diff --git a/data/.gitignore b/data/.gitignore
index 116f5ec..f05d7a5 100644
--- a/data/.gitignore
+++ b/data/.gitignore
@@ -5,5 +5,6 @@ default/
 dns/
 interactions/
 parity_mismatches/
+stats/
 patterns.json
 settings.json
diff --git a/pkg/models/models.go b/pkg/models/models.go
index 4485061..783db40 100644
--- a/pkg/models/models.go
+++ b/pkg/models/models.go
@@ -672,6 +672,21 @@ type ErrorStats struct {
 	Details      string `json:"details,omitempty" xml:"details,omitempty"`
 }
 
+// ActivityRecord is one entry in AfterTouch's local, append-only admin-UI
+// activity log (e.g. an announcement banner dismissal). Local-only: written
+// to plain JSON on disk, never transmitted automatically — the only way it
+// leaves the operator's network is an explicitly-triggered diagnostic
+// export. The same ID can recur with a new Timestamp (e.g. a dismissed
+// notification shown and dismissed again later); this is a log, not a
+// keyed map. Intentionally generic so it can back other admin-UI action
+// kinds beyond dismissals later, not just this one feature.
+type ActivityRecord struct {
+	Kind      string                 `json:"kind"`
+	ID        string                 `json:"id"`
+	Timestamp string                 `json:"timestamp"`
+	Detail    map[string]interface{} `json:"detail,omitempty"`
+}
+
 // DeviceEvent represents an event that occurred on a device.
 type DeviceEvent struct {
 	Type     string                 `json:"type"`
diff --git a/pkg/service/datastore/datastore.go b/pkg/service/datastore/datastore.go
index 0491615..5abe8dd 100644
--- a/pkg/service/datastore/datastore.go
+++ b/pkg/service/datastore/datastore.go
@@ -2738,6 +2738,82 @@ func (ds *DataStore) SaveUsageStats(stats models.UsageStats) error {
 	return ds.atomicWriteFile(path, data)
 }
 
+// RecordActivity appends one entry to the local admin-UI activity log, under
+// DataDir/stats/activity/<kind>/, one file per event (same shape as
+// SaveUsageStats/SaveErrorStats above). kind is meant to be a small,
+// developer-defined constant (e.g. "notification_dismissed") used directly
+// as a directory name — callers must not pass untrusted/user-supplied
+// values. id may recur across calls with a new timestamp each time; this is
+// an append-only log, not a keyed store. See models.ActivityRecord for the
+// local-only/never-transmitted-automatically guarantee this backs.
+func (ds *DataStore) RecordActivity(kind, id string, detail map[string]interface{}) error {
+	dir := filepath.Join(ds.DataDir, "stats", "activity", kind)
+	if err := ds.rootMkdirAll(dir, 0755); err != nil {
+		return err
+	}
+
+	now := time.Now()
+	record := models.ActivityRecord{
+		Kind:      kind,
+		ID:        id,
+		Timestamp: now.UTC().Format(time.RFC3339Nano),
+		Detail:    detail,
+	}
+
+	// The random suffix guards against two events for the same id landing in
+	// the same nanosecond (observed as flaky on coarser-resolution clocks)
+	// silently overwriting one another instead of both being recorded.
+	filename := fmt.Sprintf("%d_%d_%s.json", now.UnixNano(), rand.Int63n(1_000_000), id) //nolint:gosec
+	path := filepath.Join(dir, filename)
+
+	data, err := json.MarshalIndent(record, "", "  ")
+	if err != nil {
+		return err
+	}
+
+	return ds.atomicWriteFile(path, data)
+}
+
+// GetActivityRecords reads back every entry recorded via RecordActivity for
+// the given kind. Unreadable or malformed files are skipped rather than
+// failing the whole read — a single corrupt event shouldn't make the rest of
+// the log unreadable. Returns an empty slice (not an error) when the
+// directory doesn't exist yet, matching the "nothing recorded yet" case.
+func (ds *DataStore) GetActivityRecords(kind string) ([]models.ActivityRecord, error) {
+	dir := filepath.Join(ds.DataDir, "stats", "activity", kind)
+
+	entries, err := ds.rootReadDir(dir)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+
+		return nil, err
+	}
+
+	records := make([]models.ActivityRecord, 0, len(entries))
+
+	for _, entry := range entries {
+		if entry.IsDir() {
+			continue
+		}
+
+		data, readErr := ds.rootReadFile(filepath.Join(dir, entry.Name()))
+		if readErr != nil {
+			continue
+		}
+
+		var record models.ActivityRecord
+		if unmarshalErr := json.Unmarshal(data, &record); unmarshalErr != nil {
+			continue
+		}
+
+		records = append(records, record)
+	}
+
+	return records, nil
+}
+
 // SaveErrorStats saves error statistics to the datastore.
 func (ds *DataStore) SaveErrorStats(stats models.ErrorStats) error {
 	dir := filepath.Join(ds.DataDir, "stats", "error")
diff --git a/pkg/service/datastore/datastore_test.go b/pkg/service/datastore/datastore_test.go
index d7f727b..e857db8 100644
--- a/pkg/service/datastore/datastore_test.go
+++ b/pkg/service/datastore/datastore_test.go
@@ -458,6 +458,88 @@ func TestSettingsPersistence(t *testing.T) {
 	}
 }
 
+// TestRecordActivity_EmptyKindReturnsNilNotError verifies GetActivityRecords
+// for a kind that was never recorded returns an empty, non-error result —
+// the "nothing recorded yet" case, not a failure.
+func TestRecordActivity_EmptyKindReturnsNilNotError(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "activity-empty-test-*")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	ds := NewDataStore(tempDir)
+
+	records, err := ds.GetActivityRecords("notification_dismissed")
+	if err != nil {
+		t.Fatalf("GetActivityRecords on empty kind should not error, got: %v", err)
+	}
+	if len(records) != 0 {
+		t.Errorf("Expected no records, got %d", len(records))
+	}
+}
+
+// TestRecordActivity_SameIDRecursWithNewTimestamp is the regression test for
+// the append-only shape agreed in the #419 design: dismissing the same
+// announcement twice must produce two records, not overwrite one — this is
+// a log, not a keyed map.
+func TestRecordActivity_SameIDRecursWithNewTimestamp(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "activity-recur-test-*")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	ds := NewDataStore(tempDir)
+
+	if err := ds.RecordActivity("notification_dismissed", "admin-gate-notice", nil); err != nil {
+		t.Fatalf("First RecordActivity failed: %v", err)
+	}
+	if err := ds.RecordActivity("notification_dismissed", "admin-gate-notice", nil); err != nil {
+		t.Fatalf("Second RecordActivity failed: %v", err)
+	}
+
+	records, err := ds.GetActivityRecords("notification_dismissed")
+	if err != nil {
+		t.Fatalf("GetActivityRecords failed: %v", err)
+	}
+	if len(records) != 2 {
+		t.Fatalf("Expected 2 records for the same recurring id, got %d: %+v", len(records), records)
+	}
+	for _, r := range records {
+		if r.ID != "admin-gate-notice" || r.Kind != "notification_dismissed" || r.Timestamp == "" {
+			t.Errorf("Unexpected record shape: %+v", r)
+		}
+	}
+}
+
+// TestRecordActivity_DetailRoundTrips verifies the optional detail payload
+// survives a write/read round trip.
+func TestRecordActivity_DetailRoundTrips(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "activity-detail-test-*")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	ds := NewDataStore(tempDir)
+
+	if err := ds.RecordActivity("some_kind", "some-id", map[string]interface{}{"note": "hello"}); err != nil {
+		t.Fatalf("RecordActivity failed: %v", err)
+	}
+
+	records, err := ds.GetActivityRecords("some_kind")
+	if err != nil {
+		t.Fatalf("GetActivityRecords failed: %v", err)
+	}
+	if len(records) != 1 {
+		t.Fatalf("Expected 1 record, got %d", len(records))
+	}
+	if records[0].Detail["note"] != "hello" {
+		t.Errorf("Expected detail to round-trip, got: %+v", records[0].Detail)
+	}
+}
+
 func TestMoveDeviceMigratesData(t *testing.T) {
 	tempDir := t.TempDir()
 	ds := NewDataStore(tempDir)
diff --git a/pkg/service/handlers/handlers_activity_test.go b/pkg/service/handlers/handlers_activity_test.go
new file mode 100644
index 0000000..06122b7
--- /dev/null
+++ b/pkg/service/handlers/handlers_activity_test.go
@@ -0,0 +1,124 @@
+package handlers
+
+import (
+	"os"
+	"testing"
+
+	"github.com/gesellix/bose-soundtouch/pkg/service/datastore"
+)
+
+// TestIsAnnouncementDismissed_EmptyByDefault verifies a freshly-constructed
+// server (no prior activity log) reports nothing as dismissed.
+func TestIsAnnouncementDismissed_EmptyByDefault(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "dismissal-empty-test")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	ds := datastore.NewDataStore(tempDir)
+	_ = ds.Initialize()
+
+	server := NewServer(ds, nil, "http://127.0.0.1:8000", false, false, false)
+
+	if server.IsAnnouncementDismissed("admin-gate-notice") {
+		t.Error("Expected no announcement to be dismissed on a fresh install")
+	}
+}
+
+// TestRecordDismissal_UpdatesCacheAndPersists is a regression test for the
+// #419 design's performance requirement: after RecordDismissal, the
+// in-memory cache must reflect it immediately (no disk re-read needed), and
+// it must also be durably persisted via the activity log.
+func TestRecordDismissal_UpdatesCacheAndPersists(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "dismissal-record-test")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	ds := datastore.NewDataStore(tempDir)
+	_ = ds.Initialize()
+
+	server := NewServer(ds, nil, "http://127.0.0.1:8000", false, false, false)
+
+	if err := server.RecordDismissal("admin-gate-notice"); err != nil {
+		t.Fatalf("RecordDismissal failed: %v", err)
+	}
+
+	if !server.IsAnnouncementDismissed("admin-gate-notice") {
+		t.Error("Expected admin-gate-notice to be dismissed after RecordDismissal")
+	}
+
+	records, err := ds.GetActivityRecords(activityKindNotificationDismissed)
+	if err != nil {
+		t.Fatalf("GetActivityRecords failed: %v", err)
+	}
+	if len(records) != 1 || records[0].ID != "admin-gate-notice" {
+		t.Errorf("Expected exactly 1 persisted dismissal record, got: %+v", records)
+	}
+}
+
+// TestLoadDismissedAnnouncements_ReadsPriorHistoryAtStartup verifies a
+// restarted server picks up dismissals recorded in a previous run — the
+// startup scan, not just the live write-through path.
+func TestLoadDismissedAnnouncements_ReadsPriorHistoryAtStartup(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "dismissal-startup-test")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	ds := datastore.NewDataStore(tempDir)
+	_ = ds.Initialize()
+
+	// Simulate a dismissal recorded in a prior run, before this process's
+	// Server ever existed.
+	if err := ds.RecordActivity(activityKindNotificationDismissed, "admin-gate-notice", nil); err != nil {
+		t.Fatalf("Seeding activity record failed: %v", err)
+	}
+
+	server := NewServer(ds, nil, "http://127.0.0.1:8000", false, false, false)
+
+	if !server.IsAnnouncementDismissed("admin-gate-notice") {
+		t.Error("Expected startup scan to pick up a dismissal recorded in a prior run")
+	}
+	if server.IsAnnouncementDismissed("some-other-notice") {
+		t.Error("Expected an unrelated id to not be reported as dismissed")
+	}
+}
+
+// TestRecordDismissal_SameIDTwiceAppendsBothKeepsCacheSane verifies dismissing
+// the same announcement twice (e.g. re-shown, dismissed again) appends two
+// log entries but the in-memory cache still reports it dismissed exactly
+// once (a boolean check, not a count).
+func TestRecordDismissal_SameIDTwiceAppendsBothKeepsCacheSane(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "dismissal-recur-test")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	ds := datastore.NewDataStore(tempDir)
+	_ = ds.Initialize()
+
+	server := NewServer(ds, nil, "http://127.0.0.1:8000", false, false, false)
+
+	if err := server.RecordDismissal("admin-gate-notice"); err != nil {
+		t.Fatalf("First RecordDismissal failed: %v", err)
+	}
+	if err := server.RecordDismissal("admin-gate-notice"); err != nil {
+		t.Fatalf("Second RecordDismissal failed: %v", err)
+	}
+
+	records, err := ds.GetActivityRecords(activityKindNotificationDismissed)
+	if err != nil {
+		t.Fatalf("GetActivityRecords failed: %v", err)
+	}
+	if len(records) != 2 {
+		t.Errorf("Expected 2 append-only log entries for a recurring dismissal, got %d", len(records))
+	}
+	if !server.IsAnnouncementDismissed("admin-gate-notice") {
+		t.Error("Expected admin-gate-notice to still be reported dismissed")
+	}
+}
diff --git a/pkg/service/handlers/server.go b/pkg/service/handlers/server.go
index 9888f01..dfb8058 100644
--- a/pkg/service/handlers/server.go
+++ b/pkg/service/handlers/server.go
@@ -68,7 +68,8 @@ type Server struct {
 	RepoURL                  string
 	mgmtUsername             string
 	mgmtPassword             string
-	adminAreaAuth            string // "" (unset) / "enabled" / "disabled" — see datastore.Settings.AdminAreaAuth
+	adminAreaAuth            string               // "" (unset) / "enabled" / "disabled" — see datastore.Settings.AdminAreaAuth
+	dismissedAnnouncements   map[string]time.Time // announcement id -> most recent dismissal; see RecordDismissal
 	spotifyClientID          string
 	spotifyClientSecret      string
 	spotifyRedirectURI       string
@@ -318,6 +319,8 @@ func NewServer(ds *datastore.DataStore, sm *setup.Manager, serverURL string, red
 		},
 	)
 
+	s.dismissedAnnouncements = loadDismissedAnnouncements(ds)
+
 	return s
 }
 
@@ -1049,6 +1052,79 @@ func (s *Server) AdminAreaAuthMode() string {
 	return s.adminAreaAuth
 }
 
+// activityKindNotificationDismissed is the datastore.RecordActivity "kind"
+// used for announcement-banner dismissals (see #419 design,
+// _/i419/design-admin-area-auth-gate.md).
+const activityKindNotificationDismissed = "notification_dismissed"
+
+// loadDismissedAnnouncements scans the local activity log once at startup
+// and folds it into an id -> most-recent-dismissal-timestamp map. Called
+// from NewServer so the read path (IsAnnouncementDismissed) never touches
+// disk — only this one, scoped, boot-time scan does, regardless of how
+// large the log grows over time. Errors are logged, not fatal: a missing or
+// unreadable activity log means "nothing dismissed yet", not a startup failure.
+func loadDismissedAnnouncements(ds *datastore.DataStore) map[string]time.Time {
+	dismissed := make(map[string]time.Time)
+
+	if ds == nil {
+		return dismissed
+	}
+
+	records, err := ds.GetActivityRecords(activityKindNotificationDismissed)
+	if err != nil {
+		log.Printf("[Announcements] Failed to load dismissal history, treating as none: %v", err)
+		return dismissed
+	}
+
+	for _, record := range records {
+		ts, parseErr := time.Parse(time.RFC3339Nano, record.Timestamp)
+		if parseErr != nil {
+			continue
+		}
+
+		if existing, ok := dismissed[record.ID]; !ok || ts.After(existing) {
+			dismissed[record.ID] = ts
+		}
+	}
+
+	return dismissed
+}
+
+// RecordDismissal marks an announcement as dismissed: appends to the local
+// activity log (write-through) and updates the in-memory cache immediately,
+// so IsAnnouncementDismissed reflects it without re-reading disk. The same
+// id can be dismissed again later (e.g. if re-shown) — each call is a new
+// log entry, not an overwrite.
+func (s *Server) RecordDismissal(id string) error {
+	if s.ds != nil {
+		if err := s.ds.RecordActivity(activityKindNotificationDismissed, id, nil); err != nil {
+			return err
+		}
+	}
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if s.dismissedAnnouncements == nil {
+		s.dismissedAnnouncements = make(map[string]time.Time)
+	}
+
+	s.dismissedAnnouncements[id] = time.Now()
+
+	return nil
+}
+
+// IsAnnouncementDismissed reports whether the given announcement id has
+// been dismissed, from the in-memory cache only — never touches disk.
+func (s *Server) IsAnnouncementDismissed(id string) bool {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	_, ok := s.dismissedAnnouncements[id]
+
+	return ok
+}
+
 // SetInternalPaths sets the internal paths for the server.
 func (s *Server) SetInternalPaths(paths []string) {
 	s.mu.Lock()