The out-of-process **ACP subagent backend**: runs each child agent in a spawned subprocess, driven over the [Agent Client Protocol](https://github.com/zed-industries/agent-client-protocol) (ACP) as the *client*. Registers a `SubagentProvider` on `ctx.subagents` (the [subagent seam](../subagent)), alongside the in-process [`-spawn`](../subagent-spawn)/[`-fork`](../subagent-fork) backends — multiple backends coexist by name.
It is the direction-inverted twin of the server-side bridge in [`@deepseek-ai/dsh-acp`](../../ui/acp): that package is the ACP *agent* (it answers `initialize`/`newSession`/`prompt`); this one is the ACP *client* (it *calls* them and implements the `sessionUpdate`/`requestPermission` callbacks). Point the configured command at the `acp-agent` example to "talk to our own process".
| Context surface | What the model sees | Token effect |
|---|---|---|
| Child-agent request | The remote child receives the standalone task through ACP plus its own process's configured system prompt, tools, and fresh session. It receives no parent conversation and cannot enforce the parent's scoped persona or tool filter. | The child pays for an independent full context and its own multi-step history. These tokens never enter the parent's context. |
| Parent tool result, indirectly | Through `dsh-tool-subagent`, the parent receives only the child's final streamed assistant text or a stop-reason error, not intermediate messages or tool traffic. | Parent input grows only by the final result, which is data-dependent and retained until compaction. This provider adds no parent schema itself. |
`start(request)` spawns the configured command, wraps its stdio in an ACP `ClientSideConnection`, and drives one session: `initialize` → `newSession` → `prompt`. `run.started` resolves after `newSession` publishes the remote session and rejects when initialization fails or cancellation wins first; the service emits no start/end pair for a child that never became live. The child's streamed `agent_message_chunk` text becomes the `SubagentResult.output`; the prompt's terminal `StopReason` maps to the stop reason. `dispose()` kills the subprocess and awaits its exit.
**Fresh process per run.** Each `start` spawns a new child, runs exactly one ACP session, and disposes it. Persistent-process pooling is a future optimization (see the RFC).
Unlike the in-process backends, the child does NOT share this cordis context — it is a separate process with its own session, model client, and tools. So this backend:
- injects only `subagents` (no `ctx.agents`);
- advertises NO start-time capabilities (an out-of-process child can't enforce the parent's depth/tool-filter);
- ignores `request.parent`.
## Config
| Key | Type | Default | Notes |
|---|---|---|---|
| `providerName` | string | `acp` | Registry name on `ctx.subagents`. |
| `command` | string | — (required) | The executable to spawn for each run (the child ACP agent). |
| `permission` | `'allow' \| 'reject'` | `reject` | How to auto-answer the child's `session/request_permission` prompts. `reject` declines every prompt (answer `cancelled`); `allow` approves via the first allow-shaped option. The first cut surfaces no prompt to a human. |
| `env` | Record<string,string> | `{}` | Extra env vars for the child (e.g. its own `DEEPSEEK_API_KEY`). Forwarded on top of a credential-scrubbed copy of the parent env, so an explicit key reaches the child while ambient secrets do not leak implicitly. |
| `disposeEofGraceMs` | number | `6000` | Dispose ladder tier 1: how long the child gets to quiesce on its own after stdin EOF (flush persistence, tear down its nested subprocesses) before SIGTERM. |
| `disposeGraceMs` | number | `3000` | Dispose ladder tier 2: grace between SIGTERM and the SIGKILL escalation. |
| `max_turn_requests` | `error` (no clean equivalent; the task did not finish) |
| _(unknown)_ | `error` |
A spawn/transport/RPC failure resolves `error` (or `aborted` if a cancel was requested) — `result` never rejects on a child-level failure, per the seam contract.
The child env is built by [`buildChildEnv` from `@deepseek-ai/dsh-subagent-subprocess`](../subagent-subprocess/README.md) — the ambient env minus credential-shaped vars, with `config.env` layered on top after the scrub; the pattern and full semantics live there. For this backend that means the parent harness's own secrets never leak into the spawned agent implicitly, while the child's OWN `DEEPSEEK_API_KEY` is supplied deliberately via `config.env` and survives.
- **Keyless** (`subagent-acp.spec.ts`): spawns a scripted mock ACP server subprocess (`tests/mock-acp-server.ts`) and drives it through the real backend over real ACP stdio — connection setup, client callbacks, the prompt round-trip, stop-reason mapping, cancellation (including the early-cancel race and a torn-pipe-after-cancel), permission auto-answer, and quiescent disposal. No model, no key.
- **With-key e2e** (`subagent-acp.e2e.ts`): the harness drives ITSELF — the backend spawns the real `acp-agent` example process and a real model in that child answers a prompt and does real file work (verified on disk). Self-skips without `DEEPSEEK_API_KEY`.
`TODO(acp-subagent-replay)`: snapshot-tier coverage of an ACP child is a separate replay shape (each child is its own PROCESS with its own single-agent replay, distinct from the in-process per-session keying), deferred — see the RFC.
## Plugin export shape
Named `name` / `inject` / `Config` / `apply`, with **no default export**: the cordis Loader's `unwrapExports` does `exports.default ?? exports`, so a stray default would collapse the module to the bare function and drop the `inject` namespace (see [docs/postmortem/0001](../../../docs/postmortem/0001-acp-default-export-drops-inject.md)).
- **A fresh process per run** — persistent-process pooling is a future optimization ([the seam RFC](../../../docs/rfc/implemented/feature/2026-06-21-subagent-capability-seam.md)).
- **No start-time capability enforcement** — an out-of-process child cannot honor the parent's `outputSchema`/`depthLimit`/`toolFilter`/`persona`, so the provider advertises none and `request.parent` is ignored.
