Let notices target a lineage conversation: recipient.conversation / recipient.root matched against request.lineage

[ScriptedAlchemy]

Problem

A notice cannot be addressed to one agent conversation. AgentRecipient has actor | host | session | workspace, and the notice principal is built from the same four axes — lineage is not part of it. So a plugin that wants to tell the other subagent something ("you and I are both editing src/shared.ts") can only target:

• recipient.session — on Claude/Codex every subagent's hooks carry the root session_id, so this reaches every agent under the root, not one of them (on Cursor conversation_id doubles as the session id, so it happens to work there);
• recipient.workspace — reaches every agent whose cwd is that directory, which is what examples/worktree-proximity does today (one actor per worktree is the assumption that makes it work);
• recipient.actor — never matches in event scopes: actor is the HTTP-authenticated MCP client only (Actor principal for generated event scopes: typed unavailable reasons and the one derivable host field (parked from #233/#223) #391 closed as not planned, feat(lineage): request.lineage is the only identity-adjacent surface; scoped, self-correcting Cursor child binding #444).

The #99 scope note (2026-09-03) says directed notices "should target session, workspace, or the conversation tree via request.lineage" — but the kernel has no recipient axis for that yet.

Evidence (main @ 284141958)

• packages/rsc-runtime/src/notices/contract.ts:34-39 (AgentRecipient), :41-46 (AgentNoticePrincipal: actor/host/session/workspace).
• packages/rsc-runtime/src/agent-request.ts:493-499 — openRequest({ principal: { actor, host, session, workspace } }); lineage is on the request (:489, :505) but not on the principal.
• packages/rsc-runtime/src/notices/state.ts:185-202 — recipientMatchesPrincipal compares the four axes only.
• packages/agent-bundle/src/mcp-server-runtime.ts:134-150 — actor is http.authInfo.clientId; :759-788 — event scopes mount host/session/workspace/lineage, no actor. Same for standalone hooks, packages/agent-bundle/src/adapters/hook-contract.ts:730-741.
• packages/rsc-runtime/src/lineage-native.ts:40-54 — Claude/Codex: conversation = agent_id ?? session_id, root = session_id; Cursor: conversation = conversation_id.
• examples/worktree-proximity/src/events/tool/before.tsx:67-82 — publishes with recipient: { workspace: { root: conflict.worktreeRoot } } while dedupeKey still names the target actor.
• examples/worktree-proximity/README.md:96-102: "Generated event principals in v1 mount host, session, and workspace identity, but not actor identity. Proximity notices therefore target the recipient worktree through recipient.workspace.root … Actor-directed delivery remains future work tied to actor-principal mounting in the Add recipient-aware notices and capability-gated delivery #99/Track remaining #98 state-kernel scope: warm-runtime process state, generated-runtime state mounting, budgets, lifetime visibility #233 lineage."
• README.md:77-81: tool envelopes carry no agent_id, so peers in one directory collapse to the derived worktree:<root> identity — the workspace axis cannot separate two agents in one worktree.

Proposed shape (small)

interface AgentRecipient {
  // …existing…
  /** Matches when request.lineage.conversation === conversation (a specific agent thread). */
  readonly conversation?: string;
  /** Matches when request.lineage.root === root (every agent under one root). */
  readonly root?: string;
}
interface AgentNoticePrincipal {
  // …existing…
  readonly lineage: Observed<AgentLineage>;
}

recipientMatchesPrincipal treats an unavailable lineage like an unavailable axis today (no match). Ledger schema gains two optional recipient fields — no state-definition id bump needed if the schema is additive (state.ts notice schema). The next-event admission, inbox, resources/updated, and receipt discipline are unchanged; attempted stays the strongest state, exactly as #99 records.

Design question (from the same use-case): is a thin peer-messaging façade worth adding on top — notices.send({ to: { conversation }, content }) / notices.read() — or is the recipient axis enough and the façade stays app-level? The #99 survey already establishes that no pinned host offers a directed push channel, so the façade would only be sugar over publish + next-event admission.

Out of scope / related

• No operator/user identity axis (Actor principal for generated event scopes: typed unavailable reasons and the one derivable host field (parked from #233/#223) #391, feat(lineage): request.lineage is the only identity-adjacent surface; scoped, self-correcting Cursor child binding #444). actor keeps its current meaning.
• Add recipient-aware notices and capability-gated delivery #99 owns the notice kernel and its remaining redaction/retention item (feat(notices): redaction contract and retention policy — close out #99 acceptance item 7 #437); this is proposed as a small follow-on there, filed separately so it does not block Add recipient-aware notices and capability-gated delivery #99's close-out.
• Registry quality per host: Claude Code 2.1.257 lineage gaps: SubagentStart names no parent; a subagent's hooks name no parent #422 / Codex 0.147.0 lineage gaps: SubagentStart names no parent thread; hooks lag the MCP _meta lineage #423 / Cursor 3.18.25 lineage gaps: subagentStart omits the child conversation id; child hooks carry no parent/root; MCP _meta carries no conversation; sessionStart not dispatched to plugin hooks on desktop #424.

Acceptance

In examples/worktree-proximity: tool/before.tsx publishes with recipient: { conversation: <other actor's lineage conversation> }; the route-unit test "attempts and surfaces a notice on the recipient next event" (tests/route-unit/routes.test.ts) passes with two actors bound to the same worktree (today impossible), and the cross-process journey suite (packages/agent-bundle/tests/worktree-proximity-journeys.test.ts) still proves workspace-directed delivery for the existing journeys. A new ledger test proves a conversation-addressed notice is not admitted on a sibling's event.

[ScriptedAlchemy]

Related: #457 (read the lineage tree — supplies the peer conversation id this recipient axis targets), #459 (providers see lineage/state), #460 (publisher-scoped notice visibility). Context: #99 (notice kernel, scope note 2026-09-03), #391 / #444 (no actor-principal derivation).