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

[victorlopezsalazar]

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 }