth. fencepost's deny and ask are still enforced in this mode; only the calls fencepost would let through stop prompting.

Use an isolated environment

bypassPermissions also disables Claude Code's other safety nets (protected-path write guards, and it offers no protection against prompt injection), so Claude recommends it only in containers, VMs, or dev containers. With those nets off, fencepost is your sole gate — keep default/onError set to ask or deny (never allow). See Permission modes for the full picture.

Try it locally without installing​

To test against a clone (for development, or before committing to installing), point Claude Code at the directory (the same --permission-mode flag applies):

git clone https://github.com/ch4nn0n/fencepost.git
claude --permission-mode bypassPermissions --plugin-dir ./fencepost

What's in the plugin​

fencepost/
├── .claude-plugin/
│   ├── plugin.json          # registers PreToolUse + SessionStart hooks
│   └── marketplace.json     # lets the repo be added as a marketplace
├── hooks/
│   ├── pre-tool-use.sh      # wrapper → node dist/index.js evaluate
│   └── session-start.sh     # → node dist/index.js sessionstart
├── dist/
│   ├── index.js             # the committed JS bundle (~280 KB)
│   └── *.wasm               # tree-sitter grammars
├── presets/                 # bundled importable rule sets
└── skills/audit.md          # the /audit slash command

The two hooks:

  • PreToolUse → evaluates each tool call.
  • SessionStart → injects session guidance and prepares the /tmp/claude sandbox.

The manifest points at thin shell wrappers rather than the bundle directly. Each wrapper finds a runtime, resolves the bundle relative to its own location, and exports FENCEPOST_PRESETS_DIR so bundled presets resolve no matter where the plugin is installed:

hooks/pre-tool-use.sh
#!/usr/bin/env bash
HERE="$(cd "$(dirname "$0")" && pwd)"
export FENCEPOST_PRESETS_DIR="${FENCEPOST_PRESETS_DIR:-$HERE/../presets}"
RUNNER="$(command -v node || command -v bun || true)"
[ -z "$RUNNER" ] && exit 0   # fail open if no runtime is available
exec "$RUNNER" "$HERE/../dist/index.js" evaluate

Verify it works​

With no config present, fencepost is a no-op (it fails open to default: ask). You can run the bundle directly to confirm:

echo '{"tool_name":"Read","tool_input":{},"session_id":"t","cwd":"/tmp","hook_event_name":"PreToolUse","tool_use_id":"x"}' \
  | node dist/index.js evaluate
# (no output = allow)

Then add a config and check it compiles cleanly:

node dist/index.js verify
# exits non-zero if the config has errors — suitable for CI / pre-commit

Building from source (contributors)​

Only needed if you're changing fencepost itself. Requires Bun:

git clone https://github.com/ch4nn0n/fencepost.git
cd fencepost
bun install
bun run build     # bundles src/ → dist/index.js and copies the wasm grammars
bun test

dist/ is committed on purpose — it's the artifact the plugin ships — so rebuild and commit it whenever you change src/.

Next​

Head to the Quick start to write your first config.