[Feature] Cross-session messaging between a user's own sessions

[cwdalziel]

Problem or use case

Today a session can only message its own parent/child (sub-agent) sessions. There's no way for two sessions that aren't in a spawn hierarchy, a session on another compute, or otherwise unrelated sessions the same user owns to message each other. This blocks coordinator/worker and multi-compute workflows where peers need to hand off or nudge each other directly.

Proposed solution

Add a sys_session_message tool that delivers a message to any session the caller can already access (bounded by the existing per-user permission model), gated behind a cross_session_messaging release flag. Delivery reuses the existing message path (persist-before-forward, idle-wake) and is fire-and-forget. Messages carry a source_session_id marker so peer messages are identifiable, plus a chain-depth loop guard to bound runaway back-and-forth.

Alternatives considered

No response

Harness

Not applicable

Platform or device

Not platform-specific

Harness mode

Not applicable

Expected reach

Some users

Authentication type

Local

[github-actions]

🤖 Automated triage

• Bot assessment: Medium impact
• Priority: P2-medium
• Why: Users building coordinator/worker or multi-compute workflows cannot directly hand off between independently created sessions, while ordinary session work remains available through parent/child messaging or other manual coordination.

This automated assessment uses the issue content and repository signals. Maintainers can override the priority label.

[github-actions]

Thanks for reporting this. #6642 may be related — could you take a look in case it already covers this?

If it turns out to be the same problem, please close this one and add your details there. Otherwise leave a note and we'll pick it up here.