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Google Developers Blog — Gemini CLI hooks for policy & automation

• Category: Security

  • What shipped: Google introduced hooks in Gemini CLI (v0.26.0+), letting you run your own scripts at predefined points inside the “agent loop.”
  • How it works: hooks run synchronously (the CLI waits for them), so your organization policy can sit on the critical path of tool use.
  • Main use cases: inject project context (recent git commits, tickets, docs), validate or block risky actions, enforce compliance rules, log tool usage, and trigger notifications.
  • Important detail: hooks can be scoped with matchers (e.g., only on file write / replace tools), so you don’t pay the overhead on every action.
  • Security angle: the post explicitly frames hooks as a way to prevent accidental secret leakage by scanning content before a write/edit and returning an allow/deny decision.
  • Extension ecosystem: Gemini CLI extensions can bundle hooks, which means “installing an extension” can now imply installing enforcement logic too (good for teams, risky if you trust the wrong source).
  • Operational gotcha: because hooks execute with your user privileges, enabling untrusted project-level hooks is effectively executing untrusted code.

Why it matters

  • Policy-as-code for agents: hooks are a clean pattern for enforcing “what an agent may do” at runtime, not just “what it should do” in a prompt.
  • Practical guardrails: security controls that inspect actual tool inputs (file diffs, shell commands) are usually more reliable than content filtering alone.
  • Supply-chain implications: the agent tooling ecosystem is quickly turning into an extension ecosystem — hooks make it more powerful, and therefore more important to vet.

What to do

  1. Add a “BeforeTool” hook for high-risk actions: block or require approval for file writes, shell execution, network calls, and credential access.
  2. Start with boring rules: implement secret scanning, domain allowlists, and “no writing to prod configs” checks before fancy policy engines.
  3. Make hooks fast and deterministic: cache expensive checks; avoid network calls on the critical path unless you have tight timeouts.
  4. Treat hooks like code you deploy: version control them, code review them, and sign/verify sources for extension-bundled hooks.
  5. Log denials as security signals: repeated “blocked” actions can be a prompt injection indicator (agent being steered to do something weird).

Sources