The subagent seam — an agent delegating work to a child agent. Like [bash](bash.md) it is **one optional capability**, not part of the agent-loop spine, so its vocabulary lives here rather than in [core.md](core.md). But it differs from every other seam on one axis: **multiple provider implementations coexist** in one context, registered by name (`ctx.subagents`), where bash allows only one executor. The registry shape mirrors the [LLM adapter registry](llm-streaming.md), not the single-service bash executor.
Interface: [dsh-subagent](../../packages/subagent/subagent) (`ctx.subagents` + the vocabulary below). Implementations are sibling packages (`dsh-subagent-spawn`, `-fork`, `-acp`); the model-facing consumers are [dsh-tool-subagent](../../packages/subagent/tool-subagent) (per-provider delegation) and [dsh-tool-subagent-control](../../packages/subagent/tool-subagent-control) (the global `send_message`). Continuable-child orchestration lives on `ctx.subagentControl` in [dsh-subagent-control](../../packages/subagent/subagent-control). The proposals and rationale: [the subagent Agent Note](../../.agents/notes/implemented/feature/2026-06-21-subagent-capability-seam.md) and [the continuable background subagents Agent Note](../../.agents/notes/implemented/feature/2026-07-21-continuable-background-subagents.md).
A provider advertises its **start-time** features on a static descriptor the service checks BEFORE a run exists; a request that needs one the provider lacks is rejected loud (`SubagentError('UNSUPPORTED_CAPABILITY')`), never accepted-then-ignored. **Runtime** features are instead optional methods whose presence IS the capability, with TS narrowing as the discovery mechanism: strict live steering is [`SubagentRun.steer`](#a-live-run-subagentrun) and persisted cold resume is [`SubagentProvider.resume`](#the-provider-seam-subagentprovider).
The tool layer builds this request from the model input and its own config; the service validates it against the named provider before `start`. Required `parent` supplies the session cwd, lineage, and delegation depth. Optional output schema, depth, tool filter, and persona require matching capability flags. Unsupported schemas fail at start; in-process backends scope filters and personas to child creation and implement the supported object-rooted schema with a forced capture tool.
`signal` is the single cancellation channel before and after readiness. The [subagent composition-controls Agent Note](../../.agents/notes/implemented/feature/2026-07-12-subagent-persona-tool-filter-and-depth.md) owns the persona, live global-tool filter, absolute-depth, and visibility-not-authority rationale.
## Continuable children: `SubagentContinuation` and `SubagentResumeRequest`
A **continuable background subagent** is a durable child session with a series of Task-backed activations. `ctx.subagentControl` (`SubagentControlService` in [dsh-subagent-control](../../packages/subagent/subagent-control)) allocates the stable child id, snapshots the versioned `subagent/descriptor` payload, and passes both through the resolved start request; the provider publishes exactly that id and appends the descriptor inside the child's first turn. On follow-up, the control service loads the persisted child, authorizes the recorded `parentSession` as the direct parent, folds the descriptor, and dispatches a fully resolved resume request through `SubagentService.resume()` to `SubagentProvider.resume()`. The seam stays Task- and persistence-agnostic — descriptor lookup and Task association live only in the control service. `startContinuable()` returns a `ContinuableStart` (both identities), and `sendMessage()` returns a `SendMessageResult` reporting whether the message `steered` the running activation's existing Task or `started` a fresh one.
```ts type-equiv
/**
* The resolved continuable-child identity and durable composition record a
* control-service caller attaches to a start request.
*/
interface SubagentContinuation {
/** Control-allocated stable child session id, published verbatim. */
readonly sessionId: SessionId
/** Snapshotted descriptor persisted in the child log for cold resume. */
readonly descriptor: SubagentDescriptorData
}
```
```ts type-equiv
/**
* What a caller asks for when resuming a persisted continuable child. The
* control service loads the child log, folds and authorizes its descriptor,
* and passes this fully resolved request to
* {@link SubagentService.resume}, which dispatches to
* {@link SubagentProvider.resume}. The provider reconstructs the declared
* composition under the live parent's scope and drives one turn with `prompt`.
*/
interface SubagentResumeRequest {
/** The persisted child session id to resume. */
readonly sessionId: SessionId
/** The follow-up message that starts the resumed activation's turn. */
readonly prompt: ContentBlock[]
/**
* The live parent agent — the direct parent recorded in the persisted child
* header. In-process backends reconstruct the child under this agent's
* currently loaded scope.
*/
readonly parent: Agent
/**
* Activation-owned cancellation signal, created before descriptor lookup.
* Same pre/post-publication contract as {@link SubagentStartRequest.signal}:
* an abort before publication rejects after rollback quiescence, and an
* abort afterward cancels the published child turn.
*/
readonly signal: AbortSignal
/** The folded durable descriptor whose composition the provider reconstructs. */
readonly descriptor: SubagentDescriptorData
}
```
The descriptor (`SubagentDescriptorData` in [descriptor.ts](../../packages/subagent/subagent/src/descriptor.ts)) snapshots explicit fields — provider name, resolved child `agentOptions.provider`/`model`, optional `persona`/`toolFilter` — never the merge-extensible `AgentOptions` object, so an unrelated extension value cannot break continuation and a later composition input is a deliberate version change. It omits `subagentDepth` (cold resume trusts the persisted header's `delegationDepth` as the monotone floor) and `outputSchema` (an activation's result contract, not durable composition). The `subagent/descriptor` event is log-only: no `surfaceOp`, never in model history, and retained across compaction by the append-only log.
The outcome of a run, resolved by `SubagentRun.result`. `structured` is present only after a requested `outputSchema` was successfully satisfied; requesting a schema does not guarantee it, and a provider may return `stopReason: 'error'` when the child fails or finishes without a valid capture. A non-`completed` `stopReason` means `output` may be partial — the consumer maps it to an `isError` tool result rather than reporting partial output as success.
`SubagentStopReason` is a [merge-extensible derived union](core.md#the-map--derived-union-pattern) — a backend may add variants, so consumers branch on the known cases and treat an unknown terminal reason as a failure:
`SubagentRun` is the consumer-owned handle for a ready child — one disposable activation, never a durable child handle. Consumers await `result` and always dispose the run to reach quiescence. Child failures resolve with a non-completed stop reason; only unrepresentable infrastructure faults reject. The optional strict `steer` method advertises live delivery by presence; cold resume deliberately does NOT live here (a disposed run cannot be reconstructed after restart) — it is `SubagentProvider.resume`.
A local run MUST publish an ordinary child agent/session before `start()` fulfills, return that child session id as `SubagentRun.id`, expose the exact child as `localAgent`, and record `request.parent.session.id` in the child's `parentSession` header. Runtime ownership may place the child under the parent, provider, or root scope. A remote provider instead returns a parent-scoped lifecycle id and `localAgent: undefined`.
Each provider is a named child-agent transport, and multiple providers may coexist. The service validates requested start-time capabilities before `start()`. `inheritsParentContext` describes only conversation seeding (`fork`: true; `spawn` and `acp`: false), allowing consumers to generate accurate model-facing wording without implying inherited tools, services, or authority.
`start()` fulfills only with a ready run; `resume()` shares the same publication and lifecycle-observation contract. The service mints a unique `runId`, snapshots `local` from the provider's exact `localAgent`, observes the result, emits `subagent/start`, and returns the same run; rejection implies provider cleanup and emits no lifecycle pair. The paired `subagent/end` carries the same identity and the final output or infrastructure failure. Both events are observe-only and contain listener exceptions.
The spawn and fork backends create an ordinary agent through `parent.ctx`, pass cancellation into core creation, and dispose through `AgentHandle`. Provider removal blocks new starts without revoking accepted runs. Each child gets a new flat scope rather than inheriting parent registrations. Depth and fork seeding reuse existing agent and session vocabulary:
- **Delegation depth** is durable `SessionHeader.delegationDepth` plus the merge-extensible runtime field `AgentOptions.subagentDepth`; absence means top-level depth zero, and the greater present value is authoritative. The seam owns both fields — the loop neither sets nor reads them — so an in-process child persists parent depth + 1, resume cannot lower it, and every start rejects a derived depth outside the safe-integer domain or above a defined absolute `request.maxDepth` cap.
- **Fork seeding** uses `CreateAgentOptions.seed` (a `SessionEvent[]` prefix threaded through `AgentLoop.createAgent` → `ctx.sessions.prepare({ seed })`, the same primitive `resume` uses). The fork backend passes a *balanced completed-turn prefix* of the parent's log — the parent's events up to and including its last `turn/end` — so the seed is contiguous-from-0 and the [invariants](../../packages/support/invariants) replay accepts it (the in-flight, unbalanced turn is excluded).
