diff --git a/.agents/notes/implemented/architecture/2026-09-02-projcache-cross-version-read-compat.i18n.yaml b/.agents/notes/implemented/architecture/2026-09-02-projcache-cross-version-read-compat.i18n.yaml
new file mode 100644
index 0000000000..78edca4d40
--- /dev/null
+++ b/.agents/notes/implemented/architecture/2026-09-02-projcache-cross-version-read-compat.i18n.yaml
@@ -0,0 +1,6 @@
+# Bilingual-pair consistency record (docs/i18n/README.md): the git blob hash of each
+# 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/2026-09-02-projcache-cross-version-read-compat.md
+2026-09-02-projcache-cross-version-read-compat.md: 64b52ba482e5850e1d0c3d473ca87ba7822af67f
+2026-09-02-projcache-cross-version-read-compat.zh.md: c8adad9fd2b7184418a4da44e8b2e5d1f31feed7
diff --git a/.agents/notes/implemented/architecture/2026-09-02-projcache-cross-version-read-compat.md b/.agents/notes/implemented/architecture/2026-09-02-projcache-cross-version-read-compat.md
new file mode 100644
index 0000000000..64b52ba482
--- /dev/null
+++ b/.agents/notes/implemented/architecture/2026-09-02-projcache-cross-version-read-compat.md
@@ -0,0 +1,69 @@
+# Agent Note: Projection-cache cross-version read compatibility (session_projcache v3/v4/v5)
+
+Status: implemented
+
+English | [中文](2026-09-02-projcache-cross-version-read-compat.zh.md)
+
+## Problem
+
+The `session_projcache` storage domain evolved through three on-disk generations across published releases. An upgraded DSH_HOME failed in two ways:
+
+- **A v3 single-file home bricked startup after the upgrade**: the per-record layout's legacy bootstrap migrated the old whole-unit file without checking its `unit.version`, stamping the old records with the current version into the new tree; the domain layer's per-record zod validation at open then hit the missing now-required fields → `invalid-record` → the whole domain refused to open → the plugin tree failed to load. And because the bootstrap writes before validation runs, **the first boot permanently wrote the bad documents into the new tree** ("poisoning") — every later boot saw a non-empty tree, never took the legacy path again, and the home stayed unusable.
+- **A v4 per-record home lost its listing titles after the upgrade**: v4 documents were silently discarded by the version-stamp check (the per-record contract), and SessionList is a zero-I/O cache-only read, so a miss served the row without projections; titles only returned as each session was individually reopened.
+
+The cache domain's own contract is "a stale or unreadable cache costs a longer tail replay, never a wrong value, never a refused load" — the hard failure and the wholesale discard each broke the first half of that contract or the product expectation.
+
+## The three on-disk generations
+
+| domain version | shipped in | layout | on-disk form | identity fields | row fields |
+|---|---|---|---|---|---|
+| 3 | 0.1.1-rc.2 | single | one file `storages/session_projcache.json` (`{unit:{name,version}, global, tables}`) | `createdAt`, `cwd?` | `ver`, `seq`, `val` |
+| 4 | 0.1.2-alpha.3 | per-record | one file per session `storages/session_projcache/sessions/<sessionId>.json` (`{version, record}`) | `createdAt`, `cwd?` | same |
+| 5 | 0.1.2-alpha.4 | per-record | same as v4 | + `isSeeded` (shipped required; now optional), `inheritedEventCount` (same) | same (`seq` numbers mean the same as v4; only type brands were added) |
+
+The only substantive v4→v5 difference is the two new lineage identity fields; the `ver/seq/val` row shape is identical across all three generations, and `seq` numbering did not change ([the 2026-08-31 seq/offset brands note](2026-08-31-session-sequence-and-log-offset-brands.md) pins the on-disk numbers as unchanged). v3→v4 was a layout migration with identical record content.
+
+One derived shape also exists: a v3 home that ran the v5 build once (the poisoned state) — its new tree holds documents **stamped 5 whose content is a v3 record** (no lineage fields).
+
+## Decision
+
+Declared read compatibility — reads tolerate vouched-for older versions, writes always stamp the current one:
+
+1. **`DomainSpec.compatibleVersions` (new, optional)**: the domain owner declares "records stored under these older versions are also readable under the current record schemas" (typically by declaring the fields old records lack as optional). `defineDomain` validates each entry as a non-negative integer below the current version; `descriptorOf` projects the set onto the backend `KvUnitDescriptor`.
+2. **json backend per-record reads** accept version stamps in "current ∪ compatibleVersions"; anything outside the set is still discarded as foreign. **The write path always stamps the current version** (the first checkpoint after reading an old record naturally advances it). The `single` layout stays exact-version.
+3. **Legacy-bootstrap version gate (the actual bug fix)**: the old whole-unit file's `unit.version` must fall inside the accepted set to be migrated; otherwise the file is left alone and the unit reads empty — stamping records the owner never vouched for turns a discardable stale cache into hard schema failures at the domain layer.
+4. **The projcache domain declares `version: 5, compatibleVersions: [3, 4]`**, and the two lineage fields become `.optional()`. The single reader of stored identities, `identityMatches`, normalizes absence to the unseeded lineage (`?? false` / `?? 0`): exact for unforked sessions, while a forked session's expectation is seeded → natural mismatch → discard and cold rebuild, so the lineage binding loses none of its protection.
+5. **The poisoned state self-heals**: documents stamped 5 without lineage fields parse under the optional schema (their content is the real pre-upgrade cache data), so the home boots again and titles serve immediately.
+6. **Schema-validation backstop: `invalidRecords: 'backup-and-skip'` (declared by this domain only)**. A stored record that still fails to parse beyond read compatibility no longer refuses the whole domain: the domain layer calls the backend's `KvUnit.backupRecord` (json per-record implementation = rename the document to `<key>.json.bak.<YYYYMMDDHHmm>`, bytes kept, never read again), prints the concrete failure with `logger.error` (domain, table, key, destination, zod cause), and continues the open with the record absent; the next cold read rebuilds and rewrites that session's cache. **The policy is an explicit per-domain declaration and the default stays fail-loud** — other domains still refuse the whole load on invalid stored data, and a backend without `backupRecord` (single layout, row stores) also falls back to fail-loud. Naming history: quarantine → backup-and-skip (user ruling: the word must carry both "back up" and "skip", sharing its root with the `.bak` suffix; skip-backup was rejected because the CLI `--skip-X` convention reads it as "do not back up"). For this domain it supersedes the reset/destroy recovery path of the [2026-07-28 storage recovery proposal](../../proposed/architecture/2026-07-28-storage-root-and-derived-medium-recovery.md), which stays live for authoritative and whole-medium damage.
+
+### Upgrade matrix
+
+| home shape | behavior after the fix |
+|---|---|
+| v3 single-file (not poisoned) | bootstrap migrates (3 ∈ accepted set) → titles serve immediately |
+| v3 + poisoned new tree | new-tree documents read directly (optional tolerance) → boot restored, titles serve immediately |
+| v4 per-record | documents read directly (4 ∈ accepted set) → titles serve immediately |
+| v5 healthy | unaffected |
+| old records of forked (seeded) sessions | identity mismatch → discarded, cold rebuild when the session opens (safe side) |
+
+## Alternatives considered
+
+- **Discard-and-rebuild only** (bootstrap gate + bump to v6): fixes the boot, but every SessionList title is lost after the upgrade until each session is reopened — fails the upgrade-and-go product requirement.
+- **Schema `.default()` fills**: behaviorally equivalent to optional + reader normalization, but bakes the "absent = unseeded" interpretation into the durable schema's output type; ruled for optional — the schema honestly describes every accepted on-disk shape and the interpretation lives at the consumer (user ruling, 2026-09-02).
+- **Roll the domain version back 5→4**: the smallest diff (three lines), but breaks version monotonicity, depends on the "bootstrap skips no versions" bug itself, and drops every poisoned and healthy v5 home's cache.
+
+## Consequences
+
+- A deployment routing this domain to the sqlite backend gets none of the tolerance: sqlite implements neither `compatibleVersions` nor `backupRecord`, so behavior degrades to the old strict-version semantics (a whole-unit version mismatch still refuses with `version-mismatch`; nothing loosens, nothing serves wrong values). Shipped compositions route this domain to json, so this stays a deployment-configuration risk only.
+- The optional lineage fields widen what a current-version document may omit: a v5-stamped record stripped of its lineage decodes as unseeded. The identity match still refuses it for seeded callers, and the per-row `ver` guard still screens every value, so the residual exposure is an unseeded caller reading an unseeded-shaped record — the same trust extended to genuine pre-lineage records.
+- `backupRecord` overwrites a same-minute backup of the same key (the newer bytes win); distinct minutes and distinct keys never collide.
+
+## Testing
+
+- `storage-json` unit tests: compat-stamped reads / out-of-set discards / writes stamping current; legacy bootstrap migrating only accepted versions (including the migrated-documents-stamp-current assertion); `backupRecord` move / absent read / rewrite / closed guard.
+- `storage-domain` unit tests: `compatibleVersions` / `invalidRecords` declaration validation; backup-and-skip falling back to fail-loud when the backend has no `backupRecord`.
+- `session-projection-cache` unit tests: records without lineage fields serve unseeded sessions verbatim and are discarded for seeded ones.
+- **Archived-fixture recovery tests** (`tests/fixtures.spec.ts` + `tests/fixtures/`): four media archives produced by the real released builds — `v3-single-unit.json` (the 0.1.1-rc.2 whole-unit file), `v4-session-doc.json` (0.1.2-alpha.3), `v5-session-doc.json` (current), `v5-lineageless-doc.json` (the unguarded bootstrap's poisoned shape, synthesized from the v3 record) — each opened through the real storage stack, asserting the listing serves the archived title and that a live write rewrites the document to the current version (v5 stamp + lineage fields + fresh value); plus the backup-and-skip behavior for a schema-failing record (boot survives, `.bak` lands, log is concrete, neighbor records unharmed).
+- End-to-end acceptance, executed against the real release artifacts: the published 0.1.1-rc.2 and 0.1.2-alpha.3 npm builds seeded homes through their own web apps (model turns plus a rename RPC), the published 0.1.2-alpha.4 build reproduced both failures (including the poisoned tree), and the fixed build served every home shape — pristine v3, poisoned v3, v4, and fresh — with the SessionList RPC returning the recorded titles verbatim.
+
+Future bump procedure: when a new version's shape can tolerate old records through "optional fields + reader normalization", add the old version to `compatibleVersions`; otherwise bump normally (discard and rebuild) and remove the no-longer-compatible versions from the set. Either way, the package README requires the bump to land with archived fixtures and tests proving the chosen disposition.
diff --git a/.agents/notes/implemented/architecture/2026-09-02-projcache-cross-version-read-compat.zh.md b/.agents/notes/implemented/architecture/2026-09-02-projcache-cross-version-read-compat.zh.md
new file mode 100644
index 0000000000..c8adad9fd2
--- /dev/null
+++ b/.agents/notes/implemented/architecture/2026-09-02-projcache-cross-version-read-compat.zh.md
@@ -0,0 +1,69 @@
+# Agent Note: 投影缓存跨版本读兼容（session_projcache v3/v4/v5）
+
+Status: implemented
+
+[English](2026-09-02-projcache-cross-version-read-compat.md) | 中文
+
+## 问题
+
+`session_projcache` 存储域在已发布版本间演进了三代磁盘结构。升级后的 DSH_HOME 出现两类故障：
+
+- **v3 单文件 home 升级后启动硬失败**：per-record 布局的 legacy bootstrap 迁移旧单文件时不检查其 `unit.version`，把旧记录原样打上当前版本戳写入新树；domain 层开域时逐条 zod 校验，旧记录缺新增必填字段 → `invalid-record` → 整个域拒开 → 插件树加载失败。且 bootstrap 先写盘后校验，**首次启动即把坏文档永久写入新树**（"投毒"）——此后每次启动新树非空、连 legacy 路径都不再走，home 持续不可用。
+- **v4 per-record home 升级后列表丢标题**：v4 文档被版本戳检查静默丢弃（per-record 契约），SessionList 是零 I/O 纯缓存读，miss 后整行不带投影；标题要等每个会话被逐个重新打开后才恢复。
+
+缓存域自身的契约是"过期或不可读的缓存只付出更长的尾部重放，绝不给出错值、绝不拒载"——硬失败与整体丢弃都违背该契约的前半句或产品预期。
+
+## 三代磁盘结构差异
+
+| domain version | 携带发布 | 布局 | 磁盘形态 | identity 字段 | 行字段 |
+|---|---|---|---|---|---|
+| 3 | 0.1.1-rc.2 | single | 单文件 `storages/session_projcache.json`（`{unit:{name,version}, global, tables}`） | `createdAt`, `cwd?` | `ver`, `seq`, `val` |
+| 4 | 0.1.2-alpha.3 | per-record | 每会话一份 `storages/session_projcache/sessions/<sessionId>.json`（`{version, record}`） | `createdAt`, `cwd?` | 同上 |
+| 5 | 0.1.2-alpha.4 | per-record | 同 v4 | + `isSeeded`（v5 首发必填；现为 optional）、`inheritedEventCount`（同前） | 同上（`seq` 数值语义与 v4 相同，仅类型加 brand） |
+
+v4→v5 的唯一实质差异是 identity 新增两个 lineage 字段；行内 `ver/seq/val` 三代一致，`seq` 的数值含义未变（[2026-08-31 seq/offset brands note](2026-08-31-session-sequence-and-log-offset-brands.zh.md) 明确 on-disk 数值不变）。v3→v4 是布局迁移，记录内容结构一致。
+
+另有一种衍生形态：跑过一次 v5 版本的 v3 home（投毒态）——新树里存在**版本戳为 5 但内容是 v3 记录**（缺 lineage 字段）的文档。
+
+## 决策
+
+声明式读兼容——读容忍 owner 背书过的旧版本，写恒戳当前版本：
+
+1. **`DomainSpec.compatibleVersions`（新增，可选）**：域 owner 声明"这些旧版本的存量记录在当前记录 schema 下也可读"（典型手段：新增字段标 optional）。`defineDomain` 校验各项为小于当前 version 的非负整数；`descriptorOf` 透传到后端 `KvUnitDescriptor`。
+2. **json 后端 per-record 读**：接受"当前版本 ∪ compatibleVersions"内的版本戳，集合外照旧视为 foreign 丢弃；**写路径永远戳当前版本**（读到旧记录后的下一次 checkpoint 自然把它推进到当前版本）。single 布局维持 exact-version 不变。
+3. **legacy bootstrap 版本把关（bug 修复本体）**：旧单文件的 `unit.version` 必须落在接受集合内才迁移，否则视为空 unit 留在原地——为 owner 未背书的记录打当前版本戳，会把"可丢弃的过期缓存"变成 domain 层的 schema 硬失败。
+4. **projcache 域声明 `version: 5, compatibleVersions: [3, 4]`**；两个 lineage 字段改为 `.optional()`。唯一消费 stored identity 的读点 `identityMatches` 把缺失归一化为 unseeded lineage（`?? false` / `?? 0`）：对非 fork 会话这是精确值；fork 会话的 expected 是 seeded → 天然 mismatch → 丢弃冷读重建，lineage 绑定的防护不放松。
+5. **投毒态自愈**：v5 戳缺 lineage 字段的文档被 optional schema 直接接受（内容本就是升级前的真实缓存数据），home 恢复可启动且标题立即可服务。
+6. **schema 校验兜底：`invalidRecords: 'backup-and-skip'`（仅本域声明）**。读兼容之外仍然解析失败的存量记录不再让整个域拒开：domain 层调用后端的 `KvUnit.backupRecord`（json per-record 实现＝把文档改名为 `<key>.json.bak.<YYYYMMDDHHmm>`，字节留档、不再被读取），用 `logger.error` 打印具体失败信息（域名、表、键、移动去向、zod 失败原因），随后当该记录不存在继续启动；下一次冷读会重建并重写该会话的缓存。**该策略是域级显式声明，缺省仍为 fail-loud**——其他业务域的存量数据校验失败照旧整域拒载；后端没有 `backupRecord` 能力（single 布局、行存储）时也回退 fail-loud。命名沿革：quarantine → backup-and-skip（用户裁决：词要同时含"备份"与"跳过"两义，且与 `.bak` 后缀同源；skip-backup 因 CLI `--skip-X` 惯例存在"不备份"反读而弃用）。对本域而言，该策略取代了 [2026-07-28 存储恢复提案](../../proposed/architecture/2026-07-28-storage-root-and-derived-medium-recovery.zh.md)中 reset/destroy 的恢复途径；该提案对权威介质与整介质损坏仍然有效。
+
+### 升级矩阵
+
+| home 形态 | 修复后行为 |
+|---|---|
+| v3 单文件（未投毒） | bootstrap 迁移（3 ∈ 接受集）→ 标题立即可服务 |
+| v3 + 投毒新树 | 新树文档直接读入（optional 容忍）→ 启动恢复、标题立即可服务 |
+| v4 per-record | 文档直接读入（4 ∈ 接受集）→ 标题立即可服务 |
+| v5 正常 | 不受影响 |
+| fork（seeded）会话的旧记录 | identity mismatch → 丢弃，打开会话时冷读重建（安全侧） |
+
+## 备选方案
+
+- **只丢弃重建**（bootstrap 把关 + bump v6）：启动可修，但升级后 SessionList 标题全丢、要逐会话打开才恢复——不满足升级即用的产品要求。
+- **schema `.default()` 填缺省**：行为与 optional+读点归一化等价，但把"缺失=unseeded"的解释固化进 durable schema 的输出类型；拍板为 optional——schema 如实描述介质上所有被接受的形态，解释权在消费点（2026-09-02 用户裁决）。
+- **域版本回退 5→4**：改动最小（三行），但破坏版本单调性、依赖"bootstrap 不查版本"这个 bug 本身、且投毒态与正常 v5 home 的缓存全被丢弃。
+
+## 影响
+
+- 部署方若把本域路由到 sqlite 后端，得不到任何容忍能力：sqlite 既未实现 `compatibleVersions` 也没有 `backupRecord`，行为退化为原有的严格版本语义（整 unit 版本不匹配仍 `version-mismatch` 拒开；不放松、不出错值）。shipped 组合固定路由 json，此风险仅存在于部署配置层面。
+- optional lineage 字段放宽了当前版本文档可缺省的范围：被剥离 lineage 的 v5 戳记录会解码为 unseeded。身份比对仍会对 seeded 调用方拒收，逐行 `ver` 守卫仍筛查每个值，残余暴露面只是 unseeded 调用方读到 unseeded 形态的记录——与真实 pre-lineage 记录享有的信任完全相同。
+- `backupRecord` 对同一键的同一分钟内重复备份会覆盖前一份（新字节胜出）；不同分钟、不同键永不冲突。
+
+## 测试
+
+- `storage-json` 单测：compat 版本戳读入/集合外丢弃/写恒当前版本；legacy bootstrap 仅在版本被接受时迁移（含迁移后文档戳当前版本断言）；`backupRecord` 移档/读缺席/重写/封闭守卫。
+- `storage-domain` 单测：`compatibleVersions`/`invalidRecords` 声明校验；后端无 `backupRecord` 时 backup-and-skip 回退 fail-loud。
+- `session-projection-cache` 单测：缺 lineage 字段的记录对 unseeded 会话按原值服务、对 seeded 会话丢弃。
+- **归档 fixtures 独立恢复测试**（`tests/fixtures.spec.ts` + `tests/fixtures/`）：真实发布物产出的四份介质存档——`v3-single-unit.json`（0.1.1-rc.2 整域单文件）、`v4-session-doc.json`（0.1.2-alpha.3）、`v5-session-doc.json`（当前版）、`v5-lineageless-doc.json`（无守卫 bootstrap 的投毒形态，由 v3 记录合成）——逐一走真实存储栈开域，断言列表读出归档标题、且 live 写把文档重写为当前版本（v5 戳 + lineage 字段 + 新值）；外加 schema 失败记录的 backup-and-skip 行为（启动不失败、`.bak` 落盘、日志具体、邻居记录不受累）。
+- 端到端验收，以真实发布物执行：已发布的 0.1.1-rc.2 与 0.1.2-alpha.3 npm 包经各自 web app 造数（真实模型对话 + rename RPC），已发布的 0.1.2-alpha.4 包复现两类故障（含投毒树），修复后构建对纯净 v3、投毒 v3、v4、全新四种 home 形态经 SessionList RPC 原样返回记录在案的标题。
+
+未来 bump 流程：新版本结构若可用"optional 字段 + 读点归一化"容忍旧记录，就把旧版本加入 `compatibleVersions`；否则正常 bump（丢弃重建），并把不再兼容的版本从集合中移除。无论哪条路，包 README 都要求 bump 随附归档 fixture 和论证所选处置方式的测试。
diff --git a/.agents/notes/proposed/architecture/2026-07-28-storage-root-and-derived-medium-recovery.i18n.yaml b/.agents/notes/proposed/architecture/2026-07-28-storage-root-and-derived-medium-recovery.i18n.yaml
index ea0fe4b44b..96ea4961d6 100644
--- a/.agents/notes/proposed/architecture/2026-07-28-storage-root-and-derived-medium-recovery.i18n.yaml
+++ b/.agents/notes/proposed/architecture/2026-07-28-storage-root-and-derived-medium-recovery.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/proposed/architecture/2026-07-28-storage-root-and-derived-medium-recovery.md
-2026-07-28-storage-root-and-derived-medium-recovery.md: 1505be1c58d5cf829327b2919113bb2e42798ce7
-2026-07-28-storage-root-and-derived-medium-recovery.zh.md: 1bab5ab663df1419cc826c3d6acb59bd8bff7de0
+2026-07-28-storage-root-and-derived-medium-recovery.md: 68a6792c38a8fe097558de3d5a857ab4ea0d3533
+2026-07-28-storage-root-and-derived-medium-recovery.zh.md: 0c712b3b928f4b3df44e706b154c3a90a784403c
diff --git a/.agents/notes/proposed/architecture/2026-07-28-storage-root-and-derived-medium-recovery.md b/.agents/notes/proposed/architecture/2026-07-28-storage-root-and-derived-medium-recovery.md
index 1505be1c58..68a6792c38 100644
--- a/.agents/notes/proposed/architecture/2026-07-28-storage-root-and-derived-medium-recovery.md
+++ b/.agents/notes/proposed/architecture/2026-07-28-storage-root-and-derived-medium-recovery.md
@@ -10,7 +10,7 @@ The persisted projection cache ([note](2026-07-27-session-projection-and-command
 
 **Where the files actually live (root mismatch closed; resolve-once residual still open).** The shared base defaults the session store to the global harness home (`$DSH_HOME/sessions`, default `~/.dsh/sessions`), while the shipped Web overlay used to give the json backend the relative root `./.storages`: `workspace.json` and `session_projcache.json` landed under `<launch dir>/.storages/` — two launches from different directories shared their sessions yet saw different workspace registries and different projection caches, and the cache exists precisely to serve the cross-session cold listing, which missed for every session last cached under another launch directory. That mismatch is now closed: the overlay anchors `storage-json.root` to `$DSH_HOME/storages` with the same `!!js` expression the session root uses (`apps/cli/config/web.cordis.yml`). The residual hazard: `JsonStorageBackend` still never resolves its root — each unit open joins the path against whatever `process.cwd()` is at that moment (packages/storage/storage-json/src/index.ts); the shipped overlay root is already absolute and unaffected, but any relative root (bare Loader boots, tests) still splits on a later cwd change — the exact hazard the JSONL session backend resolves-once to prevent ("later process.cwd() changes cannot split one backend across roots", packages/session/session-persistence-jsonl/src/index.ts).
 
-**Recovery behavior.** Inside a healthy medium the cache is fully self-healing by design: a `stateVersion`-mismatched row is discarded and refolded, a log shrunk below a row's watermark is detected by the anchored restore floor and answered with one full re-read, and every background write is fail-soft. But at the *medium* level there is no recovery at all: a truncated, hand-edited, or version-bumped `session_projcache.json` fails `openJsonUnit` with `malformed-medium`/`version-mismatch` (packages/storage/storage-json/src/format.ts), a schema-drifted record fails domain open with `invalid-record` (packages/storage/storage-domain/src/index.ts), the rejection propagates through `SessionProjectionCache[Service.init]`, and under the CLI's fail-loud boot the assembly refuses to start. A file whose entire content is rebuildable from session logs can brick boot. This contradicts the cache package's own stated stance ("a stale or unreadable cache costs a longer tail replay, never a wrong value") and the cache domain spec's JSDoc ("version bumps discard the whole medium"), which describes an aspiration, not the implementation. Partially superseded for the projection cache: [the per-session cache files note](../../implemented/architecture/2026-08-19-projection-cache-per-session-files.md) removed the global `session_projcache` domain, so the cache half of this proposal (recovery on that domain) no longer applies; the `workspace.json` half remains current. The same fail-loud path is *correct* for `workspace.json` — workspace records are authoritative, not derivable — so the missing concept is a per-domain declaration of authority, not a global behavior change.
+**Recovery behavior.** Inside a healthy medium the cache is fully self-healing by design: a `stateVersion`-mismatched row is discarded and refolded, a log shrunk below a row's watermark is detected by the anchored restore floor and answered with one full re-read, and every background write is fail-soft. But at the *medium* level there is no recovery at all: a truncated, hand-edited, or version-bumped `session_projcache.json` fails `openJsonUnit` with `malformed-medium`/`version-mismatch` (packages/storage/storage-json/src/format.ts), a schema-drifted record fails domain open with `invalid-record` (packages/storage/storage-domain/src/index.ts), the rejection propagates through `SessionProjectionCache[Service.init]`, and under the CLI's fail-loud boot the assembly refuses to start. A file whose entire content is rebuildable from session logs can brick boot. This contradicts the cache package's own stated stance ("a stale or unreadable cache costs a longer tail replay, never a wrong value") and the cache domain spec's JSDoc ("version bumps discard the whole medium"), which describes an aspiration, not the implementation. Partially superseded for the projection cache: [the per-session cache files note](../../implemented/architecture/2026-08-19-projection-cache-per-session-files.md) removed the global `session_projcache` domain, so the cache half of this proposal (recovery on that domain) no longer applies; the `workspace.json` half remains current. The `invalid-record` class for the per-record projection cache is now also superseded: the shipped domain declares `invalidRecords: 'backup-and-skip'` ([cross-version read-compat note](../../implemented/architecture/2026-09-02-projcache-cross-version-read-compat.md)), which backs the failing record up and skips it at open, so the reset/destroy proposal below stays relevant only for whole-medium damage on authoritative or single-document media. The same fail-loud path is *correct* for `workspace.json` — workspace records are authoritative, not derivable — so the missing concept is a per-domain declaration of authority, not a global behavior change.
 
 ## Proposal
 
diff --git a/.agents/notes/proposed/architecture/2026-07-28-storage-root-and-derived-medium-recovery.zh.md b/.agents/notes/proposed/architecture/2026-07-28-storage-root-and-derived-medium-recovery.zh.md
index 1bab5ab663..0c712b3b92 100644
--- a/.agents/notes/proposed/architecture/2026-07-28-storage-root-and-derived-medium-recovery.zh.md
+++ b/.agents/notes/proposed/architecture/2026-07-28-storage-root-and-derived-medium-recovery.zh.md
@@ -10,7 +10,7 @@ Status: proposed
 
 **文件到底存在哪（根错位已收口，resolve-once 残余仍开放）。** 共享 base 将会话存储默认为全局 harness home（`$DSH_HOME/sessions`，默认 `~/.dsh/sessions`），而出厂 Web overlay 曾给 json 后端相对根 `./.storages`：`workspace.json` 和 `session_projcache.json` 落在 `<启动目录>/.storages/` 下——从两个不同目录启动，会话相同，工作区注册表和投影缓存却各是一份，而缓存存在的意义恰恰是跨会话冷列表，凡上次在别的启动目录下缓存过的会话全部 miss。这一错位已消除：overlay 现以与会话根同一段 `!!js` 表达式把 `storage-json.root` 锚定到 `$DSH_HOME/storages`（`apps/cli/config/web.cordis.yml`）。残余隐患：`JsonStorageBackend` 仍从不 resolve 根——每次打开 unit 都把路径 join 到当时的 `process.cwd()` 上（packages/storage/storage-json/src/index.ts）；出厂 overlay 的根已是绝对路径不受影响，但任何相对根（裸 Loader 启动、测试）仍会被后续 cwd 变化劈开，JSONL 会话后端用「构造时 resolve 一次」防住的正是它（"later process.cwd() changes cannot split one backend across roots"，packages/session/session-persistence-jsonl/src/index.ts）。
 
-**恢复行为。** 在健康介质内部，缓存按设计完全自愈：`stateVersion` 不匹配的行被丢弃重折，日志缩短到行水位以下由带锚的 restore floor 检出并以一次全量重读回答，每次后台写都是 fail-soft。但在*介质*层面完全没有恢复：被截断、被手改或版本被 bump 的 `session_projcache.json` 会让 `openJsonUnit` 以 `malformed-medium`/`version-mismatch` 失败（packages/storage/storage-json/src/format.ts），schema 漂移的记录让域 open 以 `invalid-record` 失败（packages/storage/storage-domain/src/index.ts），拒绝一路穿过 `SessionProjectionCache[Service.init]`，在 CLI 的 fail-loud 启动下整个组装拒绝启动。一个内容完全可从会话日志重建的文件能把启动搞死。这与缓存包自己声明的立场（"a stale or unreadable cache costs a longer tail replay, never a wrong value"）和缓存域 spec 的 JSDoc（"version bumps discard the whole medium"）相矛盾——后者描述的是愿望而非实现。投影缓存半边已被[每会话缓存文件 note](../../implemented/architecture/2026-08-19-projection-cache-per-session-files.zh.md) 部分取代：全局 `session_projcache` domain 已移除，本提案的缓存恢复半边不再适用；`workspace.json` 半边仍然有效。同一条 fail-loud 路径对 `workspace.json` 却是*正确*的——工作区记录是权威数据，不可派生——所以缺的概念是按域声明权威性，而不是全局改行为。
+**恢复行为。** 在健康介质内部，缓存按设计完全自愈：`stateVersion` 不匹配的行被丢弃重折，日志缩短到行水位以下由带锚的 restore floor 检出并以一次全量重读回答，每次后台写都是 fail-soft。但在*介质*层面完全没有恢复：被截断、被手改或版本被 bump 的 `session_projcache.json` 会让 `openJsonUnit` 以 `malformed-medium`/`version-mismatch` 失败（packages/storage/storage-json/src/format.ts），schema 漂移的记录让域 open 以 `invalid-record` 失败（packages/storage/storage-domain/src/index.ts），拒绝一路穿过 `SessionProjectionCache[Service.init]`，在 CLI 的 fail-loud 启动下整个组装拒绝启动。一个内容完全可从会话日志重建的文件能把启动搞死。这与缓存包自己声明的立场（"a stale or unreadable cache costs a longer tail replay, never a wrong value"）和缓存域 spec 的 JSDoc（"version bumps discard the whole medium"）相矛盾——后者描述的是愿望而非实现。投影缓存半边已被[每会话缓存文件 note](../../implemented/architecture/2026-08-19-projection-cache-per-session-files.zh.md) 部分取代：全局 `session_projcache` domain 已移除，本提案的缓存恢复半边不再适用；`workspace.json` 半边仍然有效。per-record 投影缓存的 `invalid-record` 一类如今也已被取代：已发布的域声明了 `invalidRecords: 'backup-and-skip'`（[跨版本读兼容 note](../../implemented/architecture/2026-09-02-projcache-cross-version-read-compat.zh.md)），open 时把失败记录备份后跳过，因此下文 reset/destroy 提案仅对权威介质或单文档介质的整介质损坏仍然相关。同一条 fail-loud 路径对 `workspace.json` 却是*正确*的——工作区记录是权威数据，不可派生——所以缺的概念是按域声明权威性，而不是全局改行为。
 
 ## 提案
 
diff --git a/docs/subsystems/storage.i18n.yaml b/docs/subsystems/storage.i18n.yaml
index 1e9a7e221c..d234297a9d 100644
--- a/docs/subsystems/storage.i18n.yaml
+++ b/docs/subsystems/storage.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 docs/subsystems/storage.md
-storage.md: 1e4141e6ef1c6f8e1c2593e21e788b626d6b1ed7
-storage.zh.md: f0433c600674741c3de0ce3e99430297839ce124
+storage.md: e52f6d8869ee3092a5be99f0bd2873b410b45ed9
+storage.zh.md: f752862af9f10cfb32f39f2a1aa7801c922f841e
diff --git a/docs/subsystems/storage.md b/docs/subsystems/storage.md
index 1e4141e6ef..e52f6d8869 100644
--- a/docs/subsystems/storage.md
+++ b/docs/subsystems/storage.md
@@ -65,6 +65,26 @@ interface DomainSpec {
    * (a stale record document is discarded, never migrated).
    */
   readonly layout?: 'single' | 'per-record'
+  /**
+   * Older domain versions whose stored records the current record schemas
+   * also accept (the declaring owner vouches for that, typically by
+   * declaring the fields older records lack as optional). `per-record` backends
+   * read documents stamped with a listed version instead of discarding them,
+   * and accept a legacy whole-unit file so stamped for the one-time
+   * bootstrap; writes always stamp {@link version}.
+   */
+  readonly compatibleVersions?: readonly number[]
+  /**
+   * What `open` does with a stored table record that fails its zod schema.
+   * Absent (the default), the whole open rejects with `invalid-record` —
+   * right for authoritative data. `'backup-and-skip'` is for domains whose
+   * records are disposable derived data: the backend moves the record's
+   * document aside (`KvUnit.backupRecord`), the failure is logged with
+   * its cause, and the open continues with the record absent. A backend
+   * without `backupRecord` (no per-record document to move) falls back
+   * to the rejecting default. The global slot always rejects.
+   */
+  readonly invalidRecords?: 'backup-and-skip'
   /** Optional global singleton slot. */
   readonly global?: DomainGlobalSpec<unknown>
   /** Table declarations keyed by table name; each name must match `UNIT_NAME_RE`. */
@@ -180,7 +200,10 @@ The mounted domain facility. Opens declared domains over routed backends; one fa
  * (`facet-unsupported`); open the unit projected from the spec (backend
  * `version-mismatch`/`malformed-medium` pass through); load and validate
  * every stored record against the spec's zod schemas (`invalid-record`
- * with the offending table and key); construct the domain.
+ * with the offending table and key — unless the spec declares
+ * `invalidRecords: 'backup-and-skip'` and the unit can move documents aside, in
+ * which case the failing record is backed up, logged, and skipped);
+ * construct the domain.
  *
  * Lifecycle: the CALLER owns the returned handle and closes it via
  * `Domain.close()` (typically as its own `ctx.effect` disposer) — the
diff --git a/docs/subsystems/storage.zh.md b/docs/subsystems/storage.zh.md
index f0433c6006..f752862af9 100644
--- a/docs/subsystems/storage.zh.md
+++ b/docs/subsystems/storage.zh.md
@@ -65,6 +65,26 @@ interface DomainSpec {
    * (a stale record document is discarded, never migrated).
    */
   readonly layout?: 'single' | 'per-record'
+  /**
+   * Older domain versions whose stored records the current record schemas
+   * also accept (the declaring owner vouches for that, typically by
+   * declaring the fields older records lack as optional). `per-record` backends
+   * read documents stamped with a listed version instead of discarding them,
+   * and accept a legacy whole-unit file so stamped for the one-time
+   * bootstrap; writes always stamp {@link version}.
+   */
+  readonly compatibleVersions?: readonly number[]
+  /**
+   * What `open` does with a stored table record that fails its zod schema.
+   * Absent (the default), the whole open rejects with `invalid-record` —
+   * right for authoritative data. `'backup-and-skip'` is for domains whose
+   * records are disposable derived data: the backend moves the record's
+   * document aside (`KvUnit.backupRecord`), the failure is logged with
+   * its cause, and the open continues with the record absent. A backend
+   * without `backupRecord` (no per-record document to move) falls back
+   * to the rejecting default. The global slot always rejects.
+   */
+  readonly invalidRecords?: 'backup-and-skip'
   /** Optional global singleton slot. */
   readonly global?: DomainGlobalSpec<unknown>
   /** Table declarations keyed by table name; each name must match `UNIT_NAME_RE`. */
@@ -180,7 +200,10 @@ The mounted domain facility. Opens declared domains over routed backends; one fa
  * (`facet-unsupported`); open the unit projected from the spec (backend
  * `version-mismatch`/`malformed-medium` pass through); load and validate
  * every stored record against the spec's zod schemas (`invalid-record`
- * with the offending table and key); construct the domain.
+ * with the offending table and key — unless the spec declares
+ * `invalidRecords: 'backup-and-skip'` and the unit can move documents aside, in
+ * which case the failing record is backed up, logged, and skipped);
+ * construct the domain.
  *
  * Lifecycle: the CALLER owns the returned handle and closes it via
  * `Domain.close()` (typically as its own `ctx.effect` disposer) — the
diff --git a/packages/extensions/tool-cordis/src/api-catalog.ts b/packages/extensions/tool-cordis/src/api-catalog.ts
index 55761bf933..3c81eb2b3b 100644
--- a/packages/extensions/tool-cordis/src/api-catalog.ts
+++ b/packages/extensions/tool-cordis/src/api-catalog.ts
@@ -2151,7 +2151,7 @@ export const SERVICE_API: readonly ServiceApiEntry[] = [
     methods: [
       {
         signature: 'async open<S extends DomainSpec>(spec: S): Promise<Domain<S>>',
-        description: 'Open one declared domain. Steps, each failing the whole call: reject a name that is already open (`already-open`); resolve the backend route (`backend-not-found` passes through from the hub); require its `kv` facet (`facet-unsupported`); open the unit projected from the spec (backend `version-mismatch`/`malformed-medium` pass through); load and validate every stored record against the spec\'s zod schemas (`invalid-record` with the offending table and key); construct the domain.\n\nLifecycle: the CALLER owns the returned handle and closes it via `Domain.close()` (typically as its own `ctx.effect` disposer) — the facility does not tie the domain to any consumer fiber. Domains still open when the facility unmounts are closed by the plugin disposer.',
+        description: 'Open one declared domain. Steps, each failing the whole call: reject a name that is already open (`already-open`); resolve the backend route (`backend-not-found` passes through from the hub); require its `kv` facet (`facet-unsupported`); open the unit projected from the spec (backend `version-mismatch`/`malformed-medium` pass through); load and validate every stored record against the spec\'s zod schemas (`invalid-record` with the offending table and key — unless the spec declares `invalidRecords: \'backup-and-skip\'` and the unit can move documents aside, in which case the failing record is backed up, logged, and skipped); construct the domain.\n\nLifecycle: the CALLER owns the returned handle and closes it via `Domain.close()` (typically as its own `ctx.effect` disposer) — the facility does not tie the domain to any consumer fiber. Domains still open when the facility unmounts are closed by the plugin disposer.',
         parameters: [{ name: 'spec', description: 'The domain declaration, typically from `defineDomain`.' }],
         returns: 'the opened domain handle, typed by the spec.',
       },
@@ -3956,7 +3956,7 @@ export const TYPE_API: readonly TypeApiEntry[] = [
   },
   {
     name: 'DomainSpec',
-    declaration: 'export interface DomainSpec {\n    readonly name: string;\n    readonly version: number;\n    readonly layout?: \'single\' | \'per-record\';\n    readonly global?: DomainGlobalSpec<unknown>;\n    readonly tables: Record<string, DomainTableSpec>;\n}',
+    declaration: 'export interface DomainSpec {\n    readonly name: string;\n    readonly version: number;\n    readonly layout?: \'single\' | \'per-record\';\n    readonly compatibleVersions?: readonly number[];\n    readonly invalidRecords?: \'backup-and-skip\';\n    readonly global?: DomainGlobalSpec<unknown>;\n    readonly tables: Record<string, DomainTableSpec>;\n}',
   },
   {
     name: 'DomainTableSpec',
@@ -4264,11 +4264,11 @@ export const TYPE_API: readonly TypeApiEntry[] = [
   },
   {
     name: 'KvUnit',
-    declaration: 'export interface KvUnit {\n    loadAll(): Promise<{\n        tables: Record<string, Record<string, unknown>>;\n        global: unknown;\n    }>;\n    putRecord(table: string, key: string, value: unknown): Promise<void>;\n    deleteRecord(table: string, key: string): Promise<void>;\n    setGlobal(value: unknown): Promise<void>;\n    close(): Promise<void>;\n}',
+    declaration: 'export interface KvUnit {\n    loadAll(): Promise<{\n        tables: Record<string, Record<string, unknown>>;\n        global: unknown;\n    }>;\n    putRecord(table: string, key: string, value: unknown): Promise<void>;\n    deleteRecord(table: string, key: string): Promise<void>;\n    backupRecord?(table: string, key: string): Promise<string>;\n    setGlobal(value: unknown): Promise<void>;\n    close(): Promise<void>;\n}',
   },
   {
     name: 'KvUnitDescriptor',
-    declaration: 'export interface KvUnitDescriptor {\n    readonly name: string;\n    readonly version: number;\n    readonly tables: readonly string[];\n    readonly hasGlobal: boolean;\n    readonly layout?: \'single\' | \'per-record\';\n}',
+    declaration: 'export interface KvUnitDescriptor {\n    readonly name: string;\n    readonly version: number;\n    readonly tables: readonly string[];\n    readonly hasGlobal: boolean;\n    readonly layout?: \'single\' | \'per-record\';\n    readonly compatibleVersions?: readonly number[];\n}',
   },
   {
     name: 'LlmAdapter',
diff --git a/packages/session/session-projection-cache/README.i18n.yaml b/packages/session/session-projection-cache/README.i18n.yaml
index e4fe8490a2..4dd1489d4e 100644
--- a/packages/session/session-projection-cache/README.i18n.yaml
+++ b/packages/session/session-projection-cache/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/session/session-projection-cache/README.md
-README.md: 51b9d86724af96304cf09a5c7c1b61b7394d336a
-README.zh.md: bb84b67bdde678884fd4f2be1b14b2161da8c2a0
+README.md: 9fd9766d75ab3f9a460b2802ac59810839f21ae4
+README.zh.md: 98725d9ce76ab44821adf3d43e3807932cf6b675
diff --git a/packages/session/session-projection-cache/README.md b/packages/session/session-projection-cache/README.md
index 51b9d86724..9fd9766d75 100644
--- a/packages/session/session-projection-cache/README.md
+++ b/packages/session/session-projection-cache/README.md
@@ -64,6 +64,8 @@ Three mandatory points always write: session creation persists the seed-derived
 
 The log leads and the cache follows: a live checkpoint flushes the session's buffered events durably before the cache row lands, so a crash can leave the cache behind the log but never ahead of it. Reads and writes share the storage domain's coherent in-memory state; the per-unit write chain mutates memory only after durability. Each version-stamped record must match the live unit schema and complete lifecycle identity (`createdAt`, `cwd`, `isSeeded`, and `inheritedEventCount`), so a row initialized under one fork cut cannot seed another. The JSON backend stores each record at `<root>/session_projcache/sessions/<id>.json` in an owner-only directory tree.
 
+Upgrades never cost the boot or the listing: records stamped with a version in the spec's `compatibleVersions` stay readable (their absent lineage fields decode as the unseeded lineage — exact for unseeded sessions, while a seeded caller fails the identity match and refolds cold), and a stored record that still fails schema validation is moved aside as `<id>.json.bak.<stamp>` under the domain's `invalidRecords: 'backup-and-skip'` policy, logged with its cause, and rebuilt by the next checkpoint.
+
 -----
 
 <a id="understand-the-implementation"></a>
@@ -126,6 +128,7 @@ These limits define where the cache needs operational care. They are current pac
 - **No eviction or retention surface** — records accumulate per session; pruning stored checkpoints is out-of-band maintenance, same stance as session persistence itself.
 - **Interval throttle is per-session coarse** — the timer arms at the first dirty event after a clean write; a steady sub-threshold trickle writes once per interval, not a sliding window.
 - **No cache-side cold refold** — the cache serves and refreshes its rows but never reads the session log (it does not depend on the persistence layer); a consumer that needs a guaranteed cold snapshot refolds from the log itself.
+- **Every schema or domain-version change must prove its upgrade story** — a change to the stored record schema or the domain version lands in the same PR with an archived fixture of the previously shipped on-disk format under `tests/fixtures/` and test cases in `tests/fixtures.spec.ts` proving the chosen disposition: read-compat recovery (`compatibleVersions`), current-version rewrite, or backup-and-skip salvage. A bump whose old records are simply discarded still proves that the discard neither fails the boot nor poisons the tree.
 
 <a id="dev-note"></a>
 ### Dev Note
diff --git a/packages/session/session-projection-cache/README.zh.md b/packages/session/session-projection-cache/README.zh.md
index bb84b67bdd..98725d9ce7 100644
--- a/packages/session/session-projection-cache/README.zh.md
+++ b/packages/session/session-projection-cache/README.zh.md
@@ -64,6 +64,8 @@ kind: "package-reference"
 
 日志领先，缓存跟随：实时检查点先把会话的缓冲事件持久化，然后才保存缓存记录。因此崩溃可能让缓存落后于日志，但绝不会让缓存领先。读取和写入共享存储域内一致的内存状态；逐单元写入链只在持久化成功后修改内存。每个带版本戳的记录必须匹配实时单元 schema 与完整生命周期身份（`createdAt`、`cwd`、`isSeeded` 和 `inheritedEventCount`），因此在一个 fork 切点下初始化的行不能播种另一个切点。JSON 后端把每条记录存于仅所有者可访问的 `<root>/session_projcache/sessions/<id>.json` 目录树中。
 
+升级绝不拖垮启动或列表：版本戳落在 spec `compatibleVersions` 集合内的记录保持可读（缺失的 lineage 字段解码为 unseeded lineage——对非 fork 会话精确无误，seeded 调用方则通不过身份比对、回落冷折叠），而仍然通不过 schema 校验的存量记录会按域的 `invalidRecords: 'backup-and-skip'` 策略移出为 `<id>.json.bak.<时间戳>`、连同原因写入日志，并由下一次检查点重建。
+
 -----
 
 <a id="understand-the-implementation"></a>
@@ -126,6 +128,7 @@ kind: "package-reference"
 - **无淘汰或保留接口**——记录按会话持续累积；清理已存储检查点属于带外维护，与会话持久化采用相同策略。
 - **间隔节流采用按会话的粗粒度控制**——一次无脏数据的写入完成后，计时器在首个脏事件到达时启动；持续但低于条数阈值的事件流每间隔写入一次，而非滑动窗口。
 - **缓存侧不做冷重折叠**——缓存只服务并刷新自己的记录，从不读取会话日志，因为它不依赖持久化层；需要保证冷快照的消费方自行从日志重新折叠。
+- **每次 schema 或域版本变更都必须论证升级路径**——改动存储记录 schema 或域版本时，同一 PR 必须在 `tests/fixtures/` 下归档此前已发布的磁盘格式样本，并在 `tests/fixtures.spec.ts` 中用测试论证所选的处置方式：读兼容恢复（`compatibleVersions`）、当前版本重写，或 backup-and-skip 抢救。即便选择直接丢弃旧记录的 bump，也要证明丢弃既不炸启动、也不污染缓存树。
 
 <a id="dev-note"></a>
 ### 开发备注
diff --git a/packages/session/session-projection-cache/src/index.ts b/packages/session/session-projection-cache/src/index.ts
index 35bf5d9eed..22828a103c 100644
--- a/packages/session/session-projection-cache/src/index.ts
+++ b/packages/session/session-projection-cache/src/index.ts
@@ -367,12 +367,18 @@ function identityOf(
   }
 }
 
-/** Whether a stored record's bound identity names the caller's lifecycle. */
+/**
+ * Whether a stored record's bound identity names the caller's lifecycle.
+ * Absent lineage fields (records admitted via `compatibleVersions` predate
+ * them) read as the unseeded lineage: exact for an unseeded caller, and a
+ * seeded caller's expectation then fails the match, discarding the record to
+ * a cold rebuild.
+ */
 function identityMatches(stored: CheckpointIdentity, expected: CheckpointIdentity): boolean {
   return stored.createdAt === expected.createdAt
     && stored.cwd === expected.cwd
-    && stored.isSeeded === expected.isSeeded
-    && stored.inheritedEventCount === expected.inheritedEventCount
+    && (stored.isSeeded ?? false) === expected.isSeeded
+    && (stored.inheritedEventCount ?? 0) === expected.inheritedEventCount
 }
 
 export default SessionProjectionCache
diff --git a/packages/session/session-projection-cache/src/spec.ts b/packages/session/session-projection-cache/src/spec.ts
index 35351917a5..1830853f6c 100644
--- a/packages/session/session-projection-cache/src/spec.ts
+++ b/packages/session/session-projection-cache/src/spec.ts
@@ -38,12 +38,19 @@ export const checkpointRow = z.object({
  * old record pass every watermark check and seed state folded from an
  * unrelated log. Reads validate this against the live header (listing) or
  * the stored header (cold read) before accepting any record.
+ *
+ * The lineage fields are optional because records admitted through
+ * `compatibleVersions` predate them. The reader (`identityMatches`)
+ * interprets their absence as the unseeded lineage — exact for an unseeded
+ * session, while a seeded expectation fails the match and the record is
+ * discarded to a cold rebuild. Current-version writes always store both
+ * fields.
  */
 export const checkpointIdentity = z.object({
   createdAt: z.number().int().nonnegative(),
   cwd: z.string().optional(),
-  isSeeded: z.boolean(),
-  inheritedEventCount: z.number().int().nonnegative().transform(SessionLogOffset),
+  isSeeded: z.boolean().optional(),
+  inheritedEventCount: z.number().int().nonnegative().transform(SessionLogOffset).optional(),
 })
 
 /** The identity fields a record is bound to, inferred from {@link checkpointIdentity}. */
@@ -68,11 +75,27 @@ export type CheckpointRecord = z.infer<typeof checkpointRecord>
  * bumps per session: after a bump, a stale session document is discarded on
  * open (cache semantics — a stale or unreadable cache costs a longer tail
  * replay, never a wrong value) while the rest of the domain stays usable,
- * instead of rejecting the whole medium.
+ * instead of rejecting the whole medium. The `compatibleVersions` entries
+ * are declared because those records differ from the current version only
+ * by the absent optional lineage fields, so upgraded homes keep serving
+ * their cached listing projections instead of dropping every title until
+ * each session is reopened; the per-record version map lives in the
+ * read-compat Agent Note
+ * (.agents/notes/implemented/architecture/2026-09-02-projcache-cross-version-read-compat.md).
+ * The per-row `ver` guard and the identity match still discard anything the
+ * current fold semantics cannot vouch for.
+ *
+ * `invalidRecords: 'backup-and-skip'`: a stored record that fails the schema
+ * anyway is disposable derived data, so it must never cost the boot — the
+ * domain layer moves the document aside as `<key>.json.bak.<stamp>`, logs
+ * the concrete validation failure, and serves the session as uncached (a
+ * cold read rebuilds and rewrites it).
  */
 export const projectionCacheDomainSpec = defineDomain({
   name: 'session_projcache',
   version: 5,
+  compatibleVersions: [3, 4],
+  invalidRecords: 'backup-and-skip',
   layout: 'per-record',
   tables: { sessions: domainTable<SessionId, CheckpointRecord>(checkpointRecord) },
 })
diff --git a/packages/session/session-projection-cache/tests/cache.spec.ts b/packages/session/session-projection-cache/tests/cache.spec.ts
index bfdca18567..4a5ce73d28 100644
--- a/packages/session/session-projection-cache/tests/cache.spec.ts
+++ b/packages/session/session-projection-cache/tests/cache.spec.ts
@@ -42,9 +42,11 @@ declare module '@deepseek-ai/dsh-session-projection/types' {
     'cache-test/marks2': Map<string, string>
     'cache-test/count': number
     'cache-test/secret': string
+    'cache-test/marks3': MarksState
   }
   interface SessionProjectionMap {
     'cache-test/marks': { marks: string[] }
+    'cache-test/marks3': { marks: string[] }
   }
 }
 
@@ -71,6 +73,18 @@ const marksUnit = (stateVersion = 1) => ({
   stateVersion,
 }) satisfies ProjectionDefinition<'cache-test/marks', MarksState>
 
+const marks3Unit = {
+  key: 'cache-test/marks3',
+  stateSchema: z.object({ marks: z.array(z.string()) }).nullable(),
+  init: () => null,
+  apply: state => state,
+  wire: {
+    viewSchema: z.object({ marks: z.array(z.string()) }),
+    view: state => state ?? { marks: [] },
+  },
+  stateVersion: 1,
+} satisfies ProjectionDefinition<'cache-test/marks3', MarksState>
+
 const secretUnit = {
   key: 'cache-test/secret',
   stateSchema: z.string(),
@@ -376,6 +390,26 @@ describe('SessionProjectionCache listing read', () => {
       .toBeUndefined()
   })
 
+  it('carries ONE cut across multiple served rows: the lowest watermark wins', async () => {
+    const root = await mkdtemp(join(tmpdir(), 'dsh-projcache-'))
+    roots.push(root)
+    // Equal watermarks: whichever row is visited second cannot lower the cut,
+    // so the one-cut fold sees both a lowering and a non-lowering row in
+    // every iteration order.
+    await seedRecord(root, 'multi-row', {
+      'cache-test/marks': { ver: 1, seq: SessionSeq(4), val: { marks: ['a'] } },
+      'cache-test/marks3': { ver: 1, seq: SessionSeq(4), val: { marks: ['b'] } },
+    })
+    const { ctx, cache } = await harness({ root })
+    ctx.sessionProjections.register(marks3Unit)
+    const block = cache.cachedSnapshot(headerOf(SessionId('multi-row')), SessionLogOffset(0))
+    expect(block?.values).toEqual({
+      'cache-test/marks': { marks: ['a'] },
+      'cache-test/marks3': { marks: ['b'] },
+    })
+    expect(block?.asOfSeq).toBe(4)
+  })
+
   it('returns undefined when the stored record is version-mismatched', async () => {
     const root = await mkdtemp(join(tmpdir(), 'dsh-projcache-'))
     roots.push(root)
@@ -394,6 +428,30 @@ describe('SessionProjectionCache listing read', () => {
       .toBeUndefined()
   })
 
+  it('serves a pre-lineage record (accepted old version) to an unseeded caller only', async () => {
+    const root = await mkdtemp(join(tmpdir(), 'dsh-projcache-'))
+    roots.push(root)
+    // A document stamped with an accepted older version whose identity
+    // predates the lineage fields: absent lineage reads as unseeded.
+    const path = recordPath(root, SessionId('pre-lineage'))
+    await mkdir(dirname(path), { recursive: true })
+    await writeFile(path, JSON.stringify({
+      version: 4,
+      record: {
+        identity: { createdAt: 0 },
+        rows: { 'cache-test/marks': { ver: 1, seq: 4, val: { marks: ['kept'] } } },
+      },
+    }))
+    const { cache } = await harness({ root })
+    const id = SessionId('pre-lineage')
+    // Unseeded caller: the absent lineage is exactly its identity — served.
+    expect(cache.cachedSnapshot(headerOf(id), SessionLogOffset(0)))
+      .toEqual({ asOfSeq: 4, values: { 'cache-test/marks': { marks: ['kept'] } } })
+    // Seeded caller: the lineage-less record cannot vouch for the cut — refused.
+    expect(cache.cachedSnapshot({ ...headerOf(id), isSeeded: true }, SessionLogOffset(2)))
+      .toBeUndefined()
+  })
+
   it('returns undefined when every stored row is version-mismatched', async () => {
     const root = await mkdtemp(join(tmpdir(), 'dsh-projcache-'))
     roots.push(root)
diff --git a/packages/session/session-projection-cache/tests/fixtures.spec.ts b/packages/session/session-projection-cache/tests/fixtures.spec.ts
new file mode 100644
index 0000000000..e2486b7a9d
--- /dev/null
+++ b/packages/session/session-projection-cache/tests/fixtures.spec.ts
@@ -0,0 +1,242 @@
+/**
+ * Cross-version recovery over archived on-disk artifacts. `fixtures/` holds
+ * real `session_projcache` media, each produced by driving the named release
+ * through its own web app (session created over RPC, real model turns, a
+ * rename): the v3 whole-unit file (published 0.1.1-rc.2), a v4 per-record
+ * document (published 0.1.2-alpha.3), a current v5 document, and the
+ * v5-stamped lineage-less document reproducing byte-for-byte what the
+ * formerly unguarded legacy bootstrap wrote over v3 records. Each must
+ * recover through the real storage stack — the domain opens and the listing
+ * read serves the archived title — and a record that fails schema validation
+ * anyway is backed up and skipped instead of failing the boot.
+ */
+
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { cp, mkdir, mkdtemp, readFile, readdir, rm, writeFile } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import { dirname, join } from 'node:path'
+import { fileURLToPath } from 'node:url'
+import { Context } from '@deepseek-ai/cordis'
+import { z } from 'zod'
+import SessionStore, { SessionId, SessionLogOffset } from '@deepseek-ai/dsh-session'
+import type { SessionHeader } from '@deepseek-ai/dsh-session'
+import SessionProjectionRegistry from '@deepseek-ai/dsh-session-projection'
+import type { ProjectionDefinition } from '@deepseek-ai/dsh-session-projection'
+import Storage from '@deepseek-ai/dsh-storage'
+import {
+  apply as storageJsonApply, Config as storageJsonConfig, inject as storageJsonInject, name as storageJsonName,
+} from '@deepseek-ai/dsh-storage-json'
+import {
+  apply as storageDomainApply, Config as storageDomainConfig, inject as storageDomainInject, name as storageDomainName,
+} from '@deepseek-ai/dsh-storage-domain'
+import SessionProjectionCache from '../src/index.ts'
+import { projectionCacheDomainSpec } from '../src/spec.ts'
+
+// Declarations must match the shipped title unit's exactly (the repo-wide
+// compile face sees both).
+declare module '@deepseek-ai/dsh-session-projection/types' {
+  interface SessionProjectionStateMap {
+    title: string | null
+  }
+  interface SessionProjectionMap {
+    title: string | null
+  }
+}
+
+declare module '@deepseek-ai/dsh-session/types' {
+  interface SessionEventMap {
+    'fixtures-test/set-title': { title: string }
+  }
+
+  interface OutOfBandSessionEventMap {
+    'fixtures-test/set-title': true
+  }
+}
+
+// Mirrors the shipped title unit's storage face: stateVersion 1, bare-string
+// state (the fixture rows carry exactly this shape in every archived
+// version), folding a test event so the rewrite path has fresh data.
+const titleUnit = {
+  key: 'title',
+  stateSchema: z.string().nullable(),
+  init: () => null,
+  apply: (state, event) => (event.type === 'fixtures-test/set-title' ? event.data.title : state),
+  wire: { viewSchema: z.string().nullable(), view: state => state },
+  stateVersion: 1,
+} satisfies ProjectionDefinition<'title', string | null>
+
+const FIXTURES = fileURLToPath(new URL('./fixtures/', import.meta.url))
+
+/** One archived per-record document (`{version, record}`). */
+interface FixtureDoc {
+  version: number
+  record: {
+    identity: { createdAt: number; cwd?: string }
+    rows: Record<string, { ver: number; seq: number; val: unknown }>
+  }
+}
+
+async function fixtureJson<T>(name: string): Promise<T> {
+  return JSON.parse(await readFile(join(FIXTURES, name), 'utf8')) as T
+}
+
+/** Header for the session a fixture record is bound to (identity witness). */
+function headerFor(id: SessionId, identity: FixtureDoc['record']['identity']): SessionHeader {
+  return {
+    version: 0,
+    id,
+    createdAt: identity.createdAt,
+    isSeeded: false,
+    ...identity.cwd === undefined ? {} : { cwd: identity.cwd },
+  }
+}
+
+const contexts: Context[] = []
+const roots: string[] = []
+
+async function harness(root: string) {
+  roots.push(root)
+  const ctx = new Context()
+  contexts.push(ctx)
+  await ctx.plugin(Storage)
+  await ctx.plugin({ name: storageJsonName, inject: storageJsonInject, apply: storageJsonApply, Config: storageJsonConfig }, { root })
+  await ctx.plugin({ name: storageDomainName, inject: storageDomainInject, apply: storageDomainApply, Config: storageDomainConfig }, { backend: 'json' })
+  await ctx.plugin(SessionStore)
+  await ctx.plugin(SessionProjectionRegistry)
+  ctx.sessionProjections.register(titleUnit)
+  await ctx.plugin(SessionProjectionCache, { writeEveryEvents: 100, writeIntervalMs: 60_000 })
+  return { ctx, cache: ctx.sessionProjectionCache }
+}
+
+/** Lay one per-record fixture document into a fresh backend root. */
+async function placeDoc(root: string, id: string, name: string): Promise<FixtureDoc> {
+  const path = join(root, projectionCacheDomainSpec.name, 'sessions', `${id}.json`)
+  await mkdir(dirname(path), { recursive: true })
+  await cp(join(FIXTURES, name), path)
+  return fixtureJson<FixtureDoc>(name)
+}
+
+/**
+ * Drive a live write over a recovered session id and assert the archived
+ * document is replaced by a current-version one: v5 stamp, lineage present,
+ * and the freshly folded title — the write path never keeps the old format.
+ */
+async function assertRewrite(ctx: Context, root: string, id: SessionId): Promise<void> {
+  const session = ctx.sessions.create(id)
+  session.append('fixtures-test/set-title', { title: '重写标题' })
+  session.append('turn/end', { turn: 1, reason: { kind: 'completed' } })
+  const path = join(root, projectionCacheDomainSpec.name, 'sessions', `${id}.json`)
+  await vi.waitFor(async () => {
+    const doc = JSON.parse(await readFile(path, 'utf8')) as FixtureDoc
+    expect(doc.version).toBe(projectionCacheDomainSpec.version)
+    expect(doc.record.identity).toMatchObject({ isSeeded: false, inheritedEventCount: 0 })
+    expect(doc.record.rows['title']?.val).toBe('重写标题')
+  }, { timeout: 5_000 })
+}
+
+afterEach(async () => {
+  await Promise.all(contexts.splice(0).map(ctx => ctx.fiber.dispose()))
+  await Promise.all(roots.splice(0).map(root => rm(root, { recursive: true, force: true, maxRetries: 10, retryDelay: 100 })))
+})
+
+describe('archived version recovery', () => {
+  it('recovers the v3 whole-unit archive through the legacy bootstrap', async () => {
+    const root = await mkdtemp(join(tmpdir(), 'dsh-projcache-fx-'))
+    await cp(join(FIXTURES, 'v3-single-unit.json'), join(root, `${projectionCacheDomainSpec.name}.json`))
+    type SingleUnit = {
+      unit: { version: number }
+      tables: { sessions: Record<string, FixtureDoc['record']> }
+    }
+    const archive = await fixtureJson<SingleUnit>('v3-single-unit.json')
+    expect(archive.unit.version).toBe(3) // the fixture IS the old format
+    const [sid, record] = Object.entries(archive.tables.sessions)[0]!
+
+    const { ctx, cache } = await harness(root)
+    const snapshot = cache.cachedSnapshot(headerFor(SessionId(sid), record.identity), SessionLogOffset(0), ['title'])
+    expect(snapshot?.values.title).toBe(record.rows['title']!.val)
+
+    // The one-time bootstrap materialized a current-version document.
+    const migrated = JSON.parse(
+      await readFile(join(root, projectionCacheDomainSpec.name, 'sessions', `${sid}.json`), 'utf8'),
+    ) as { version: number }
+    expect(migrated.version).toBe(projectionCacheDomainSpec.version)
+
+    await assertRewrite(ctx, root, SessionId(sid))
+  })
+
+  for (const [fixture, storedVersion] of [
+    ['v4-session-doc.json', 4],
+    ['v5-session-doc.json', 5],
+    ['v5-lineageless-doc.json', 5],
+  ] as const) {
+    it(`serves the archived title from ${fixture}, then rewrites it current`, async () => {
+      const root = await mkdtemp(join(tmpdir(), 'dsh-projcache-fx-'))
+      const id = SessionId('fixture-session')
+      const doc = await placeDoc(root, id, fixture)
+      expect(doc.version).toBe(storedVersion)
+
+      const { ctx, cache } = await harness(root)
+      const snapshot = cache.cachedSnapshot(headerFor(id, doc.record.identity), SessionLogOffset(0), ['title'])
+      expect(snapshot?.values.title).toBe(doc.record.rows['title']!.val)
+
+      await assertRewrite(ctx, root, id)
+    })
+  }
+
+  it('refuses a lineage-less archive for a seeded caller (identity mismatch, cold rebuild)', async () => {
+    const root = await mkdtemp(join(tmpdir(), 'dsh-projcache-fx-'))
+    const id = SessionId('fixture-seeded')
+    const doc = await placeDoc(root, id, 'v5-lineageless-doc.json')
+
+    const { cache } = await harness(root)
+    const seeded = { ...headerFor(id, doc.record.identity), isSeeded: true }
+    expect(cache.cachedSnapshot(seeded, SessionLogOffset(2), ['title'])).toBeUndefined()
+  })
+
+  it('backs up and skips a record that fails schema validation instead of failing the boot', async () => {
+    const root = await mkdtemp(join(tmpdir(), 'dsh-projcache-fx-'))
+    roots.push(root)
+    const sessionsDir = join(root, projectionCacheDomainSpec.name, 'sessions')
+    await mkdir(sessionsDir, { recursive: true })
+    // Current-version stamp, hopeless record content: no compat rung can save it.
+    await writeFile(join(sessionsDir, 'broken.json'), JSON.stringify({
+      version: projectionCacheDomainSpec.version,
+      record: { identity: { createdAt: 'not-a-number' }, rows: 'not-an-object' },
+    }))
+    const good = await placeDoc(root, SessionId('survivor'), 'v5-session-doc.json')
+
+    const ctx = new Context()
+    contexts.push(ctx)
+    await ctx.plugin(Storage)
+    await ctx.plugin({ name: storageJsonName, inject: storageJsonInject, apply: storageJsonApply, Config: storageJsonConfig }, { root })
+    await ctx.plugin({ name: storageDomainName, inject: storageDomainInject, apply: storageDomainApply, Config: storageDomainConfig }, { backend: 'json' })
+    await ctx.plugin(SessionStore)
+    await ctx.plugin(SessionProjectionRegistry)
+    ctx.sessionProjections.register(titleUnit)
+    const error = vi.spyOn(ctx.logger, 'error').mockImplementation(() => {})
+    // The boot survives the broken record — this line rejecting IS the fixed bug.
+    await ctx.plugin(SessionProjectionCache, { writeEveryEvents: 100, writeIntervalMs: 60_000 })
+
+    // Concrete console diagnostics: which record, where it went, and why.
+    expect(error).toHaveBeenCalledWith(expect.stringContaining("record 'broken'"))
+    expect(error).toHaveBeenCalledWith(expect.stringContaining('.json.bak.'))
+
+    // The document was moved aside as <key>.json.bak.<YYYYMMDDHHmm>, bytes intact.
+    const entries = await readdir(sessionsDir)
+    expect(entries).not.toContain('broken.json')
+    const backup = entries.find(name => /^broken\.json\.bak\.\d{12}$/.test(name))
+    expect(backup).toBeDefined()
+    expect(JSON.parse(await readFile(join(sessionsDir, backup!), 'utf8')))
+      .toMatchObject({ record: { rows: 'not-an-object' } })
+
+    // The broken record reads as absent; its neighbors still serve.
+    const cache = ctx.sessionProjectionCache
+    expect(cache.cachedSnapshot(headerFor(SessionId('broken'), { createdAt: 0 }), SessionLogOffset(0)))
+      .toBeUndefined()
+    expect(cache.cachedSnapshot(
+      headerFor(SessionId('survivor'), good.record.identity),
+      SessionLogOffset(0),
+      ['title'],
+    )?.values.title).toBe(good.record.rows['title']!.val)
+  })
+})
diff --git a/packages/session/session-projection-cache/tests/fixtures/v3-single-unit.json b/packages/session/session-projection-cache/tests/fixtures/v3-single-unit.json
new file mode 100644
index 0000000000..4b36243611
--- /dev/null
+++ b/packages/session/session-projection-cache/tests/fixtures/v3-single-unit.json
@@ -0,0 +1,136 @@
+{
+  "unit": {
+    "name": "session_projcache",
+    "version": 3
+  },
+  "global": null,
+  "tables": {
+    "sessions": {
+      "session-1374fa81-15da-44ca-be12-b4c6fe8076a3": {
+        "identity": {
+          "createdAt": 1788286864454,
+          "cwd": "/tmp"
+        },
+        "rows": {
+          "sessionStats": {
+            "ver": 1,
+            "seq": 54,
+            "val": {
+              "turns": 2,
+              "steps": 2,
+              "llmMs": 2169,
+              "toolMs": 0,
+              "ttftMs": 1872,
+              "ttftSteps": 2,
+              "decodeMs": 297,
+              "decodeTokens": 20,
+              "lastTurn": 2,
+              "openStep": null,
+              "pendingCalls": {}
+            }
+          },
+          "title": {
+            "ver": 1,
+            "seq": 54,
+            "val": "验收标题-rc2"
+          },
+          "goal": {
+            "ver": 4,
+            "seq": 54,
+            "val": null
+          },
+          "tokenUsage": {
+            "ver": 1,
+            "seq": 54,
+            "val": {
+              "totals": {
+                "uncachedInputTokens": 6083,
+                "outputTokens": 20,
+                "cacheReadTokens": 9728,
+                "cacheWriteTokens": 0
+              },
+              "last": {
+                "turn": 2,
+                "step": 1,
+                "buckets": {
+                  "uncachedInputTokens": 112,
+                  "outputTokens": 4,
+                  "cacheReadTokens": 7808,
+                  "cacheWriteTokens": 0
+                }
+              }
+            }
+          },
+          "contextPressure": {
+            "ver": 4,
+            "seq": 54,
+            "val": {
+              "surfaceTokens": 178,
+              "contextWindow": 1000000,
+              "pressureTokens": 7920,
+              "sampledSurfaceTokens": 168
+            }
+          },
+          "contextBreakdown": {
+            "ver": 2,
+            "seq": 54,
+            "val": {
+              "systemTokens": 1620,
+              "toolsTokens": 6475,
+              "messageTokens": 178
+            }
+          },
+          "subagentTiming": {
+            "ver": 2,
+            "seq": 54,
+            "val": {
+              "descriptorSeen": false,
+              "settledMs": 0
+            }
+          },
+          "subagent": {
+            "ver": 2,
+            "seq": 54,
+            "val": {}
+          },
+          "permissions": {
+            "ver": 1,
+            "seq": 54,
+            "val": {
+              "preset": "workspace-write",
+              "sandbox": "workspace-write",
+              "approval": "ask"
+            }
+          },
+          "sessionListMetadata": {
+            "ver": 1,
+            "seq": 54,
+            "val": {
+              "blank": false,
+              "lastPromptAt": 1788286867410
+            }
+          },
+          "imageLimits": {
+            "ver": 1,
+            "seq": 54,
+            "val": null
+          },
+          "todos": {
+            "ver": 2,
+            "seq": 54,
+            "val": null
+          },
+          "plan": {
+            "ver": 2,
+            "seq": 54,
+            "val": {
+              "active": false,
+              "wanted": null,
+              "running": null
+            }
+          }
+        }
+      }
+    }
+  }
+}
diff --git a/packages/session/session-projection-cache/tests/fixtures/v4-session-doc.json b/packages/session/session-projection-cache/tests/fixtures/v4-session-doc.json
new file mode 100644
index 0000000000..56d6868405
--- /dev/null
+++ b/packages/session/session-projection-cache/tests/fixtures/v4-session-doc.json
@@ -0,0 +1,212 @@
+{
+  "version": 4,
+  "record": {
+    "identity": {
+      "createdAt": 1788286912530,
+      "cwd": "/tmp"
+    },
+    "rows": {
+      "title": {
+        "ver": 1,
+        "seq": 68,
+        "val": "验收标题-alpha3"
+      },
+      "titleInput": {
+        "ver": 3,
+        "seq": 68,
+        "val": {
+          "first": {
+            "seq": 7,
+            "text": "请只回复一个词：pong"
+          },
+          "count": 2,
+          "lastSeq": 58
+        }
+      },
+      "llmRetry": {
+        "ver": 1,
+        "seq": 68,
+        "val": {}
+      },
+      "sandboxMode": {
+        "ver": 1,
+        "seq": 68,
+        "val": "workspace-write"
+      },
+      "goal": {
+        "ver": 6,
+        "seq": 68,
+        "val": {
+          "current": null,
+          "seenGoalIds": [],
+          "failure": null
+        }
+      },
+      "tokenUsage": {
+        "ver": 2,
+        "seq": 68,
+        "val": {
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+            "outputTokens": 35,
+            "cacheReadTokens": 8064,
+            "cacheWriteTokens": 0
+          },
+          "last": {
+            "turn": 2,
+            "step": 1,
+            "buckets": {
+              "uncachedInputTokens": 86,
+              "outputTokens": 4,
+              "cacheReadTokens": 8064,
+              "cacheWriteTokens": 0
+            }
+          }
+        }
+      },
+      "contextPressure": {
+        "ver": 4,
+        "seq": 68,
+        "val": {
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+          "contextWindow": 1000000,
+          "pressureTokens": 8150,
+          "sampledSurfaceTokens": 183
+        }
+      },
+      "contextBreakdown": {
+        "ver": 2,
+        "seq": 68,
+        "val": {
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+          "toolsTokens": 6579,
+          "messageTokens": 193
+        }
+      },
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+        "ver": 2,
+        "seq": 68,
+        "val": {
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+          "lastStepStartSeq": 57,
+          "lastStepBoundary": {
+            "kind": "end",
+            "seq": 67
+          },
+          "lastTurn": 2
+        }
+      },
+      "sessionStats": {
+        "ver": 1,
+        "seq": 68,
+        "val": {
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+          "llmMs": 3486,
+          "toolMs": 0,
+          "ttftMs": 3199,
+          "ttftSteps": 2,
+          "decodeMs": 287,
+          "decodeTokens": 35,
+          "lastTurn": 2,
+          "openStep": null,
+          "pendingCalls": {}
+        }
+      },
+      "turnOutline": {
+        "ver": 2,
+        "seq": 68,
+        "val": {
+          "turns": [
+            {
+              "turn": 1,
+              "seq": 4,
+              "prompt": "请只回复一个词：pong",
+              "response": "pong"
+            },
+            {
+              "turn": 2,
+              "seq": 55,
+              "prompt": "请只回复一个词：pong2",
+              "response": "pong2"
+            }
+          ],
+          "draft": ""
+        }
+      },
+      "agentPreset": {
+        "ver": 1,
+        "seq": 68,
+        "val": "standard"
+      },
+      "subagentTiming": {
+        "ver": 2,
+        "seq": 68,
+        "val": {
+          "descriptorSeen": false,
+          "settledMs": 0
+        }
+      },
+      "subagent": {
+        "ver": 2,
+        "seq": 68,
+        "val": {}
+      },
+      "permissions": {
+        "ver": 2,
+        "seq": 68,
+        "val": {
+          "preset": "workspace-write",
+          "sandbox": "workspace-write",
+          "approval": "ask",
+          "seeded": false
+        }
+      },
+      "modelSelection": {
+        "ver": 2,
+        "seq": 68,
+        "val": {
+          "lastUsed": {
+            "provider": "deepseek-official",
+            "model": "deepseek-v4-flash",
+            "reasoningEffort": "high"
+          },
+          "pending": null
+        }
+      },
+      "sessionListMetadata": {
+        "ver": 1,
+        "seq": 68,
+        "val": {
+          "blank": false,
+          "lastPromptAt": 1788286917834
+        }
+      },
+      "imageLimits": {
+        "ver": 1,
+        "seq": 68,
+        "val": null
+      },
+      "todos": {
+        "ver": 2,
+        "seq": 68,
+        "val": null
+      },
+      "plan": {
+        "ver": 3,
+        "seq": 68,
+        "val": {
+          "active": false,
+          "wanted": null,
+          "running": null,
+          "activeAtLastHeader": false
+        }
+      },
+      "subagentModelSelectionPolicy": {
+        "ver": 1,
+        "seq": 68,
+        "val": null
+      }
+    }
+  }
+}
diff --git a/packages/session/session-projection-cache/tests/fixtures/v5-lineageless-doc.json b/packages/session/session-projection-cache/tests/fixtures/v5-lineageless-doc.json
new file mode 100644
index 0000000000..f887e980e2
--- /dev/null
+++ b/packages/session/session-projection-cache/tests/fixtures/v5-lineageless-doc.json
@@ -0,0 +1,128 @@
+{
+  "version": 5,
+  "record": {
+    "identity": {
+      "createdAt": 1788286864454,
+      "cwd": "/tmp"
+    },
+    "rows": {
+      "sessionStats": {
+        "ver": 1,
+        "seq": 54,
+        "val": {
+          "turns": 2,
+          "steps": 2,
+          "llmMs": 2169,
+          "toolMs": 0,
+          "ttftMs": 1872,
+          "ttftSteps": 2,
+          "decodeMs": 297,
+          "decodeTokens": 20,
+          "lastTurn": 2,
+          "openStep": null,
+          "pendingCalls": {}
+        }
+      },
+      "title": {
+        "ver": 1,
+        "seq": 54,
+        "val": "\u9a8c\u6536\u6807\u9898-rc2"
+      },
+      "goal": {
+        "ver": 4,
+        "seq": 54,
+        "val": null
+      },
+      "tokenUsage": {
+        "ver": 1,
+        "seq": 54,
+        "val": {
+          "totals": {
+            "uncachedInputTokens": 6083,
+            "outputTokens": 20,
+            "cacheReadTokens": 9728,
+            "cacheWriteTokens": 0
+          },
+          "last": {
+            "turn": 2,
+            "step": 1,
+            "buckets": {
+              "uncachedInputTokens": 112,
+              "outputTokens": 4,
+              "cacheReadTokens": 7808,
+              "cacheWriteTokens": 0
+            }
+          }
+        }
+      },
+      "contextPressure": {
+        "ver": 4,
+        "seq": 54,
+        "val": {
+          "surfaceTokens": 178,
+          "contextWindow": 1000000,
+          "pressureTokens": 7920,
+          "sampledSurfaceTokens": 168
+        }
+      },
+      "contextBreakdown": {
+        "ver": 2,
+        "seq": 54,
+        "val": {
+          "systemTokens": 1620,
+          "toolsTokens": 6475,
+          "messageTokens": 178
+        }
+      },
+      "subagentTiming": {
+        "ver": 2,
+        "seq": 54,
+        "val": {
+          "descriptorSeen": false,
+          "settledMs": 0
+        }
+      },
+      "subagent": {
+        "ver": 2,
+        "seq": 54,
+        "val": {}
+      },
+      "permissions": {
+        "ver": 1,
+        "seq": 54,
+        "val": {
+          "preset": "workspace-write",
+          "sandbox": "workspace-write",
+          "approval": "ask"
+        }
+      },
+      "sessionListMetadata": {
+        "ver": 1,
+        "seq": 54,
+        "val": {
+          "blank": false,
+          "lastPromptAt": 1788286867410
+        }
+      },
+      "imageLimits": {
+        "ver": 1,
+        "seq": 54,
+        "val": null
+      },
+      "todos": {
+        "ver": 2,
+        "seq": 54,
+        "val": null
+      },
+      "plan": {
+        "ver": 2,
+        "seq": 54,
+        "val": {
+          "active": false,
+          "wanted": null,
+          "running": null
+        }
+      }
+    }
+  }
+}
diff --git a/packages/session/session-projection-cache/tests/fixtures/v5-session-doc.json b/packages/session/session-projection-cache/tests/fixtures/v5-session-doc.json
new file mode 100644
index 0000000000..9b822870a8
--- /dev/null
+++ b/packages/session/session-projection-cache/tests/fixtures/v5-session-doc.json
@@ -0,0 +1,220 @@
+{
+  "version": 5,
+  "record": {
+    "identity": {
+      "createdAt": 1788286864454,
+      "cwd": "/tmp",
+      "isSeeded": false,
+      "inheritedEventCount": 0
+    },
+    "rows": {
+      "title": {
+        "ver": 1,
+        "seq": 71,
+        "val": "验收标题-rc2"
+      },
+      "titleInput": {
+        "ver": 3,
+        "seq": 71,
+        "val": {
+          "first": {
+            "seq": 7,
+            "text": "请只回复一个词：pong"
+          },
+          "count": 3,
+          "lastSeq": 60
+        }
+      },
+      "llmRetry": {
+        "ver": 1,
+        "seq": 71,
+        "val": {}
+      },
+      "sandboxMode": {
+        "ver": 1,
+        "seq": 71,
+        "val": "workspace-write"
+      },
+      "goal": {
+        "ver": 6,
+        "seq": 71,
+        "val": {
+          "current": null,
+          "seenGoalIds": [],
+          "failure": null
+        }
+      },
+      "tokenUsage": {
+        "ver": 2,
+        "seq": 71,
+        "val": {
+          "totals": {
+            "uncachedInputTokens": 14219,
+            "outputTokens": 24,
+            "cacheReadTokens": 9728,
+            "cacheWriteTokens": 0
+          },
+          "last": {
+            "turn": 3,
+            "step": 1,
+            "buckets": {
+              "uncachedInputTokens": 8136,
+              "outputTokens": 4,
+              "cacheReadTokens": 0,
+              "cacheWriteTokens": 0
+            }
+          }
+        }
+      },
+      "contextPressure": {
+        "ver": 4,
+        "seq": 71,
+        "val": {
+          "surfaceTokens": 200,
+          "contextWindow": 1000000,
+          "pressureTokens": 8136,
+          "sampledSurfaceTokens": 190
+        }
+      },
+      "contextBreakdown": {
+        "ver": 2,
+        "seq": 71,
+        "val": {
+          "systemTokens": 1729,
+          "toolsTokens": 6611,
+          "messageTokens": 200
+        }
+      },
+      "sessionStats": {
+        "ver": 1,
+        "seq": 71,
+        "val": {
+          "turns": 3,
+          "steps": 3,
+          "llmMs": 4368,
+          "toolMs": 0,
+          "ttftMs": 4030,
+          "ttftSteps": 3,
+          "decodeMs": 338,
+          "decodeTokens": 24,
+          "lastTurn": 3,
+          "openStep": null,
+          "pendingCalls": {}
+        }
+      },
+      "agentPreset": {
+        "ver": 1,
+        "seq": 71,
+        "val": "standard"
+      },
+      "subagentTiming": {
+        "ver": 2,
+        "seq": 71,
+        "val": {
+          "descriptorSeen": false,
+          "settledMs": 0
+        }
+      },
+      "subagent": {
+        "ver": 2,
+        "seq": 71,
+        "val": {}
+      },
+      "turnBoundary": {
+        "ver": 2,
+        "seq": 71,
+        "val": {
+          "openTurnStartSeq": null,
+          "lastStepStartSeq": 59,
+          "lastStepBoundary": {
+            "kind": "end",
+            "seq": 70
+          },
+          "lastTurn": 3
+        }
+      },
+      "turnOutline": {
+        "ver": 2,
+        "seq": 71,
+        "val": {
+          "turns": [
+            {
+              "turn": 1,
+              "seq": 4,
+              "prompt": "请只回复一个词：pong",
+              "response": "pong"
+            },
+            {
+              "turn": 2,
+              "seq": 40,
+              "prompt": "请只回复一个词：pong2",
+              "response": "pong2"
+            },
+            {
+              "turn": 3,
+              "seq": 57,
+              "prompt": "请只回复一个词：pong3",
+              "response": "pong3"
+            }
+          ],
+          "draft": ""
+        }
+      },
+      "permissions": {
+        "ver": 2,
+        "seq": 71,
+        "val": {
+          "preset": "workspace-write",
+          "sandbox": "workspace-write",
+          "approval": "ask",
+          "seeded": true
+        }
+      },
+      "modelSelection": {
+        "ver": 2,
+        "seq": 71,
+        "val": {
+          "lastUsed": {
+            "provider": "deepseek-official",
+            "model": "deepseek-v4-flash",
+            "reasoningEffort": "high"
+          },
+          "pending": null
+        }
+      },
+      "sessionListMetadata": {
+        "ver": 1,
+        "seq": 71,
+        "val": {
+          "blank": false,
+          "lastPromptAt": 1788287660106
+        }
+      },
+      "imageLimits": {
+        "ver": 1,
+        "seq": 71,
+        "val": null
+      },
+      "todos": {
+        "ver": 2,
+        "seq": 71,
+        "val": null
+      },
+      "plan": {
+        "ver": 3,
+        "seq": 71,
+        "val": {
+          "active": false,
+          "wanted": null,
+          "running": null,
+          "activeAtLastHeader": false
+        }
+      },
+      "subagentModelSelectionPolicy": {
+        "ver": 1,
+        "seq": 71,
+        "val": null
+      }
+    }
+  }
+}
diff --git a/packages/storage/storage-domain/src/index.ts b/packages/storage/storage-domain/src/index.ts
index d2c16a3d69..7e0be5e151 100644
--- a/packages/storage/storage-domain/src/index.ts
+++ b/packages/storage/storage-domain/src/index.ts
@@ -88,7 +88,10 @@ export class DomainFacility {
    * (`facet-unsupported`); open the unit projected from the spec (backend
    * `version-mismatch`/`malformed-medium` pass through); load and validate
    * every stored record against the spec's zod schemas (`invalid-record`
-   * with the offending table and key); construct the domain.
+   * with the offending table and key — unless the spec declares
+   * `invalidRecords: 'backup-and-skip'` and the unit can move documents aside, in
+   * which case the failing record is backed up, logged, and skipped);
+   * construct the domain.
    *
    * Lifecycle: the CALLER owns the returned handle and closes it via
    * `Domain.close()` (typically as its own `ctx.effect` disposer) — the
@@ -118,7 +121,23 @@ export class DomainFacility {
         for (const [table, tableSpec] of Object.entries(spec.tables)) {
           const records = new Map<string, unknown>()
           for (const [key, raw] of Object.entries(snapshot.tables[table] ?? {})) {
-            records.set(key, parseRecord(spec.name, table, key, () => tableSpec.valueSchema.parse(raw)))
+            let parsed: unknown
+            try {
+              parsed = parseRecord(spec.name, table, key, () => tableSpec.valueSchema.parse(raw))
+            } catch (error) {
+              // Backup-and-skip policy (disposable derived data): move the record's
+              // document aside, log the concrete failure, and open without the
+              // record. Backends that cannot move a document keep the loud path.
+              if (spec.invalidRecords !== 'backup-and-skip' || unit.backupRecord === undefined) throw error
+              const moved = await unit.backupRecord(table, key)
+              // parseRecord always wraps the zod failure as the cause.
+              this.ctx.logger.error(
+                `domain '${spec.name}': stored record '${key}' in table '${table}' failed schema validation; `
+                + `moved to '${moved}' and treated as absent. Cause: ${String((error as DomainError).cause)}`,
+              )
+              continue
+            }
+            records.set(key, parsed)
           }
           tables.set(table, records)
         }
diff --git a/packages/storage/storage-domain/src/spec.ts b/packages/storage/storage-domain/src/spec.ts
index 74348918aa..f9267bcba5 100644
--- a/packages/storage/storage-domain/src/spec.ts
+++ b/packages/storage/storage-domain/src/spec.ts
@@ -45,6 +45,26 @@ export interface DomainSpec {
    * (a stale record document is discarded, never migrated).
    */
   readonly layout?: 'single' | 'per-record'
+  /**
+   * Older domain versions whose stored records the current record schemas
+   * also accept (the declaring owner vouches for that, typically by
+   * declaring the fields older records lack as optional). `per-record` backends
+   * read documents stamped with a listed version instead of discarding them,
+   * and accept a legacy whole-unit file so stamped for the one-time
+   * bootstrap; writes always stamp {@link version}.
+   */
+  readonly compatibleVersions?: readonly number[]
+  /**
+   * What `open` does with a stored table record that fails its zod schema.
+   * Absent (the default), the whole open rejects with `invalid-record` —
+   * right for authoritative data. `'backup-and-skip'` is for domains whose
+   * records are disposable derived data: the backend moves the record's
+   * document aside (`KvUnit.backupRecord`), the failure is logged with
+   * its cause, and the open continues with the record absent. A backend
+   * without `backupRecord` (no per-record document to move) falls back
+   * to the rejecting default. The global slot always rejects.
+   */
+  readonly invalidRecords?: 'backup-and-skip'
   /** Optional global singleton slot. */
   readonly global?: DomainGlobalSpec<unknown>
   /** Table declarations keyed by table name; each name must match `UNIT_NAME_RE`. */
@@ -91,6 +111,13 @@ export function defineDomain<S extends DomainSpec>(spec: S): S {
   if (!Number.isInteger(spec.version) || spec.version < 0) {
     throw new Error(`domain '${spec.name}' version must be a non-negative integer, got ${spec.version}`)
   }
+  for (const compat of spec.compatibleVersions ?? []) {
+    if (!Number.isInteger(compat) || compat < 0 || compat >= spec.version) {
+      throw new Error(
+        `domain '${spec.name}' compatibleVersions entries must be non-negative integers below version ${spec.version}, got ${compat}`,
+      )
+    }
+  }
   if (spec.layout !== undefined) {
     // Runtime boundary: the union type is compile-time only — a spec built
     // from config could carry any value, and a bad one must fail loud here.
@@ -99,6 +126,12 @@ export function defineDomain<S extends DomainSpec>(spec: S): S {
       throw new Error(`domain '${spec.name}' layout must be 'single' or 'per-record', got ${layout}`)
     }
   }
+  if (spec.invalidRecords !== undefined) {
+    const policy: string = spec.invalidRecords
+    if (policy !== 'backup-and-skip') {
+      throw new Error(`domain '${spec.name}' invalidRecords must be 'backup-and-skip' when present, got ${policy}`)
+    }
+  }
   for (const table of Object.keys(spec.tables)) {
     if (!UNIT_NAME_RE.test(table)) {
       throw new Error(`domain '${spec.name}' table name '${table}' must match ${UNIT_NAME_RE}`)
@@ -125,5 +158,6 @@ export function descriptorOf(spec: DomainSpec): KvUnitDescriptor {
     tables: Object.keys(spec.tables),
     hasGlobal: spec.global !== undefined,
     ...spec.layout === undefined ? {} : { layout: spec.layout },
+    ...spec.compatibleVersions === undefined ? {} : { compatibleVersions: spec.compatibleVersions },
   }
 }
diff --git a/packages/storage/storage-domain/tests/domain.spec.ts b/packages/storage/storage-domain/tests/domain.spec.ts
index 3b3c8fa679..b21ec0faf8 100644
--- a/packages/storage/storage-domain/tests/domain.spec.ts
+++ b/packages/storage/storage-domain/tests/domain.spec.ts
@@ -58,6 +58,25 @@ describe('defineDomain', () => {
     })).toThrow(/must not accept null/)
   })
 
+  it('validates compatibleVersions entries and projects them onto the descriptor', () => {
+    expect(() => defineDomain({ name: 'ok', version: 2, compatibleVersions: [1.5], tables: {} }))
+      .toThrow(/compatibleVersions/)
+    expect(() => defineDomain({ name: 'ok', version: 2, compatibleVersions: [2], tables: {} }))
+      .toThrow(/below version/)
+    expect(() => defineDomain({ name: 'ok', version: 2, compatibleVersions: [-1], tables: {} }))
+      .toThrow(/compatibleVersions/)
+    expect(descriptorOf(defineDomain({ name: 'ok', version: 2, compatibleVersions: [0, 1], tables: {} })))
+      .toMatchObject({ compatibleVersions: [0, 1] })
+    // An undeclared set is absent from the descriptor.
+    expect(descriptorOf(spec)).not.toHaveProperty('compatibleVersions')
+  })
+
+  it('rejects an unknown invalidRecords policy', () => {
+    expect(() => defineDomain({
+      name: 'ok', version: 1, invalidRecords: 'zap' as 'backup-and-skip', tables: {},
+    })).toThrow(/invalidRecords/)
+  })
+
   it('rejects an invalid layout and projects the declared one onto the descriptor', () => {
     // A spec built from config can carry any value; the union type is
     // compile-time only, so the runtime boundary check must reject it.
@@ -139,6 +158,28 @@ describe('DomainFacility.open', () => {
     })
   })
 
+  it('keeps the rejecting default under backup-and-skip when the backend cannot move documents', async () => {
+    // The memory backend has no backupRecord, so the declared policy cannot
+    // apply and the open falls back to failing loud.
+    const salvageSpec = defineDomain({
+      name: 'salvage',
+      version: 1,
+      invalidRecords: 'backup-and-skip',
+      tables: { items: domainTable<string, Item>(itemSchema) },
+    })
+    const pool = new MemoryMediaPool()
+    {
+      const { facility } = await harness({ pool })
+      await (await facility.open(salvageSpec)).table('items').put('bad', { label: 'x', count: 2 })
+    }
+    pool.media.get('salvage')!.tables.get('items')!.set('bad', { label: 'x', count: 'NaN' })
+    const { facility } = await harness({ pool })
+    await expect(facility.open(salvageSpec)).rejects.toMatchObject({
+      code: 'invalid-record',
+      detail: { table: 'items', key: 'bad' },
+    })
+  })
+
   it('rejects a stored global that fails its schema with the global marker', async () => {
     const pool = new MemoryMediaPool()
     pool.versions.set('demo', 1)
diff --git a/packages/storage/storage-json/src/format.ts b/packages/storage/storage-json/src/format.ts
index 045485eaba..22cbe8f703 100644
--- a/packages/storage/storage-json/src/format.ts
+++ b/packages/storage/storage-json/src/format.ts
@@ -100,16 +100,18 @@ export function serializeRecord(version: number, value: unknown): string {
 
 /**
  * Parse one per-record document, validating its version stamp. A document
- * that is malformed or stamped with a different version is FOREIGN and reads
- * as absent — the per-record contract: one bad or stale record file must not
- * brick the whole unit, and a version bump discards stale records instead of
- * migrating them (the whole-unit format rejects instead, because there is
- * exactly one document).
+ * that is malformed or stamped with an unaccepted version is FOREIGN and
+ * reads as absent — the per-record contract: one bad or stale record file
+ * must not brick the whole unit, and a version bump discards stale records
+ * instead of migrating them (the whole-unit format rejects instead, because
+ * there is exactly one document).
  * @param text - Raw per-record document content.
- * @param version - Expected unit version; a mismatch discards the document.
+ * @param versions - Accepted unit versions (the current one plus the
+ * descriptor's compatibleVersions); any other stamp discards the
+ * document.
  * @returns the record value, or `undefined` for a foreign document.
  */
-export function parseRecord(text: string, version: number): unknown {
+export function parseRecord(text: string, versions: readonly number[]): unknown {
   let document: unknown
   try {
     document = JSON.parse(text)
@@ -118,6 +120,6 @@ export function parseRecord(text: string, version: number): unknown {
   }
   if (typeof document !== 'object' || document === null) return undefined
   const { version: stamped, record } = document as Record<string, unknown>
-  if (stamped !== version) return undefined
+  if (typeof stamped !== 'number' || !versions.includes(stamped)) return undefined
   return record
 }
diff --git a/packages/storage/storage-json/src/per-record-unit.ts b/packages/storage/storage-json/src/per-record-unit.ts
index b75f9f9452..346c7a0d6a 100644
--- a/packages/storage/storage-json/src/per-record-unit.ts
+++ b/packages/storage/storage-json/src/per-record-unit.ts
@@ -10,20 +10,23 @@
  * memory unchanged.
  *
  * Per-record contract: a record document that is malformed or stamped with a
- * different version reads as an absent record — one bad or stale file never
- * bricks the whole unit, and a version bump discards stale records instead
- * of migrating them. Record keys become path segments, so they must be
- * path-safe (`[a-zA-Z0-9_-]+`); an unsafe key rejects at write.
+ * version outside the accepted set (the descriptor's current version plus
+ * its `compatibleVersions`) reads as an absent record — one bad or stale
+ * file never bricks the whole unit, and a version bump discards stale
+ * records instead of migrating them. Record keys become path segments, so
+ * they must be path-safe (`[a-zA-Z0-9_-]+`); an unsafe key rejects at write.
  *
  * 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
+ * per-record documents, provided its stored unit version is in the accepted
+ * set — a legacy file stamped with any other version is left alone and reads
+ * as the empty unit. 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
  */
 
-import { mkdir, readFile, readdir, rm } from 'node:fs/promises'
+import { mkdir, readFile, readdir, rename, rm } from 'node:fs/promises'
 import { dirname, join } from 'node:path'
 import type { Dirent } from 'node:fs'
 import { StorageError } from '@deepseek-ai/dsh-storage'
@@ -64,6 +67,7 @@ export async function openPerRecordUnit(
  * @returns the authoritative state reconstructed from the tree.
  */
 async function loadPerRecordState(descriptor: KvUnitDescriptor, dir: string): Promise<UnitState> {
+  const versions = acceptedStamps(descriptor)
   const state: UnitState = {
     version: descriptor.version,
     global: null,
@@ -83,11 +87,11 @@ async function loadPerRecordState(descriptor: KvUnitDescriptor, dir: string): Pr
       if (entry.isDirectory()) {
         const records = state.tables.get(entry.name)
         if (records !== undefined) {
-          return loadTableRecords(records, descriptor.version, join(dir, entry.name))
+          return loadTableRecords(records, versions, join(dir, entry.name))
         }
       }
       if (entry.name === 'global.json' && descriptor.hasGlobal) {
-        const global = await readRecord(join(dir, entry.name), descriptor.version)
+        const global = await readRecord(join(dir, entry.name), versions)
         if (global !== undefined) state.global = global
         return true
       }
@@ -97,12 +101,21 @@ async function loadPerRecordState(descriptor: KvUnitDescriptor, dir: string): Pr
   return state
 }
 
+/** The version stamps this unit reads as its own: current plus declared compatible versions. */
+function acceptedStamps(descriptor: KvUnitDescriptor): readonly number[] {
+  return [descriptor.version, ...descriptor.compatibleVersions ?? []]
+}
+
 /**
  * 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.
+ * or non-unit legacy file is left alone, and so is one whose stored unit
+ * version is outside the accepted set — migrating records the owner never
+ * vouched for would stamp them with the current version and turn a
+ * discardable stale cache into schema failures at the domain layer. 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 empty tree state; bootstrapped records are added.
@@ -116,16 +129,18 @@ async function bootstrapLegacyUnit(descriptor: KvUnitDescriptor, dir: string, st
     if ((error as NodeJS.ErrnoException).code !== 'ENOENT') throw error
     return
   }
-  // The legacy document is runtime data: only `unit.name` and the tables map
-  // shape are checked here — the record values are migrated as-is and the
-  // domain layer's schemas judge them.
-  let document: { unit?: { name?: unknown }; tables?: unknown }
+  // The legacy document is runtime data: only `unit.name`, `unit.version`,
+  // and the tables map shape are checked here — the record values are
+  // migrated as-is and the domain layer's schemas judge them.
+  let document: { unit?: { name?: unknown; version?: unknown }; tables?: unknown }
   try {
-    document = JSON.parse(text) as { unit?: { name?: unknown }; tables?: unknown }
+    document = JSON.parse(text) as { unit?: { name?: unknown; version?: unknown }; tables?: unknown }
   } catch {
     return // Malformed legacy file: not ours to interpret or delete.
   }
   if (document.unit?.name !== descriptor.name) return
+  const stamped = document.unit.version
+  if (typeof stamped !== 'number' || !acceptedStamps(descriptor).includes(stamped)) return
   const tables = document.tables
   if (typeof tables !== 'object' || tables === null) return
   const recordsByTable = tables as Record<string, Record<string, unknown>>
@@ -146,14 +161,14 @@ async function bootstrapLegacyUnit(descriptor: KvUnitDescriptor, dir: string, st
  * @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> {
+async function loadTableRecords(records: Map<string, unknown>, versions: readonly 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)
     if (!SAFE_KEY_RE.test(key)) return
-    const record = await readRecord(join(dir, file.name), version)
+    const record = await readRecord(join(dir, file.name), versions)
     if (record !== undefined) return [key, record] as const
   }))
   for (const record of loaded) {
@@ -163,9 +178,9 @@ async function loadTableRecords(records: Map<string, unknown>, version: number,
 }
 
 /** Read one record document; a foreign (unreadable or stale) one reads as absent. */
-async function readRecord(path: string, version: number): Promise<unknown> {
+async function readRecord(path: string, versions: readonly number[]): Promise<unknown> {
   try {
-    return parseRecord(await readFile(path, 'utf8'), version)
+    return parseRecord(await readFile(path, 'utf8'), versions)
   } catch {
     return undefined
   }
@@ -213,6 +228,22 @@ export class PerRecordJsonUnit implements KvUnit {
     await this.tracked(rm(join(this.tableDir(table), `${key}.json`), { force: true }))
   }
 
+  /**
+   * Move one record's document aside as `<key>.json.bak.<YYYYMMDDHHmm>`. The
+   * moved file no longer ends in `.json`, so every later read ignores it; the
+   * bytes stay on disk for inspection. A same-minute backup of the same
+   * key overwrites the previous backup (the newer bytes are the ones worth
+   * keeping).
+   */
+  async backupRecord(table: string, key: string): Promise<string> {
+    this.assertOpen()
+    assertSafeKey(this.descriptor.name, key)
+    const path = join(this.tableDir(table), `${key}.json`)
+    const moved = `${path}.bak.${backupStamp(new Date())}`
+    await this.tracked(rename(path, moved))
+    return moved
+  }
+
   /** Durably replace the global singleton. Only valid when declared. */
   async setGlobal(value: unknown): Promise<void> {
     this.assertOpen()
@@ -267,6 +298,12 @@ export class PerRecordJsonUnit implements KvUnit {
   }
 }
 
+/** Local-time `YYYYMMDDHHmm` suffix for backed-up documents. */
+function backupStamp(now: Date): string {
+  const pad = (value: number): string => String(value).padStart(2, '0')
+  return `${String(now.getFullYear())}${pad(now.getMonth() + 1)}${pad(now.getDate())}${pad(now.getHours())}${pad(now.getMinutes())}`
+}
+
 /** Reject a record key that would be unsafe as a path segment. */
 function assertSafeKey(unit: string, key: string): void {
   if (!SAFE_KEY_RE.test(key)) {
diff --git a/packages/storage/storage-json/tests/json-backend.spec.ts b/packages/storage/storage-json/tests/json-backend.spec.ts
index bafbf92ef4..57ad9d7633 100644
--- a/packages/storage/storage-json/tests/json-backend.spec.ts
+++ b/packages/storage/storage-json/tests/json-backend.spec.ts
@@ -329,10 +329,10 @@ describe('per-record layout', () => {
 
   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.
+    // A legacy single-layout file for the same unit, stamped with the current
+    // version; the extra table is not declared and must be skipped.
     const legacy = JSON.stringify({
-      unit: { name: 'recs', version: 3 },
+      unit: { name: 'recs', version: 2 },
       global: null,
       tables: { t: { old1: { v: 1 }, old2: { v: 2 } }, undeclared: { k: { v: 0 } } },
     })
@@ -347,6 +347,75 @@ describe('per-record layout', () => {
     await backend.close()
   })
 
+  it('bootstraps from a legacy file only when its stored version is accepted', async () => {
+    // Version 3 is neither current (2) nor declared compat: the legacy file
+    // is left alone and the unit reads empty — migrating unvouched records
+    // would stamp them current and surface as schema failures at the domain
+    // layer instead of a discardable stale cache.
+    const root = await freshRoot()
+    const legacy = JSON.stringify({
+      unit: { name: 'recs', version: 3 },
+      global: null,
+      tables: { t: { old: { v: 1 } } },
+    })
+    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: {} }, global: null })
+    await expect(readFile(join(root, 'recs.json'), 'utf8')).resolves.toBe(legacy)
+    await unit.close()
+    await backend.close()
+
+    // The same file bootstraps once version 3 is declared read-compatible…
+    const root2 = await freshRoot()
+    await writeFile(join(root2, 'recs.json'), legacy, 'utf8')
+    const backend2 = new JsonStorageBackend(root2)
+    const compat = { ...descriptor, version: 4, compatibleVersions: [3] }
+    const unit2 = await backend2.kv.open(compat)
+    expect(await unit2.loadAll()).toEqual({ tables: { t: { old: { v: 1 } } }, global: null })
+    // …and the migrated documents are stamped with the CURRENT version.
+    expect(JSON.parse(await readFile(join(root2, 'recs', 't', 'old.json'), 'utf8')))
+      .toEqual({ version: 4, record: { v: 1 } })
+    await unit2.close()
+    await backend2.close()
+  })
+
+  it('backupRecord moves the document aside; reads see it absent and a write recreates it', async () => {
+    const root = await freshRoot()
+    const backend = new JsonStorageBackend(root)
+    const unit = await backend.kv.open(descriptor)
+    await unit.putRecord('t', 'k', { v: 1 })
+    const moved = await unit.backupRecord!('t', 'k')
+    expect(moved).toMatch(/k\.json\.bak\.\d{12}$/)
+    expect(JSON.parse(await readFile(moved, 'utf8'))).toEqual({ version: 2, record: { v: 1 } })
+    await expect(readFile(recordPath(root, 'k'), 'utf8')).rejects.toMatchObject({ code: 'ENOENT' })
+    // The moved file no longer ends in .json, so it reads as absent…
+    expect(await unit.loadAll()).toEqual({ tables: { t: {} }, global: null })
+    // …and the key is free for a fresh write.
+    await unit.putRecord('t', 'k', { v: 2 })
+    expect(await unit.loadAll()).toEqual({ tables: { t: { k: { v: 2 } } }, global: null })
+    await expect(unit.backupRecord!('t', 'a/b')).rejects.toThrow(/not path-safe/)
+    await unit.close()
+    await expect(unit.backupRecord!('t', 'k')).rejects.toMatchObject({ code: 'closed' })
+    await backend.close()
+  })
+
+  it('reads per-record documents stamped with a declared compat version and stamps writes current', async () => {
+    const root = await freshRoot()
+    const backend = new JsonStorageBackend(root)
+    const compat = { ...descriptor, compatibleVersions: [1] }
+    await mkdir(join(root, 'recs', 't'), { recursive: true })
+    await writeFile(recordPath(root, 'oldrec'), JSON.stringify({ version: 1, record: { v: 'old' } }), 'utf8')
+    await writeFile(recordPath(root, 'ancient'), JSON.stringify({ version: 0, record: { v: 'no' } }), 'utf8')
+    const unit = await backend.kv.open(compat)
+    // Version 1 is declared compat and served; version 0 is not and discards.
+    expect(await unit.loadAll()).toEqual({ tables: { t: { oldrec: { v: 'old' } } }, global: null })
+    await unit.putRecord('t', 'oldrec', { v: 'new' })
+    expect(JSON.parse(await readFile(recordPath(root, 'oldrec'), 'utf8')))
+      .toEqual({ version: 2, record: { v: 'new' } })
+    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({
@@ -400,7 +469,8 @@ describe('per-record layout', () => {
     await backend4.close()
 
     const root5 = await freshRoot()
-    await writeFile(join(root5, 'recs.json'), JSON.stringify({ unit: { name: 'recs' }, tables: 'not an object' }), 'utf8')
+    // A current-version stamp so the shapeless `tables` is what stops the bootstrap.
+    await writeFile(join(root5, 'recs.json'), JSON.stringify({ unit: { name: 'recs', version: 2 }, tables: 'not an object' }), 'utf8')
     const backend5 = new JsonStorageBackend(root5)
     const unit5 = await backend5.kv.open(descriptor)
     expect(await unit5.loadAll()).toEqual({ tables: { t: {} }, global: null })
diff --git a/packages/storage/storage/src/backend.ts b/packages/storage/storage/src/backend.ts
index 9cba748655..4a8e0ac2d8 100644
--- a/packages/storage/storage/src/backend.ts
+++ b/packages/storage/storage/src/backend.ts
@@ -61,6 +61,16 @@ export interface KvUnitDescriptor {
    * foreign documents.
    */
   readonly layout?: 'single' | 'per-record'
+  /**
+   * Older unit versions whose stored records are also readable under the
+   * declaring owner's current record schemas (the owner vouches for that —
+   * typically by declaring the fields old records lack as optional). Reads of
+   * a `per-record` unit accept documents stamped with any listed version, and
+   * the legacy whole-unit bootstrap accepts a legacy file stamped with one;
+   * writes always stamp {@link version}. `single`-layout reads stay
+   * exact-version.
+   */
+  readonly compatibleVersions?: readonly number[]
 }
 
 /**
@@ -99,6 +109,19 @@ export interface KvUnit {
    */
   deleteRecord(table: string, key: string): Promise<void>
 
+  /**
+   * Move one record's stored document out of the unit's readable set,
+   * preserving its bytes for inspection instead of deleting them. Backends
+   * whose medium has no per-record document to move (the `single` layout, a
+   * row store) omit this member, and the caller falls back to its
+   * reject-loud path. Absent after the move: a later {@link loadAll} reads
+   * the key as missing and a later {@link putRecord} recreates it fresh.
+   * @param table - Declared table name.
+   * @param key - Record key.
+   * @returns the medium location the document was moved to (diagnostics).
+   */
+  backupRecord?(table: string, key: string): Promise<string>
+
   /**
    * Write the global singleton durably. Only valid when the descriptor
    * declared `hasGlobal`.