fix: _is_plain_int on every weekday_set path (from_dict + post-set); shared scan function probes the real guard

- direct list construction validates entries before set() coercion
  (bool collapse and unhashable items both raise RuleValidationError)
- unknown-key error sorts by repr (mixed-type keys never leak TypeError)
- string rule_type accepted ONLY as exact model names: storage aliases
  ('monthly'/'custom'), whitespace padding, and case variants rejected;
  the DAO owns storage-string mapping
- purity guard checks ImportFrom alias names so 'from . import db' is
  caught, not just node.module
- _is_plain_int requires type(value) is int: unhashable int subclasses
  cannot construct

custom_components/nestquest/recurrence.py

@@ -0,0 +1,389 @@
+"""The ScheduleRule model: rule types, validation, and serialization.
+
+A :class:`ScheduleRule` describes WHEN a recurring chore fires — a pure
+frozen dataclass with no Home Assistant and no database imports.  The
+evaluation engine (Feature 04) consumes it; the DAO layer (Feature 02)
+serializes its dict form into the ``schedule_rules`` table.
+
+Type model: one enum member per recurrence shape.  ``MONTHLY_DAY``
+("the 15th of every month") and ``MONTHLY_WEEKDAY`` ("the second
+Tuesday of every month") are distinct types rather than one monthly
+type with optional fields, so validation is total: a rule either
+constructs, or raises :class:`RuleValidationError` naming the offending
+field.  Every type carries ONLY the fields its shape needs; carrying
+any other shape's field is a construction error.
+
+Storage mapping (Feature 02, ``schedule_rules`` table): the enum's
+``storage_value`` maps model types onto the schema's CHECK values —
+MONTHLY_DAY and MONTHLY_WEEKDAY both store as ``'monthly'`` and are
+disambiguated by which fields are populated; CUSTOM_DAYS stores as
+``'custom'``.  ``weekday_set`` round-trips as the sorted CSV list the
+table stores (one element for MONTHLY_WEEKDAY, which names the
+position's weekday); ``nth_weekday_weekday`` is a MODEL-ONLY field
+carried through from_dict/to_dict for losslessness and folded into
+``weekday_set`` by the DAO mapping.  ``to_dict``/``from_dict`` are the
+model-side lossless pair; the DB mapping lives in the DAO layer and is
+exercised there.
+
+Validation policy (fail fast at construction — the schema's CHECK
+constraints are a second line of defense, but invalid rules must never
+reach storage):
+
+- ``interval`` >= 1 for every rule.
+- ``weekday_set`` is required for WEEKLY and CUSTOM_DAYS, forbidden
+  elsewhere; entries are 0 (Monday) .. 6 (Sunday), no booleans.
+- ``day_of_month`` 1..31 is required for MONTHLY_DAY.
+- ``nth_weekday`` 1..5 or -1 (0 is meaningless as a position and is
+  rejected) with a weekday 0..6 for MONTHLY_WEEKDAY.
+- ``month`` 1..12 is required for YEARLY.
+- ``start_date`` is a strict ISO date; ``end_date`` is optional and,
+  when present, on or after ``start_date``.
+
+Month-end policy (documented for Feature 04): a MONTHLY_DAY rule for
+the 31st does NOT fire in a 30-day month, and a YEARLY rule for
+February 29th does NOT fire in a non-leap year — the engine skips
+instead of clamping to month end, because clamping silently moves
+"the 31st" to "the 30th" and double-fires tasks.
+"""
+from __future__ import annotations
+
+import datetime
+import enum
+from dataclasses import dataclass, field
+
+#: Date-shape policy: strict YYYY-MM-DD, matching the DAO layers.
+_DATE_FORMAT = "%Y-%m-%d"
+
+#: Weekday-name policy: 0 = Monday .. 6 = Sunday (ISO order).
+
+
+class RuleType(enum.Enum):
+    """The six recurrence shapes the engine supports.
+
+    ``storage_value`` is the string written to the ``schedule_rules``
+    table's ``rule_type`` column (its CHECK whitelist).  MONTHLY_DAY
+    and MONTHLY_WEEKDAY are distinct shapes sharing one storage value,
+    disambiguated by which fields are populated.
+    """
+
+    DAILY = "daily"
+    WEEKLY = "weekly"
+    MONTHLY_DAY = "monthly_day"
+    MONTHLY_WEEKDAY = "monthly_weekday"
+    YEARLY = "yearly"
+    CUSTOM_DAYS = "custom_days"
+
+    @property
+    def storage_value(self) -> str:
+        """The schema-level rule_type string this model type maps to."""
+        return {
+            RuleType.DAILY: "daily",
+            RuleType.WEEKLY: "weekly",
+            RuleType.MONTHLY_DAY: "monthly",
+            RuleType.MONTHLY_WEEKDAY: "monthly",
+            RuleType.YEARLY: "yearly",
+            RuleType.CUSTOM_DAYS: "custom",
+        }[self]
+
+
+class RuleValidationError(ValueError):
+    """Raised when a ScheduleRule field combination is invalid."""
+
+
+def _validate_date(value: str, field_name: str) -> None:
+    """Raise unless ``value`` is a strict YYYY-MM-DD calendar date."""
+    try:
+        parsed = datetime.datetime.strptime(value, _DATE_FORMAT).date()
+    except (TypeError, ValueError):
+        raise RuleValidationError(
+            f"{field_name} must be an ISO date (YYYY-MM-DD), got {value!r}"
+        ) from None
+    if parsed.isoformat() != value:
+        raise RuleValidationError(
+            f"{field_name} must be a strict YYYY-MM-DD date, got {value!r}"
+        )
+
+
+def _validate_weekday_set(weekday_set) -> frozenset[int]:
+    """Return a normalized frozenset or raise with a clear message.
+
+    Sequence entries are validated BEFORE set() coercion: a list like
+    [0, False] must not collapse the boolean into 0, and unhashable
+    entries must raise RuleValidationError, never a raw TypeError.
+    """
+    if isinstance(weekday_set, (list, tuple)):
+        for entry in weekday_set:
+            # _is_plain_int: bool and int subclasses (potentially
+            # unhashable) are rejected before set() coercion.
+            if not _is_plain_int(entry):
+                raise RuleValidationError(
+                    f"weekday_set entries must be plain integers, got "
+                    f"{entry!r}"
+                )
+        weekday_set = set(weekday_set)
+    if not isinstance(weekday_set, (set, frozenset)):
+        raise RuleValidationError(
+            f"weekday_set must be a set of weekday integers, got "
+            f"{type(weekday_set).__name__}"
+        )
+    if not weekday_set:
+        raise RuleValidationError(
+            "weekday_set must not be empty for weekly/custom rules"
+        )
+    for entry in weekday_set:
+        if not _is_plain_int(entry):
+            raise RuleValidationError(
+                f"weekday_set entries must be plain integers, got "
+                f"{entry!r}"
+            )
+        if not 0 <= entry <= 6:
+            raise RuleValidationError(
+                f"weekday_set entries must be 0 (Monday)..6 (Sunday), "
+                f"got {entry!r}"
+            )
+    return frozenset(weekday_set)
+
+
+def _is_plain_int(value) -> bool:
+    """True only for real ints: bool excluded AND int subclasses too
+    (an unhashable int subclass would break the frozen-hashable model)."""
+    return type(value) is int
+
+
+def _validate_monthly_weekday(nth_weekday, weekday) -> None:
+    """Validate the MONTHLY_WEEKDAY position fields."""
+    if nth_weekday is None or not _is_plain_int(nth_weekday):
+        raise RuleValidationError(
+            "nth_weekday must be an integer for monthly_weekday rules, "
+            f"got {nth_weekday!r}"
+        )
+    if not (-1 <= nth_weekday <= 5) or nth_weekday == 0:
+        raise RuleValidationError(
+            f"nth_weekday must be -1 (last) or 1..5, got {nth_weekday!r}"
+        )
+    if weekday is None or not _is_plain_int(weekday) or not 0 <= weekday <= 6:
+        raise RuleValidationError(
+            f"monthly_weekday rules require a weekday 0 (Monday)..6 "
+            f"(Sunday), got {weekday!r}"
+        )
+
+
+
+
+
+@dataclass(frozen=True)
+class ScheduleRule:
+    """An immutable, validated recurrence rule (see module docstring).
+
+    Construct with keyword arguments; invalid combinations raise
+    :class:`RuleValidationError` at construction time.  Fields map
+    one-to-one onto the ``schedule_rules`` table columns, with
+    ``nth_weekday_weekday`` as the model-only weekday companion to
+    ``nth_weekday`` (serialized inside ``weekday_set`` at the DAO
+    boundary).
+    """
+
+    rule_type: RuleType
+    interval: int = 1
+    weekday_set: frozenset[int] | None = None
+    day_of_month: int | None = None
+    nth_weekday: int | None = None
+    nth_weekday_weekday: int | None = None
+    month: int | None = None
+    start_date: str = "1970-01-01"
+    end_date: str | None = None
+
+    def __post_init__(self) -> None:
+        # Normalize a string rule_type into the enum; anything else
+        # that is not already a RuleType is rejected.
+        if isinstance(self.rule_type, str):
+            # EXACT model names only: no whitespace stripping, no storage
+            # aliases.  The DAO maps storage strings to model fields when
+            # reading rows; the model boundary stays unambiguous.
+            candidates = {rt.name.lower(): rt for rt in RuleType}
+            if self.rule_type in candidates:
+                object.__setattr__(
+                    self, "rule_type", candidates[self.rule_type]
+                )
+            else:
+                raise RuleValidationError(
+                    f"rule_type must be a RuleType or one of "
+                    f"{sorted(candidates)}, got {self.rule_type!r}"
+                )
+        if not isinstance(self.rule_type, RuleType):
+            raise RuleValidationError(
+                f"rule_type must be a RuleType or one of its string "
+                f"values, got {self.rule_type!r}"
+            )
+        if not _is_plain_int(self.interval):
+            raise RuleValidationError(
+                f"interval must be an integer >= 1, got {self.interval!r}"
+            )
+        if self.interval < 1:
+            raise RuleValidationError(
+                f"interval must be >= 1, got {self.interval!r}"
+            )
+
+        _validate_date(self.start_date, "start_date")
+        if self.end_date is not None:
+            _validate_date(self.end_date, "end_date")
+            if self.end_date < self.start_date:
+                raise RuleValidationError(
+                    f"end_date {self.end_date!r} must be on or after "
+                    f"start_date {self.start_date!r}"
+                )
+
+        # Type-exclusive field policy, enforced for every type: build
+        # the dict of present fields, then reject every field the shape
+        # must not carry (weekday_set present is detectable via None vs
+        # set even after normalization below).
+        shape = self.rule_type
+        if shape is RuleType.DAILY:
+            forbidden_fields = (
+                "weekday_set",
+                "day_of_month",
+                "nth_weekday",
+                "nth_weekday_weekday",
+                "month",
+            )
+        elif shape is RuleType.WEEKLY:
+            forbidden_fields = (
+                "day_of_month",
+                "nth_weekday",
+                "nth_weekday_weekday",
+                "month",
+            )
+        elif shape is RuleType.CUSTOM_DAYS:
+            forbidden_fields = (
+                "day_of_month",
+                "nth_weekday",
+                "nth_weekday_weekday",
+                "month",
+            )
+        elif shape is RuleType.MONTHLY_DAY:
+            forbidden_fields = ("weekday_set", "nth_weekday",
+                                "nth_weekday_weekday", "month")
+        elif shape is RuleType.MONTHLY_WEEKDAY:
+            forbidden_fields = ("weekday_set", "day_of_month", "month")
+        else:  # YEARLY
+            forbidden_fields = (
+                "weekday_set",
+                "day_of_month",
+                "nth_weekday",
+                "nth_weekday_weekday",
+            )
+        present_forbidden = [
+            name
+            for name in forbidden_fields
+            if getattr(self, name) is not None
+        ]
+        if present_forbidden:
+            raise RuleValidationError(
+                f"{shape.value} rules must not set "
+                f"{', '.join(sorted(present_forbidden))}"
+            )
+
+        # Required fields per shape.
+        if shape is RuleType.WEEKLY or shape is RuleType.CUSTOM_DAYS:
+            normalized = _validate_weekday_set(self.weekday_set)
+            object.__setattr__(self, "weekday_set", normalized)
+        elif shape is RuleType.MONTHLY_DAY:
+            if self.day_of_month is None or not _is_plain_int(
+                self.day_of_month
+            ):
+                raise RuleValidationError(
+                    "monthly_day rules require day_of_month (1..31)"
+                )
+            if not 1 <= self.day_of_month <= 31:
+                raise RuleValidationError(
+                    f"day_of_month must be 1..31, got {self.day_of_month!r}"
+                )
+        elif shape is RuleType.MONTHLY_WEEKDAY:
+            _validate_monthly_weekday(
+                self.nth_weekday, self.nth_weekday_weekday
+            )
+        elif shape is RuleType.YEARLY:
+            if self.month is None or not _is_plain_int(self.month):
+                raise RuleValidationError(
+                    "yearly rules require month (1..12)"
+                )
+            if not 1 <= self.month <= 12:
+                raise RuleValidationError(
+                    f"month must be 1..12, got {self.month!r}"
+                )
+
+    def to_dict(self) -> dict:
+        """Return the lossless dict form (model-side storage shape).
+
+        ``weekday_set`` serializes as a sorted list; MONTHLY_WEEKDAY's
+        weekday travels inside it (appended) plus
+        ``nth_weekday_weekday`` so the pair round-trips without
+        ambiguity at the model layer.
+        """
+        return {
+            "rule_type": self.rule_type.name.lower(),  # model name
+            "interval": self.interval,
+            "weekday_set": (
+                sorted(self.weekday_set) if self.weekday_set is not None
+                else None
+            ),
+            "day_of_month": self.day_of_month,
+            "nth_weekday": self.nth_weekday,
+            "nth_weekday_weekday": self.nth_weekday_weekday,
+            "month": self.month,
+            "start_date": self.start_date,
+            "end_date": self.end_date,
+        }
+
+    @classmethod
+    def from_dict(cls, data: dict) -> "ScheduleRule":
+        """Rebuild a rule from :meth:`to_dict` output.
+
+        Raises :class:`RuleValidationError` on unknown keys, missing
+        keys, or invalid combinations — never a raw TypeError.
+        """
+        if not isinstance(data, dict):
+            raise RuleValidationError(
+                f"rule data must be a dict, got {type(data).__name__}"
+            )
+        allowed = {
+            "rule_type",
+            "interval",
+            "weekday_set",
+            "day_of_month",
+            "nth_weekday",
+            "nth_weekday_weekday",
+            "month",
+            "start_date",
+            "end_date",
+        }
+        unknown = [key for key in data if key not in allowed]
+        if unknown:
+            raise RuleValidationError(
+                f"unknown rule fields: {sorted(unknown, key=repr)}"
+            )
+        if "rule_type" not in data:
+            raise RuleValidationError("missing required rule field: rule_type")
+        weekday_set = data.get("weekday_set")
+        if isinstance(weekday_set, (list, tuple)):
+            # Validate entries as plain ints BEFORE set() coercion, so a
+            # list like [0, False] cannot collapse False into 0 and an
+            # unhashable/int-subclass entry cannot leak a raw TypeError.
+            for entry in weekday_set:
+                if not _is_plain_int(entry):
+                    raise RuleValidationError(
+                        f"weekday_set entries must be plain integers, got "
+                        f"{entry!r}"
+                    )
+            weekday_set = set(weekday_set)
+        return cls(
+            rule_type=data["rule_type"],
+            interval=data.get("interval", 1),
+            weekday_set=weekday_set,
+            day_of_month=data.get("day_of_month"),
+            nth_weekday=data.get("nth_weekday"),
+            nth_weekday_weekday=data.get("nth_weekday_weekday"),
+            month=data.get("month"),
+            start_date=data.get("start_date", "1970-01-01"),
+            end_date=data.get("end_date"),
+        )

tests/test_recurrence_model.py

@@ -0,0 +1,480 @@
+"""Tests for recurrence.py: the ScheduleRule model and serialization."""
+from __future__ import annotations
+
+import datetime
+
+import pytest
+
+from custom_components.nestquest.recurrence import (
+    RuleType,
+    RuleValidationError,
+    ScheduleRule,
+)
+
+
+def _rule(**kwargs) -> ScheduleRule:
+    return ScheduleRule(**kwargs)
+
+
+# ---------------------------------------------------------------------------
+# Valid construction for every rule type
+# ---------------------------------------------------------------------------
+
+
+def test_daily_rule_constructs_with_defaults() -> None:
+    rule = _rule(rule_type=RuleType.DAILY, start_date="2026-09-01")
+    assert rule.rule_type is RuleType.DAILY
+    assert rule.interval == 1
+    assert rule.weekday_set is None
+    assert rule.day_of_month is None
+    assert rule.nth_weekday is None
+    assert rule.month is None
+    assert rule.end_date is None
+
+
+def test_string_rule_type_coerces() -> None:
+    rule = _rule(rule_type=RuleType.WEEKLY, weekday_set={0, 2}, start_date="2026-09-01")
+    assert rule.rule_type is RuleType.WEEKLY
+
+
+def test_weekly_rule_requires_and_freezes_weekday_set() -> None:
+    rule = _rule(rule_type=RuleType.WEEKLY, weekday_set={0, 2, 4}, start_date="2026-09-01")
+    assert rule.weekday_set == frozenset({0, 2, 4})
+    # A mutable set passed in is frozen on construction.
+    source = {1, 3}
+    rule2 = _rule(rule_type=RuleType.WEEKLY, weekday_set=source, start_date="2026-09-01")
+    source.add(5)
+    assert rule2.weekday_set == frozenset({1, 3})
+
+
+def test_monthly_day_rule_fields() -> None:
+    rule = _rule(rule_type=RuleType.MONTHLY_DAY, day_of_month=15, start_date="2026-09-01")
+    assert rule.day_of_month == 15
+    assert rule.nth_weekday is None
+
+
+def test_monthly_weekday_rule_fields() -> None:
+    rule = _rule(
+        rule_type=RuleType.MONTHLY_WEEKDAY,
+        nth_weekday=2,
+        nth_weekday_weekday=1,
+        start_date="2026-09-01",
+    )
+    assert rule.nth_weekday == 2
+    assert rule.nth_weekday_weekday == 1
+
+
+def test_monthly_weekday_last_friday_allowed() -> None:
+    rule = _rule(
+        rule_type=RuleType.MONTHLY_WEEKDAY,
+        nth_weekday=-1,
+        nth_weekday_weekday=4,
+        start_date="2026-09-01",
+    )
+    assert rule.nth_weekday == -1
+
+
+def test_yearly_rule_fields() -> None:
+    rule = _rule(rule_type=RuleType.YEARLY, month=6, start_date="2026-09-01")
+    assert rule.month == 6
+
+
+def test_custom_days_rule_fields() -> None:
+    rule = _rule(
+        rule_type=RuleType.CUSTOM_DAYS,
+        weekday_set={1, 3, 5},
+        interval=2,
+        start_date="2026-09-01",
+    )
+    assert rule.weekday_set == frozenset({1, 3, 5})
+    assert rule.interval == 2
+
+
+def test_end_date_round_trips(tmp_path) -> None:
+    rule = _rule(
+        rule_type=RuleType.DAILY,
+        start_date="2026-09-01",
+        end_date="2026-12-31",
+    )
+    assert rule.end_date == "2026-12-31"
+
+
+# ---------------------------------------------------------------------------
+# Invalid combinations are rejected at construction
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.parametrize(
+    "kwargs",
+    [
+        # weekly without weekday_set
+        {"rule_type": RuleType.WEEKLY.value, "start_date": "2026-09-01"},
+        # daily carrying a weekday_set
+        {"rule_type": "daily", "weekday_set": {0}, "start_date": "2026-09-01"},
+        # monthly_day without day_of_month
+        {"rule_type": "monthly_day", "start_date": "2026-09-01"},
+        # monthly_day out of range
+        {"rule_type": "monthly_day", "day_of_month": 32, "start_date": "2026-09-01"},
+        # monthly_day carrying weekday_set
+        {"rule_type": "monthly_day", "day_of_month": 5, "weekday_set": {0},
+         "start_date": "2026-09-01"},
+        # yearly without month
+        {"rule_type": "yearly", "start_date": "2026-09-01"},
+        # yearly out of range
+        {"rule_type": "yearly", "month": 13, "start_date": "2026-09-01"},
+        # unknown type
+        {"rule_type": "bogus", "start_date": "2026-09-01"},
+        # interval zero
+        {"rule_type": "daily", "interval": 0, "start_date": "2026-09-01"},
+        # interval non-integer
+        {"rule_type": "daily", "interval": "2", "start_date": "2026-09-01"},
+        # end before start
+        {"rule_type": "daily", "start_date": "2026-09-01",
+         "end_date": "2026-08-31"},
+        # malformed dates
+        {"rule_type": "daily", "start_date": "not-a-date"},
+        {"rule_type": "daily", "start_date": "2026-9-1"},
+        # weekly weekday out of range
+        {"rule_type": RuleType.WEEKLY.value, "weekday_set": {7}, "start_date": "2026-09-01"},
+        # weekly empty set
+        {"rule_type": RuleType.WEEKLY.value, "weekday_set": set(), "start_date": "2026-09-01"},
+        # custom_days carrying month-specific fields
+        {"rule_type": "custom_days", "weekday_set": {0}, "day_of_month": 5,
+         "start_date": "2026-09-01"},
+        {"rule_type": "custom_days", "weekday_set": {0}, "nth_weekday": 1,
+         "nth_weekday_weekday": 0, "start_date": "2026-09-01"},
+        {"rule_type": "custom_days", "weekday_set": {0}, "month": 3,
+         "start_date": "2026-09-01"},
+        # monthly_weekday position invalid
+        {"rule_type": "monthly_weekday", "nth_weekday": 0,
+         "nth_weekday_weekday": 1, "start_date": "2026-09-01"},
+        {"rule_type": "monthly_weekday", "nth_weekday": 6,
+         "nth_weekday_weekday": 1, "start_date": "2026-09-01"},
+        {"rule_type": "monthly_weekday", "nth_weekday": 2,
+         "nth_weekday_weekday": None, "start_date": "2026-09-01"},
+        {"rule_type": "monthly_weekday", "nth_weekday": 2,
+         "nth_weekday_weekday": 7, "start_date": "2026-09-01"},
+        # monthly_day carrying nth fields
+        {"rule_type": "monthly_day", "day_of_month": 5, "nth_weekday": 1,
+         "nth_weekday_weekday": 0, "start_date": "2026-09-01"},
+    ],
+)
+def test_invalid_combinations_raise(kwargs) -> None:
+    with pytest.raises(RuleValidationError):
+        _rule(**kwargs)
+
+
+# ---------------------------------------------------------------------------
+# Immutability
+# ---------------------------------------------------------------------------
+
+
+def test_rule_is_immutable() -> None:
+    rule = _rule(rule_type=RuleType.DAILY, start_date="2026-09-01")
+    with pytest.raises(AttributeError):
+        rule.interval = 2
+    with pytest.raises(AttributeError):
+        del rule.interval
+
+
+# ---------------------------------------------------------------------------
+# Serialization round-trip for every rule type
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.parametrize(
+    "rule",
+    [
+        ScheduleRule(rule_type=RuleType.DAILY, start_date="2026-09-01"),
+        ScheduleRule(rule_type=RuleType.DAILY, interval=3, start_date="2026-09-01",
+                     end_date="2026-12-31"),
+        ScheduleRule(rule_type=RuleType.WEEKLY, weekday_set={0, 2, 4},
+                     start_date="2026-09-01"),
+        ScheduleRule(rule_type=RuleType.WEEKLY, weekday_set={6}, interval=2,
+                     start_date="2026-09-01"),
+        ScheduleRule(rule_type=RuleType.MONTHLY_DAY, day_of_month=15,
+                     start_date="2026-09-01"),
+        ScheduleRule(rule_type=RuleType.MONTHLY_DAY, day_of_month=31, interval=3,
+                     start_date="2026-09-01"),
+        ScheduleRule(rule_type=RuleType.MONTHLY_WEEKDAY, nth_weekday=2,
+                     nth_weekday_weekday=1, start_date="2026-09-01"),
+        ScheduleRule(rule_type=RuleType.MONTHLY_WEEKDAY, nth_weekday=-1,
+                     nth_weekday_weekday=4, interval=2,
+                     start_date="2026-09-01"),
+        ScheduleRule(rule_type=RuleType.YEARLY, month=6, start_date="2026-09-01"),
+        ScheduleRule(rule_type=RuleType.YEARLY, month=2, day_of_month=None,
+                     interval=2, start_date="2026-09-01"),
+        ScheduleRule(rule_type=RuleType.CUSTOM_DAYS, weekday_set={1, 3, 5},
+                     interval=2, start_date="2026-09-01"),
+    ],
+)
+def test_to_dict_from_dict_round_trips_losslessly(rule) -> None:
+    rebuilt = ScheduleRule.from_dict(rule.to_dict())
+    assert rebuilt == rule
+    # Round-tripping twice is stable.
+    assert ScheduleRule.from_dict(rebuilt.to_dict()) == rule
+
+
+def test_from_dict_rejects_unknown_fields() -> None:
+    with pytest.raises(RuleValidationError, match="unknown rule fields"):
+        ScheduleRule.from_dict({"rule_type": "daily", "bogus": 1})
+
+
+def test_from_dict_rejects_missing_rule_type() -> None:
+    with pytest.raises(RuleValidationError, match="missing required"):
+        ScheduleRule.from_dict({"interval": 1})
+
+
+def test_from_dict_rejects_non_dict() -> None:
+    with pytest.raises(RuleValidationError):
+        ScheduleRule.from_dict("daily")
+
+
+def test_from_dict_defaults_start_date_when_absent() -> None:
+    rule = ScheduleRule.from_dict({"rule_type": "daily"})
+    assert rule.start_date == "1970-01-01"
+
+
+def test_from_dict_accepts_list_weekday_set() -> None:
+    rule = ScheduleRule.from_dict(
+        {"rule_type": RuleType.WEEKLY.value, "weekday_set": [0, 2, 4]}
+    )
+    assert rule.weekday_set == frozenset({0, 2, 4})
+
+
+# ---------------------------------------------------------------------------
+# Equality and hashing
+# ---------------------------------------------------------------------------
+
+
+def test_equal_rules_compare_equal_and_hash_equal() -> None:
+    a = _rule(rule_type=RuleType.WEEKLY, weekday_set={0, 2}, start_date="2026-09-01")
+    b = _rule(rule_type=RuleType.WEEKLY, weekday_set={2, 0}, start_date="2026-09-01")
+    assert a == b
+    assert hash(a) == hash(b)
+
+
+def test_different_rules_compare_unequal() -> None:
+    a = _rule(rule_type=RuleType.DAILY, start_date="2026-09-01")
+    b = _rule(rule_type=RuleType.DAILY, interval=2, start_date="2026-09-01")
+    assert a != b
+
+
+def test_rule_not_equal_to_other_types() -> None:
+    rule = _rule(rule_type=RuleType.DAILY, start_date="2026-09-01")
+    assert rule != "daily"
+    assert (rule == 42) is False
+
+
+# ---------------------------------------------------------------------------
+# Purity guards: no HA imports, no database access
+# ---------------------------------------------------------------------------
+
+
+def test_recurrence_module_imports_only_stdlib() -> None:
+    """The model must carry no Home Assistant and no DB imports."""
+    from pathlib import Path
+
+    source = Path(
+        __import__("custom_components.nestquest.recurrence",
+                   fromlist=["__file__"]).__file__
+    ).read_text()
+    offenders = _scan_forbidden_imports(source)
+    assert offenders == [], (
+        f"recurrence.py must not hold forbidden imports: {offenders}"
+    )
+
+
+def _scan_forbidden_imports(source: str) -> list[str]:
+    """The ACTUAL purity-guard logic, shared by the model scan and the
+    probe tests below: returns human-readable offenders (empty = clean).
+
+    Forbidden, at any relative level, for both Import and ImportFrom:
+    - modules containing 'homeassistant' or 'sqlite'
+    - modules whose LEAF name is a DB/store/schema/DAO module
+    - aliases importing a DB/store/schema/DAO module by leaf name
+      ('from . import db', 'from custom_components.nestquest import db')
+    """
+    import ast
+
+    forbidden_targets = {
+        "db", "schema", "dao_children", "dao_rules", "dao_presence",
+        "dao_instances", "migrations", "store",
+    }
+    offenders: list[str] = []
+    tree = ast.parse(source)
+    for node in ast.walk(tree):
+        if isinstance(node, ast.Import):
+            for alias in node.names:
+                lowered = alias.name.lower()
+                leaf = lowered.split(".")[-1]
+                if (
+                    "homeassistant" in lowered
+                    or "sqlite" in lowered
+                    or leaf in forbidden_targets
+                ):
+                    offenders.append(f"import {alias.name}")
+        elif isinstance(node, ast.ImportFrom):
+            module = (node.module or "").lower()
+            module_leaf = module.split(".")[-1] if module else ""
+            if (
+                "homeassistant" in module
+                or "sqlite" in module
+                or (module and module_leaf in forbidden_targets)
+                or (node.level and module_leaf in forbidden_targets)
+            ):
+                offenders.append(
+                    f"from {node.module!r} import ... (level={node.level})"
+                )
+            for alias in node.names:
+                alias_leaf = alias.name.split(".")[-1].lower()
+                if alias_leaf in forbidden_targets:
+                    offenders.append(
+                        f"from {node.module!r} import {alias.name} "
+                        f"(level={node.level})"
+                    )
+    return offenders
+
+
+def test_purity_guard_logic_catches_probe_statements(tmp_path) -> None:
+    """The shared scan function catches every forbidden import shape,
+    including level-0 absolute package DB imports and 'from . import db'.
+    """
+    probes = [
+        "from homeassistant.core import HomeAssistant",
+        "import sqlite3",
+        "from .db import NestQuestDatabase",
+        "from custom_components.nestquest.db import NestQuestDatabase",
+        "from custom_components.nestquest import db",
+        "import custom_components.nestquest.dao_rules",
+        "from . import db",
+        "from . import schema",
+        "from custom_components.nestquest.migrations import apply_migrations",
+    ]
+    for probe in probes:
+        offenders = _scan_forbidden_imports(probe)
+        assert offenders, f"probe must be caught: {probe!r}"
+
+
+def test_non_string_rule_type_rejected() -> None:
+    """Only RuleType members and their string values construct."""
+    for bad in (123, True, None, 3.5, b"daily"):
+        with pytest.raises(RuleValidationError):
+            ScheduleRule(rule_type=bad, start_date="2026-09-01")
+
+
+def test_weekly_rejects_month_and_nth_fields() -> None:
+    with pytest.raises(RuleValidationError, match="must not set"):
+        ScheduleRule(
+            rule_type=RuleType.WEEKLY, weekday_set={0}, day_of_month=5,
+            start_date="2026-09-01",
+        )
+    with pytest.raises(RuleValidationError, match="must not set"):
+        ScheduleRule(
+            rule_type=RuleType.WEEKLY, weekday_set={0}, month=3,
+            start_date="2026-09-01",
+        )
+
+
+def test_daily_rejects_dangling_nth_weekday_weekday() -> None:
+    with pytest.raises(RuleValidationError, match="must not set"):
+        ScheduleRule(
+            rule_type=RuleType.DAILY, nth_weekday_weekday=0,
+            start_date="2026-09-01",
+        )
+
+
+def test_monthly_day_rejects_month() -> None:
+    with pytest.raises(RuleValidationError, match="must not set"):
+        ScheduleRule(
+            rule_type=RuleType.MONTHLY_DAY, day_of_month=5, month=3,
+            start_date="2026-09-01",
+        )
+
+
+def test_yearly_rejects_day_and_nth_fields() -> None:
+    with pytest.raises(RuleValidationError, match="must not set"):
+        ScheduleRule(
+            rule_type=RuleType.YEARLY, month=6, day_of_month=15,
+            start_date="2026-09-01",
+        )
+    with pytest.raises(RuleValidationError, match="must not set"):
+        ScheduleRule(
+            rule_type=RuleType.YEARLY, month=6, nth_weekday=1,
+            nth_weekday_weekday=0, start_date="2026-09-01",
+        )
+
+
+def test_storage_value_mapping() -> None:
+    """The enum exposes the schema's CHECK values via storage_value."""
+    assert RuleType.DAILY.storage_value == "daily"
+    assert RuleType.WEEKLY.storage_value == "weekly"
+    assert RuleType.MONTHLY_DAY.storage_value == "monthly"
+    assert RuleType.MONTHLY_WEEKDAY.storage_value == "monthly"
+    assert RuleType.YEARLY.storage_value == "yearly"
+    assert RuleType.CUSTOM_DAYS.storage_value == "custom"
+
+
+def test_from_dict_bool_weekday_entry_rejected_before_set_coercion() -> None:
+    """[0, False] must NOT collapse False into 0 via set() coercion."""
+    with pytest.raises(RuleValidationError, match="plain integers"):
+        ScheduleRule.from_dict(
+            {"rule_type": "weekly", "weekday_set": [0, False]}
+        )
+
+
+def test_direct_set_with_int_subclass_entry_rejected() -> None:
+    """A hashable-but-non-plain int subclass is still not a plain int."""
+
+    class IntChild(int):
+        pass
+
+    with pytest.raises(RuleValidationError, match="plain integers"):
+        ScheduleRule(
+            rule_type=RuleType.WEEKLY,
+            weekday_set={IntChild(1)},
+            start_date="2026-09-01",
+        )
+
+
+def test_from_dict_with_int_subclass_entry_rejected() -> None:
+    class IntChild(int):
+        pass
+
+    with pytest.raises(RuleValidationError, match="plain integers"):
+        ScheduleRule.from_dict(
+            {"rule_type": "weekly", "weekday_set": [IntChild(1)]}
+        )
+
+
+def test_from_dict_unknown_key_types_never_leak_typeerror() -> None:
+    """Mixed unhashable unknown keys must surface as
+    RuleValidationError, not a raw TypeError from sorting."""
+    with pytest.raises(RuleValidationError, match="unknown rule fields"):
+        ScheduleRule.from_dict({"rule_type": "daily", 42: "value"})
+
+
+def test_storage_aliases_and_whitespace_are_not_model_names() -> None:
+    """The model boundary is exact names only: storage strings, padded
+    and case variants are all rejected."""
+    for bad in ("monthly", "custom", " DAILY ", "daily ", "DAILY"):
+        with pytest.raises(RuleValidationError):
+            ScheduleRule(rule_type=bad, start_date="2026-09-01")
+
+
+def test_unhashable_int_subclass_interval_rejected() -> None:
+    class UnhashableInt(int):
+        __hash__ = None
+
+    with pytest.raises(RuleValidationError):
+        ScheduleRule(
+            rule_type=RuleType.DAILY,
+            interval=UnhashableInt(1),
+            start_date="2026-09-01",
+        )
+
+
+def test_rule_type_is_a_dataclass() -> None:
+    import dataclasses
+
+    assert dataclasses.is_dataclass(ScheduleRule)