ADR 0164: Parent agents collaborate across conversations

• Status: Superseded by ADR 0165
• Date: 2026-09-05
• Deciders: PI-Desktop core
• Related: D321, D318, ADR 0147, ADR 0162, ADR 0062, 03-runtime/02-agent-runtime.md §5f.2, 03-runtime/06-host-rpc-protocol.md §4, E2E-165d
• Amends: ADR 0147 (parent must not call A2A) and ADR 0162 (no messaging with the parent). Subagent sibling coordination is unchanged.

Context

ADR 0147 kept the A2A tool out of the parent catalog so the parent could not open a second, weaker channel to its own delegates. ADR 0162 then let subagents address peers in other sessions, but still not the parent.

That split does not match how users collaborate: they want two conversations in the sidebar to coordinate, not two nested workers. The parent is the session's only agent that already has the user's context. Asking the user to spawn a discussant in each chat just to pass a note is the wrong seam.

The broker is already process-global. Each session runtime already lives in the sidecar map after the first prompt. The missing piece is that the parent never registers as an A2A client.

The remaining hazard is the original one: if the parent can address its own subagents, Task's brief and report boundary collapses.

Decision

1. Each Agent-mode session runtime registers a parent A2A agent for its lifetime (first prompt through dispose). The card uses kind: "parent", name parent (host-uniquified to parent-2, …), and a description that carries the session title. Plan/Goal runtimes do not register.
2. A2A is a core Agent-mode parent tool, present in the catalog from runtime construction — not after broker register. The first call (or the first prompt) mints the token. It stays out of Plan/Goal.
3. Kinds do not mix. Agent cards carry kind: "parent" | "subagent" (default subagent). a2a.agents.list and recipient resolution only consider the caller's kind. A parent cannot send to a subagent; a subagent cannot send to a parent. Same-session parent-to-delegate backchannel is therefore impossible.
4. Inbound notes join the next parent turn. Events addressed to the parent queue on its peer id. A parent that called A2A(wait) wakes as today. If the parent is idle, the next prompt() prepends a short A2A inbox to the user text so the model sees the other conversation's note without a separate auto-started turn.
5. Reachability is runtime-lifetime. A conversation is discoverable after it has an Agent runtime this process (after the first agent prompt) and until that runtime is disposed. Sidebar rows with no live runtime are not in the registry.

Unchanged: capability tokens, counterpart event routing, recipientContextId delivery, parent still uses Task* for its own delegates, no remote A2A, no nested delegation.

Consequences

• Two Agent conversations on the same host can discover / send / wait / complete with each other.
• A parent that omits to addresses the unique other live parent, or A2A_NO_PEERS when several exist — same rule ADR 0162 used for remote subagents.
• Subagent A2A no longer lists parent cards, so a roundtable cannot page the user through A2A.
• An idle conversation does not start a provider turn on inbound A2A; the note waits for the user's next message in that session.
• Not addressed: auto-waking an idle conversation, addressing a session that has never run this process, and parent-to-foreign-subagent.

Alternatives considered

• Keep parent A2A-less; users spawn discussants: rejected. That is not cross-conversation collaboration; it is nested worker coordination the user did not ask for.
• Let parents address every live agent: rejected. It reopens the parent→own-delegate backchannel ADR 0147 closed.
• Auto-start a turn in the idle session: rejected for this change. It spends a provider request the user did not send and needs a durable host turn the sidecar cannot mint. Inbox-on-next-prompt is enough to collaborate once both conversations are in use.