diff --git a/freedays/README.md b/freedays/README.md
index 4e38345..1f59e38 100644
--- a/freedays/README.md
+++ b/freedays/README.md
@@ -63,13 +63,19 @@ guarded by a drift test against this package.
 
 ## Free days are not sick days
 
-screen-locker's sick days stay exactly where they are, with their rolling
-windows, their 120-character justifications and their escalating countdown.
-They are *designed to be hard to take*.
+Sick days are a separate pool: `sickdays` (`../sickdays`), uncapped, picked
+per date, with its own file (`~/.local/share/sickdays/`) and its own remote
+node (`sickdays-sync/`). Nothing is shared with this pool — not the budget,
+not the file, not the sync path. It replaced screen-locker's capped sick day.
 
-These are the opposite, and merging the two would have to ruin one of them:
-rolling windows forbid consecutive days, which is the entire point of a
-holiday. So the policies stay separate. Only the storage is shared.
+A date is never in both pools. Since 0.4.0 `mark` refuses a sick day
+(`SickDayConflictError`), importing `sickdays` at call time, by name —
+sickdays depends on freedays, never the reverse, so there is no import
+cycle. Where sickdays is not installed (the gaming daemon's root env, CI)
+the mark is **allowed** and a warning is logged: there is no local sick-day
+pool there to conflict with, and refusing every free day would break a
+load-bearing package for nothing. The reverse direction is sickdays' own
+check, which fails closed.
 
 ## How it syncs
 
diff --git a/freedays/freedays/__init__.py b/freedays/freedays/__init__.py
index 8a94798..d803ec3 100644
--- a/freedays/freedays/__init__.py
+++ b/freedays/freedays/__init__.py
@@ -20,9 +20,9 @@ Two properties are deliberate and load-bearing:
 * **One global pool.** A free day is free for every app at once. There is no
   per-app dimension to mark, and none to forget to mark.
 
-Free days are *not* sick days. Sick days stay where they are, with their
-rolling windows and their justification friction, because they exist to be
-hard to take. These exist to be easy: declared in advance, no reason
+Free days are *not* sick days. Sick days are a separate, uncapped pool
+(``sickdays``), and a date is never in both: :func:`mark` refuses a sick day
+(:mod:`freedays._sick_guard`). Free days are declared in advance, no reason
 required, consecutive runs allowed, capped only by an annual budget.
 """
 
@@ -48,6 +48,7 @@ from freedays._errors import (
     FreeDayError,
     NotFreeError,
     PastDateError,
+    SickDayConflictError,
 )
 from freedays._model import FreeDay
 from freedays._paths import Paths
@@ -65,6 +66,7 @@ __all__ = [
     "NotFreeError",
     "PastDateError",
     "Paths",
+    "SickDayConflictError",
     "Status",
     "SyncUnavailableError",
     "actor_id",
diff --git a/freedays/freedays/_api.py b/freedays/freedays/_api.py
index f8d1994..21e0c2d 100644
--- a/freedays/freedays/_api.py
+++ b/freedays/freedays/_api.py
@@ -41,6 +41,7 @@ from freedays._model import (
     to_free_day,
 )
 from freedays._paths import Paths, resolve_paths
+from freedays._sick_guard import refuse_if_sick
 
 if TYPE_CHECKING:
     from datetime import date
@@ -140,6 +141,7 @@ def mark(
         PastDateError: If ``day`` has already passed.
         AlreadyFreeError: If ``day`` is already free.
         BudgetExhaustedError: If that year has no days left.
+        SickDayConflictError: If ``day`` is a sick day (``sickdays``).
     """
     resolved = resolve_paths(paths)
     reference = today(now)
@@ -153,6 +155,7 @@ def mark(
     if existing is not None and is_free_record(existing):
         msg = f"{to_iso(day)} is already a free day"
         raise AlreadyFreeError(msg)
+    refuse_if_sick(day)
     if is_exhausted(log, day.year, budget=budget):
         msg = f"no free days left in {day.year} -- all {budget} are spent"
         raise BudgetExhaustedError(msg)
diff --git a/freedays/freedays/_errors.py b/freedays/freedays/_errors.py
index 94d5445..283c15d 100644
--- a/freedays/freedays/_errors.py
+++ b/freedays/freedays/_errors.py
@@ -33,3 +33,7 @@ class AlreadyFreeError(FreeDayError):
 
 class NotFreeError(FreeDayError):
     """The day is not marked free, so there is nothing to release."""
+
+
+class SickDayConflictError(FreeDayError):
+    """The day is a sick day (``sickdays``); a date is never in both pools."""
diff --git a/freedays/freedays/_sick_guard.py b/freedays/freedays/_sick_guard.py
new file mode 100644
index 0000000..c450a09
--- /dev/null
+++ b/freedays/freedays/_sick_guard.py
@@ -0,0 +1,57 @@
+# Copyright (c) 2026 Krzysztof Rudnicki
+"""A date is never both a free day and a sick day (0.4.0).
+
+Sick days are a separate pool (``sickdays``, its own file and remote node).
+sickdays depends on freedays, never the other way round, so this module
+imports it *at call time, by name* -- no module-level import, no cycle.
+
+If sickdays is not installed the mark is **allowed** and a warning logged.
+freedays is load-bearing on hosts that never get sickdays (the gaming
+daemon's root env, CI), and on such a host there is no local sick-day pool
+to conflict with; refusing every free day there would be a regression for
+nothing. The other direction (a sick day on a free date) is refused by
+sickdays itself, which fails closed.
+"""
+
+from __future__ import annotations
+
+import importlib
+import logging
+from typing import TYPE_CHECKING, cast
+
+from freedays._day import to_iso
+from freedays._errors import SickDayConflictError
+
+if TYPE_CHECKING:
+    from collections.abc import Callable
+    from datetime import date
+
+_logger = logging.getLogger(__name__)
+
+
+def is_sick_predicate() -> Callable[[date], bool] | None:
+    """``sickdays.is_sick_day``, imported now; ``None`` (logged) if absent."""
+    try:
+        module = importlib.import_module("sickdays")
+    except ImportError:
+        _logger.warning(
+            "sickdays is not installed: cannot check the sick-day pool, "
+            "so the free day is marked unchecked"
+        )
+        return None
+    return cast("Callable[[date], bool]", module.is_sick_day)
+
+
+def refuse_if_sick(day: date) -> None:
+    """Refuse to mark ``day`` free when it is a sick day.
+
+    Raises:
+        SickDayConflictError: ``day`` is a sick day.
+    """
+    is_sick = is_sick_predicate()
+    if is_sick is not None and is_sick(day):
+        msg = (
+            f"{to_iso(day)} is a sick day -- remove it with "
+            "'sickdays remove' before marking it free"
+        )
+        raise SickDayConflictError(msg)
diff --git a/freedays/freedays/tests/conftest.py b/freedays/freedays/tests/conftest.py
index 5c9e831..c80fa7e 100644
--- a/freedays/freedays/tests/conftest.py
+++ b/freedays/freedays/tests/conftest.py
@@ -37,6 +37,8 @@ def _redirect_state(monkeypatch: pytest.MonkeyPatch, tmp_path: Path) -> None:
 
     for module in ("freedays._api", "freedays._audit", "freedays._sync"):
         monkeypatch.setattr(f"{module}.resolve_paths", _resolve)
+    # Never consult the real sick-day pool; test_sick_guard covers the check.
+    monkeypatch.setattr("freedays._sick_guard.is_sick_predicate", lambda: None)
 
 
 @pytest.fixture
diff --git a/freedays/freedays/tests/test_hmac.py b/freedays/freedays/tests/test_hmac.py
index 19e5918..93e146e 100644
--- a/freedays/freedays/tests/test_hmac.py
+++ b/freedays/freedays/tests/test_hmac.py
@@ -24,7 +24,10 @@ def test_a_generated_key_round_trips(tmp_path: Path) -> None:
     key_file = tmp_path / "hmac.key"
     created = generate_key(key_file)
     assert created is not None
-    assert load_key(key_file) == created
+    # load_key strips whitespace, and a random key can begin or end with a
+    # whitespace byte (~5% of keys): signing and verifying both strip, so
+    # the stripped key is the one that must round-trip.
+    assert load_key(key_file) == created.strip()
 
 
 def test_two_generated_keys_differ(tmp_path: Path) -> None:
diff --git a/freedays/freedays/tests/test_sick_guard.py b/freedays/freedays/tests/test_sick_guard.py
new file mode 100644
index 0000000..4948a5a
--- /dev/null
+++ b/freedays/freedays/tests/test_sick_guard.py
@@ -0,0 +1,60 @@
+# Copyright (c) 2026 Krzysztof Rudnicki
+"""A sick day cannot be marked free; no sickdays means the mark is allowed."""
+
+from __future__ import annotations
+
+from datetime import date
+import sys
+import types
+
+import pytest
+
+from freedays._api import is_free_day, mark
+from freedays._errors import SickDayConflictError
+from freedays._sick_guard import is_sick_predicate
+
+TODAY = date(2026, 10, 16)
+SICK = date(2026, 10, 20)
+
+
+def _fake_sickdays(monkeypatch: pytest.MonkeyPatch) -> None:
+    fake = types.ModuleType("sickdays")
+
+    def _is_sick_day(day: date) -> bool:
+        return day == SICK
+
+    vars(fake)["is_sick_day"] = _is_sick_day
+    monkeypatch.setitem(sys.modules, "sickdays", fake)
+
+
+def test_the_predicate_is_sickdays_own(monkeypatch: pytest.MonkeyPatch) -> None:
+    _fake_sickdays(monkeypatch)
+    predicate = is_sick_predicate()
+    assert predicate is not None
+    assert predicate(SICK)
+    assert not predicate(TODAY)
+
+
+def test_no_sickdays_is_logged_and_allowed(
+    monkeypatch: pytest.MonkeyPatch, caplog: pytest.LogCaptureFixture
+) -> None:
+    monkeypatch.setitem(sys.modules, "sickdays", None)
+    with caplog.at_level("WARNING"):
+        assert is_sick_predicate() is None
+    assert "sickdays is not installed" in caplog.text
+
+
+def test_a_sick_day_cannot_be_marked_free(monkeypatch: pytest.MonkeyPatch) -> None:
+    _fake_sickdays(monkeypatch)
+    predicate = is_sick_predicate()
+    monkeypatch.setattr("freedays._sick_guard.is_sick_predicate", lambda: predicate)
+    with pytest.raises(SickDayConflictError, match="sickdays remove"):
+        mark(SICK, now=TODAY)
+    assert not is_free_day(SICK, now=TODAY)
+    mark(TODAY, now=TODAY)
+    assert is_free_day(TODAY, now=TODAY)
+
+
+def test_without_sickdays_the_mark_goes_through() -> None:
+    mark(SICK, now=TODAY)
+    assert is_free_day(SICK, now=TODAY)
diff --git a/freedays/pyproject.toml b/freedays/pyproject.toml
index 9ff8260..60b069d 100644
--- a/freedays/pyproject.toml
+++ b/freedays/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "freedays"
-version = "0.3.1"
+version = "0.4.0"
 description = "Shared pre-declared free-day pool: one global list of days on which every gate app stands down"
 requires-python = ">=3.14"
 # Deliberately NOT gatelock. Its __init__ imports tkinter for the lock-window
diff --git a/sickdays/sickdays/tests/test_hmac.py b/sickdays/sickdays/tests/test_hmac.py
index 14f0543..ffd39a8 100644
--- a/sickdays/sickdays/tests/test_hmac.py
+++ b/sickdays/sickdays/tests/test_hmac.py
@@ -24,7 +24,10 @@ def test_a_generated_key_round_trips(tmp_path: Path) -> None:
     key_file = tmp_path / "hmac.key"
     created = generate_key(key_file)
     assert created is not None
-    assert load_key(key_file) == created
+    # load_key strips whitespace, and a random key can begin or end with a
+    # whitespace byte (~5% of keys): signing and verifying both strip, so
+    # the stripped key is the one that must round-trip.
+    assert load_key(key_file) == created.strip()
 
 
 def test_two_generated_keys_differ(tmp_path: Path) -> None: