DCG — Destructive Command Guard Skill

DCG — Destructive Command Guard

A high-performance Claude Code hook that intercepts and blocks destructive commands before they execute. Written in Rust with SIMD-accelerated filtering for sub-millisecond latency.

Why This Exists

AI coding agents are powerful but fallible. They can accidentally run destructive commands:

"Let me clean up the build artifacts" → rm -rf ./src (typo)
"I'll reset to the last commit" → git reset --hard (destroys uncommitted changes)
"Let me fix the merge conflict" → git checkout -- . (discards all modifications)
"I'll clean up untracked files" → git clean -fd (permanently deletes untracked files)

DCG intercepts dangerous commands before execution and blocks them with a clear explanation, giving you a chance to stash your changes first.

Critical Design Principles

1. Whitelist-First Architecture

Safe patterns are checked before destructive patterns. This ensures explicitly safe commands are never accidentally blocked:

git checkout -b feature    →  Matches SAFE "checkout-new-branch"  →  ALLOW
git checkout -- file.txt   →  No safe match, matches DESTRUCTIVE  →  DENY

2. Fail-Safe Defaults (Default-Allow)

Unrecognized commands are allowed by default. This ensures:

The hook never breaks legitimate workflows
Only known dangerous patterns are blocked
New git commands work until explicitly categorized

3. Zero False Negatives Philosophy

The pattern set prioritizes never allowing dangerous commands over avoiding false positives. A few extra prompts for manual confirmation are acceptable; lost work is not.

What It Blocks

Git Commands That Destroy Uncommitted Work

| Command | Reason | |---------|--------| | git reset --hard | Destroys uncommitted changes | | git reset --merge | Destroys uncommitted changes | | git checkout -- <file> | Discards file modifications | | git restore <file> (without --staged) | Discards uncommitted changes | | git clean -f | Permanently deletes untracked files |

Git Commands That Destroy Remote History

| Command | Reason | |---------|--------| | git push --force / -f | Overwrites remote commits | | git branch -D | Force-deletes without merge check |

Git Commands That Destroy Stashed Work

| Command | Reason | |---------|--------| | git stash drop | Permanently deletes a stash | | git stash clear | Permanently deletes all stashes |

Filesystem Commands

| Command | Reason | |---------|--------| | rm -rf (outside /tmp, /var/tmp, $TMPDIR) | Recursive deletion is dangerous |

What It ALLOWS

Safe operations pass through silently:

Always Safe Git Operations

git status, git log, git diff, git add, git commit, git push, git pull, git fetch, git branch -d (safe delete with merge check), git stash, git stash pop, git stash list

Explicitly Safe Patterns

| Pattern | Why Safe | |---------|----------| | git checkout -b <branch> | Creating new branches | | git checkout --orphan <branch> | Creating orphan branches | | git restore --staged <file> | Unstaging only, doesn't touch working tree | | git restore -S <file> | Short flag for staged | | git clean -n / --dry-run | Preview mode, no actual deletion | | rm -rf /tmp/* | Temp directories are ephemeral | | rm -rf $TMPDIR/* | Shell variable forms |

Safe Alternative: `--force-with-lease`

git push --force-with-lease   # ALLOWED - refuses if remote has unseen commits
git push --force              # BLOCKED - can overwrite others' work

Modular Pack System

DCG uses a modular "pack" system to organize patterns by category:

Core Packs (Always Enabled)

| Pack | Description | |------|-------------| | core.git | Destructive git commands | | core.filesystem | Dangerous rm -rf outside temp |

Database Packs

| Pack | Description | |------|-------------| | database.postgresql | DROP/TRUNCATE in PostgreSQL | | database.mysql | DROP/TRUNCATE in MySQL/MariaDB | | database.mongodb | dropDatabase, drop() | | database.redis | FLUSHALL/FLUSHDB | | database.sqlite | DROP in SQLite |

Container Packs

| Pack | Description | |------|-------------| | containers.docker | docker system prune, docker rm -f | | containers.compose | docker-compose down --volumes | | containers.podman | podman system prune |

Kubernetes Packs

| Pack | Description | |------|-------------| | kubernetes.kubectl | kubectl delete namespace | | kubernetes.helm | helm uninstall | | kubernetes.kustomize | kustomize delete patterns |

Cloud Provider Packs

| Pack | Description | |------|-------------| | cloud.aws | Destructive AWS CLI commands | | cloud.gcp | Destructive gcloud commands | | cloud.azure | Destructive az commands |

Infrastructure Packs

| Pack | Description | |------|-------------| | infrastructure.terraform | terraform destroy | | infrastructure.ansible | Dangerous ansible patterns | | infrastructure.pulumi | pulumi destroy |

System Packs

| Pack | Description | |------|-------------| | system.disk | dd, mkfs, fdisk operations | | system.permissions | Dangerous chmod/chown patterns | | system.services | systemctl stop/disable patterns |

Other Packs

| Pack | Description | |------|-------------| | strict_git | Extra paranoid git protections | | package_managers | npm unpublish, cargo yank |

Configuring Packs

# ~/.config/dcg/config.toml
[packs]
enabled = [
    "database.postgresql",
    "containers.docker",
    "kubernetes",  # Enables all kubernetes sub-packs
]

Environment Variables

| Variable | Description | |----------|-------------| | DCG_PACKS="containers.docker,kubernetes" | Enable packs (comma-separated) | | DCG_DISABLE="kubernetes.helm" | Disable packs/sub-packs | | DCG_VERBOSE=1 | Verbose output | | DCG_COLOR=auto\|always\|never | Color mode | | DCG_BYPASS=1 | Bypass DCG entirely (escape hatch) |

Installation

Quick Install (Recommended)

curl -fsSL "https://raw.githubusercontent.com/Dicklesworthstone/destructive_command_guard/master/install.sh?$(date +%s)" | bash

# Easy mode: auto-update PATH
curl -fsSL "https://raw.githubusercontent.com/Dicklesworthstone/destructive_command_guard/master/install.sh?$(date +%s)" | bash -s -- --easy-mode

# System-wide (requires sudo)
curl -fsSL "https://raw.githubusercontent.com/Dicklesworthstone/destructive_command_guard/master/install.sh?$(date +%s)" | sudo bash -s -- --system

From Source (Requires Rust Nightly)

cargo +nightly install --git https://github.com/Dicklesworthstone/destructive_command_guard

Prebuilt Binaries

Available for: Linux x86_64, Linux ARM64, macOS Intel, macOS Apple Silicon, Windows

Claude Code Configuration

Add to ~/.claude/settings.json:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "dcg"
          }
        ]
      }
    ]
  }
}

Important: Restart Claude Code after adding the hook.

How It Works

Processing Pipeline

┌─────────────────────────────────────────────────────────────────┐
│                        Claude Code                               │
│  Agent executes `rm -rf ./build`                                │
└─────────────────────┬───────────────────────────────────────────┘
                      │
                      ▼ PreToolUse hook (stdin: JSON)
┌─────────────────────────────────────────────────────────────────┐
│                          dcg                                     │
│  ┌──────────────┐    ┌──────────────┐    ┌──────────────┐       │
│  │    Parse     │───▶│  Normalize   │───▶│ Quick Reject │       │
│  │    JSON      │    │   Command    │    │   Filter     │       │
│  └──────────────┘    └──────────────┘    └──────┬───────┘       │
│                                                  │               │
│                      ┌───────────────────────────┘               │
│                      ▼                                           │
│  ┌──────────────────────────────────────────────────────────┐   │
│  │                   Pattern Matching                        │   │
│  │   1. Check SAFE_PATTERNS (whitelist) ──▶ Allow if match  │   │
│  │   2. Check DESTRUCTIVE_PATTERNS ──────▶ Deny if match    │   │
│  │   3. No match ────────────────────────▶ Allow (default)  │   │
│  └──────────────────────────────────────────────────────────┘   │
└─────────────────────┬───────────────────────────────────────────┘
                      │
                      ▼ stdout: JSON (deny) or empty (allow)

Stage 1: JSON Parsing

Reads hook input from stdin
Validates Claude Code's PreToolUse format
Non-Bash tools immediately allowed

Stage 2: Command Normalization

Strips absolute paths: /usr/bin/git status → git status
Preserves argument paths

Stage 3: Quick Rejection Filter

SIMD-accelerated substring search for "git" or "rm"
Commands without these bypass regex entirely (99%+ of commands)

Stage 4: Pattern Matching

Safe patterns checked first (short-circuit on match → allow)
Destructive patterns checked second (match → deny)
No match → default allow

Exit Codes

| Code | Meaning | |------|---------| | 0 | Command is safe, proceed | | 2 | Command is blocked, do not execute |

CLI Usage

Test commands manually:

# Show version with build metadata
dcg --version

# Test a command
echo '{"tool_name":"Bash","tool_input":{"command":"git reset --hard"}}' | dcg

Example Block Message

════════════════════════════════════════════════════════════════════════
BLOCKED  dcg
────────────────────────────────────────────────────────────────────────
Reason:  git reset --hard destroys uncommitted changes. Use 'git stash' first.

Command:  git reset --hard HEAD~1

Tip: If you need to run this command, execute it manually in a terminal.
     Consider using 'git stash' first to save your changes.
════════════════════════════════════════════════════════════════════════

Contextual Suggestions

| Command Type | Suggestion | |-------------|------------| | git reset, git checkout -- | "Consider using 'git stash' first" | | git clean | "Use 'git clean -n' first to preview" | | git push --force | "Consider using '--force-with-lease'" | | rm -rf | "Verify the path carefully before running manually" |

Edge Cases Handled

Path Normalization

/usr/bin/git reset --hard          # Blocked
/usr/local/bin/git checkout -- .   # Blocked
/bin/rm -rf /home/user             # Blocked

Flag Ordering Variants

rm -rf /path          # Combined flags
rm -fr /path          # Reversed order
rm -r -f /path        # Separate flags
rm --recursive --force /path    # Long flags

All variants are handled.

Shell Variable Expansion

rm -rf $TMPDIR/build           # Allowed (temp)
rm -rf ${TMPDIR}/build         # Allowed
rm -rf "$TMPDIR/build"         # Allowed
rm -rf "${TMPDIR:-/tmp}/build" # Allowed

Staged vs Worktree Restore

git restore --staged file.txt    # Allowed (unstaging only)
git restore -S file.txt          # Allowed (short flag)
git restore file.txt             # BLOCKED (discards changes)
git restore --worktree file.txt  # BLOCKED (explicit worktree)
git restore -S -W file.txt       # BLOCKED (includes worktree)

Performance Optimizations

DCG is designed for zero perceived latency:

| Optimization | Technique | |--------------|-----------| | Lazy Static | Regex patterns compiled once via LazyLock | | SIMD Quick Reject | memchr crate for CPU vector instructions | | Early Exit | Safe match returns immediately | | Zero-Copy JSON | serde_json operates on input buffer | | Zero-Allocation | Cow<str> for path normalization | | Release Profile | opt-level="z", LTO, single codegen unit |

Result: Sub-millisecond execution for typical commands.

Pattern Counts

| Type | Count | |------|-------| | Safe patterns (whitelist) | 34 | | Destructive patterns (blacklist) | 16 |

Security Considerations

What DCG Protects Against

Accidental data loss from git checkout -- or git reset --hard
Remote history destruction from force pushes
Stash loss from git stash drop/clear
Filesystem accidents from rm -rf outside temp directories

What DCG Does NOT Protect Against

Malicious actors (can bypass the hook)
Non-Bash commands (Python/JavaScript file writes, API calls)
Committed but unpushed work
Commands inside scripts (./deploy.sh contents not inspected)

Threat Model

DCG assumes the AI agent is well-intentioned but fallible. It catches honest mistakes, not adversarial attacks.

Troubleshooting

Hook not blocking commands

Verify ~/.claude/settings.json has hook configuration
Restart Claude Code
Test manually: echo '{"tool_name":"Bash","tool_input":{"command":"git reset --hard"}}' | dcg

Hook blocking safe commands

Check if there's an edge case not covered
File a GitHub issue
Temporary bypass: DCG_BYPASS=1 or run command manually

FAQ

Q: Why block git branch -D but allow git branch -d?

Lowercase -d only deletes branches fully merged. Uppercase -D force-deletes regardless of merge status, potentially losing commits.

Q: Why is git push --force-with-lease allowed?

Force-with-lease refuses to push if the remote has commits you haven't seen, preventing accidental overwrites.

Q: Why block all rm -rf outside temp directories?

Recursive forced deletion is extremely dangerous. A typo or wrong variable can delete critical files. Temp directories are designed to be ephemeral.

Q: What if I really need to run a blocked command?

DCG instructs the agent to ask for permission. Run the command manually in a separate terminal after making a conscious decision.

Integration with Flywheel

| Tool | Integration | |------|-------------| | Claude Code | Native PreToolUse hook | | Agent Mail | Agents can report blocked commands to coordinator | | BV | Flag tasks that repeatedly trigger DCG | | CASS | Search DCG block patterns across sessions | | RU | DCG protects agent-sweep from destructive commits |

Agent Skills: DCG — Destructive Command Guard

Skill Files