[Feature] Compact a running background session from outside (no external trigger exists)

What I'm trying to do

I run long-lived background agent sessions (claude --agent <role> --bg) as workers. A session finishes a task, reports, and stays alive to take the next one. Between tasks is exactly the "natural break" the docs recommend compacting at:

To choose when its overhead happens, run /compact at a natural break in your work, such as between tasks, instead of waiting for auto-compaction to trigger mid-task. — https://code.claude.com/docs/en/prompt-caching (Compacting the conversation)

The gap

There is no way to compact a running session from outside it, so that advice cannot be followed for background agents:

• /compact is a slash command typed by a human inside an interactive session. A background session has no human at its prompt.
• claude agents has no compact verb (checked claude agents --help, v2.1.241).
• The "Resume from summary" dialog (https://code.claude.com/docs/en/sessions) applies to claude --resume of a session that is not running, and is interactive.
• A message sent to the session over cross-session messaging is just a user turn; it cannot invoke a slash command.

That leaves --autocompact <tokens> as the only lever, and a threshold fires wherever the context happens to cross it, which is mid-task by construction. Concretely: my lanes measure 134k–188k tokens per task, so a threshold at 250k lands partway through the next task, every time. No value of the threshold changes that — lower fires earlier inside the task, higher fires later inside a subsequent one.

What I'd like

Any external trigger for compaction of a running session. In rough order of usefulness to me:

1. claude agents compact <session-id> (optionally with focus instructions, like /compact <instructions>).
2. A control message over the cross-session messaging channel that the receiving session interprets as /compact rather than as user text.
3. A hook or SDK surface that can request compaction on the next turn boundary.

Why it matters

The cost case is already in your docs: after compaction, later requests carry the summary instead of the full history. For a worker that lives for hours across many tasks, being able to compact at a task boundary is the difference between every later turn carrying one task's history and carrying all of them. Today that saving is only reachable by killing the session and starting a new one, which throws away the state that made it worth keeping alive.

Environment

Claude Code 2.1.241, macOS (darwin 25.3), Claude subscription (1-hour cache TTL).