Sub-agent tool calls bypass PreToolUse hooks and mod beforeToolCall hooks — only permission rules apply

首先重現 settings.json PreToolUse/PostToolUse 對主迴圈和一般子代理的行為，然後將其與 mod beforeToolCall 和 permissions.deny 進行比較。追蹤工具呼叫攔截的進入點，並判斷預期結果是對子代理進行一致的強制執行，還是在 hooks 和 mods 文件中提供明確的涵蓋矩陣。文件記載或觀察到的行為與所選預期一致，即表示完成。

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Summary

Tool calls issued by sub-agents (the child runs created by the agent tool) do not trigger either of the two mechanisms documented for intercepting tool calls:

• PreToolUse / PostToolUse hooks configured in settings.json — never fire for sub-agent calls.
• A mod's beforeToolCall hook — never fire for sub-agent calls.

Only permissions deny/ask rules are enforced for sub-agent calls.

The practical consequence: any guard built on hooks or on mod hooks fences the main loop only. The same action re-issued through a sub-agent passes unfenced, so delegation becomes a one-call workaround for the guard rather than a blocked path. This is easy to miss because the guard appears to work — it fires reliably when the action is performed directly.

Expected Behavior

Either:

1. A mechanism documented as intercepting tool calls intercepts the same tool calls regardless of which agent issues them; or
2. The documentation states explicitly and prominently which execution contexts each mechanism covers (main loop / sub-agent / plan mode), so authors do not build guards that only appear to work.

The hooks page notes that reacting to broader lifecycle activity — including sub-agent activity — is what mods are for, and the mods page describes beforeToolCall generically ("fires per tool call"). Read together, these imply mod hooks cover sub-agent calls. Observationally, they do not.

Actual Behavior

With a PreToolUse hook and a mod beforeToolCall both configured to block a distinctive path/command:

• Main loop: blocked as configured. Hooks fire and are logged.
• Sub-agent (general), identical action: succeeds. Neither hook fires. No log entry, no block, no injected context.

Permission rules behave consistently in both contexts: a deny rule blocks the same call whether it comes from the main loop or from a sub-agent.

Also worth noting: the main loop's beforeToolCall does receive the entire agent spawn call, including the full prompt text, and can block the spawn outright. So the delegation boundary is already a possible interception point — it just is not covered by the hook mechanisms, and injection into the sub-agent prompt is advisory text rather than enforcement.

Steps to reproduce

1. Configure a PreToolUse hook in settings.json that blocks a specific path or command, and separately load a mod implementing beforeToolCall with a similar rule.
2. Perform the guarded action from the main loop. It is blocked; hooks fire.
3. Start a fresh session (so the mod is loaded) and delegate the identical action to the general sub-agent.
4. The action succeeds. Neither hook fires — verified both by absence of the block and by absence of any log written by the hook.

Repeat step 4 with a permissions.deny rule matching the same call: the sub-agent is blocked.

Command Code Version

1.54.0

Operating System

Linux

Additional context

The two mechanisms have different coverage, which is not obvious from the docs:

Suggest documenting this matrix directly, since it determines whether a user's guard is security-relevant or cosmetic. If sub-agent coverage for mod hooks is intended, the hook contracts page would be the natural place to state the guarantee.