diff --git a/backend/app/retrieval/routed_vectors.py b/backend/app/retrieval/routed_vectors.py
index 8ea10cd..ab959be 100644
--- a/backend/app/retrieval/routed_vectors.py
+++ b/backend/app/retrieval/routed_vectors.py
@@ -190,9 +190,49 @@ async def search_remote(query: str, *, top_k: int, accept_local=False, strict=Fa
     if batch is None:
         return None
 
+    if not await _prepare_for_search([batch], strict):
+        return None
     return await asyncio.to_thread(_search_space, batch, top_k, strict)
 
 
+async def _prepare_indexes(batches):
+    from app.services.coordination import vault_mutation_lock
+    def prepare(check_only=False):
+        conn = connect()
+        try:
+            if check_only:
+                return space_index.is_ready(conn, batches)
+            space_index.prepare(conn, batches)
+        finally:
+            conn.close()
+    if await asyncio.to_thread(prepare, True):
+        return
+    # Share the cooperative gate with saves: never block the event loop on a
+    # SQLite write lock while a migration owns it in another thread.
+    async with vault_mutation_lock():
+        work = asyncio.create_task(asyncio.to_thread(prepare))
+        cancelled = False
+        while not work.done():
+            try:
+                await asyncio.shield(work)
+            except asyncio.CancelledError:
+                cancelled = True
+        work.result()
+        if cancelled:
+            raise asyncio.CancelledError
+
+
+async def _prepare_for_search(batches, strict):
+    try:
+        await _prepare_indexes(batches)
+        return True
+    except Exception as exc:
+        record_embedding(fallback_reason='REMOTE_INDEX_UNAVAILABLE')
+        if strict:
+            raise ApiError(409, 'SEMANTIC_INDEX_UNAVAILABLE', '向量索引准备失败，请检查索引状态。') from exc
+        return False
+
+
 def _search_space(batch, top_k, strict):
     record_embedding(attempted_space={"model_id": batch.space_id, "dimensions": batch.dimensions})
     try:
@@ -209,7 +249,6 @@ def _search_space(batch, top_k, strict):
                     if strict:
                         raise ValueError("semantic index missing")
                     return None
-                _ensure_table(conn)
                 result = space_index.search(conn, batch, top_k)
                 record_embedding(source=batch.source, model_id=batch.space_id,
                                  dimensions=batch.dimensions, fallback_reason=None)
@@ -234,6 +273,8 @@ async def _search_partitioned(query: str, policies: set[bool], *, top_k: int, st
         if batch is None:
             return None
         batches[policy] = batch
+    if not await _prepare_for_search(list(batches.values()), strict):
+        return None
     return await asyncio.to_thread(_search_partitions, batches, policies, top_k, strict)
 
 
@@ -247,7 +288,6 @@ def _search_partitions(batches, policies, top_k, strict):
                 raise ValueError("embedding policies changed while querying")
             ranked = []
             for policy, batch in batches.items():
-                _ensure_table(conn)
                 ranked.append(space_index.search(conn, batch, top_k, policy))
             spaces = [{"source": b.source, "model_id": b.space_id, "dimensions": b.dimensions,
                        "local_only": policy} for policy, b in batches.items()]
diff --git a/backend/app/retrieval/space_index.py b/backend/app/retrieval/space_index.py
index ddf9e0f..1947c5b 100644
--- a/backend/app/retrieval/space_index.py
+++ b/backend/app/retrieval/space_index.py
@@ -1,12 +1,42 @@
 """Persistent vec0 indexes derived from durable routed vectors, one per space/dimension."""
 import hashlib
 import json
+import threading
 
 import sqlite_vec
 
 from app.retrieval.vectorstore import VectorHit
 
 
+_migration_lock = threading.Lock()
+
+
+def is_ready(conn, batches):
+    return all(conn.execute('SELECT 1 FROM sqlite_master WHERE name=?',
+               (table_name(batch.space_id, batch.dimensions),)).fetchone() for batch in batches)
+
+
+def prepare(conn, batches):
+    """Finish lazy writes before opening a search snapshot. Warm searches do not write."""
+    from app.retrieval.routed_vectors import _ensure_table
+    batches = list(batches)
+    if is_ready(conn, batches):
+        return
+    # Waiting holds no read transaction, so a concurrent migration can commit.
+    with _migration_lock:
+        if is_ready(conn, batches):
+            return
+        conn.execute('BEGIN IMMEDIATE')
+        try:
+            _ensure_table(conn)
+            for batch in batches:
+                ensure(conn, batch.space_id, batch.dimensions)
+            conn.execute('COMMIT')
+        except BaseException:
+            conn.execute('ROLLBACK')
+            raise
+
+
 def table_name(space, dimensions):
     return 'routed_vec_' + hashlib.sha256(json.dumps([space, dimensions]).encode()).hexdigest()
 
@@ -40,7 +70,7 @@ def upsert(conn, block_ids, batch):
 
 
 def search(conn, batch, top_k, policy=None):
-    table = ensure(conn, batch.space_id, batch.dimensions)
+    table = table_name(batch.space_id, batch.dimensions)
     # Coverage checks stay relational; no JSON decoding or Python dot products on the hot path.
     where = '' if policy is None else ' AND b.embedding_local_only=?'
     params = () if policy is None else (int(policy),)
diff --git a/backend/app/services/coordination.py b/backend/app/services/coordination.py
index b6dd0e2..6452249 100644
--- a/backend/app/services/coordination.py
+++ b/backend/app/services/coordination.py
@@ -1,7 +1,14 @@
 import asyncio
 from functools import wraps
+from weakref import WeakKeyDictionary
 
-_vault_mutation_lock = asyncio.Lock()
+_vault_locks = WeakKeyDictionary()
+
+
+def vault_mutation_lock():
+    # Service/test lifecycle restarts must not reuse a lock bound to a closed loop.
+    loop = asyncio.get_running_loop()
+    return _vault_locks.setdefault(loop, asyncio.Lock())
 
 
 def serialized_vault_mutation(operation):
@@ -9,7 +16,7 @@ def serialized_vault_mutation(operation):
 
     @wraps(operation)
     async def wrapped(*args, **kwargs):
-        async with _vault_mutation_lock:
+        async with vault_mutation_lock():
             return await operation(*args, **kwargs)
 
     return wrapped
diff --git a/backend/app/services/index_service.py b/backend/app/services/index_service.py
index 2534b3d..db80a59 100644
--- a/backend/app/services/index_service.py
+++ b/backend/app/services/index_service.py
@@ -17,7 +17,7 @@ from app.errors import ApiError
 from app.knowledge.parser import parse_note
 from app.services.note_service import index_note, prepare_note_index
 from app.database.db import connect, transaction
-from app.services.coordination import _vault_mutation_lock
+from app.services.coordination import vault_mutation_lock
 from app.retrieval.vectorstore import SqliteVecStore
 from app.local_models.runtime import LocalEmbedding
 from app.services import note_service
@@ -116,7 +116,7 @@ async def rebuild(request: IndexRebuildRequest) -> IndexJob:
             prepared_notes.append((parsed, prepared))
         # All network/model awaits precede the transaction. The concrete SQLite
         # methods below complete synchronously despite their async interfaces.
-        async with _vault_mutation_lock:
+        async with vault_mutation_lock():
             if _scan_vault() != docs or saved_records != {key: repository.get_note_record(key) for key in _pending_notes()}:
                 raise ApiError(409, "INDEX_SNAPSHOT_CHANGED", "笔记在计算期间发生变化，稍后重新计算。")
             conn = connect()
@@ -263,7 +263,7 @@ async def _refresh_saved_note(note_id: str) -> None:
         prepared = await prepare_note_index(parsed, strict=True)
         if isinstance(note_service.embedding, LocalEmbedding) and parsed.blocks and prepared[1] is None:
             raise ApiError(503, "EMBEDDING_UNAVAILABLE", "笔记已保存，后台向量计算未完成。")
-        async with _vault_mutation_lock:
+        async with vault_mutation_lock():
             current = repository.get_note_record(note_id)
             if current != record or note_service._read_markdown(record.file_path) != markdown:
                 # Another save or rename won the race; leave the durable queue entry intact.
diff --git a/backend/tests/test_routed_retrieval.py b/backend/tests/test_routed_retrieval.py
index 282b8ca..d647100 100644
--- a/backend/tests/test_routed_retrieval.py
+++ b/backend/tests/test_routed_retrieval.py
@@ -112,6 +112,115 @@ def test_legacy_vectors_migrate_without_document_embedding(runtime):
     asyncio.run(scenario())
 
 
+@pytest.mark.parametrize('partitioned', [False, True])
+def test_concurrent_first_search_serializes_migration_and_warm_search_is_read_only(runtime, monkeypatch, partitioned):
+    import threading
+    from app.retrieval import space_index
+    async def scenario():
+        await seed()
+        table = space_index.table_name('space-a', 3)
+        conn = connect()
+        try:
+            with transaction(conn):
+                conn.execute(f'DROP TRIGGER {table}_delete')
+                conn.execute(f'DROP TRIGGER {table}_update')
+                conn.execute(f'DROP TABLE {table}')
+        finally:
+            conn.close()
+        entered, release, second = threading.Event(), threading.Event(), threading.Event()
+        original_ensure, original_prepare = space_index.ensure, space_index.prepare
+        calls = []
+        def ensure(*args):
+            calls.append(1)
+            entered.set()
+            assert release.wait(5)
+            return original_ensure(*args)
+        def prepare(*args):
+            if entered.is_set():
+                second.set()
+            return original_prepare(*args)
+        monkeypatch.setattr(space_index, 'ensure', ensure)
+        monkeypatch.setattr(space_index, 'prepare', prepare)
+        batch = routed_vectors.RemoteEmbeddings('space-a', 3, [[1., 0., 0.]])
+        async def search():
+            if entered.is_set():
+                second.set()
+            await routed_vectors._prepare_indexes([batch])
+            if partitioned:
+                return await asyncio.to_thread(routed_vectors._search_partitions, {False: batch}, {False}, 2, True)
+            return await asyncio.to_thread(routed_vectors._search_space, batch, 2, True)
+        tasks = []
+        try:
+            tasks.append(asyncio.create_task(search()))
+            assert await asyncio.to_thread(entered.wait, 5)
+            tasks.append(asyncio.create_task(search()))
+            assert await asyncio.to_thread(second.wait, 5)
+            release.set()
+            first, other = await asyncio.gather(*tasks)
+            assert first == other and len(first) == 2
+            assert len(calls) == 1
+            # Prepared indexes are reusable even with SQLite query_only enforced.
+            original_connect = routed_vectors.connect
+            def read_only():
+                connection = original_connect()
+                connection.execute('PRAGMA query_only=ON')
+                return connection
+            monkeypatch.setattr(routed_vectors, 'connect', read_only)
+            assert await search() == first
+        finally:
+            release.set()
+            await asyncio.gather(*tasks, return_exceptions=True)
+    asyncio.run(scenario())
+
+
+@pytest.mark.parametrize('cancel_search', [False, True])
+def test_save_waits_for_migration_even_when_search_is_cancelled(runtime, monkeypatch, cancel_search):
+    import threading
+    from app.retrieval import space_index
+    async def scenario():
+        apple, _ = await seed()
+        table = space_index.table_name('space-a', 3)
+        conn = connect()
+        try:
+            with transaction(conn):
+                conn.execute(f'DROP TRIGGER {table}_delete')
+                conn.execute(f'DROP TRIGGER {table}_update')
+                conn.execute(f'DROP TABLE {table}')
+        finally:
+            conn.close()
+        entered, release = threading.Event(), threading.Event()
+        original = space_index.ensure
+        def slow(*args):
+            entered.set()
+            assert release.wait(5)
+            return original(*args)
+        monkeypatch.setattr(space_index, 'ensure', slow)
+        # Keep the subsequent vector job queued; test saving and its durable marker.
+        monkeypatch.setattr(index_service, 'schedule_workspace_rebuild', lambda: None)
+        query = asyncio.create_task(routed_vectors.search_remote('apple orchard', top_k=2, strict=True))
+        save = None
+        try:
+            assert await asyncio.to_thread(entered.wait, 5)
+            if cancel_search:
+                query.cancel()
+            save = asyncio.create_task(note_service.update_note(apple.note_id, markdown='Saved during migration', defer_vectors=True))
+            await asyncio.sleep(0.02)
+            assert not save.done()
+            release.set()
+            saved = await asyncio.wait_for(save, 5)
+            assert saved.markdown == 'Saved during migration'
+            assert (await note_service.get_note(apple.note_id)).markdown == saved.markdown
+            assert repository.get_index_meta()[f'note_vectors_pending:{apple.note_id}'] == '1'
+            # Query may observe the saved revision's pending index, but saving must succeed.
+            result = (await asyncio.gather(query, return_exceptions=True))[0]
+            if cancel_search:
+                assert isinstance(result, asyncio.CancelledError)
+        finally:
+            release.set()
+            await asyncio.gather(*([query, save] if save else [query]), return_exceptions=True)
+    asyncio.run(scenario())
+
+
 @pytest.mark.parametrize("outcome", ["api", "api_failure", "missing_space"])
 def test_benchmark_reports_actual_embedding_and_fallback(runtime, outcome):
     from app.benchmarks import service
diff --git a/docs/development/模型隔离向量索引与增量登记.md b/docs/development/模型隔离向量索引与增量登记.md
index aff56b4..b36339a 100644
--- a/docs/development/模型隔离向量索引与增量登记.md
+++ b/docs/development/模型隔离向量索引与增量登记.md
@@ -10,6 +10,10 @@
 
 数据库搜索及旧空间转换在线程中运行，不阻塞异步服务事件循环。首次转换仍会占用 SQLite 写事务；超大库应进一步评估迁移耗时。新增空间保留原空间索引，暂不自动清理历史空间。
 
+首次转换使用进程内迁移锁串行执行，在打开检索读快照之前以 `BEGIN IMMEDIATE` 获取写事务；等待迁移时不持有读事务，避免多个搜索从读锁升级写锁发生冲突。迁移完成后二次检查即可复用。普通检索和隐私分区检索共用该流程，已有索引的检索仅执行读取。
+
+首次转换同时通过 Vault 的异步写入锁与笔记保存、后台索引协调，避免保存事务读取旧 Block 后与迁移争抢写锁。等待是异步的；取消检索时，仍等待迁移线程结束后才释放锁。已就绪的空间直接走只读检查，不进入写入队列。写入锁按事件循环生命周期创建，避免重启后复用已关闭循环的锁。
+
 ## 外部新增文件
 
 文件树检测到新增 Markdown 后先登记元数据与全文索引，然后为每条新笔记持久化 `note_vectors_pending:<note_id>`，逐笔记在后台计算。不再因新增文件设置全库重建标记；已存在的全库待处理标记仍保留，以免跳过之前未完成的任务。
@@ -20,5 +24,7 @@
 
 - 隔离数据库运行后端全套测试：637 项通过；最终覆盖检查优化另运行检索及后台工作区回归，96 项通过。
 - 新增用例验证同模型不同维度并存、重新连接复用且查询不解码向量 JSON、旧表迁移只计算查询向量，以及外部新增文件不触发全量重建。
+- 并发迁移回归用同步事件暂停第一个请求的转换，同时发起第二个请求；验证普通/隐私分区路径均只迁移一次、两个请求结果一致，并开启 SQLite `query_only` 验证后续检索不会写入。相关检索与后台工作区测试 91 项通过。
+- 并发保存回归暂停迁移后发起实际 `update_note(..., defer_vectors=True)`，验证保存等待、事件循环仍可运行、放行迁移后正文成功落盘并保留新内容的向量待处理标记；另覆盖搜索被取消时不得提前放行保存。
 - `backend/scripts/vector-index-benchmark.py` 使用临时数据库、固定随机种子，比较 4000 条 384 维向量的 top-20，旧新路径结果顺序一致。5 次采样中位数：Python 扫描约 1289.64 ms，sqlite-vec 约 19.67 ms；首次转换约 1329.51 ms。该结果只测向量检索，不包含查询 Embedding、重排与 HTTP 耗时，不代表端到端加速比例。
 - 原始结果：[2026-09-06-sqlite-vec-search.json](performance/2026-09-06-sqlite-vec-search.json)。基准可用后端虚拟环境 Python 直接执行上述脚本，不读写用户 Vault。