DCG — Destructive Command Guard

A pre-tool-use hook that intercepts every shell command the agent is about to run. If the command matches a known destructive pattern, the hook returns a non-zero exit and a stopReason payload, blocking execution.

What it blocks

• rm -rf / — at root
• sudo ... rm -rf / — same with sudo
• git reset --hard — irreversible discard of local work
• git clean -fd — force-delete untracked files (commonly nukes .env, generated configs)
• git push --force to main / master — overwriting upstream history on protected branches
• rm of .env / credentials / .pem / SSH keys
• shred / dd — disk destruction
• :(){ :|:& };: — fork bomb
• chmod 777 on $HOME — permission disaster
• find $HOME -delete — bulk deletion in home

The full pattern list is in hooks/dcg.sh. New patterns can be added by appending to the patterns array.

What it does NOT block

By design DCG is a floor, not a ceiling:

• rm -rf <project>/build — project-local cleanup is allowed
• git reset HEAD~1 — non---hard resets are recoverable
• rm .DS_Store — junk-file removal is allowed
• Anything outside the agent's shell tool

This is intentional. Over-blocking causes alert fatigue; agents start either circumventing or losing trust. DCG focuses on the truly unrecoverable cases.

How install wires it up

jutsu install dcg registers a pre-tool-use hook with matcher Bash for every active agent:

Each entry is tagged with _kaijutsu: skill:dcg:hook:block-destructive-shell so jutsu remove dcg can clean up exactly what we added without disturbing user-authored hooks.

The script at hooks/dcg.sh is platform-agnostic POSIX bash — it reads the hook JSON from stdin, extracts the command field via jq (with a regex fallback if jq is absent), and matches against the pattern list.

How agents experience a block

When DCG fires:

{
  "continue": false,
  "stopReason": "dcg blocked: rm -rf /",
  "matched_command": "rm -rf /"
}

Plus a non-zero exit. Each agent's hook framework propagates this differently:

• Claude Code: surfaces stopReason to the model so the assistant knows why and can react (apologize, propose a safer alternative).
• Codex: returns the "blocked" status to the model.
• Antigravity: similar — the BeforeTool interception aborts the call.

In every case the destructive shell never runs.

Customization

Edit hooks/dcg.sh to add or remove patterns:

patterns+=(
  "rm of database backups:rm.*\\.sql\\.bak"
)

The format is "<label>:<regex>". The script grep -Es each regex against the proposed command.

Testing locally

After install, in any project where DCG is registered:

# In any agent session, ask:
> please run `rm -rf /tmp/foo`

# DCG should let it through (limited to /tmp/foo, not /).

> please run `rm -rf /`

# DCG should block. The agent will report "dcg blocked: rm -rf /".

Hard rules

• Never bypass DCG by editing settings.json directly while an agent is running. Restart the agent after disabling.
• DCG is not a security boundary against malicious skills. A malicious skill could ship its own hooks that disable DCG. Treat DCG as a guardrail against agent mistakes, not against adversaries with skill-install access.
• The hook script timeout is 5 seconds. If jq is slow or stdin is malformed, the hook exits and the action proceeds (fail-open). v0.4 may switch to fail-closed.
• DCG patterns are not exhaustive. Use it alongside, not instead of, repository-level safeguards (commit signing, branch protection, CI checks).

v0 limitations

• Patterns only match shell strings — agents that use direct file-edit tools (Claude's Edit tool, Codex's apply_patch) bypass DCG. Future v0.4: extend matchers to Edit / Write tools with sensitive-path patterns (.env, credentials, id_rsa).
• No per-project pattern overrides yet. v0.4 will read <project>/.kaijutsu/dcg-patterns.txt as an addendum.
• jq dependency is best-effort. If jq isn't installed, the script greps over the full JSON payload — works but slightly less precise.