A Claude Code hook that runs a JavaScript safety script before Bash commands. Bash is a common command-line shell.

Use it to run bouncer.js before Bash tool calls on systems with Node.js installed.

It provides a checkpoint before shell commands run, while allowing the command to continue when Node.js is unavailable.

PreToolUse — Hook by karanb192

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods

npx agentmods add hooks/karanb192/bouncer/pre-tool-use

Clone the repo

git clone --depth 1 https://github.com/karanb192/bouncer

Per session not measured What this adds to a session before it is invoked.

When invoked not measured Not applicable: nothing here is loaded into a session.

Security scan A 0 findings. Scan, not verified.

Origin original No closer match found in the catalogue.

Security

Grade A, and why

PreToolUse 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 yesterday.

A static scan of the commands this hook runs, not an audit. A hook is shell that executes on your machine at the event it names, which is why every command in it is printed with what was found.

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.

claude/hooks/hooks.json · 15 lines

What it actually says

{
  "PreToolUse": [
    {
      "matcher": "Bash",
      "hooks": [
        {
          "type": "command",
          "command": "command -v node >/dev/null 2>&1 && node \"${CLAUDE_PLUGIN_ROOT:-${PLUGIN_ROOT}}/bouncer.js\" || exit 0",
          "commandWindows": "$r = if ($env:CLAUDE_PLUGIN_ROOT) { $env:CLAUDE_PLUGIN_ROOT } else { $env:PLUGIN_ROOT }; if (Get-Command node -ErrorAction SilentlyContinue) { node \"$r\\bouncer.js\" }",
          "timeout": 5
        }
      ]
    }
  ]
}

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.

yesterday First seen · 15 lines scan A d5280b106e6e

Subscribe to this mod's changes

PreToolUse is a hook published in the GitHub repository karanb192/bouncer (5 stars, last pushed 14d ago), licensed MIT. Its token cost is not measured: a hook is shell that never enters the context. 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 2026-08-31.

Guardrails Languages Security ai-agents claude-code developer-tools hooks safety

PreToolUse

Getting it into your agent

Grade A, and why

Nothing flagged

What it actually says

What this file has done since we first saw it

Other hooks, from other repositories

PreToolUse

Notification

Stop

PreToolUse

PostToolUse

SubagentStop