Merge branch 'codex/simp-hide-concrete-agent-loop' into codex/simp-hide-subagent-internals

packages/subagent/subagent-acp/README.md

@@ -58,3 +58,25 @@ The child environment is built by [`buildChildEnv`](../subagent-subprocess/READM
 The package has no default export. Cordis loader unwrapping would otherwise hide the named `inject` metadata; see [postmortem 0001](../../../docs/postmortem/0001-acp-default-export-drops-inject.md).
 
 Keyless tests drive a scripted ACP subprocess over real stdio. The with-key e2e drives the repository's real ACP agent and self-skips without `DEEPSEEK_API_KEY`.
+
+## Model Experience
+
+### Child-agent request
+
+**What the model sees**: The remote child receives the standalone task content through ACP plus its own process's configured system prompt, tools, and fresh session. It receives no parent conversation. This provider advertises no optional start-time capabilities, so the local service rejects requests for persona, tool filtering, depth enforcement, or structured output instead of silently omitting them.
+
+**Token effect**: 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
+
+**What the model sees**: Through `dsh-tool-subagent`, the parent receives only the child's final streamed assistant text or that consumer's exact stop-reason error, not intermediate messages or tool traffic. A request already cancelled before publication becomes exactly `Error: subagent request was aborted before the ACP child started`; other start failures pass through as `Error: <message>`.
+
+**Token effect**: Parent input grows only by the final result or error, which is data-dependent and retained until compaction. This provider adds no parent schema itself.
+
+## Known Limitations and Deferred Work
+
+- **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 optional start-time capabilities** — this provider cannot apply the local harness's `outputSchema`, depth cap, tool filter, or persona inside the remote process, so it advertises none and the service rejects requests that require them.
+- **Only `agent_message_chunk` text is collected** — the child's tool-call activity, thought chunks, and plan updates are not surfaced to the parent.
+- **Permission prompts are auto-answered** (`permission: allow | reject`) — no human is surfaced a child's `session/request_permission` in this cut.
+- **No snapshot-tier replay coverage** (`TODO(acp-subagent-replay)`) — an ACP child is its own process with its own replay shape, deferred.