fix(subagent-acp): resolve child cwd from the parent session

The ACP backend resolved an unset cwd to the harness process's launch
directory, so a delegated child ran — and announced its ACP session —
in the server's cwd instead of the delegating session's workspace.
Resolve the working directory explicitly: the config cwd override
(made absolute and validated at load), else the parent session
header's cwd (validated at start), failing loud before spawning when
neither exists.

packages/subagent/subagent-acp/src/index.ts

@@ -1,11 +1,14 @@
 /**
  * Out-of-process ACP subagent backend. Each child has its own process, session, model, and
- * tools, so it shares no Cordis context, ignores `request.parent`, and advertises no parent-
- * enforced start capabilities. This plugin uses named exports only; a default would hide its
+ * tools, so it shares no Cordis context and advertises no parent-enforced start capabilities;
+ * the ONE thing it reads off `request.parent` is the session's workspace cwd (see
+ * {@link resolveCwd}). This plugin uses named exports only; a default would hide its
  * loader metadata (see `docs/postmortem/0001-acp-default-export-drops-inject.md`).
  * @module @deepseek-ai/dsh-subagent-acp
  */
 
+import { statSync } from 'node:fs'
+import { isAbsolute, resolve } from 'node:path'
 import type { Context } from 'cordis'
 import z from 'schemastery'
 import type { SubagentCapabilities, SubagentProvider, SubagentStartRequest } from '@deepseek-ai/dsh-subagent'
@@ -23,8 +26,11 @@ export interface Config {
   /** Arguments passed to {@link command}. */
   args: string[]
   /**
-   * Working directory for the child process and its ACP session. Defaults to
-   * the parent process's cwd when omitted.
+   * Working directory override for the child process and its ACP session. A
+   * relative path resolves against the harness launch directory at load, and
+   * the result must be an existing directory. When omitted, each child
+   * inherits its delegating parent session's cwd — and starting one from a
+   * parent session that has no cwd fails.
    */
   cwd?: string
   /**
@@ -71,6 +77,53 @@ function assertPositiveFinite(name: string, value: number): void {
 /** The shape after schemastery applied the defaults (cwd has none). */
 type ResolvedConfig = Required<Omit<Config, 'cwd'>> & Pick<Config, 'cwd'>
 
+/** Whether `path` names an existing, accessible directory. */
+function isDirectory(path: string): boolean {
+  try {
+    return statSync(path).isDirectory()
+  } catch {
+    // statSync throws only filesystem access errors here (ENOENT/EACCES/ENOTDIR/…),
+    // and every one of them means the path cannot serve as the child's cwd.
+    return false
+  }
+}
+
+/**
+ * Assert `cwd` can actually host the child: absolute (it doubles as the ACP
+ * session workspace, and a relative path would be re-anchored to the server
+ * process's launch directory) and an existing directory (fail here, before the
+ * process boundary, instead of as an ambiguous spawn ENOENT).
+ * @param label - which source supplied the value, for the diagnostic.
+ * @param cwd - the candidate working directory.
+ * @returns `cwd`, validated.
+ */
+function assertUsableCwd(label: string, cwd: string): string {
+  if (!isAbsolute(cwd)) {
+    throw new Error(`subagent-acp: ${label} must be an absolute path: ${cwd}`)
+  }
+  if (!isDirectory(cwd)) {
+    throw new Error(`subagent-acp: ${label} is not an accessible directory: ${cwd}`)
+  }
+  return cwd
+}
+
+/**
+ * Resolve the child's working directory: the deployment `cwd` override when
+ * configured (already validated at load), else the parent session's workspace
+ * cwd (validated here, its earliest resolvable point). Fails loud when neither
+ * exists — falling back to the harness process cwd would silently bind the
+ * child to the server's launch directory instead of the delegating session's
+ * workspace (one server process serves many sessions, each with its own cwd).
+ */
+function resolveCwd(configured: string | undefined, request: SubagentStartRequest): string {
+  if (configured !== undefined) return configured
+  const parentCwd = request.parent.session.header.cwd
+  if (parentCwd === undefined) {
+    throw new Error('subagent-acp: no working directory for the child — configure `cwd` or delegate from a parent session that has one')
+  }
+  return assertUsableCwd('parent session cwd', parentCwd)
+}
+
 /**
  * The ACP provider. Advertises NO start-time capabilities: an out-of-process
  * child cannot honor `outputSchema`/`maxDepth`/`toolFilter` (the service rejects
@@ -87,7 +140,7 @@ class AcpProvider implements SubagentProvider {
     const spec: AcpRunSpec = {
       command: this.config.command,
       args: this.config.args,
-      cwd: this.config.cwd ?? process.cwd(),
+      cwd: resolveCwd(this.config.cwd, request),
       permission: this.config.permission,
       env: this.config.env,
       disposeEofGraceMs: this.config.disposeEofGraceMs,
@@ -107,5 +160,10 @@ export function apply(ctx: Context, config: Config): void {
   const resolved = config as ResolvedConfig
   assertPositiveFinite('disposeEofGraceMs', resolved.disposeEofGraceMs)
   assertPositiveFinite('disposeGraceMs', resolved.disposeGraceMs)
-  ctx.subagents.registerProvider(new AcpProvider(resolved.providerName, ctx, resolved))
+  // Interpret a relative configured cwd against the harness launch directory
+  // ONCE, at load, and fail a misconfigured directory here — not per start.
+  const validated: ResolvedConfig = resolved.cwd === undefined
+    ? resolved
+    : { ...resolved, cwd: assertUsableCwd('config cwd', resolve(resolved.cwd)) }
+  ctx.subagents.registerProvider(new AcpProvider(validated.providerName, ctx, validated))
 }