Does onPostToolUse hook additionalContext get injected into sub-agent context?

記載されている SDK バージョンを使用し、onPostToolUse hook、task tool、session.send({ mode: "immediate" }) のエントリーポイントから始めます。動作を再現しながら、セッションジャーナルで hook.start と hook.end のイベント、およびサブエージェントが受け取ったコンテキストを確認します。コンテキストのスコープ、メッセージの配信動作、ターン途中でステアリングするための推奨方法が明確に文書化されていれば完了です。

Context

We're building a qa agent. It has an orchestrator agent that dispatches work to custom sub-agents via the task tool. We use session-level onPostToolUse hooks to enforce guardrails (retry limits on failing tests). The hooks fire correctly during sub-agent execution — we can see hook.start/hook.end events in the session journal, and onPreToolUse denials work as expected inside sub-agents.

What we observe

We have a guardrail that tracks test failures per spec. After 3 failures, we want to tell the sub-agent "stop retrying, mark as fixme, move on." Currently our hook calls session.send({ prompt: text, mode: "immediate" }) separately. The sub-agent ignores this and keeps retrying (7+ attempts observed).

Before max attempts, we return { additionalContext: feedbackText } from the hook (per-attempt guidance like "attempt 2/3, try a different approach"). We haven't been able to confirm whether this feedback actually reaches the sub-agent.

Questions

1. Does { additionalContext } returned from onPostToolUse get injected into the active sub-agent's context, or only the parent session?
2. Does session.send({ mode: "immediate" }) reach a running sub-agent, or does it queue on the parent session?
3. What is the recommended way to steer a sub-agent mid-turn from a hook?

Environment

• @github/copilot-sdk@1.0.6
• @github/copilot@1.0.70
• Custom agents with tools: null (all tools)
• Hooks registered at session creation via hooks: { onPreToolUse, onPostToolUse }