test(session): exercise the source-change retry through the CLI

The parent-child integration case can succeed without observing a race, while scheduler-only tests do not exercise the actual error classification or CLI output. Inject one genuine source-change error at the persistence open boundary in an isolated child process, then delegate to the real backend. Assert DEFERRED, completion of the other input, one RETRY, preserved source bytes and the resulting parent catalog. This adds no production test hook or timing-based race.

scripts/migrate-sessions-to-v4.spec.ts

@@ -3,6 +3,7 @@ import { existsSync, mkdtempSync, readFileSync, statSync } from 'node:fs'
 import { mkdir, readFile, symlink, unlink, writeFile } from 'node:fs/promises'
 import { availableParallelism, tmpdir } from 'node:os'
 import { dirname, join, resolve } from 'node:path'
+import { pathToFileURL } from 'node:url'
 import { afterEach, describe, expect, it } from 'vitest'
 import { execa } from 'execa'
 import type { SessionFormatJsonObject } from '@deepseek-ai/dsh-session-format'
@@ -323,6 +324,46 @@ describe('one-time V4 migration command', () => {
     })
   })
 
+  it('reports a source-change deferral and retries after the other input completes', async () => {
+    const root = temporaryRoot()
+    const parent = await fixture(root, 'a-parent', 3, 'none')
+    const child = await fixture(root, 'b-child', 3, 'none', [{
+      type: 'subagent/descriptor', data: { version: 3, mode: 'continuable', provider: 'spawn', label: 'fixture child' },
+    }], { origin: 'subagent', parentSession: 'a-parent', createdAt: 2, delegationDepth: 1 })
+    const entrypoint = join(temporaryRoot(), 'retry-once.mjs')
+    const backendUrl = pathToFileURL(join(repository, 'packages/session/session-persistence-jsonl/src/index.ts')).href
+    const generationUrl = pathToFileURL(join(repository, 'packages/session/session-persistence-jsonl/src/generation.ts')).href
+    await writeFile(entrypoint, `
+      const { default: Backend } = await import(${JSON.stringify(backendUrl)});
+      const { JsonlGenerationSourceChangedError } = await import(${JSON.stringify(generationUrl)});
+      const open = Backend.prototype.open;
+      let changed = false;
+      Backend.prototype.open = async function(id, access, options) {
+        if (id === 'a-parent' && access === 'write' && !changed) {
+          changed = true;
+          throw new JsonlGenerationSourceChangedError(${JSON.stringify(child.path)});
+        }
+        return open.call(this, id, access, options);
+      };
+      process.argv[1] = ${JSON.stringify(script)};
+      await import(${JSON.stringify(pathToFileURL(script).href)});
+    `)
+    const result = await runAt(entrypoint, '--sessions-dir', root, '--jobs', '2')
+    expect(result.status, result.stdout + result.stderr).toBe(0)
+    const lines = result.stdout.split('\n')
+    const deferred = lines.findIndex(line => line.includes('[1/2]') && line.includes('DEFERRED'))
+    const childDone = lines.findIndex(line => line.includes('[2/2]') && line.includes('completed=1/2'))
+    const retry = lines.findIndex(line => line.includes('[1/2]') && line.includes('RETRY'))
+    expect(deferred).toBeGreaterThanOrEqual(0)
+    expect(childDone).toBeGreaterThan(deferred)
+    expect(retry).toBeGreaterThan(childDone)
+    expect(lines.filter(line => line.includes('RETRY'))).toHaveLength(1)
+    expect(result.summary).toMatchObject({ totals: { converted: 2, alreadyV4: 0, failed: 0, skipped: 0 }, failureGroups: [] })
+    const target = await readFile(join(parent.directory, 'session.v4.jsonl'), 'utf8')
+    expect(target).toContain('"type":"subagent/catalog"')
+    for (const source of [parent, child]) expect(await readFile(source.path)).toEqual(source.bytes)
+  })
+
   it('opens an existing V4 torn tail without repairing its bytes', async () => {
     const root = temporaryRoot()
     const current = await fixture(root, 'current', 4, 'none')