Subagent ends its turn right after starting background work; parent receives "I'll wait" as the final report and stalls

Summary

A subagent starts background work, either a nested Agent call or a Bash call with run_in_background: true. It then ends its turn with text such as "I'll wait for the notification." The turn end counts as the subagent's completion. The parent receives "I'll wait" as the subagent's final report. The background work finishes later, but nothing resumes the subagent, so the parent never gets the result.

Environment

• Claude Code 2.1.294
• macOS (Darwin 25.5.0)
• Subagents are custom agent types with pinned effort (haiku-medium, sonnet-low, opus-low)

Reproduction

1. The main session starts subagent A with the Agent tool (foreground).
2. Subagent A starts a nested Agent call and omits run_in_background. The transcript shows no run_in_background key on that call. The nested agent's metadata records requestShape: "background".
3. Subagent A ends its turn with "I'll wait for the result."
4. The parent receives that text as subagent A's final output. The nested agent reports later, and nothing reads it.

The same happens when subagent A starts Bash with run_in_background: true and then ends its turn.

Evidence

• A heuristic scan of local transcripts (June to October 2026) found 37 subagent runs whose last tool call was a background Agent launch, and 6 whose last tool call was a background Bash launch. Not every run stalled. I did not verify each case by hand.
• Two of these runs on 2026-10-08 stalled in front of the user.

Workaround tested (not a fix)

• A PreToolUse hook on Agent|Task returned updatedInput with run_in_background: false when the hook input contained agent_id. agent_id appears only in hook input from inside a subagent. The nested call then ran in the foreground, and its result returned to the subagent.
• A SubagentStop hook that blocks the first stop when the last tool call launched background work made a subagent wait inside its turn and report the result. I tested this with a background Bash task.

Expected

One of:

• A subagent's final message does not go to the parent while work the subagent launched is still pending.
• Background work can resume a subagent that has ended its turn.
• Nested Agent calls inside a subagent default to foreground.

Related

• #83848: stalls with status: completed and no final text. This report adds the trigger: the turn end after a background launch.
• #74317: fabricated "waiting for background agent" stall.
• #91225 and #100117: run_in_background handling under the fork-subagent path. These may share a cause. Unverified.