[BUG] Invoking a sub-agent does not return agentId

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?

Per https://docs.claude.com/en/docs/claude-code/sub-agents#resumable-subagents when we invoke a sub-agent we are supposed to get back the agentId of the sub-agent.

What Should Happen?

agentId should be returned

Error Messages/Logs

Steps to Reproduce

Ask claude to invoke a command using a sub-agent, such as the command mentioned at https://docs.claude.com/en/docs/claude-code/sub-agents#resumable-subagents

Claude Model

Sonnet (default)

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.0.31

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

WSL (Windows Subsystem for Linux)

Additional Information

Per https://docs.claude.com/en/docs/claude-code/sub-agents#resumable-subagents the system is supposed to return the agentId but this value is not visible by hitting CTRL+O nor is it returned in function_results from the Task tool.

Expected behavior: Return agentId as part of function_results so claude will be able to programmatically resume execution of the sub-agent for future operations.