diff --git a/routes/calendar_routes.py b/routes/calendar_routes.py
index 6e0ee124c..b9c3b0a52 100644
--- a/routes/calendar_routes.py
+++ b/routes/calendar_routes.py
@@ -10,6 +10,7 @@ from typing import Optional, List
 from fastapi import APIRouter, HTTPException, Request, UploadFile, File
 from pydantic import BaseModel
 from sqlalchemy import or_, and_
+from sqlalchemy.exc import IntegrityError
 from dateutil.rrule import rrulestr
 
 from core.database import SessionLocal, CalendarCal, CalendarDeletedEvent, CalendarEvent
@@ -221,22 +222,125 @@ class EventUpdate(BaseModel):
 
 # ── Helpers ──
 
+_DEFAULT_CALENDAR_NAMESPACE = uuid.UUID("4840613a-9847-4a3b-bd75-19e6bc5fc3ce")
+
+
+def _default_calendar_id(owner: str, collision_index: int = 0) -> str:
+    """Return one stable primary-key candidate for an owner's lazy default.
+
+    Slot zero preserves the original owner-derived identifier.  Later slots
+    let a username be reused after its prior calendar was migrated to another
+    owner during a rename, without making concurrent first use choose random
+    and therefore divergent identifiers.
+    """
+    if collision_index == 0:
+        candidate_name = owner
+    else:
+        candidate_name = json.dumps(
+            [owner, collision_index],
+            ensure_ascii=False,
+            separators=(",", ":"),
+        )
+    return str(uuid.uuid5(_DEFAULT_CALENDAR_NAMESPACE, candidate_name))
+
+
+def _begin_sqlite_default_write(db) -> None:
+    """Serialize an absent-default check with other SQLite writers.
+
+    SQLite's default deferred transactions allow two workers to both read an
+    empty calendar set before either writes.  ``BEGIN IMMEDIATE`` acquires the
+    writer reservation before the second, authoritative lookup.  We issue it
+    only when the driver has not already opened a write transaction; a caller
+    with a pending write already owns the required reservation.
+    """
+    connection = db.connection()
+    dbapi_connection = connection.connection
+    driver_connection = getattr(
+        dbapi_connection,
+        "driver_connection",
+        dbapi_connection,
+    )
+    if not getattr(driver_connection, "in_transaction", False):
+        connection.exec_driver_sql("BEGIN IMMEDIATE")
+
+
 def _ensure_default_calendar(db, owner: str = None) -> CalendarCal:
-    """Create default calendar if none exist for this owner."""
+    """Return the owner's calendar, staging a default in the caller's transaction.
+
+    A stable owner-derived primary key makes concurrent first-use inserts
+    converge on one row on every SQL backend.  SQLite additionally serializes
+    the absent-row check because its deferred transactions otherwise permit
+    both workers to read the gap before either writes.  Other backends recover
+    a lost insert race inside a savepoint so the caller's event transaction
+    remains usable and atomic.
+    """
     owner = owner or FALLBACK_OWNER
     cal = db.query(CalendarCal).filter(CalendarCal.owner == owner).first()
-    if not cal:
+    if cal:
+        return cal
+
+    dialect = db.get_bind().dialect.name
+    if dialect == "sqlite":
+        _begin_sqlite_default_write(db)
+        # Another worker may have committed while BEGIN IMMEDIATE waited.
+        cal = db.query(CalendarCal).filter(CalendarCal.owner == owner).first()
+        if cal:
+            return cal
+
+    collision_index = 0
+    while True:
+        default_id = _default_calendar_id(owner, collision_index)
+
+        if dialect == "sqlite":
+            # BEGIN IMMEDIATE above makes this occupancy check authoritative:
+            # another SQLite writer cannot rename, delete, or claim this slot
+            # until the caller commits or rolls back.
+            occupant = db.query(CalendarCal).filter(
+                CalendarCal.id == default_id,
+            ).first()
+            if occupant is not None:
+                if occupant.owner == owner:
+                    return occupant
+                collision_index += 1
+                continue
+
         cal = CalendarCal(
-            id=str(uuid.uuid4()),
+            id=default_id,
             owner=owner,
             name="Personal",
             color="#5b8abf",
             source="local",
         )
-        db.add(cal)
-        db.commit()
-        db.refresh(cal)
-    return cal
+
+        if dialect == "sqlite":
+            db.add(cal)
+            db.flush()
+            return cal
+
+        try:
+            # A uniqueness failure rolls back only this savepoint, not an event
+            # or reminder already staged by the caller's outer transaction.
+            with db.begin_nested():
+                db.add(cal)
+                db.flush()
+            return cal
+        except IntegrityError:
+            # Use a locking/current read so repeatable-read backends can observe
+            # the row that won after our transaction's original empty snapshot.
+            occupant = db.query(CalendarCal).filter(
+                CalendarCal.id == default_id,
+            ).with_for_update().first()
+            if occupant is None:
+                # Do not misclassify an unrelated integrity failure as an ID
+                # collision and loop forever. A concurrently deleted winner is
+                # safe for the caller to retry as a fresh transaction.
+                raise
+            if occupant.owner == owner:
+                return occupant
+            # A renamed calendar owns this deterministic slot. Advance to the
+            # next stable slot; concurrent callers for this owner will still
+            # converge there.
+            collision_index += 1
 
 
 # Per-request user time context. chat_routes sets this from browser timezone
@@ -1015,6 +1119,9 @@ def setup_calendar_routes(upload_handler=None) -> APIRouter:
         db = SessionLocal()
         try:
             _ensure_default_calendar(db, owner)
+            # Listing calendars intentionally lazily creates a durable default.
+            # Other callers commit it with the event they are creating.
+            db.commit()
             cals = db.query(CalendarCal).filter(CalendarCal.owner == owner).all()
             return {"calendars": [
                 {"name": c.name, "href": c.id, "color": c.color, "source": c.source}
@@ -1023,6 +1130,7 @@ def setup_calendar_routes(upload_handler=None) -> APIRouter:
         except HTTPException:
             raise
         except Exception as e:
+            db.rollback()
             logger.error("Failed to list calendars: %s", e)
             raise HTTPException(500, "Failed to list calendars")
         finally:
diff --git a/src/tools/calendar.py b/src/tools/calendar.py
index e6572ba40..6dda5a0e3 100644
--- a/src/tools/calendar.py
+++ b/src/tools/calendar.py
@@ -196,6 +196,9 @@ async def do_manage_calendar(content: str, owner: Optional[str] = None) -> Dict:
     try:
         if action == "list_calendars":
             _ensure_default_calendar(db, owner)
+            # This read path intentionally persists the lazily-created default;
+            # event creation commits it in the event's transaction instead.
+            db.commit()
             cals = _calendar_query().all()
             result = [{"name": c.name, "href": c.id} for c in cals]
             if result:
diff --git a/tests/test_calendar_default_transaction.py b/tests/test_calendar_default_transaction.py
new file mode 100644
index 000000000..ffd981e51
--- /dev/null
+++ b/tests/test_calendar_default_transaction.py
@@ -0,0 +1,538 @@
+"""Default calendar creation belongs to the caller's transaction.
+
+Before this regression, ``_ensure_default_calendar`` committed independently.
+If event persistence then failed, the event rolled back but a new ``Personal``
+calendar remained (``calendar_count=1``, ``event_count=0``).
+"""
+
+import json
+import threading
+from contextlib import contextmanager
+from datetime import datetime, timedelta
+from types import SimpleNamespace
+
+import pytest
+from fastapi import HTTPException
+from sqlalchemy import create_engine, event
+from sqlalchemy.exc import IntegrityError
+from sqlalchemy.orm import Session, sessionmaker
+from sqlalchemy.pool import NullPool
+
+from tests.helpers.import_state import clear_fake_database_modules
+
+clear_fake_database_modules()
+
+import core.database as cdb  # noqa: E402
+import routes.calendar_routes as calendar_routes  # noqa: E402
+from core.database import CalendarCal, CalendarEvent  # noqa: E402
+from routes.calendar_routes import EventCreate  # noqa: E402
+from routes.calendar_routes import (  # noqa: E402
+    _default_calendar_id,
+    _ensure_default_calendar,
+)
+
+
+class _RejectEventCommit(Session):
+    """Reproduce an event commit failure after default-calendar creation."""
+
+    def commit(self):
+        if any(isinstance(row, CalendarEvent) for row in self.new):
+            raise RuntimeError("commit guard rejected event commit")
+        return super().commit()
+
+
+@pytest.fixture
+def session_factory(tmp_path, monkeypatch):
+    engine = create_engine(
+        f"sqlite:///{tmp_path / 'calendar.db'}",
+        connect_args={"check_same_thread": False},
+        poolclass=NullPool,
+    )
+    cdb.Base.metadata.create_all(engine)
+    factory = sessionmaker(
+        bind=engine,
+        autoflush=False,
+        autocommit=False,
+        class_=_RejectEventCommit,
+    )
+    monkeypatch.setattr(cdb, "SessionLocal", factory)
+    monkeypatch.setattr(calendar_routes, "SessionLocal", factory)
+    try:
+        yield factory
+    finally:
+        engine.dispose()
+
+
+def _request():
+    return SimpleNamespace(state=SimpleNamespace(current_user="alice"))
+
+
+def _endpoint(method, suffix):
+    router = calendar_routes.setup_calendar_routes()
+    for route in router.routes:
+        if route.path.endswith(suffix) and method in route.methods:
+            return route.endpoint
+    raise RuntimeError(f"{method} *{suffix} not found")
+
+
+def _counts(factory):
+    db = factory()
+    try:
+        return db.query(CalendarCal).count(), db.query(CalendarEvent).count()
+    finally:
+        db.close()
+
+
+async def test_route_event_failure_rolls_back_new_default_calendar(session_factory):
+    create_event = _endpoint("POST", "/events")
+
+    with pytest.raises(HTTPException) as caught:
+        await create_event(
+            _request(),
+            EventCreate(summary="Planning", dtstart="2126-07-20T09:00:00Z"),
+        )
+
+    assert caught.value.status_code == 500
+    assert _counts(session_factory) == (0, 0)
+
+
+async def test_route_event_validation_failure_rolls_back_new_default_calendar(
+    session_factory,
+):
+    create_event = _endpoint("POST", "/events")
+
+    with pytest.raises(HTTPException) as caught:
+        await create_event(
+            _request(),
+            EventCreate(summary="Planning", dtstart="not-a-datetime"),
+        )
+
+    assert caught.value.status_code == 500
+    assert _counts(session_factory) == (0, 0)
+
+
+async def test_tool_event_failure_rolls_back_new_default_calendar(session_factory):
+    from src.tools.calendar import do_manage_calendar
+
+    result = await do_manage_calendar(
+        json.dumps({
+            "action": "create_event",
+            "summary": "Planning",
+            "dtstart": "2126-07-20T09:00:00Z",
+        }),
+        owner="alice",
+    )
+
+    assert result["exit_code"] == 1
+    assert "commit guard rejected event commit" in result["error"]
+    assert _counts(session_factory) == (0, 0)
+
+
+async def test_tool_event_validation_failure_rolls_back_new_default_calendar(
+    session_factory,
+):
+    from src.tools.calendar import do_manage_calendar
+
+    result = await do_manage_calendar(
+        json.dumps({
+            "action": "create_event",
+            "summary": "Planning",
+            "dtstart": "not-a-datetime",
+        }),
+        owner="alice",
+    )
+
+    assert result["exit_code"] == 1
+    assert "Could not parse dtstart" in result["error"]
+    assert _counts(session_factory) == (0, 0)
+
+
+async def test_route_list_calendars_persists_lazy_default(session_factory):
+    list_calendars = _endpoint("GET", "/calendars")
+
+    result = await list_calendars(_request())
+
+    assert [calendar["name"] for calendar in result["calendars"]] == ["Personal"]
+    assert _counts(session_factory) == (1, 0)
+
+
+async def test_tool_list_calendars_persists_lazy_default(session_factory):
+    from src.tools.calendar import do_manage_calendar
+
+    result = await do_manage_calendar(
+        json.dumps({"action": "list_calendars"}),
+        owner="alice",
+    )
+
+    assert result["exit_code"] == 0
+    assert [calendar["name"] for calendar in result["calendars"]] == ["Personal"]
+    assert _counts(session_factory) == (1, 0)
+
+
+def test_repeated_rename_and_reuse_uses_stable_fallback_ids(tmp_path):
+    engine = create_engine(
+        f"sqlite:///{tmp_path / 'renamed-calendar.db'}",
+        connect_args={"check_same_thread": False},
+        poolclass=NullPool,
+    )
+    cdb.Base.metadata.create_all(engine)
+    factory = sessionmaker(bind=engine, autoflush=False, autocommit=False)
+    db = factory()
+    try:
+        first = _ensure_default_calendar(db, "alice")
+        assert first.id == _default_calendar_id("alice")
+        db.commit()
+
+        # The supported user-rename migration changes owner columns while
+        # deliberately preserving durable row identifiers.
+        first.owner = "bob"
+        db.commit()
+
+        second = _ensure_default_calendar(db, "alice")
+        assert second.id == _default_calendar_id("alice", 1)
+        db.commit()
+
+        # Repeating the same lifecycle must advance deterministically instead
+        # of failing or choosing a random identifier.
+        second.owner = "carol"
+        db.commit()
+
+        third = _ensure_default_calendar(db, "alice")
+        assert third.id == _default_calendar_id("alice", 2)
+        db.commit()
+
+        rows = db.query(CalendarCal).order_by(CalendarCal.owner).all()
+        assert [(row.owner, row.id) for row in rows] == [
+            ("alice", _default_calendar_id("alice", 2)),
+            ("bob", _default_calendar_id("alice")),
+            ("carol", _default_calendar_id("alice", 1)),
+        ]
+    finally:
+        db.close()
+        engine.dispose()
+
+
+def _assert_concurrent_first_use(tmp_path, occupied_owner=None):
+    engine = create_engine(
+        f"sqlite:///{tmp_path / 'concurrent-calendar.db'}",
+        connect_args={"check_same_thread": False, "timeout": 10},
+        poolclass=NullPool,
+    )
+    cdb.Base.metadata.create_all(engine)
+    factory = sessionmaker(bind=engine, autoflush=False, autocommit=False)
+    expected_collision_index = 0
+    if occupied_owner is not None:
+        seed = factory()
+        try:
+            seed.add(CalendarCal(
+                id=_default_calendar_id("alice"),
+                owner=occupied_owner,
+                name="Personal",
+                source="local",
+            ))
+            seed.commit()
+            expected_collision_index = 1
+        finally:
+            seed.close()
+    first_staged = threading.Event()
+    second_selected = threading.Event()
+    errors = []
+
+    @event.listens_for(engine, "after_cursor_execute")
+    def observe_second_gap(conn, cursor, statement, parameters, context, executemany):
+        if (
+            threading.current_thread().name == "calendar-worker-second"
+            and statement.lstrip().upper().startswith("SELECT")
+            and "FROM calendars" in statement
+        ):
+            second_selected.set()
+
+    def create_default(worker, hold=False):
+        db = factory()
+        try:
+            if not hold:
+                assert first_staged.wait(5)
+            cal = _ensure_default_calendar(db, "alice")
+            start = datetime(2126, 7, 20, 9 if hold else 10)
+            db.add(CalendarEvent(
+                uid=worker,
+                calendar_id=cal.id,
+                summary=f"Event {worker}",
+                dtstart=start,
+                dtend=start + timedelta(hours=1),
+            ))
+            if hold:
+                first_staged.set()
+                # The second session has observed the uncommitted gap before
+                # this transaction releases its writer reservation.
+                assert second_selected.wait(5)
+            db.commit()
+            assert cal.id == _default_calendar_id("alice", expected_collision_index)
+        except BaseException as exc:  # pragma: no cover - asserted below
+            errors.append((worker, exc))
+            db.rollback()
+        finally:
+            db.close()
+
+    first = threading.Thread(
+        target=create_default,
+        args=("first", True),
+        name="calendar-worker-first",
+    )
+    second = threading.Thread(
+        target=create_default,
+        args=("second",),
+        name="calendar-worker-second",
+    )
+    first.start()
+    second.start()
+    first.join(10)
+    second.join(10)
+
+    try:
+        assert not first.is_alive() and not second.is_alive()
+        assert errors == []
+        db = factory()
+        try:
+            rows = db.query(CalendarCal).filter(CalendarCal.owner == "alice").all()
+            assert [(row.id, row.name) for row in rows] == [
+                (_default_calendar_id("alice", expected_collision_index), "Personal")
+            ]
+            assert db.query(CalendarEvent).count() == 2
+            if occupied_owner is not None:
+                occupied = db.query(CalendarCal).filter(
+                    CalendarCal.id == _default_calendar_id("alice"),
+                ).one()
+                assert occupied.owner == occupied_owner
+        finally:
+            db.close()
+    finally:
+        engine.dispose()
+
+
+def test_concurrent_first_use_creates_one_sqlite_default(tmp_path):
+    _assert_concurrent_first_use(tmp_path)
+
+
+def test_concurrent_first_use_after_rename_creates_one_fallback_default(tmp_path):
+    _assert_concurrent_first_use(tmp_path, occupied_owner="bob")
+
+
+def test_sqlite_default_stays_in_callers_transaction(session_factory):
+    db = session_factory()
+    try:
+        cal = _ensure_default_calendar(db, "rollback-owner")
+        assert cal.id == _default_calendar_id("rollback-owner")
+        db.rollback()
+    finally:
+        db.close()
+
+    verify = session_factory()
+    try:
+        assert (
+            verify.query(CalendarCal)
+            .filter(CalendarCal.owner == "rollback-owner")
+            .count()
+            == 0
+        )
+    finally:
+        verify.close()
+
+
+def test_sqlite_fallback_default_stays_in_callers_transaction(session_factory):
+    seed = session_factory()
+    try:
+        seed.add(CalendarCal(
+            id=_default_calendar_id("alice"),
+            owner="bob",
+            name="Personal",
+            source="local",
+        ))
+        seed.commit()
+    finally:
+        seed.close()
+
+    db = session_factory()
+    try:
+        cal = _ensure_default_calendar(db, "alice")
+        assert cal.id == _default_calendar_id("alice", 1)
+        db.rollback()
+    finally:
+        db.close()
+
+    verify = session_factory()
+    try:
+        assert verify.query(CalendarCal).filter(CalendarCal.owner == "alice").count() == 0
+        assert verify.query(CalendarCal).filter(CalendarCal.owner == "bob").count() == 1
+    finally:
+        verify.close()
+
+
+class _FakeDialect:
+    name = "postgresql"
+
+
+class _FakeBind:
+    dialect = _FakeDialect()
+
+
+class _FakeQuery:
+    def __init__(self, session):
+        self.session = session
+
+    def filter(self, *conditions):
+        return self
+
+    def with_for_update(self):
+        self.session.locking_read = True
+        return self
+
+    def first(self):
+        self.session.query_count += 1
+        if self.session.query_count == 1:
+            return None
+        return self.session.winner
+
+
+class _GenericRaceSession:
+    """Minimal non-SQLite session that loses the deterministic-ID race."""
+
+    def __init__(self):
+        self.query_count = 0
+        self.nested_entries = 0
+        self.locking_read = False
+        self.candidate = None
+        self.winner = CalendarCal(
+            id=_default_calendar_id("alice"),
+            owner="alice",
+            name="Personal",
+            source="local",
+        )
+
+    def get_bind(self):
+        return _FakeBind()
+
+    def query(self, model):
+        assert model is CalendarCal
+        return _FakeQuery(self)
+
+    @contextmanager
+    def begin_nested(self):
+        self.nested_entries += 1
+        yield
+
+    def add(self, row):
+        self.candidate = row
+
+    def flush(self):
+        raise IntegrityError("insert", {}, RuntimeError("duplicate primary key"))
+
+
+def test_generic_backend_lost_race_recovers_inside_savepoint():
+    db = _GenericRaceSession()
+
+    winner = _ensure_default_calendar(db, "alice")
+
+    assert winner is db.winner
+    assert db.nested_entries == 1
+    assert db.locking_read is True
+    assert db.candidate.id == db.winner.id
+
+
+def test_generic_backend_unattributed_integrity_error_is_not_retried():
+    db = _GenericRaceSession()
+    db.winner = None
+
+    with pytest.raises(IntegrityError):
+        _ensure_default_calendar(db, "alice")
+
+    assert db.nested_entries == 1
+
+
+class _GenericRenamedSlotSession(_GenericRaceSession):
+    """A different owner occupies slot zero; slot one remains available."""
+
+    def __init__(self):
+        super().__init__()
+        self.candidates = []
+        self.winner = CalendarCal(
+            id=_default_calendar_id("alice"),
+            owner="bob",
+            name="Personal",
+            source="local",
+        )
+
+    def add(self, row):
+        self.candidate = row
+        self.candidates.append(row)
+
+    def flush(self):
+        if len(self.candidates) == 1:
+            raise IntegrityError("insert", {}, RuntimeError("duplicate primary key"))
+
+
+def test_generic_backend_renamed_slot_advances_inside_savepoint():
+    db = _GenericRenamedSlotSession()
+
+    fallback = _ensure_default_calendar(db, "alice")
+
+    assert fallback is db.candidates[-1]
+    assert fallback.id == _default_calendar_id("alice", 1)
+    assert fallback.owner == "alice"
+    assert db.nested_entries == 2
+    assert db.locking_read is True
+    assert db.winner.owner == "bob"
+
+
+def test_generic_backend_fallback_keeps_outer_transaction_usable(tmp_path):
+    engine = create_engine(
+        f"sqlite:///{tmp_path / 'generic-savepoint-calendar.db'}",
+        poolclass=NullPool,
+    )
+    cdb.Base.metadata.create_all(engine)
+    # SQLite supplies a lightweight local SQL executor here; changing only the
+    # dispatch name exercises the real Session/savepoint branch used by
+    # PostgreSQL-style backends without pretending to validate their dialect.
+    engine.dialect.name = "postgresql"
+    factory = sessionmaker(bind=engine, autoflush=False, autocommit=False)
+
+    seed = factory()
+    try:
+        seed.add(CalendarCal(
+            id=_default_calendar_id("alice"),
+            owner="bob",
+            name="Personal",
+            source="local",
+        ))
+        seed.commit()
+    finally:
+        seed.close()
+
+    db = factory()
+    try:
+        cal = _ensure_default_calendar(db, "alice")
+        start = datetime(2126, 7, 20, 9)
+        db.add(CalendarEvent(
+            uid="after-fallback",
+            calendar_id=cal.id,
+            summary="Atomic",
+            dtstart=start,
+            dtend=start + timedelta(hours=1),
+        ))
+        db.commit()
+    finally:
+        db.close()
+
+    verify = factory()
+    try:
+        assert [
+            (row.owner, row.id)
+            for row in verify.query(CalendarCal).order_by(CalendarCal.owner).all()
+        ] == [
+            ("alice", _default_calendar_id("alice", 1)),
+            ("bob", _default_calendar_id("alice")),
+        ]
+        assert verify.query(CalendarEvent).count() == 1
+    finally:
+        verify.close()
+        engine.dispose()