diff --git a/.agents/notes/implemented/architecture/2026-08-19-projection-cache-per-session-files.i18n.yaml b/.agents/notes/implemented/architecture/2026-08-19-projection-cache-per-session-files.i18n.yaml
index 46aafdd528..7409f997a1 100644
--- a/.agents/notes/implemented/architecture/2026-08-19-projection-cache-per-session-files.i18n.yaml
+++ b/.agents/notes/implemented/architecture/2026-08-19-projection-cache-per-session-files.i18n.yaml
@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write .agents/notes/implemented/architecture/2026-08-19-projection-cache-per-session-files.md
-2026-08-19-projection-cache-per-session-files.md: e32a74de3beea2fbe2dfff8835c8d1ef86983358
-2026-08-19-projection-cache-per-session-files.zh.md: 9c6c9b250282de059b649db1723a5f1f2e9b3945
+2026-08-19-projection-cache-per-session-files.md: 9e102e786a6c06d82d1a0f45cc2f96a50c8abcd8
+2026-08-19-projection-cache-per-session-files.zh.md: d875c3f57800936f66fbf65233637df9bf300e2d
diff --git a/.agents/notes/implemented/architecture/2026-08-19-projection-cache-per-session-files.md b/.agents/notes/implemented/architecture/2026-08-19-projection-cache-per-session-files.md
index e32a74de3b..9e102e786a 100644
--- a/.agents/notes/implemented/architecture/2026-08-19-projection-cache-per-session-files.md
+++ b/.agents/notes/implemented/architecture/2026-08-19-projection-cache-per-session-files.md
@@ -20,7 +20,7 @@ Reads and writes share ONE coherent state: every read (`cachedSnapshot`) is a sy
 - Listing is a synchronous in-memory read; a session without a record document simply lacks the projection column.
 - ACP, headless, SDK, and Web sessions publish cache rows for later consumers. The log-leading durability barrier may flush a covered prefix at the cache cadence and split otherwise coalesced physical JSONL runs; recorded profile snapshots re-pack the logical event stream so cache timing does not define fixture layout.
 - The per-record contract scopes failure: a malformed or stale-version document reads as an absent record at open, so one bad file never bricks the cache, and a checkpoint schema bump discards stale sessions per record instead of rejecting the whole domain.
-- No migration: the cache is derived data, never an authority. An obsolete cache (any earlier format) is never read — the first cold read refolds from the log and writes the current format.
+- The json backend bootstraps the per-record tree from the legacy whole-unit cache only when enumeration finds no new-layout document path. Any new document path, including an unreadable or stale file, suppresses the bootstrap for the whole unit; missing session rows refold from the log. The legacy file remains untouched.
 - The cache record is bound to the same log lifecycle as before: the stored `{createdAt, cwd}` identity guards against a recreated id.
 
 ## Alternatives considered
diff --git a/.agents/notes/implemented/architecture/2026-08-19-projection-cache-per-session-files.zh.md b/.agents/notes/implemented/architecture/2026-08-19-projection-cache-per-session-files.zh.md
index 9c6c9b2502..d875c3f578 100644
--- a/.agents/notes/implemented/architecture/2026-08-19-projection-cache-per-session-files.zh.md
+++ b/.agents/notes/implemented/architecture/2026-08-19-projection-cache-per-session-files.zh.md
@@ -20,7 +20,7 @@ Status: implemented
 - 列表读取是同步内存读；没有记录文档的会话只是缺少投影列。
 - ACP、headless、SDK 与 Web 会话都会发布缓存行，供后续消费方使用。确保日志领先的持久性屏障可能按缓存节奏 flush 已覆盖的前缀，并拆分原本会合并的物理 JSONL 行；各 profile 的录制快照会重新 pack 逻辑事件流，因此缓存时序不会决定 fixture 布局。
 - per-record 契约把故障范围缩小到单记录：畸形或过期版本的文档在打开时读作"无此记录"，单个坏文件不会拖垮整个缓存；检查点 schema 升级按会话丢弃过期行，而不是拒绝整个域。
-- 无需迁移：缓存是派生数据，绝非权威。过时的缓存（任何更早格式）从不被读取——首次冷读从日志重折叠并写出当前格式。
+- json 后端仅在枚举时没有发现任何新布局文档路径，才从旧整单元缓存引导 per-record 目录树。只要存在任意新文档路径，即使文件不可读或版本陈旧，也会对整个单元禁用引导；缺失的会话行从日志重折叠。旧文件保持不变。
 - 缓存记录仍绑定同一日志生命周期：存储的 `{createdAt, cwd}` 身份防止被重建的 id 误导。
 
 ## Alternatives considered
diff --git a/packages/storage/storage-json/README.i18n.yaml b/packages/storage/storage-json/README.i18n.yaml
index 8f40753305..69853e8521 100644
--- a/packages/storage/storage-json/README.i18n.yaml
+++ b/packages/storage/storage-json/README.i18n.yaml
@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/storage/storage-json/README.md
-README.md: 1a9873d2a3300998de67efa9be566ebc1250b0bf
-README.zh.md: e01ed29c5464e2f36472936db4a88db1bc4e7612
+README.md: d23362be7ad9d884a60b941663dfdd33668c300d
+README.zh.md: b90c78fa41302ce5f14a897b07cf1eb548e1843b
diff --git a/packages/storage/storage-json/README.md b/packages/storage/storage-json/README.md
index 1a9873d2a3..d23362be7a 100644
--- a/packages/storage/storage-json/README.md
+++ b/packages/storage/storage-json/README.md
@@ -13,6 +13,7 @@ Design: [domain KV storage Agent Note](../../../.agents/notes/proposed/architect
 
 - `single` layout: the in-memory unit state is authoritative; every write primitive republishes the whole file via temp-write + fsync + atomic `rename()` replace. A unit file is always the complete current net state — legibility is this backend's reason to exist; scale is the SQLite backend's job.
 - A missing file (or unit directory, for `per-record`) opens as an empty unit and materializes on the first write. In `single` layout a foreign or unparsable file rejects with `malformed-medium`, and a stored version differing from the descriptor rejects with `version-mismatch` (no migration, pre-release stance). In `per-record` layout the contract is per record instead: a document that is malformed or stamped with another version reads as an absent record, so one bad or stale file never bricks the unit, and a version bump discards stale records rather than rejecting the whole unit.
+- An empty `per-record` tree bootstraps its declared-table records from a legacy `<unit>.json` whole-unit file and retains that file unchanged. Any new-layout document path in a declared table, or `global.json` for a declared global, suppresses the bootstrap for the whole unit even when the document is unreadable or stale; absent records remain absent instead of being filled from legacy state.
 - Write ordering across calls belongs to the caller (the domain layer's write chain); each single call is atomic and durable once resolved.
 
 ## Config
diff --git a/packages/storage/storage-json/README.zh.md b/packages/storage/storage-json/README.zh.md
index e01ed29c54..b90c78fa41 100644
--- a/packages/storage/storage-json/README.zh.md
+++ b/packages/storage/storage-json/README.zh.md
@@ -13,6 +13,7 @@
 
 - `single` 布局：内存中的单元状态具有最终决定权；每个写入原语都会通过临时文件写入 + fsync + 原子 `rename()` 替换重新发布整个文件。单元文件始终是完整的当前状态：可读性是该后端存在的理由，规模问题则属于 SQLite 后端。
 - 缺失文件（`per-record` 时是缺失单元目录）会作为空单元打开，并在第一次写入时物化。`single` 布局下外来或无法解析的文件以 `malformed-medium` 拒绝；已存版本与描述符不同时以 `version-mismatch` 拒绝（预发布立场，不迁移）。`per-record` 布局的契约改为按记录：畸形或版本戳不符的文档读作"无此记录"，单个坏文件或过期文件不会拖垮整个单元，版本升级按记录丢弃过期行而不是拒绝整个单元。
+- 空的 `per-record` 目录树会从旧 `<unit>.json` 整单元文件引导其已声明表的记录，并原样保留旧文件。已声明表中只要存在任意新布局文档路径，或已声明 global 对应的 `global.json` 存在，就会对整个单元禁用引导，即使该文档不可读或版本陈旧；缺失记录仍保持缺失，不从旧状态补入。
 - 跨调用的写入顺序属于调用方（领域层的写入链）；每次调用都具备原子性，并在完成时已达到持久状态。
 
 ## 配置
diff --git a/packages/storage/storage-json/src/per-record-unit.ts b/packages/storage/storage-json/src/per-record-unit.ts
index e05cec8c9c..b75f9f9452 100644
--- a/packages/storage/storage-json/src/per-record-unit.ts
+++ b/packages/storage/storage-json/src/per-record-unit.ts
@@ -15,9 +15,11 @@
  * of migrating them. Record keys become path segments, so they must be
  * path-safe (`[a-zA-Z0-9_-]+`); an unsafe key rejects at write.
  *
- * One-time migration: a legacy whole-unit file `<root>/<name>.json` (the
- * pre-per-record layout) is split into per-record documents on first open,
- * new records winning, and deleted once every record migrated.
+ * Legacy bootstrap: when the new tree has no document path, a legacy
+ * whole-unit file `<root>/<name>.json` (the pre-per-record layout) seeds
+ * per-record documents. Any new document path, including one whose contents
+ * are unreadable or stale, suppresses the bootstrap for the whole unit. The
+ * legacy file is never changed or deleted.
  * @module @deepseek-ai/dsh-storage-json/src/per-record-unit
  */
 
@@ -72,39 +74,40 @@ async function loadPerRecordState(descriptor: KvUnitDescriptor, dir: string): Pr
     entries = await readdir(dir, { withFileTypes: true })
   } catch (error) {
     if ((error as NodeJS.ErrnoException).code !== 'ENOENT') throw error
-    // Missing directory = empty unit; the legacy migration below still runs
+    // Missing directory = empty unit; the legacy bootstrap below still runs
     // (the fresh-upgrade shape is exactly an absent new tree).
   }
-  if (entries !== undefined) {
-    await Promise.all(entries.map(async (entry) => {
+  const hasNewDocuments = entries === undefined
+    ? false
+    : (await Promise.all(entries.map(async (entry) => {
       if (entry.isDirectory()) {
         const records = state.tables.get(entry.name)
         if (records !== undefined) {
-          await loadTableRecords(records, descriptor.version, join(dir, entry.name))
+          return loadTableRecords(records, descriptor.version, join(dir, entry.name))
         }
-      } else if (entry.isFile() && entry.name === 'global.json' && descriptor.hasGlobal) {
+      }
+      if (entry.name === 'global.json' && descriptor.hasGlobal) {
         const global = await readRecord(join(dir, entry.name), descriptor.version)
         if (global !== undefined) state.global = global
+        return true
       }
-    }))
-  }
-  await migrateLegacyUnit(descriptor, dir, state)
+      return false
+    }))).some(Boolean)
+  if (!hasNewDocuments) await bootstrapLegacyUnit(descriptor, dir, state)
   return state
 }
 
 /**
- * One-time migration of a legacy whole-unit file (`<root>/<name>.json`, the
- * pre-per-record layout). Every record it holds that the new tree lacks is
- * written as a per-record document — an already-present new record wins —
- * and the legacy file is deleted only after all records migrated. A missing
- * legacy file, or one that is unreadable, foreign (another unit's name), or
- * not a unit document, is left alone: the migration is idempotent, and the
- * legacy file's absence is the "migrated" marker.
+ * Bootstrap an empty per-record tree from a legacy whole-unit file
+ * (`<root>/<name>.json`, the pre-per-record layout). Every declared-table
+ * record is copied into a current-version document, while the legacy file is
+ * retained unchanged. A missing, foreign (another unit's name), malformed,
+ * or non-unit legacy file is left alone; other read failures propagate.
  * @param descriptor - Static identity and shape of the unit.
  * @param dir - The per-record unit directory (`<root>/<name>`).
- * @param state - The tree state loaded so far; migrated records are added.
+ * @param state - The empty tree state; bootstrapped records are added.
  */
-async function migrateLegacyUnit(descriptor: KvUnitDescriptor, dir: string, state: UnitState): Promise<void> {
+async function bootstrapLegacyUnit(descriptor: KvUnitDescriptor, dir: string, state: UnitState): Promise<void> {
   const legacyPath = join(dirname(dir), `${descriptor.name}.json`)
   let text: string | undefined
   try {
@@ -130,19 +133,22 @@ async function migrateLegacyUnit(descriptor: KvUnitDescriptor, dir: string, stat
     const target = state.tables.get(table)
     if (target === undefined) continue
     for (const [key, value] of Object.entries(records)) {
-      if (target.has(key)) continue // An existing new record wins.
       const path = join(dir, table, `${key}.json`)
       await mkdir(dirname(path), { recursive: true, mode: 0o700 })
       await writeAtomic(path, serializeRecord(descriptor.version, value))
       target.set(key, value)
     }
   }
-  await rm(legacyPath, { force: true }) // Every record migrated: drop the legacy file.
 }
 
-/** Read one declared table's record documents into `records`. */
-async function loadTableRecords(records: Map<string, unknown>, version: number, dir: string): Promise<void> {
+/**
+ * Read one declared table's record documents into `records`.
+ * @returns whether the directory contains any `.json` document path,
+ * independently of key safety, readability, or stored version.
+ */
+async function loadTableRecords(records: Map<string, unknown>, version: number, dir: string): Promise<boolean> {
   const files = await readdir(dir, { withFileTypes: true })
+  const hasDocuments = files.some(file => file.name.endsWith('.json'))
   const loaded = await Promise.all(files.map(async (file) => {
     if (!file.name.endsWith('.json')) return
     const key = file.name.slice(0, -'.json'.length)
@@ -153,6 +159,7 @@ async function loadTableRecords(records: Map<string, unknown>, version: number,
   for (const record of loaded) {
     if (record !== undefined) records.set(...record)
   }
+  return hasDocuments
 }
 
 /** Read one record document; a foreign (unreadable or stale) one reads as absent. */
diff --git a/packages/storage/storage-json/tests/json-backend.spec.ts b/packages/storage/storage-json/tests/json-backend.spec.ts
index 59260b8c03..41ad071ea0 100644
--- a/packages/storage/storage-json/tests/json-backend.spec.ts
+++ b/packages/storage/storage-json/tests/json-backend.spec.ts
@@ -338,26 +338,41 @@ describe('per-record layout', () => {
     await backend.close()
   })
 
-  it('migrates a legacy whole-unit file once, new records win, then deletes it', async () => {
+  it('bootstraps an empty per-record tree from a legacy whole-unit file and preserves it', async () => {
     const root = await freshRoot()
     // A legacy single-layout file for the same unit (any older version);
     // the extra table is not declared and must be skipped.
-    await writeFile(join(root, 'recs.json'), JSON.stringify({
+    const legacy = JSON.stringify({
       unit: { name: 'recs', version: 3 },
       global: null,
       tables: { t: { old1: { v: 1 }, old2: { v: 2 } }, undeclared: { k: { v: 0 } } },
-    }), 'utf8')
-    // A new per-record row that must win over its legacy namesake.
-    await mkdir(join(root, 'recs', 't'), { recursive: true })
-    await writeFile(join(root, 'recs', 't', 'old1.json'), JSON.stringify({ version: 2, record: { v: 9 } }), 'utf8')
+    })
+    await writeFile(join(root, 'recs.json'), legacy, 'utf8')
     const backend = new JsonStorageBackend(root)
     const unit = await backend.kv.open(descriptor)
-    expect(await unit.loadAll()).toEqual({ tables: { t: { old1: { v: 9 }, old2: { v: 2 } } }, global: null })
-    // The legacy file was deleted; a reopen reads the migrated tree.
-    await expect(readFile(join(root, 'recs.json'), 'utf8')).rejects.toMatchObject({ code: 'ENOENT' })
+    expect(await unit.loadAll()).toEqual({ tables: { t: { old1: { v: 1 }, old2: { v: 2 } } }, global: null })
+    await expect(readFile(join(root, 'recs.json'), 'utf8')).resolves.toBe(legacy)
     await unit.close()
     const unit2 = await backend.kv.open(descriptor)
-    expect(await unit2.loadAll()).toEqual({ tables: { t: { old1: { v: 9 }, old2: { v: 2 } } }, global: null })
+    expect(await unit2.loadAll()).toEqual({ tables: { t: { old1: { v: 1 }, old2: { v: 2 } } }, global: null })
+    await backend.close()
+  })
+
+  it('ignores the legacy whole-unit file when any new document path exists', async () => {
+    const root = await freshRoot()
+    const legacy = JSON.stringify({
+      unit: { name: 'recs', version: 1 },
+      global: null,
+      tables: { t: { old: { v: 1 } } },
+    })
+    await writeFile(join(root, 'recs.json'), legacy, 'utf8')
+    await mkdir(join(root, 'recs', 't'), { recursive: true })
+    await writeFile(recordPath(root, 'broken'), '{oops', 'utf8')
+    const backend = new JsonStorageBackend(root)
+    const unit = await backend.kv.open(descriptor)
+    expect(await unit.loadAll()).toEqual({ tables: { t: {} }, global: null })
+    await expect(readFile(recordPath(root, 'old'), 'utf8')).rejects.toMatchObject({ code: 'ENOENT' })
+    await expect(readFile(join(root, 'recs.json'), 'utf8')).resolves.toBe(legacy)
     await backend.close()
   })