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

まず、メインループと一般的なサブエージェントについて settings.json PreToolUse/PostToolUse の動作を再現し、次に mod beforeToolCall および permissions.deny と比較します。ツール呼び出しのインターセプトのエントリーポイントを追跡し、意図した結果がサブエージェントに対する一貫した適用なのか、それとも hooks と mods のドキュメントにおける明示的なカバレッジマトリクスなのかを判断します。ドキュメント化された、または観測された動作が選択した期待値と一致すれば完了です。

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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.