diff --git a/CHANGELOG.md b/CHANGELOG.md
index 35ac9182..93ea830f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -488,6 +488,23 @@ policies are unchanged.
 
 ### Fixes
 
+- A cache key too long to be a filename is now cached. The calendar
+  plugin's key joins every calendar id the user picked; on a real install
+  it passed 300 bytes, ext4 refuses names over 255, and every write failed
+  with `File name too long` — logged as "(permission denied)", so it read
+  like a cache-directory ownership problem. `DiskCache.get_cache_path` now
+  keeps a key of up to 200 UTF-8 bytes as its filename, as before, and
+  turns a longer one into its first bytes plus a hash of the whole key. The
+  web UI's cache list and delete keep working, because the shortened name
+  maps back to the same file. A failed write now names the real error.
+- The cache's memory tier no longer serves data older than the reader asked
+  for. A record loaded from disk was timed in memory from the load, not
+  from when it was written, so `get(key, max_age=300)` could return data
+  close to 600 s old (after a restart, after the hourly memory sweep, or in
+  the other process, which only ever loads the record from disk), and a
+  stored `ttl` was stretched the same way. A memory hit is now also checked against
+  the record's own timestamp, and a stale one falls through to disk, which
+  returns a newer write if there is one.
 - The garbage-collection timer (`GcMonitor`, above) no longer prints
   `Exception ignored while calling GC callback ... 'NoneType' object has no
   attribute 'perf_counter'` when the display service or a test run exits.
diff --git a/src/cache/disk_cache.py b/src/cache/disk_cache.py
index a31782b9..e66f4b10 100644
--- a/src/cache/disk_cache.py
+++ b/src/cache/disk_cache.py
@@ -4,6 +4,7 @@ Disk Cache
 Handles persistent disk-based caching with atomic writes and error recovery.
 """
 
+import hashlib
 import json
 import math
 import os
@@ -31,6 +32,35 @@ except ImportError:  # pragma: no cover - exercised on hosts without the wheel
 # useful, and a half-written file was never useful.
 _ORPHAN_TEMP_MAX_AGE_SECONDS = 3600
 
+# Longest key, in UTF-8 bytes, used verbatim as a filename stem. ext4 caps a
+# name at 255 bytes and set()'s temp file is ".<stem>.json.<8 random>", 15
+# bytes longer than the stem, so anything near the cap could never be written:
+# the calendar plugin's key joins every calendar id and passed 300 bytes on a
+# real install, failing every write with ENAMETOOLONG. Longer keys keep this
+# many bytes as a readable prefix and end in a hash of the whole key.
+_MAX_KEY_FILENAME_BYTES = 200
+_KEY_HASH_CHARS = 16
+
+
+def _filename_stem(key: str) -> str:
+    """The filename stem for a key that is already a safe path component.
+
+    Short keys are used as they are, so every file already on disk keeps its
+    name. A long one becomes its first bytes plus a hash of the full key: the
+    prefix keeps the stem recognisable (and keeps the data-type words that
+    cleanup's retention lookup reads from it), the hash keeps two keys that
+    share a long prefix apart. The result is itself short, so a stem read back
+    from a filename -- which is how the web UI names a key it deletes -- maps to
+    the same file.
+    """
+    encoded = key.encode('utf-8')
+    if len(encoded) <= _MAX_KEY_FILENAME_BYTES:
+        return key
+    digest = hashlib.sha256(encoded).hexdigest()[:_KEY_HASH_CHARS]
+    keep = _MAX_KEY_FILENAME_BYTES - _KEY_HASH_CHARS - 1
+    prefix = encoded[:keep].decode('utf-8', errors='ignore')
+    return f"{prefix}-{digest}"
+
 
 
 class CacheStrategyProtocol(Protocol):
@@ -343,6 +373,8 @@ class DiskCache:
         derives them), so rejecting anything with a path component turns
         away only inputs that could never have been written here.
 
+        A key too long to be a filename is shortened by _filename_stem.
+
         Args:
             key: Cache key
 
@@ -356,7 +388,7 @@ class DiskCache:
         if safe_key is None:
             self.logger.warning("Rejected unsafe cache key %r", key)
             return None
-        return os.path.join(self.cache_dir, f"{safe_key}.json")
+        return os.path.join(self.cache_dir, f"{_filename_stem(safe_key)}.json")
     
     def get(self, key: str, max_age: Optional[int] = 300) -> Optional[Dict[str, Any]]:
         """
@@ -561,7 +593,7 @@ class DiskCache:
                             # If direct write also fails, try fallback location
                             self.logger.warning("Direct write failed for key '%s' to %s: %s", key, cache_path, write_error)
                             raise  # Re-raise to trigger fallback logic
-                except (IOError, OSError, PermissionError):
+                except (IOError, OSError, PermissionError) as primary_error:
                     # Attempt one-time fallback write to user's home cache directory
                     try:
                         # Try user's home cache directory as fallback
@@ -587,11 +619,14 @@ class DiskCache:
                         self.logger.debug("Fallback cache write also failed for key '%s': %s", key, e2)
                     
                     # If all write attempts failed, log warning but don't raise exception
-                    # Cache is a performance optimization, not critical for operation
+                    # Cache is a performance optimization, not critical for operation.
+                    # Name the real error: this used to say "permission denied"
+                    # whatever happened, which sent a too-long filename off to
+                    # be debugged as a directory-ownership problem.
                     self.logger.warning(
-                        "Could not write cache for key '%s' to %s (permission denied). "
+                        "Could not write cache for key '%s' to %s (%s). "
                         "Cache will be unavailable for this key, but application will continue.",
-                        key, cache_path
+                        key, cache_path, primary_error.strerror or primary_error
                     )
                     return  # Exit gracefully without raising exception
         
diff --git a/src/cache_manager.py b/src/cache_manager.py
index 56fbdf9a..f18d2380 100644
--- a/src/cache_manager.py
+++ b/src/cache_manager.py
@@ -46,6 +46,32 @@ from src.cache.disk_cache import DateTimeEncoder  # noqa: F401 - deliberate re-e
 # CacheManager.config_manager not built yet (None means "not available").
 _UNSET: Any = object()
 
+
+def _outlived(record: Any, max_age: Optional[float], now: float) -> bool:
+    """Whether a record's own timestamp puts it past max_age.
+
+    The memory tier times an entry from when it was put there, and a record
+    loaded from disk is put there when it is read, not when it was written: a
+    record 290 s old, read after a restart, could be served for another
+    max_age from memory. This is the age check DiskCache.get makes, with the
+    same rule that a stored ttl wins over the caller's max_age. A record that
+    carries no timestamp is left to the memory tier's own clock.
+    """
+    if not isinstance(record, dict):
+        return False
+    stored_ttl = record.get('ttl')
+    if isinstance(stored_ttl, (int, float)) and not isinstance(stored_ttl, bool) \
+            and stored_ttl >= 0:
+        max_age = stored_ttl
+    stamp = record.get('timestamp')
+    if max_age is None or stamp is None or isinstance(stamp, bool):
+        return False
+    try:
+        return now - float(stamp) > max_age
+    except (TypeError, ValueError):
+        return False
+
+
 class CacheManager:
     """Manages caching of API responses to reduce API calls."""
 
@@ -284,7 +310,11 @@ class CacheManager:
         # 1) Memory cache
         cached = self._memory_cache_component.get(key, max_age=in_memory_ttl)
         if cached is not None:
-            return cached
+            if not _outlived(cached, max_age, time.time()):
+                return cached
+            # Too old for this reader. Disk may hold a newer write (from the
+            # other process), and if it does not, the miss is the right answer.
+            self._memory_cache_component.clear(key)
 
         # 2) Disk cache
         record = self._disk_cache_component.get(key, max_age=max_age)
@@ -318,7 +348,9 @@ class CacheManager:
         # Check memory cache first (1 minute TTL)
         cached = self._memory_cache_component.get(key, max_age=60)
         if cached is not None:
-            return cached
+            if not _outlived(cached, 3600, time.time()):
+                return cached
+            self._memory_cache_component.clear(key)
         
         # Check disk cache
         data = self._disk_cache_component.get(key, max_age=3600)  # 1 hour for load_cache
diff --git a/test/test_cache_long_keys.py b/test/test_cache_long_keys.py
new file mode 100644
index 00000000..22f4714f
--- /dev/null
+++ b/test/test_cache_long_keys.py
@@ -0,0 +1,99 @@
+"""A cache key too long to be a filename still gets a cache file.
+
+The calendar plugin's key joins every calendar id the user picked; on a real
+install it passed 300 bytes, and since ext4 caps a filename at 255 every write
+failed with ENAMETOOLONG -- logged as "permission denied", every update.
+"""
+
+import logging
+import os
+from unittest.mock import patch
+
+from src.cache.disk_cache import DiskCache, _MAX_KEY_FILENAME_BYTES, _filename_stem
+from src.cache_manager import CacheManager
+
+# The shape of the key that failed on hdpi, ids anonymised.
+CALENDAR_KEY = (
+    "calendar_events_someone@example.com_en.usa#holiday@group.v.calendar.google.com_"
+    "family13997378751670666433@group.calendar.google.com_ncaaf_-m-07kbp5_"
+    "%47eorgia+%42ulldogs+football#sports@group.v.calendar.google.com_nfl_-m-07l24_"
+    "%54ampa+%42ay+%42uccaneers#sports@group.v.calendar.google.com_primary"
+)
+
+# ext4/xfs/btrfs NAME_MAX; set()'s temp file adds 15 bytes to the stem.
+NAME_MAX = 255
+TEMP_OVERHEAD = len(".") + len(".json") + len(".") + 8
+
+
+def test_the_real_key_was_too_long_to_write():
+    assert len((CALENDAR_KEY + ".json").encode()) > NAME_MAX - 10
+
+
+def test_a_long_key_round_trips(tmp_path):
+    cache = DiskCache(str(tmp_path))
+    cache.set(CALENDAR_KEY, {"events": [1, 2, 3]})
+
+    assert cache.get(CALENDAR_KEY, max_age=None) == {"events": [1, 2, 3]}
+    path = cache.get_cache_path(CALENDAR_KEY)
+    assert os.path.isfile(path)
+    stem = os.path.basename(path)[:-len(".json")]
+    assert len(stem.encode()) + TEMP_OVERHEAD <= NAME_MAX
+
+
+def test_short_keys_keep_their_filename(tmp_path):
+    cache = DiskCache(str(tmp_path))
+    exactly = "k" * _MAX_KEY_FILENAME_BYTES
+    assert cache.get_cache_path("weather_current") == str(tmp_path / "weather_current.json")
+    assert cache.get_cache_path(exactly) == str(tmp_path / f"{exactly}.json")
+    assert cache.get_cache_path(exactly + "k") != str(tmp_path / f"{exactly}k.json")
+
+
+def test_long_keys_sharing_a_prefix_stay_apart(tmp_path):
+    cache = DiskCache(str(tmp_path))
+    first, second = CALENDAR_KEY + "_a", CALENDAR_KEY + "_b"
+    cache.set(first, {"which": "a"})
+    cache.set(second, {"which": "b"})
+
+    assert cache.get_cache_path(first) != cache.get_cache_path(second)
+    assert cache.get(first, max_age=None) == {"which": "a"}
+    assert cache.get(second, max_age=None) == {"which": "b"}
+
+
+def test_the_prefix_never_splits_a_character():
+    key = "news_" + "é" * 300  # two bytes each, so the cut lands mid-character
+    stem = _filename_stem(key)
+
+    assert stem.startswith("news_é")
+    assert len(stem.encode("utf-8")) <= _MAX_KEY_FILENAME_BYTES
+    stem.encode("utf-8").decode("utf-8")  # well-formed
+
+
+def test_a_stem_listed_by_the_web_ui_deletes_the_same_file(tmp_path):
+    with patch('src.cache_manager.CacheManager._get_writable_cache_dir', return_value=str(tmp_path)):
+        manager = CacheManager()
+    try:
+        manager.save_cache(CALENDAR_KEY, {"events": []})
+
+        listed = [entry["key"] for entry in manager.list_cache_files()]
+        assert len(listed) == 1
+        manager.clear_cache(listed[0])
+
+        assert [n for n in os.listdir(tmp_path) if n.endswith(".json")] == []
+    finally:
+        manager.stop_cleanup_thread()
+
+
+def test_a_failed_write_names_the_real_error(tmp_path, monkeypatch, caplog):
+    blocker = tmp_path / "a-file"
+    blocker.write_text("")
+    # No writable fallback either, so set() gives up and says why.
+    monkeypatch.setattr(os.path, "expanduser", lambda _p: str(blocker / "home"))
+    cache = DiskCache(str(tmp_path / "missing"))
+
+    with caplog.at_level(logging.WARNING):
+        cache.set("weather_current", {"t": 1})
+
+    gave_up = [r.getMessage() for r in caplog.records if "Could not write cache" in r.getMessage()]
+    assert len(gave_up) == 1
+    assert "permission denied" not in gave_up[0]
+    assert os.strerror(2) in gave_up[0]  # ENOENT: the directory does not exist
diff --git a/test/test_cache_memory_tier_age.py b/test/test_cache_memory_tier_age.py
new file mode 100644
index 00000000..ca53df90
--- /dev/null
+++ b/test/test_cache_memory_tier_age.py
@@ -0,0 +1,95 @@
+"""The memory tier never serves a record older than the reader asked for.
+
+A record read from disk went into the memory tier timed from the read, not
+from when it was written, so get(max_age=300) could hand out data up to twice
+that old: after a restart, after the memory sweep, or in a second process that
+loaded a record once and kept serving it.
+"""
+
+import time
+from unittest.mock import patch
+
+import pytest
+
+from src.cache_manager import CacheManager
+
+
+class Clock:
+    def __init__(self, now):
+        self.now = now
+
+    def __call__(self):
+        return self.now
+
+
+@pytest.fixture
+def clock(monkeypatch):
+    fake = Clock(1_800_000_000.0)
+    monkeypatch.setattr(time, "time", fake)
+    return fake
+
+
+def _manager(path):
+    # No disk sweep: it judges files by their real mtime against the fake
+    # clock and would delete them as months old.
+    with patch('src.cache_manager.CacheManager._get_writable_cache_dir',
+               return_value=str(path)), \
+            patch('src.cache_manager.CacheManager.start_cleanup_thread'):
+        return CacheManager()
+
+
+def test_a_record_loaded_late_expires_on_its_own_timestamp(tmp_path, clock):
+    writer = _manager(tmp_path)
+    writer.set("weather_current", {"t": 1})
+
+    reader = _manager(tmp_path)  # a restart, or the other process
+    clock.now += 250
+    assert reader.get("weather_current", max_age=300) == {"t": 1}
+
+    clock.now += 100  # the data is 350 s old; it sat in memory for 100 s
+    assert reader.get("weather_current", max_age=300) is None
+
+
+def test_a_stored_ttl_bounds_the_memory_copy_too(tmp_path, clock):
+    writer = _manager(tmp_path)
+    writer.set("odds_espn_football_nfl_401", {"spread": 6.5}, ttl=60)
+
+    reader = _manager(tmp_path)
+    clock.now += 55
+    assert reader.get("odds_espn_football_nfl_401", max_age=3600) == {"spread": 6.5}
+
+    clock.now += 60
+    assert reader.get("odds_espn_football_nfl_401", max_age=3600) is None
+
+
+def test_a_stale_memory_copy_gives_way_to_a_newer_write_on_disk(tmp_path, clock):
+    writer = _manager(tmp_path)
+    reader = _manager(tmp_path)
+    writer.set("stocks_AAPL", {"price": 1})
+    assert reader.get("stocks_AAPL", max_age=300) == {"price": 1}
+
+    clock.now += 280
+    writer.set("stocks_AAPL", {"price": 2})
+    clock.now += 40  # reader's copy: 40 s in memory, 320 s old
+
+    assert reader.get("stocks_AAPL", max_age=300) == {"price": 2}
+
+
+def test_fresh_records_are_still_served_from_memory(tmp_path, clock):
+    manager = _manager(tmp_path)
+    manager.set("news_NFL", {"items": []})
+    clock.now += 100
+
+    with patch.object(manager._disk_cache_component, "get") as disk_get:
+        assert manager.get("news_NFL", max_age=300) == {"items": []}
+    disk_get.assert_not_called()
+
+
+def test_max_age_none_and_records_without_a_timestamp_never_expire(tmp_path, clock):
+    manager = _manager(tmp_path)
+    manager.set("plugin_health_x", {"ok": True})
+    manager.save_cache("raw_record", {"no": "timestamp"})
+    clock.now += 10 ** 6
+
+    assert manager.get("plugin_health_x", max_age=None) == {"ok": True}
+    assert manager.get_cached_data("raw_record", max_age=None) == {"no": "timestamp"}