test(subagent-sdk): keyless Loader-composition e2e across the SDK wire

A test-only cordis.yml under examples/jsonrpc-agent boots the headless app
through the Loader; a scripted model delegates once to the SDK backend, whose
child is a COMPLETE second harness runtime (own cordis.yml, jsonrpc serving
surface, scripted cwd-echo model, own JSONL persistence). Asserts the parent
tool result AND the child's own persisted transcript both carry the parent
session's workspace cwd; child launch resolves through the shared
example-launch resolver so src/lib modes both hold.

packages/subagent/subagent-sdk/tests/loader-composition.e2e.ts

@@ -0,0 +1,101 @@
+/**
+ * Keyless REAL-composition coverage for parent-session cwd inheritance across
+ * the SDK wire: a test-only cordis.yml boots the headless app through the
+ * Loader with the SDK backend's `cwd` omitted, a scripted model delegates
+ * once, and the child — a COMPLETE second harness runtime booted from its own
+ * cordis.yml and driven over stdio JSON-RPC — echoes where it actually ran.
+ * Both the parent's tool result and the child's own persisted session log
+ * must carry the parent session's cwd. Mock-only composition, so only this
+ * keyless tier applies (the with-key tier lives in subagent-sdk.e2e.ts).
+ */
+
+import { realpathSync } from 'node:fs'
+import { readFile, readdir } from 'node:fs/promises'
+import { join } from 'node:path'
+import { fileURLToPath } from 'node:url'
+import { describe, expect, it } from 'vitest'
+import { type SessionEvent } from '@deepseek-ai/dsh-session'
+import { LOADER_SMOKE_TEST_TIMEOUT_MS, resolveExampleLaunch, runLoaderSmoke } from '@deepseek-ai/dsh-loader-smoke'
+
+const fixtureDir = new URL('../../../../examples/jsonrpc-agent/tests/fixtures/subagent/subagent-sdk/', import.meta.url)
+const driver = fileURLToPath(new URL('driver.ts', fixtureDir))
+const configPath = fileURLToPath(new URL('cordis.yml', fixtureDir))
+const childConfigPath = fileURLToPath(new URL('child.cordis.yml', fixtureDir))
+const runtimeBin = fileURLToPath(new URL('../../../../packages/examples/jsonrpc-demo/src/bin.ts', import.meta.url))
+const repoTsconfig = fileURLToPath(new URL('../../../../tsconfig.json', import.meta.url))
+
+async function jsonlFiles(dir: string): Promise<string[]> {
+  const entries = await readdir(dir, { withFileTypes: true })
+  const paths = await Promise.all(entries.map(async (entry) => {
+    const path = join(dir, entry.name)
+    if (entry.isDirectory()) return jsonlFiles(path)
+    return entry.isFile() && entry.name.endsWith('.jsonl') ? [path] : []
+  }))
+  return paths.flat()
+}
+
+async function sessionEvents(log: string): Promise<SessionEvent[]> {
+  const lines = (await readFile(log, 'utf8')).trimEnd().split('\n')
+  return lines.slice(1).map(line => JSON.parse(line) as SessionEvent)
+}
+
+describe('SDK subagent cwd inheritance through a real cordis.yml', () => {
+  it('runs the child runtime in the parent session workspace', async () => {
+    // The child launch honors the same src/lib mode as the driving harness,
+    // per the shared example-launch resolver (testing policy forbids
+    // hand-written `--import tsx` argv for example subprocesses).
+    const childLaunch = resolveExampleLaunch({
+      srcBin: runtimeBin,
+      configArgs: [childConfigPath],
+      tsconfigPath: repoTsconfig,
+    })
+
+    let events: SessionEvent[] = []
+    let childEvents: SessionEvent[] = []
+    let workspace = ''
+    const { stderr } = await runLoaderSmoke({
+      label: 'sdk-subagent cwd composition smoke',
+      tempDirPrefix: 'sdk-subagent-cwd-e2e-',
+      binScript: driver,
+      libBinScript: driver,
+      configPath,
+      tsconfigPath: repoTsconfig,
+      env: {
+        DSH_TEST_CHILD_COMMAND: childLaunch.command,
+        DSH_TEST_CHILD_ARGS: JSON.stringify(childLaunch.args),
+        DSH_TEST_CHILD_ENV: JSON.stringify({
+          ...Object.fromEntries(Object.entries(childLaunch.env).filter(([, value]) => value !== undefined)),
+        }),
+      },
+      inspect: async (cwd) => {
+        // The child reports realpaths; canonicalize the temp workspace to match.
+        workspace = realpathSync(cwd)
+        const parentLogs = await jsonlFiles(join(cwd, '.sessions'))
+        expect(parentLogs).toHaveLength(1)
+        events = await sessionEvents(parentLogs[0] as string)
+        // The child runtime persisted its own transcript in ITS cwd — which
+        // must be the parent session's workspace for the inheritance to hold.
+        const childLogs = await jsonlFiles(join(cwd, '.child-sessions'))
+        expect(childLogs).toHaveLength(1)
+        childEvents = await sessionEvents(childLogs[0] as string)
+      },
+    })
+    expect(stderr).not.toContain('UNHANDLED')
+
+    // The parent's tool result carries the child model's echo of its real
+    // process.cwd() — the parent session's workspace, never the harness
+    // process's launch directory.
+    const results = events.filter(event => event.type === 'tool/result')
+    expect(results).toHaveLength(1)
+    const resultText = results[0]!.data.content
+      .filter(block => block.type === 'text')
+      .map(block => block.text)
+      .join('')
+    expect(resultText).toBe(`child cwd: ${workspace}`)
+
+    // The child ran a real turn of its own: user message in, assistant out.
+    expect(childEvents.some(event => event.type === 'user/message')).toBe(true)
+    const childAnswers = childEvents.filter(event => event.type === 'assistant/message')
+    expect(childAnswers.length).toBeGreaterThan(0)
+  }, LOADER_SMOKE_TEST_TIMEOUT_MS)
+})