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Allow tree-wide agent communication

Status: accepted

Current decision

Agents may address any retained agent in the same root session tree by canonical path. SendMessage may target the root or any agent. Agent-originated mail to /root can wake the idle root through the Monitor Background event scheduler; self-mail never starts a turn. Mail to idle non-root recipients starts their turn when capacity allows. WaitAgent wakes for mailbox activity from any source. The spawn tree defines identity, capacity ownership, and automatic child-completion delivery, but does not restrict explicit communication to parent-child edges.

Rationale and consequences

Direct peer communication avoids parent relay turns while retaining one session authority boundary. Idle-root notification follows the session's existing background-wake holds and current permissions, not sender-controlled dispatch. Automatic results still go to the spawn parent and can wake it, so a peer that requests work needs an explicit reply when it needs the result. See Agents.

Decision history

  • Idle-recipient wake (2026-10-19). SendMessage absorbed FollowupAgent and ordinary results now wake non-root parents too. ADR 0035 records the evidence and delivery contract; tree-wide addressing is unchanged.
  • Idle-root notification (2026-10). Root-directed messages and results previously stayed queued until another root turn, stranding late reports until the user spoke again. The root-only background-wake exception replaces the blanket no-start rule; explicit child activation and tree-wide addressing are unchanged. ADR 0035 owns delivery, workflow-consumption, and recovery semantics.

ADR 0027 restricted messages and follow-ups to tree edges to avoid hidden cross-branch dependencies and cycles. Its acknowledged cost was relaying through parents. ADR 0037 replaced that routing restriction with tree-wide addressing; identity and automatic result ownership still follow the spawn tree. The original records contain no measurement or incident explaining the reversal beyond this tradeoff.