ADR 0165: Withdraw the A2A / Peer coordination stack

• Status: Accepted
• Date: 2026-09-05
• Deciders: PI-Desktop core
• Related: D326, D277, D318, D321, D325, ADR 0062, ADR 0089, 03-runtime/02-agent-runtime.md §5f.2, 03-runtime/03-tools-and-permissions.md §10.2, 03-runtime/06-host-rpc-protocol.md §3–§4, E2E-165
• Supersedes: ADR 0147, ADR 0162, and ADR 0164. ADR 0138 and ADR 0140 remain superseded by ADR 0147; this ADR withdraws the replacement those records described. Historical ADR files are retained.

Context

ADR 0138 introduced in-process subagent peer messaging (PeerSend / PeerInbox / PeerWait). ADR 0140 folded those into one Peer tool. ADR 0147 replaced that mailbox with a host-core Agent2Agent (A2A) broker, an a2a.* RPC domain, durable a2a_* SQLite tables, and a delegate A2A tool. ADR 0162 then lifted same-session-only addressing; ADR 0164 / D325 gave Agent-mode parents a core A2A tool so conversations could coordinate.

That stack is no longer wanted. Sibling and parent-to-parent channels sat beside the existing Task / TaskWait / TaskList / TaskStop contract, added a protocol capability the rest of the app did not use, and were easy for the model to miss or misuse. Concurrent delegates already report through the parent; that is enough.

Decision

Remove the A2A / Peer coordination stack from the product:

1. No A2A or Peer tool. Neither name is a core Agent tool, an assignable subagent tool, or a ToolSearch result. A definition that lists A2A or Peer is treated like any other unknown tool name: dropped with a parse warning.
2. No broker. Delete the host-core a2a module, the in-memory registry, the leftover in-process SubagentMailbox, and every a2a.* RPC method and a2a.task.event / a2a.push notification.
3. No parent or sibling channel. Concurrent delegates coordinate only by writing self-contained reports the parent collects through Task*. Parents do not discover, message, or wait on other conversations.
4. Protocol v11. Handshake PROTOCOL_VERSION moves from 10 to 11. The capabilities array no longer advertises "a2a". A v10 host or client is rejected at handshake, the same exact-match rule as every prior bump.
5. Schema v13. SCHEMA_VERSION moves from 12 to 13. migrate_v12_to_v13 drops the A2A tables (a2a_tasks, a2a_messages, a2a_artifacts, a2a_push_configs). Fresh databases never create them. The v11→v12 step remains in the chain as a version-only bump so existing backups and chained migrations keep working; it no longer creates tables that v13 would immediately drop.

Delegation itself (ADR 0062 / ADR 0089) is unchanged: Task still fans out bounded workers, reports still stay out of the parent model context until TaskWait, and builtins stay read-only by default.

Consequences

• Cross-conversation and sibling messaging are gone. Users who want two sessions to share facts do so in the prompt, not through a tool.
• The roundtable example plugin no longer tells delegates to call Peer or A2A. It runs independent concurrent Tasks and has the parent synthesize the reports, optionally feeding earlier reports into a later round's brief.
• Historical changelog entries for 0.11.3 (peer messaging) and 0.12.0 (A2A) stay as shipped history. Product READMEs drop current A2A claims.
• ADRs 0138, 0140, 0147, 0162, and 0164 remain on disk as records.