diff --git a/src/basic_memory/db.py b/src/basic_memory/db.py
index d9f56a05..81500511 100644
--- a/src/basic_memory/db.py
+++ b/src/basic_memory/db.py
@@ -19,7 +19,7 @@ from sqlalchemy.ext.asyncio import (
     AsyncEngine,
     async_scoped_session,
 )
-from sqlalchemy.pool import NullPool
+from sqlalchemy.pool import AsyncAdaptedQueuePool, NullPool
 
 from basic_memory.repository.postgres_search_repository import PostgresSearchRepository
 from basic_memory.repository.sqlite_search_repository import SQLiteSearchRepository
@@ -216,19 +216,31 @@ def _create_sqlite_engine(db_url: str, db_type: DatabaseType) -> AsyncEngine:
                 "isolation_level": None,  # Use autocommit mode
             }
         )
+
+    if db_type == DatabaseType.MEMORY:
+        # Trigger: an in-memory SQLite URL would default to StaticPool, which hands the
+        # same DBAPI connection to every concurrently checked-out session.
+        # Why: concurrent asyncio tasks then share one transaction scope — a rollback
+        # issued by one session (scoped_session exception handling or the pool's
+        # reset-on-return) silently destroys another session's uncommitted writes (#940).
+        # Outcome: a single-connection blocking queue pool keeps the in-memory database
+        # alive for the engine's lifetime while serializing sessions at transaction
+        # granularity, restoring the isolation the repositories assume.
+        engine = create_async_engine(
+            db_url,
+            connect_args=connect_args,
+            poolclass=AsyncAdaptedQueuePool,
+            pool_size=1,
+            max_overflow=0,
+        )
+    elif os.name == "nt":
         # Use NullPool for Windows filesystem databases to avoid connection pooling issues
-        # Important: Do NOT use NullPool for in-memory databases as it will destroy the database
-        # between connections
-        if db_type == DatabaseType.FILESYSTEM:
-            engine = create_async_engine(
-                db_url,
-                connect_args=connect_args,
-                poolclass=NullPool,  # Disable connection pooling on Windows
-                echo=False,
-            )
-        else:
-            # In-memory databases need connection pooling to maintain state
-            engine = create_async_engine(db_url, connect_args=connect_args)
+        engine = create_async_engine(
+            db_url,
+            connect_args=connect_args,
+            poolclass=NullPool,  # Disable connection pooling on Windows
+            echo=False,
+        )
     else:
         engine = create_async_engine(db_url, connect_args=connect_args)
 
diff --git a/tests/db/test_memory_db_session_isolation.py b/tests/db/test_memory_db_session_isolation.py
new file mode 100644
index 00000000..1e25b4d9
--- /dev/null
+++ b/tests/db/test_memory_db_session_isolation.py
@@ -0,0 +1,93 @@
+"""Regression test for issue #940: lost writes on the in-memory SQLite engine.
+
+The in-memory SQLite URL (``sqlite+aiosqlite://``) used to fall back to
+SQLAlchemy's StaticPool, which hands the same DBAPI connection to every
+concurrently checked-out session. Concurrent asyncio tasks then share one
+transaction scope: a rollback issued by one session — scoped_session's
+exception handling, or the pool's reset-on-return at checkin — silently rolls
+back another session's uncommitted writes. During sync this manifested as a
+freshly inserted relation row vanishing without any error, which is how
+``test_sync_entity_circular_relations`` failed on CI with
+``len(entity_b.outgoing_relations) == 0``.
+
+This test pins the isolation invariant directly: a session that rolls back in
+one task must never destroy an uncommitted write of a session in another task.
+"""
+
+import asyncio
+from pathlib import Path
+
+import pytest
+from sqlalchemy import text
+
+from basic_memory import db
+from basic_memory.models import Base
+
+
+class _SimulatedIndexingFailure(Exception):
+    """Stand-in for any per-file error that _run_bounded swallows during sync."""
+
+
+@pytest.mark.asyncio
+async def test_concurrent_session_rollback_does_not_destroy_uncommitted_writes():
+    """A rolled-back session in one task must not erase another task's writes."""
+    async with db.engine_session_factory(
+        db_path=Path("unused.db"), db_type=db.DatabaseType.MEMORY
+    ) as (engine, session_maker):
+        async with engine.begin() as conn:
+            await conn.run_sync(Base.metadata.create_all)
+
+        # Seed a project and entity so a relation row satisfies its FK constraints.
+        async with db.scoped_session(session_maker) as session:
+            await session.execute(
+                text(
+                    "INSERT INTO project (id, external_id, name, description, path, is_active,"
+                    " is_default, created_at, updated_at, permalink) "
+                    "VALUES (1, 'px', 'p', '', '/tmp', 1, 1, '2024-01-01', '2024-01-01', 'p')"
+                )
+            )
+            await session.execute(
+                text(
+                    "INSERT INTO entity (id, external_id, title, note_type, content_type,"
+                    " project_id, file_path, created_at, updated_at) "
+                    "VALUES (1, 'ex', 'E', 'note', 'text/markdown', 1, 'e.md',"
+                    " '2024-01-01', '2024-01-01')"
+                )
+            )
+
+        write_in_flight = asyncio.Event()
+
+        async def writer() -> None:
+            # Mirrors RelationRepository.add_all_ignore_duplicates: INSERT executed,
+            # commit only happens at scoped_session exit several awaits later.
+            async with db.scoped_session(session_maker) as session:
+                await session.execute(
+                    text(
+                        "INSERT INTO relation (project_id, from_id, to_id, to_name,"
+                        " relation_type) VALUES (1, 1, NULL, 'target', 'depends_on')"
+                    )
+                )
+                write_in_flight.set()
+                # Real DB roundtrips (not sleeps) keep this transaction open across
+                # await points, exactly like the multi-statement sessions in sync.
+                for _ in range(10):
+                    await session.execute(text("SELECT 1"))
+
+        async def failing_reader() -> None:
+            # Mirrors any per-file failure during batch indexing: scoped_session
+            # rolls back on the exception path while sibling tasks are mid-write.
+            await write_in_flight.wait()
+            with pytest.raises(_SimulatedIndexingFailure):
+                async with db.scoped_session(session_maker) as session:
+                    await session.execute(text("SELECT 1"))
+                    raise _SimulatedIndexingFailure()
+
+        await asyncio.gather(writer(), failing_reader())
+
+        async with db.scoped_session(session_maker) as session:
+            count = (await session.execute(text("SELECT count(*) FROM relation"))).scalar()
+
+        assert count == 1, (
+            "writer's committed INSERT was rolled back by a concurrent session — "
+            "the in-memory engine is sharing one transaction scope across sessions"
+        )
diff --git a/tests/repository/test_entity_repository.py b/tests/repository/test_entity_repository.py
index 194722af..8464b42c 100644
--- a/tests/repository/test_entity_repository.py
+++ b/tests/repository/test_entity_repository.py
@@ -291,9 +291,11 @@ async def test_delete_entity_with_relations(entity_repository: EntityRepository,
         remaining_relations = result.scalars().all()
         assert len(remaining_relations) == 0
 
-        # Verify target entity still exists
-        target_exists = await entity_repository.find_by_id(target.id)
-        assert target_exists is not None
+    # Verify target entity still exists. Runs outside the session block above:
+    # find_by_id opens its own scoped session, and the serialized in-memory pool
+    # (one connection, see #940) deadlocks on nested session checkouts.
+    target_exists = await entity_repository.find_by_id(target.id)
+    assert target_exists is not None
 
 
 @pytest.mark.asyncio