diff --git a/Sources/CalendarrCore/Snapshot/SnapshotFilter.swift b/Sources/CalendarrCore/Snapshot/SnapshotFilter.swift
new file mode 100644
index 0000000..a7c8afe
--- /dev/null
+++ b/Sources/CalendarrCore/Snapshot/SnapshotFilter.swift
@@ -0,0 +1,32 @@
+import Foundation
+
+extension CalendarrSnapshot {
+    /// Keep only the listed calendars. An empty set means "every calendar",
+    /// which is what an unconfigured widget or complication asks for.
+    ///
+    /// Filtering narrows the events but deliberately not the window: the
+    /// snapshot still speaks for the same range, it just has fewer calendars
+    /// in it.
+    public func filtered(toCalendarKeys keys: Set<String>) -> CalendarrSnapshot {
+        guard !keys.isEmpty else { return self }
+        return replacingEvents(events.filter { keys.contains($0.calendarKey) })
+    }
+
+    /// Remove the listed calendars, keeping everything else. An empty set is a
+    /// no-op.
+    public func filtered(excludingCalendarKeys keys: Set<String>) -> CalendarrSnapshot {
+        guard !keys.isEmpty else { return self }
+        return replacingEvents(events.filter { !keys.contains($0.calendarKey) })
+    }
+
+    private func replacingEvents(_ events: [SnapshotEvent]) -> CalendarrSnapshot {
+        CalendarrSnapshot(writtenAt: writtenAt,
+                          coverageStart: coverageStart,
+                          coverageEnd: coverageEnd,
+                          isLoggedIn: isLoggedIn,
+                          writerVersion: writerVersion,
+                          events: events,
+                          theme: theme,
+                          language: language)
+    }
+}
diff --git a/Tests/CalendarrCoreTests/SnapshotFilterTests.swift b/Tests/CalendarrCoreTests/SnapshotFilterTests.swift
new file mode 100644
index 0000000..10edc7a
--- /dev/null
+++ b/Tests/CalendarrCoreTests/SnapshotFilterTests.swift
@@ -0,0 +1,51 @@
+import Foundation
+import Testing
+@testable import CalendarrCore
+
+struct SnapshotFilterTests {
+
+    private static let now = Date(timeIntervalSince1970: 1_789_000_000)
+
+    private static func event(_ id: String, key: String) -> SnapshotEvent {
+        SnapshotEvent(id: id, title: id, start: now, end: now.addingTimeInterval(3600),
+                      isAllDay: false, colorHex: "#4285f4", location: "", calendarKey: key)
+    }
+
+    private static func snapshot(_ events: [SnapshotEvent]) -> CalendarrSnapshot {
+        CalendarrSnapshot(writtenAt: now,
+                          coverageStart: now.addingTimeInterval(-86400),
+                          coverageEnd: now.addingTimeInterval(86400),
+                          isLoggedIn: true, writerVersion: "1.0", events: events,
+                          theme: SnapshotTheme(today: "#1", text: "#2", background: "#3",
+                                               line: "#4", primary: "#5", accent: "#6"),
+                          language: "de")
+    }
+
+    @Test("An empty allow-list means every calendar, not no calendar")
+    func emptyAllowListKeepsEverything() {
+        let snap = Self.snapshot([Self.event("a", key: "local:1"), Self.event("b", key: "caldav:2")])
+        #expect(snap.filtered(toCalendarKeys: []).events.count == 2)
+    }
+
+    @Test("An allow-list keeps only the listed calendars")
+    func allowListNarrows() {
+        let snap = Self.snapshot([Self.event("a", key: "local:1"), Self.event("b", key: "caldav:2")])
+        #expect(snap.filtered(toCalendarKeys: ["local:1"]).events.map(\.id) == ["a"])
+    }
+
+    @Test("A deny-list removes only the listed calendars")
+    func denyListNarrows() {
+        let snap = Self.snapshot([Self.event("a", key: "local:1"), Self.event("b", key: "caldav:2")])
+        #expect(snap.filtered(excludingCalendarKeys: ["local:1"]).events.map(\.id) == ["b"])
+    }
+
+    @Test("Filtering narrows the events but never the window")
+    func windowIsUntouched() {
+        let snap = Self.snapshot([Self.event("a", key: "local:1")])
+        let filtered = snap.filtered(toCalendarKeys: ["nothing:0"])
+
+        #expect(filtered.events.isEmpty)
+        #expect(filtered.coverageStart == snap.coverageStart)
+        #expect(filtered.coverageEnd == snap.coverageEnd)
+    }
+}