subagent-read-only-guard — Claude Code hook: setup and features

{
  "description": "Enforces read-only subagents. A PreToolUse hook denies Edit, Write, MultiEdit, NotebookEdit and PowerShell, and any Bash or Monitor command that is not read-only, when the call comes from a subagent (detected by the agent_id field Claude Code adds only inside subagents; tested on Claude Code 2.1.296). Shell commands pass only when every command is a bare program from a short read-only allowlist (ls, cat, grep, rg, find, jq, sort, sed -n 'N,Mp', cd, git log/diff/status/show/blame/branch/tag and similar) without the options that make it write or run other programs, including abbreviated long options. Redirection to files, command substitution, shell expansion ($VAR, {a,b}), subshells, env-var prefixes, git -c and anything the guard cannot classify are denied, including its own errors. The main conversation is never affected. Not covered: MCP tools, and git diff/blame running diff or textconv drivers already configured in the repo. Reading secrets (.env, printenv) is still allowed, so pair it with env-file-protection. Pairs with the hooks/subagents-explore-only setting, which tells subagents up front that they are exploration-only.",
  "supportingFiles": [
    {
      "source": "subagent-read-only-guard.py",
      "destination": ".claude/hooks/subagent-read-only-guard.py",
      "executable": true
    }
  ],
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Edit|Write|MultiEdit|NotebookEdit|PowerShell|Bash|Monitor",
        "hooks": [
          {
            "type": "command",
            "command": "python3 .claude/hooks/subagent-read-only-guard.py"
          }
        ]
      }
    ]
  }
}