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

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

Hook blocking safe commands

1. Check if there's an edge case not covered
2. File a GitHub issue
3. 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