[BUG] SendMessage-resumed background subagent never appears in the background tasks UI

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When a background subagent is resumed via the SendMessage tool (the response reads "resumed from transcript in the background"), it never shows up in the client's background tasks tab. The tab stays empty even though the agent is actively running — editing files and eventually firing a completion notification. So there's no live indication that any background work is in progress.

This breaks visibility parity: an Agent-tool-spawned background agent is expected to be trackable in that tab, but a SendMessage-resumed one isn't, so I can't see it's running, tell whether it stalled, or confirm it's still alive until the completion notification arrives.

What Should Happen?

Expected

Resumed agent shows in the background tasks tab, same as an Agent-tool-spawned
background agent.

Actual

The agent's work runs correctly and a task-notification fires on completion,
but it never appears in the background tasks tab. The initially Agent-spawned
run also did not appear.

Error Messages/Logs

Steps to Reproduce

Steps to reproduce

1. Spawn a subagent in the background via the Agent tool.
2. After it completes, resume it several times via SendMessage({to: agentId}).

(Each returns "resumed from transcript in the background".)

1. Open the client's "background tasks" tab while it runs.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.179

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

Environment

• Claude Code: 2.1.179
• Node: v20.20.2
• OS: macOS 26.5.1 (25F80)
• Session type: Claude Agent SDK (not interactive CLI)
• Relevant flag: CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS (SendMessage worked in this session)

Notes / possible mechanism

• Persisted agent meta.json is structurally identical for tracked vs untracked

agents (agentType/description/toolUseId/spawnDepth) — no "tracked" flag.

• On SendMessage resume, meta.json keeps the ORIGINAL spawn toolUseId; the resume

itself is a separate tool_use. If the tab maps live work to the current Agent
tool_use, resumes would be missed.

• SendMessage may be gated as experimental — this could be incomplete UI

integration rather than a regression.