Feature: Expose sub-agent context window usage in statusline / parent session telemetry

Summary

When using agent teams, the statusline command only receives context window data for the team lead (main session). Sub-agent context windows are isolated and not surfaced anywhere the parent session can observe.

Current behaviour

The statusline JSON input contains context_window.used_percentage and related fields scoped to the current session only. Spawned sub-agents have no mechanism to report their context usage back to the team lead or to the statusline renderer.

Desired behaviour

One or more of the following would address this:

1. Statusline JSON includes a sub_agents array with per-agent { name, context_window } entries when a team is active, allowing the statusline command to render e.g. agents: planner 45% | implementer 12%.

1. A shared telemetry file or IPC channel that sub-agents can write their context snapshots to, and the statusline (or any script) can read from.

1. An environment variable or flag injected into sub-agent shells containing the agent's own used_percentage, so agents could self-report to a shared file.

Use case

Working with multi-agent teams on long tasks, it is useful to know which sub-agent is approaching its context limit before it silently degrades or loses earlier context. Currently the only signal is when a sub-agent starts producing lower-quality output.

Workaround attempted

Sub-agents could write to /tmp files manually, but they have no access to their own context usage percentage — that data is only injected into the statusline JSON by Claude Code, not available inside a running agent's environment.

Environment

• Claude Code v2.1.63
• Platform: Windows 11 / Git Bash
• Feature: agent teams (CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1)