docs(subagent): align the parent contract with ACP cwd inheritance

SubagentStartRequest.parent, its core-data type-equiv mirror, and the
ACP backend Agent Note still said the out-of-process backend ignores
request.parent — no longer true: with cwd unconfigured it reads exactly
the session header's cwd, and a parent session without one now rejects
at start. State the one-field read at each owning contract, record the
workspace-resolution decision in the Agent Note, and align the README's
accessibility wording with the search-permission guard.

.agents/notes/implemented/feature/2026-06-22-acp-subagent-backend.md

@@ -20,7 +20,11 @@ The client advertises NO optional capabilities (no `fs`, no `terminal`): the chi
 
 ### No start-time capabilities
 
-The provider's `capabilities` are all `false`. An out-of-process child cannot honor the parent's `maxDepth` (it has no access to `parent.options.subagentDepth`) or `toolFilter` (it owns its own tool registry), and the first cut does not implement `outputSchema`. The service rejects a request needing any of them before `start` runs. The backend injects only `subagents` (not `ctx.agents`) and ignores `request.parent`.
+The provider's `capabilities` are all `false`. An out-of-process child cannot honor the parent's `maxDepth` (it has no access to `parent.options.subagentDepth`) or `toolFilter` (it owns its own tool registry), and the first cut does not implement `outputSchema`. The service rejects a request needing any of them before `start` runs. The backend injects only `subagents` (not `ctx.agents`); the ONE thing it reads off `request.parent` is the session header's cwd (see the workspace resolution below) — no conversation context, depth, or tool state crosses the process boundary.
+
+### Workspace cwd resolution
+
+The child's working directory is an explicit resolution, never the harness process cwd: the deployment `cwd` override when configured (made absolute against the launch directory and validated at load), else the parent session header's cwd (validated at start), and a loud rejection before anything spawns when neither exists. One ACP server process serves sessions from many workspaces, so `process.cwd()` cannot stand in for a session's workspace — the old implicit fallback ran children in the server's launch directory. A candidate must be an absolute path naming a directory the harness can ENTER (`X_OK` — `statSync().isDirectory()` alone accepts a mode-600 directory that spawn would fail with EACCES), and the same resolved path becomes both the subprocess cwd and the ACP `session/new` workspace.
 
 ### StopReason mapping
 
@@ -33,6 +37,7 @@ The child is a separate process, so it inherits an environment. Credential-shape
 ## Testing
 
 - **Keyless unit/integration:** A scripted ACP subprocess exercises real stdio for prompt/output flow, every stop-reason mapping, signal and disposal cancellation (including pre-abort, pre-session race, and torn-pipe cases), both permission policies, ignored non-message updates, missing-command cleanup, provider reload, and namespace exports.
+- **Keyless Loader composition:** A test-only cordis.yml boots the stdio app through the real Loader with the backend's `cwd` omitted; a scripted model delegates once and the scripted child proves it ran in — and was announced — the parent session's workspace (the cwd-inheritance branch end to end).
 - **With-key e2e:** The backend spawns the real ACP example; its model answers `PONG`, writes `proof.txt`, and the parent verifies the file.
 - **Snapshot gap:** Each ACP child is a separate process with its own replay session, unlike in-process per-session replay. Deterministic mock-server coverage exists, while `TODO(acp-subagent-replay)` tracks parent replay against a replaying child.
 

docs/core-data-structures/subagent.md

@@ -49,7 +49,10 @@ interface SubagentStartRequest {
    * The spawning ("parent") agent — the one whose tool call started this
    * subagent. REQUIRED: in-process backends read `parent.session.header` for
    * the working directory, the `parentSession` lineage to stamp on the child,
-   * and the parent's delegation depth. Out-of-process backends (ACP) ignore it.
+   * and the parent's delegation depth. The out-of-process backend (ACP) reads
+   * exactly one field — the session header's cwd, the child's workspace when
+   * no deployment `cwd` override is configured; nothing else crosses the
+   * process boundary.
    */
   readonly parent: Agent
   /**

packages/subagent/subagent-acp/README.md

@@ -6,7 +6,7 @@ The ACP provider runs each subagent in a fresh subprocess and drives it as an Ag
 
 `start(request)` resolves the child's working directory, then performs `spawn` → ACP `initialize` → `newSession` before it fulfills. Fulfillment therefore means a remote session is ready and ownership has transferred to the caller. A spawn, initialization, new-session, or pre-publication cancellation failure rejects only after the subprocess has been reaped; a working-directory resolution failure rejects before anything is spawned.
 
-The working directory is the configured `cwd` override when set, else the delegating parent session's cwd — never the server process's own cwd, because one server process serves sessions from many workspaces. The parent-derived value must be an absolute path naming an existing directory, and the same resolved path becomes both the subprocess cwd and the ACP `session/new` workspace.
+The working directory is the configured `cwd` override when set, else the delegating parent session's cwd — never the server process's own cwd, because one server process serves sessions from many workspaces. The parent-derived value must be an absolute path naming a directory the harness can enter (search permission — what a subprocess cwd needs), and the same resolved path becomes both the subprocess cwd and the ACP `session/new` workspace.
 
 The returned run id is minted in the parent namespace. The child server's session id remains private to ACP wire calls because ACP guarantees it only within that fresh child process; using it as the parent lifecycle id could collide with another remote run or a local agent.
 
@@ -25,7 +25,7 @@ ACP advertises no start-time capabilities because this process cannot enforce th
 | `providerName` | `acp` | Registry name on `ctx.subagents`. |
 | `command` | required | Executable spawned for each run. |
 | `args` | `[]` | Command arguments. |
-| `cwd` | parent session cwd | Working-directory override for the child process and its ACP session; must be non-empty, a relative value resolves against the harness launch directory at load, and the result must name an existing directory. |
+| `cwd` | parent session cwd | Working-directory override for the child process and its ACP session; must be non-empty, a relative value resolves against the harness launch directory at load, and the result must name a directory the harness can enter. |
 | `permission` | `reject` | Auto-answer permission requests by rejecting or choosing the first allow-shaped option. |
 | `env` | `{}` | Explicit child environment layered over a credential-scrubbed parent environment. |
 | `disposeEofGraceMs` | `6000` | Grace after stdin EOF before SIGTERM. |
@@ -59,7 +59,7 @@ 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`.
+Keyless tests drive a scripted ACP subprocess over real stdio, including a Loader-composed stdio app proving parent-session cwd inheritance end to end. The with-key e2e drives the repository's real ACP agent and self-skips without `DEEPSEEK_API_KEY`.
 
 ## Model Experience
 

packages/subagent/subagent/src/types.ts

@@ -56,7 +56,10 @@ export interface SubagentStartRequest {
    * The spawning ("parent") agent — the one whose tool call started this
    * subagent. REQUIRED: in-process backends read `parent.session.header` for
    * the working directory, the `parentSession` lineage to stamp on the child,
-   * and the parent's delegation depth. Out-of-process backends (ACP) ignore it.
+   * and the parent's delegation depth. The out-of-process backend (ACP) reads
+   * exactly one field — the session header's cwd, the child's workspace when
+   * no deployment `cwd` override is configured; nothing else crosses the
+   * process boundary.
    */
   readonly parent: Agent
   /**