diff --git a/docs/rfc/proposed/feature/2026-06-21-subagent-capability-seam.md b/docs/rfc/proposed/feature/2026-06-21-subagent-capability-seam.md
index 02c4fa36b4..d99637b550 100644
--- a/docs/rfc/proposed/feature/2026-06-21-subagent-capability-seam.md
+++ b/docs/rfc/proposed/feature/2026-06-21-subagent-capability-seam.md
@@ -6,7 +6,7 @@ Status: proposed
 
 ## Problem
 
-The harness has a long-deferred seam for **subagents** — an agent delegating work to another agent. The intent is sketched in two `TODO(sub-agents)` markers ([packages/core/agent/src/types.ts](../../../../packages/core/agent/src/types.ts), [packages/core/agent-loop/src/index.ts](../../../../packages/core/agent-loop/src/index.ts)): a creation option referencing a parent agent (fork = seed the child session with the parent's event log; spawn = fresh session), with the child returned as an `Agent` handle so steering and event subscription work uniformly. No service, vocabulary, or implementation exists yet.
+The harness has a long-deferred seam for **subagents** — an agent delegating work to another agent. The intent was sketched in the `Agent`/`AgentLoop` interfaces ([packages/core/agent/src/types.ts](../../../../packages/core/agent/src/types.ts), [packages/core/agent-loop/src/index.ts](../../../../packages/core/agent-loop/src/index.ts)): a creation option referencing a parent agent (fork = seed the child session with the parent's event log; spawn = fresh session), with the child returned as an `Agent` handle so steering and event subscription work uniformly. This RFC realizes that seam (see the implementation-status banner above for what has landed); the design below is the proposal it was argued from, when no service, vocabulary, or implementation yet existed.
 
 The distinctive requirement — the one that shapes the whole design — is that **multiple subagent implementations must coexist at runtime**. A parent may want a cheap in-process child for a scoped subtask AND an isolated out-of-process child (over ACP) in the same session. The transports we foresee:
 
diff --git a/docs/rfc/proposed/simplification/2026-06-20-unify-agent-and-session-id.md b/docs/rfc/proposed/simplification/2026-06-20-unify-agent-and-session-id.md
index 19bde62d01..2455981ba8 100644
--- a/docs/rfc/proposed/simplification/2026-06-20-unify-agent-and-session-id.md
+++ b/docs/rfc/proposed/simplification/2026-06-20-unify-agent-and-session-id.md
@@ -46,7 +46,7 @@ The genuine risks of collapsing the two ids into one (the case AGAINST this prop
 
 - **It forecloses a one-agent-resumes-many-sessions / one-session-driven-by-many-agents future.** Today the separate ids leave room for an agent (a stable actor) to detach from one session and attach to another, or for a handoff where a new agent process adopts an existing session under a new actor handle. Unifying makes "agent" and "session" the same lifetime, so any such future needs a NEW seam (e.g. an explicit `actorId` distinct from the session) — re-introducing the very separation we removed. We judge this generality currently unused, but it is a door this change closes.
 
-- **Sub-agents / fork / spawn (an explicitly deferred seam) may WANT a stable actor id across forked sessions.** `AgentLoop.create`'s `TODO(sub-agents)` envisions a child agent seeded from a parent's event log. If the design wants "the same agent identity across a fork" (parent and child share an actor but have distinct session logs), a unified id blocks it. The implementing PR must check the intended fork/spawn model BEFORE unifying, or accept that fork always mints a fresh combined id.
+- **Subagents / fork / spawn may WANT a stable actor id across forked sessions.** The [subagent seam](../feature/2026-06-21-subagent-capability-seam.md) runs a child agent seeded from a parent's event log (fork). If a future design wants "the same agent identity across a fork" (parent and child share an actor but have distinct session logs), a unified id blocks it. The implementing PR must check the intended fork/spawn model BEFORE unifying, or accept that fork always mints a fresh combined id. (As shipped, each subagent child mints its own distinct agent id — `parentSession` records lineage — so the seam does not currently rely on a shared actor id, but unifying would foreclose adding one.)
 
 - **The config-driven resume-or-create policy becomes load-bearing, not cosmetic.** Today the per-run-uuid session id quietly sidesteps the "a fixed id collides with its own on-disk log on the second run" problem. Once the id is unified and stable, a config agent restarting MUST decide resume-vs-fresh deliberately — there is no longer a throwaway session id to hide behind. Getting this wrong reintroduces the create-collision the uuid was avoiding (a durable backend refuses to re-create an id whose log exists). This is the one real design decision the implementing PR owns, and it is easy to get subtly wrong.
 
diff --git a/packages/subagent/subagent-spawn/src/in-process.ts b/packages/subagent/subagent-spawn/src/in-process.ts
index 3b04b55c86..e121611697 100644
--- a/packages/subagent/subagent-spawn/src/in-process.ts
+++ b/packages/subagent/subagent-spawn/src/in-process.ts
@@ -143,6 +143,11 @@ export function startInProcessRun(
 
   const result: Promise<SubagentResult> = (async () => {
     try {
+      // A signal already aborted BEFORE the run starts never fires an `abort`
+      // event (`addEventListener` only fires on the transition), so the listener
+      // above won't catch it — settle `aborted` without running the child rather
+      // than completing an already-cancelled request.
+      if (request.signal?.aborted) return { output: [], stopReason: 'aborted' }
       child.send(request.prompt)
       await child.whenIdle()
       return readResult(child, seedLength, cancelled)
diff --git a/packages/subagent/subagent-spawn/tests/subagent-spawn.spec.ts b/packages/subagent/subagent-spawn/tests/subagent-spawn.spec.ts
index 4cbf1ca9d3..c57819d86a 100644
--- a/packages/subagent/subagent-spawn/tests/subagent-spawn.spec.ts
+++ b/packages/subagent/subagent-spawn/tests/subagent-spawn.spec.ts
@@ -128,6 +128,22 @@ describe('dsh-subagent-spawn', () => {
     await run.dispose()
   })
 
+  it('settles aborted (without running the child) when the request signal is ALREADY aborted', async () => {
+    // Regression: a signal aborted BEFORE the run starts never fires an `abort`
+    // event, so the listener can't catch it. The driver must check the
+    // already-aborted case up front and settle `aborted` without running the
+    // child — otherwise an already-cancelled request runs to `completed`. The
+    // empty script proves the child's model is never called.
+    const controller = new AbortController()
+    controller.abort()
+    const { ctx, parent } = await setup([])
+    const run = ctx.subagents.start('spawn', { prompt: [{ type: 'text', text: 'p' }], parent, signal: controller.signal })
+    const result = await run.result
+    expect(result.stopReason).toBe('aborted')
+    expect(result.output).toEqual([])
+    await run.dispose()
+  })
+
   it('cancelling BEFORE the child turn starts settles aborted, not error', async () => {
     // Regression: a cancel landing in the pre-turn window clears the queued
     // prompt before any `turn/end` is logged. Deriving the stop reason from