# subagent/ — subagent capability family

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The subagent seam: an agent delegating work to a child agent. Like the [bash](../bash/README.md) and [llm](../llm/README.md) families this is a capability seam (see [capability seams](../../.agents/notes/implemented/architecture/2026-06-13-capability-seams.md)) — but with one defining difference: **multiple provider implementations coexist in one context**, registered by name, rather than the single-implementation bash shape. The registry mirrors the LLM adapter registry.

| Package | Role | ctx key |
|---|---|---|
| `subagent/` | Abstract subagent seam: named-provider registry + vocabulary + the durable child descriptor | `ctx.subagents` |
| `subagent-inprocess/` | Shared in-process run driver (no provider; one cleanup effect per run) | — |
| `subagent-spawn/` | In-process backend: a fresh child agent, with cold resume | (registers on `ctx.subagents`) |
| `subagent-fork/` | In-process backend: a child seeded with the parent's completed-turn prefix, with cold resume | (registers on `ctx.subagents`) |
| `subagent-acp/` | Out-of-process backend: a child agent in a spawned subprocess, driven over ACP (one-shot) | (registers on `ctx.subagents`) |
| `subagent-dsh-sdk/` | Out-of-process backend: a child harness runtime in a spawned subprocess, driven over stdio JSON-RPC through the TypeScript SDK client | (registers on `ctx.subagents`) |
| `subagent-control/` | Continuable-child orchestration: stable ids, descriptor lookup, Task-backed activation, steer-or-resume routing | `ctx.subagentControl` |
| `tool-subagent/` | Model-facing `subagent` delegation tool over `ctx.subagents` | (registers on `ctx.tools`) |
| `tool-subagent-control/` | The one globally named `send_message` follow-up tool over `ctx.subagentControl` | (registers on `ctx.tools`) |

The interface lives at `subagent/subagent/`. The in-process `subagent-spawn` / `subagent-fork` backends share the `subagent-inprocess` driver (a library with no provider of its own — both depend on it, neither on the other), and the out-of-process `subagent-acp` / `subagent-dsh-sdk` backends spawn their children through the [`subprocess/`](../subprocess/README.md) seam (the shared credential scrub, tree-scoped teardown, and dispose ladder). `subagent-control` sits above the seam: it binds one durable child session to a series of disposable Task-backed activations, and both model tools and human-facing adapters route through its one contract. Tests replace only the child boundary with package-local fixtures.

The proposals and design rationale: [.agents/notes/implemented/feature/2026-06-21-subagent-capability-seam.md](../../.agents/notes/implemented/feature/2026-06-21-subagent-capability-seam.md) and [.agents/notes/implemented/feature/2026-07-21-continuable-background-subagents.md](../../.agents/notes/implemented/feature/2026-07-21-continuable-background-subagents.md).