diff --git a/docs/architecture.md b/docs/architecture.md
index 2b05e613b1..612e73ddce 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -116,7 +116,7 @@ Tool schemas are deliberately **part of the assembly**: "what the model is told
 - `whenIdle()` — resolves once the agent reaches quiescence after settling out of `running` (resolves immediately when already idle; awaits the loop exit when disposed). A non-owner's quiescence-observation hook: it lets a consumer await the current work settling **without** disposing the agent. It is NOT teardown — it does not stop queued work, unregister the agent, or detach the session; a lifecycle owner tears an agent down with `await AgentHandle.dispose()` (which stops the loop, awaits its exit, and unregisters).
 - `session`, `status`, `options`
 
-**TODO(sub-agents)**: `spawn`/`fork` land on `AgentLoop.create()` — fork seeds the child Session with the parent's event log, spawn starts fresh; children are ordinary `Agent` handles so `steer()` and event subscription work uniformly. Inter-agent channels beyond these primitives are deliberately deferred.
+**Subagents**: `spawn`/`fork` are realized by the [`@deepseek-ai/dsh-subagent`](../packages/subagent/subagent) seam (a named-provider registry on `ctx.subagents`), not a method on `Agent`. The in-process backends create the child via `ctx.agents.create` — fork seeds the child Session with a balanced completed-turn prefix of the parent's log (`CreateAgentOptions.seed`), spawn starts fresh; children are ordinary `Agent` handles so `steer()` and event subscription work uniformly. Out-of-process transports (ACP, and later A2A / Codex app-server / Claude Code SDK) register as sibling providers. See [docs/core-data-structures/subagent.md](core-data-structures/subagent.md) and [the subagent RFC](rfc/proposed/feature/2026-06-21-subagent-capability-seam.md). Inter-agent channels beyond delegation remain deferred.
 
 ### Loop lifecycle (session / turn / step)
 
diff --git a/docs/cordis-catalog/events-and-services.md b/docs/cordis-catalog/events-and-services.md
index 36edfae505..e0fe1cd6c2 100644
--- a/docs/cordis-catalog/events-and-services.md
+++ b/docs/cordis-catalog/events-and-services.md
@@ -25,7 +25,7 @@ An agent was registered in the AgentRegistry and is ready to receive messages.
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:136`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:137`](../../packages/core/agent/src/types.ts)
 
 #### `agent/disposed` — emit
 
@@ -37,7 +37,7 @@ An agent was disposed and removed from the registry; its fiber and any in-flight
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:142`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:143`](../../packages/core/agent/src/types.ts)
 
 #### `agent/error` — emit
 
@@ -49,7 +49,7 @@ A step or turn errored. The loop reports a failure here (plus the logger) even w
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:219`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:220`](../../packages/core/agent/src/types.ts)
 
 #### `agent/queued` — emit
 
@@ -61,7 +61,7 @@ A message entered the agent's inbox (queued or steering). `source` is the resolv
 
 Types: [Agent](../core-data-structures/core.md) · [ContentBlock](../core-data-structures/core.md) · [MessageSource](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:155`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:156`](../../packages/core/agent/src/types.ts)
 
 #### `agent/request` — waterfall
 
@@ -73,7 +73,7 @@ Waterfall: mutate the fully-assembled GenerateOptions before the model call (hoo
 
 Types: [Agent](../core-data-structures/core.md) · [GenerateOptions](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:188`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:189`](../../packages/core/agent/src/types.ts)
 
 #### `agent/status` — emit
 
@@ -85,7 +85,7 @@ Agent status changed (`idle` ⇄ `running`, or → `disposed`). Drive lifecycle
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:149`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:150`](../../packages/core/agent/src/types.ts)
 
 #### `agent/steering` — emit
 
@@ -97,7 +97,7 @@ Steering content was injected into a running turn.
 
 Types: [Agent](../core-data-structures/core.md) · [ContentBlock](../core-data-structures/core.md) · [MessageSource](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:213`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:214`](../../packages/core/agent/src/types.ts)
 
 #### `agent/step-end` — emit
 
@@ -109,7 +109,7 @@ A step ended.
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:179`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:180`](../../packages/core/agent/src/types.ts)
 
 #### `agent/step-result` — waterfall
 
@@ -121,7 +121,7 @@ Waterfall: post-process the assembled assistant Message before tool dispatch (va
 
 Types: [Agent](../core-data-structures/core.md) · [Message](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:194`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:195`](../../packages/core/agent/src/types.ts)
 
 #### `agent/step-start` — emit
 
@@ -133,7 +133,7 @@ A step (one model call plus its tool dispatch) began. `step` is 1-based within t
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:174`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:175`](../../packages/core/agent/src/types.ts)
 
 #### `agent/stream-chunk` — emit
 
@@ -145,7 +145,7 @@ A raw StreamChunk arrived from the model (token-level UI/log feed).
 
 Types: [Agent](../core-data-structures/core.md) · [StreamChunk](../core-data-structures/llm-streaming.md)
 
-Source: [`packages/core/agent/src/types.ts:208`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:209`](../../packages/core/agent/src/types.ts)
 
 #### `agent/turn-continuation` — waterfall
 
@@ -157,7 +157,7 @@ Waterfall: override the turn-continuation decision. The default (computed by the
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:201`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:202`](../../packages/core/agent/src/types.ts)
 
 #### `agent/turn-end` — emit
 
@@ -169,7 +169,7 @@ A turn ended. `reason` distinguishes a clean stop from a truncated or aborted on
 
 Types: [Agent](../core-data-structures/core.md) · [TurnEndReason](../core-data-structures/session.md)
 
-Source: [`packages/core/agent/src/types.ts:168`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:169`](../../packages/core/agent/src/types.ts)
 
 #### `agent/turn-start` — emit
 
@@ -181,7 +181,7 @@ A turn began. `turn` is the 1-based turn number within the session.
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:162`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:163`](../../packages/core/agent/src/types.ts)
 
 ### `llm/*`
 
diff --git a/docs/core-data-structures/core.md b/docs/core-data-structures/core.md
index 117d1e3606..49c1559c40 100644
--- a/docs/core-data-structures/core.md
+++ b/docs/core-data-structures/core.md
@@ -273,11 +273,12 @@ interface Agent {
    */
   whenIdle(): Promise<void>
 
-  // TODO(sub-agents): spawn/fork seams — semantics deliberately deferred.
-  // The intended shape: 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 steer()
-  // and event subscription work uniformly. See docs/architecture.md.
+  // Subagent delegation is realized on top of this interface by the
+  // `@deepseek-ai/dsh-subagent` seam, not by a method here: a backend creates
+  // the child through `ctx.agents.create` (fork seeds the child Session with a
+  // balanced prefix of the parent's log via `CreateAgentOptions.seed`; spawn
+  // starts fresh) and drives it as an ordinary Agent handle, so steer() and
+  // event subscription work uniformly. See docs/core-data-structures/subagent.md.
 }
 ```
 
diff --git a/examples/coding-agent/cordis.yml b/examples/coding-agent/cordis.yml
index c8371d3cba..136031062a 100644
--- a/examples/coding-agent/cordis.yml
+++ b/examples/coding-agent/cordis.yml
@@ -44,7 +44,7 @@
     # under ./.sessions); unset starts a fresh session each run.
     resumeSessionId: !!js process.env.RESUME_SESSION_ID
     persistenceRoot: './.sessions'
-    welcome: 'coding-agent ready. Give it a coding task (bash is its only tool).'
+    welcome: 'coding-agent ready. Give it a coding task (its tools are bash and subagent).'
     systemPrompt: |
       You are coding-agent, a CLI coding assistant.
 
diff --git a/packages/core/agent-loop/src/index.ts b/packages/core/agent-loop/src/index.ts
index 3179674366..e5393ed0aa 100644
--- a/packages/core/agent-loop/src/index.ts
+++ b/packages/core/agent-loop/src/index.ts
@@ -121,10 +121,6 @@ export class AgentLoop extends Service implements AgentFactory {
    * deliberate resume-or-create policy (resume the prior session if one exists,
    * else start fresh) or an explicit caller-chosen session id — revisit when the
    * UI/ACP path owns session selection.
-   *
-   * TODO(sub-agents): spawn/fork land here — accept a parent agent reference;
-   * fork seeds the new Session with the parent's event log, spawn starts
-   * fresh; the child is returned as a regular Agent handle.
    */
   create(id: AgentId, options: AgentOptions = {}): ReactLoopAgent {
     this.assertAgentIdFree(id)
diff --git a/packages/core/agent/src/types.ts b/packages/core/agent/src/types.ts
index 6d27bf7256..efe392155c 100644
--- a/packages/core/agent/src/types.ts
+++ b/packages/core/agent/src/types.ts
@@ -118,11 +118,12 @@ export interface Agent {
    */
   whenIdle(): Promise<void>
 
-  // TODO(sub-agents): spawn/fork seams — semantics deliberately deferred.
-  // The intended shape: 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 steer()
-  // and event subscription work uniformly. See docs/architecture.md.
+  // Subagent delegation is realized on top of this interface by the
+  // `@deepseek-ai/dsh-subagent` seam, not by a method here: a backend creates
+  // the child through `ctx.agents.create` (fork seeds the child Session with a
+  // balanced prefix of the parent's log via `CreateAgentOptions.seed`; spawn
+  // starts fresh) and drives it as an ordinary Agent handle, so steer() and
+  // event subscription work uniformly. See docs/core-data-structures/subagent.md.
 }
 
 declare module 'cordis' {
diff --git a/packages/subagent/subagent-fork/tests/subagent-fork.spec.ts b/packages/subagent/subagent-fork/tests/subagent-fork.spec.ts
index d550101cb9..96cdd55141 100644
--- a/packages/subagent/subagent-fork/tests/subagent-fork.spec.ts
+++ b/packages/subagent/subagent-fork/tests/subagent-fork.spec.ts
@@ -10,11 +10,16 @@ import AgentLoop from '@deepseek-ai/dsh-agent-loop'
 import * as Invariants from '@deepseek-ai/dsh-invariants'
 import SubagentService from '@deepseek-ai/dsh-subagent'
 import { MockAdapter, textResponse } from '../../../core/agent-loop/tests/mock-adapter.ts'
+import type { StreamChunk } from '@deepseek-ai/dsh-llm'
 import * as fork from '../src/index.ts'
 import { completedTurnPrefix } from '../src/index.ts'
 
 type Script = ConstructorParameters<typeof MockAdapter>[0]
 
+/** A bare `stop` finish that streams no content → the turn ends `completed`
+ * with NO `assistant/message` of its own. */
+const emptyStop: StreamChunk[] = [{ type: 'finish', reason: { kind: 'stop' } }]
+
 /**
  * Drives the REAL fork backend with a real loop + scripted mock MODEL + the
  * real dsh-invariants plugin. The invariants plugin re-replays a seeded child
@@ -132,6 +137,26 @@ describe('dsh-subagent-fork', () => {
     await run.dispose()
   })
 
+  it('does NOT return the seeded parent output when the child produces no message of its own', async () => {
+    // Regression: readResult must scope to the child's OWN events (after the
+    // seed). The parent completes a turn with a distinctive assistant message,
+    // then the fork child's own turn finishes with a bare `stop` and NO
+    // assistant/message. Scanning the whole (seeded) log would return the
+    // parent's "parent stale" message with stopReason 'completed'; scoped to the
+    // child's own events the output is empty.
+    const { ctx, parent } = await setup([textResponse('parent stale'), emptyStop])
+    parent.send([{ type: 'text', text: 'parent question' }])
+    await parent.whenIdle()
+
+    const run = ctx.subagents.start('fork', { prompt: [{ type: 'text', text: 'child question' }], parent })
+    const result = await run.result
+    // The child completed its own (empty) turn — completed, but with NO output
+    // borrowed from the seeded parent prefix.
+    expect(result.stopReason).toBe('completed')
+    expect(result.output).toEqual([])
+    await run.dispose()
+  })
+
   it('advertises depthLimit but not outputSchema/toolFilter', async () => {
     const { ctx } = await setup([])
     expect(ctx.subagents.getProvider('fork')!.capabilities).toEqual({ outputSchema: false, depthLimit: true, toolFilter: false })
diff --git a/packages/subagent/subagent-spawn/src/in-process.ts b/packages/subagent/subagent-spawn/src/in-process.ts
index 4b40bb667d..3b04b55c86 100644
--- a/packages/subagent/subagent-spawn/src/in-process.ts
+++ b/packages/subagent/subagent-spawn/src/in-process.ts
@@ -99,6 +99,11 @@ export function startInProcessRun(
   }
 
   const childId = AgentId(randomUUID())
+  // The child's OWN events begin after the seed (fork seeds the parent's
+  // completed-turn prefix; spawn seeds nothing). `readResult` scopes to this
+  // boundary so a child that produces no message of its own never returns the
+  // SEEDED parent's last assistant message as its result.
+  const seedLength = options.seed?.length ?? 0
   const parentHeader = request.parent.session.header
   // Inherit the parent's model by default (a child with no model cannot run);
   // an explicit `request.agentOptions.model` overrides it. The parent's
@@ -124,14 +129,23 @@ export function startInProcessRun(
 
   // Bridge the request's abort signal to the child (the consumer also bridges
   // its own exec.signal, but a backend-level bridge keeps the contract local).
-  const onAbort = (): void => { child.cancel('subagent cancelled') }
+  // `cancelled` records that a cancel was requested at all, so the pre-turn
+  // cancel window — where the child clears the queued prompt before any
+  // `turn/end` is logged — settles as `aborted` (honoring the cancel contract)
+  // rather than falling through to the no-turn `error` mapping.
+  let cancelled = false
+  const requestCancel = (reason: string): void => {
+    cancelled = true
+    child.cancel(reason)
+  }
+  const onAbort = (): void => { requestCancel('subagent cancelled') }
   request.signal?.addEventListener('abort', onAbort, { once: true })
 
   const result: Promise<SubagentResult> = (async () => {
     try {
       child.send(request.prompt)
       await child.whenIdle()
-      return readResult(child)
+      return readResult(child, seedLength, cancelled)
     } finally {
       request.signal?.removeEventListener('abort', onAbort)
     }
@@ -141,7 +155,7 @@ export function startInProcessRun(
     id: childId,
     result,
     cancel(reason?: string): void {
-      child.cancel(reason ?? 'subagent cancelled')
+      requestCancel(reason ?? 'subagent cancelled')
     },
     async dispose(): Promise<void> {
       request.signal?.removeEventListener('abort', onAbort)
@@ -151,14 +165,22 @@ export function startInProcessRun(
 }
 
 /**
- * Read a settled child's terminal result from its session log: the last
- * `assistant/message` content (deep-cloned — the log is frozen) and the last
- * `turn/end` reason mapped to a {@link SubagentStopReason}.
+ * Read a settled child's terminal result from its session log, scoped to the
+ * child's OWN events (everything at or after `seedLength` — fork seeds the
+ * parent's completed-turn prefix, so a child that produced no message of its
+ * own must NOT return the seeded parent's last assistant message). The output
+ * is the child's last `assistant/message` content (deep-cloned — the log is
+ * frozen); the stop reason is the child's last `turn/end` reason mapped to a
+ * {@link SubagentStopReason}. When `cancelled` is set but no `turn/end` was
+ * logged (a cancel landed in the pre-turn window, before any turn ran), the
+ * run settles `aborted` per the {@link SubagentRun.cancel} contract rather than
+ * the generic no-turn `error`.
  */
-function readResult(child: Agent): SubagentResult {
-  const events = child.session.events
-  const lastMessage = events.findLast((e): e is SessionEvent<'assistant/message'> => e.type === 'assistant/message')
-  const lastEnd = events.findLast((e): e is SessionEvent<'turn/end'> => e.type === 'turn/end')
+function readResult(child: Agent, seedLength: number, cancelled: boolean): SubagentResult {
+  const own = child.session.events.slice(seedLength)
+  const lastMessage = own.findLast((e): e is SessionEvent<'assistant/message'> => e.type === 'assistant/message')
+  const lastEnd = own.findLast((e): e is SessionEvent<'turn/end'> => e.type === 'turn/end')
   const output: ContentBlock[] = lastMessage ? structuredClone(lastMessage.data.content) : []
+  if (lastEnd === undefined && cancelled) return { output, stopReason: 'aborted' }
   return { output, stopReason: toStopReason(lastEnd?.data.reason) }
 }
diff --git a/packages/subagent/subagent-spawn/tests/subagent-spawn.spec.ts b/packages/subagent/subagent-spawn/tests/subagent-spawn.spec.ts
index 830aa81ec8..4cbf1ca9d3 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('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
+    // `turn/end` alone then mis-maps the no-turn case to `error`; the run must
+    // honor the cancel contract and settle `aborted`. The cancel is synchronous
+    // (same tick as start, before the loop's queued-wait continuation runs), so
+    // the turn is dropped and the empty script is never consumed.
+    const { ctx, parent } = await setup([])
+    const run = ctx.subagents.start('spawn', { prompt: [{ type: 'text', text: 'p' }], parent })
+    run.cancel('early')
+    const result = await run.result
+    expect(result.stopReason).toBe('aborted')
+    expect(result.output).toEqual([])
+    await run.dispose()
+  })
+
   it('cancelling a running child settles the run as aborted (the abort bridge + cancel())', async () => {
     // 'hang' makes the child's model stream one chunk then wait until aborted.
     const controller = new AbortController()
diff --git a/packages/subagent/subagent/src/index.ts b/packages/subagent/subagent/src/index.ts
index c7d954f09c..356ad60a00 100644
--- a/packages/subagent/subagent/src/index.ts
+++ b/packages/subagent/subagent/src/index.ts
@@ -70,7 +70,7 @@ declare module 'cordis' {
 export interface SubagentRunInfo {
   /** The provider that started the run. */
   provider: string
-  /** The child agent/session id. */
+  /** The child agent's id. */
   id: AgentId
 }
 
@@ -78,7 +78,7 @@ export interface SubagentRunInfo {
 export interface SubagentRunEndInfo {
   /** The provider that ran it. */
   provider: string
-  /** The child agent/session id. */
+  /** The child agent's id. */
   id: AgentId
   /** The terminal stop reason. */
   stopReason: SubagentResult['stopReason']
diff --git a/packages/subagent/subagent/src/types.ts b/packages/subagent/subagent/src/types.ts
index 0e04acb317..fb60d5667c 100644
--- a/packages/subagent/subagent/src/types.ts
+++ b/packages/subagent/subagent/src/types.ts
@@ -122,7 +122,7 @@ export interface SubagentResult {
  * presence of the method IS the capability — narrow before calling.
  */
 export interface SubagentRun {
-  /** The child agent's id (also its session id token, for correlation). */
+  /** The child agent's id (use `ctx.agents.get(id)` to reach the live child). */
   readonly id: AgentId
   /**
    * Resolves with the child's terminal {@link SubagentResult} when the run