Security overview

An agent acts through tools, so the kit's guardrails sit on the tools: which exist, which are approved, and what each call may touch. They're enforced in the SDK's options and in PreToolUse hooks, which run before every tool call, the main agent's and its subagents'. A hook's denial can't be talked around by the model; a line in the prompt can.

Guardrail	Protects against	Where	Page
Minimal tool set	the model using tools the agent has no business with (the CLI has dozens)	tools / allowedTools	Session options
disallowedTools	a specific tool, including an MCP server's dangerous one	SDK, before anything else	AgentSpec
File scope	writes outside the agent's folders, edits to originals, searches over the whole project, reading protected files	PreToolUse hook	File scope
Subagent type gate	the SDK's built-in general-purpose subagent, which inherits every tool	PreToolUse hook	Subagent gates
Subagent Bash gate	the main agent running shell commands	PreToolUse hook	Subagent gates
Subagent foreground gate	subagents launched in the background and silently lost	PreToolUse hook	Subagent gates
MCP approval	nothing: it approves every mcp__* call so project servers work	canUseTool	Permissions and isolation
Setting sources, auto-memory, git context	the runner's personal Claude Code configuration leaking into the agent	SDK options	Permissions and isolation
Step gate, approvals	actions a person should see first	hook and tool	Human in the loop
Transcript redaction	secrets written to logs	PreToolUse/PostToolUse hook	Transcript

What the kit does not protect

• Bash. A subagent with Bash can reach any path and run any command the user can. The file scope doesn't apply to it. Keep Bash opt-in and prefer a dedicated tool.
• Your MCP tools. A tool you register runs with your process's permissions. Validate its inputs, and put destructive ones behind an approval or in disallowedTools.
• Web content. WebFetch and WebSearch bring untrusted text into the context (prompt injection). The guardrails above limit what an injected instruction can make the agent do; they don't stop the agent from reading it.
• The run folders. They hold whole conversations and tool results, unredacted. Keep them out of version control and out of shared folders.
• Prompts. "Never do X" in a prompt is a request. Anything that must never happen belongs in disallowedTools or a hook.

A checklist for a new agent

1. Give it the narrowest folders: knowledgeDir for notes, sourcesDir for originals, nothing more unless needed.
2. Put every secret file in deniedPaths and every secret value in secrets.
3. Disallow every tool you don't want, especially from third-party MCP servers.
4. Prefer small tools to Bash; if a subagent really needs Bash, make it opt-in.
5. Start new agents in interactive or guided mode.
6. Keep settingSources at its default (["project"]) or narrower.
7. Add .env and your runs folder to .gitignore.