diff --git a/CalendarrKit/.gitignore b/CalendarrKit/.gitignore
new file mode 100644
index 0000000..000dfed
--- /dev/null
+++ b/CalendarrKit/.gitignore
@@ -0,0 +1,5 @@
+.build/
+.swiftpm/
+DerivedData/
+*.xcuserstate
+.DS_Store
diff --git a/CalendarrKit/Package.swift b/CalendarrKit/Package.swift
new file mode 100644
index 0000000..84ebb17
--- /dev/null
+++ b/CalendarrKit/Package.swift
@@ -0,0 +1,37 @@
+// swift-tools-version: 6.0
+import PackageDescription
+
+let package = Package(
+    name: "CalendarrKit",
+    platforms: [
+        .iOS(.v17),
+        // String form, not `.v26`: that enum case needs PackageDescription 6.2,
+        // and raising swift-tools-version for one constant would change
+        // defaults across the whole manifest.
+        .watchOS("26.0"),
+        .macOS(.v14),
+        .macCatalyst(.v17),
+    ],
+    products: [
+        // Foundation only — deliberately no SwiftUI, no UIKit, no AppKit, so it
+        // compiles for iOS, Mac Catalyst and native macOS from one source.
+        .library(name: "CalendarrCore", targets: ["CalendarrCore"]),
+    ],
+    targets: [
+        .target(
+            name: "CalendarrCore",
+            // Matches the apps, which build in Swift 5 language mode. Note that
+            // this package deliberately does NOT set defaultIsolation(MainActor),
+            // even though the Calendarr iOS app target does: the widget extension
+            // and the read-only consumer app have no MainActor default, and making
+            // the package MainActor-by-default would force `await` at every
+            // SnapshotStore.read() call site.
+            swiftSettings: [.swiftLanguageMode(.v5)]
+        ),
+        .testTarget(
+            name: "CalendarrCoreTests",
+            dependencies: ["CalendarrCore"],
+            swiftSettings: [.swiftLanguageMode(.v5)]
+        ),
+    ]
+)
diff --git a/CalendarrKit/README.md b/CalendarrKit/README.md
new file mode 100644
index 0000000..e206dd2
--- /dev/null
+++ b/CalendarrKit/README.md
@@ -0,0 +1,105 @@
+# CalendarrKit
+
+Shared code between Calendarr (iOS / Mac Catalyst) and other apps from the same
+developer team that want to **read** the user's calendar.
+
+`CalendarrCore` depends on Foundation only — no SwiftUI, no UIKit, no AppKit — so
+it compiles for iOS, Mac Catalyst and native macOS from one source. That
+constraint is load-bearing: `swift build` against the native macOS SDK is what
+catches a UIKit import before it reaches a consumer, because a Mac Catalyst build
+would compile it happily.
+
+## What a reading app gets
+
+```swift
+import CalendarrCore
+
+let store = SnapshotStore()          // uses CalendarrAppGroup.current
+
+switch store.read() {
+case .ok(let snap):
+    snap.events          // [SnapshotEvent]
+    snap.theme           // the colours Calendarr is currently rendering with
+    snap.writtenAt       // "as of 14:32"
+    snap.coverageStart   // see "Coverage" below
+    snap.coverageEnd
+case .neverWritten:      // Calendarr has never run here → "open Calendarr once"
+case .loggedOut:         // signed out → "sign in to Calendarr", discard old events
+case .incompatible:      // Calendarr is newer than this package → "update the app"
+case .unreadable(let why): // almost always a wrong App Group; log `why`
+}
+
+store.readCalendars()    // [SnapshotCalendar] for a filter UI
+```
+
+React to changes without polling:
+
+```swift
+let token = SnapshotChangeNotifier.observe { /* hop to your actor, read again */ }
+// keep `token` alive; releasing it removes the observer
+```
+
+Also reload on window activation — the notification is a best-effort signal, and
+missing one should leave you slightly stale, never wrong.
+
+## Coverage — the part that bites
+
+The snapshot is not the whole calendar. Outside `coverageStart..<coverageEnd` it
+carries **no information**, which is not the same as "no events". Rendering an
+empty March because it is outside the window produces a screen that looks correct
+and is false. Use `snapshot.covers(date)` and grey out what you cannot speak for.
+
+| | |
+|---|---|
+| Window | ~7 days back, ~42 days ahead, frozen at write time |
+| Cap | 500 events, sorted by start |
+| Filters | hidden and "banished" calendars are already excluded — you see the user's filtered view |
+| Scope | personal only; group overlays are never published, deliberately |
+| Dropped | notes, url, source, creator, privacy, read-only, reminders, recurrence |
+
+What remains is enough to *display* a calendar and deliberately not enough to
+edit one or reason about permissions.
+
+## Setup for a consuming app
+
+Same developer team (`PP34X97WS3`) is required — that is what makes the container
+and the keychain group shared.
+
+```xml
+<key>com.apple.security.app-sandbox</key><true/>
+<key>com.apple.security.application-groups</key>
+<array><string>PP34X97WS3.group.com.scarriffleservices.calendarr</string></array>
+```
+
+The Team ID prefix is mandatory on macOS and forbidden on iOS. `CalendarrAppGroup.current`
+resolves the right one; it must match the entitlement byte for byte, or
+`containerURL(…)` returns nil and every read silently yields nothing.
+
+Never sign in from a consuming app. Credentials, 2FA and token storage belong to
+Calendarr alone.
+
+## Wire format rules
+
+`widget-cache.json`, `widget-calendars.json` and `session.json` live in the group
+container and are read by apps that ship on independent schedules.
+
+1. Filenames do not change within a major version, so an older build keeps reading
+   after the writer updates.
+2. Readers refuse `schemaVersion > maxSupportedSchema` instead of guessing.
+3. Writers only ever *add* optional fields. Never rename, remove or retype one.
+4. A breaking change means a new major version *and* a new filename, with the old
+   file still written for one full release cycle.
+5. The package version is the contract: `CalendarrKit 1.x` ⇔ snapshot schema 2.
+   Depend on `.upToNextMajor(from: "1.0.0")`.
+
+Rule 3 is why the six colour values sit flat at the top level instead of nested
+under `theme`, which would read better: a v1 reader requires them flat, so nesting
+would have broken every older build on the first v2 write. `theme` is a computed
+view over them. `SnapshotSchemaTests` pins this down in both directions.
+
+## Verify
+
+```bash
+DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer swift build   # native macOS — catches UIKit leaks
+DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer swift test
+```
diff --git a/CalendarrKit/Sources/CalendarrCore/AppGroup.swift b/CalendarrKit/Sources/CalendarrCore/AppGroup.swift
new file mode 100644
index 0000000..1a6acc6
--- /dev/null
+++ b/CalendarrKit/Sources/CalendarrCore/AppGroup.swift
@@ -0,0 +1,31 @@
+import Foundation
+
+/// The App Group the Calendarr apps share.
+///
+/// The identifier registered in the Apple Developer portal never changes. What
+/// changes is the string the *runtime* expects: macOS — including Mac Catalyst —
+/// requires the Team ID prefix, and iOS forbids it.
+///
+/// Getting this wrong fails silently. `containerURL(forSecurityApplication‑
+/// GroupIdentifier:)` returns nil, every read and write no-ops, and widgets show
+/// placeholder content forever with no error raised anywhere. `current` must
+/// therefore stay byte-identical to `com.apple.security.application-groups` in
+/// the entitlements file used for the platform being built.
+public enum CalendarrAppGroup {
+    /// As registered in the Apple Developer portal.
+    public static let unprefixed = "group.com.scarriffleservices.calendarr"
+
+    /// Team ID — the value `$(AppIdentifierPrefix)` expands to at build time.
+    public static let teamID = "PP34X97WS3"
+
+    /// The value to hand to `FileManager.containerURL(forSecurityApplicationGroupIdentifier:)`.
+    #if os(macOS) || targetEnvironment(macCatalyst)
+    public static let current = "\(teamID).\(unprefixed)"
+    #else
+    public static let current = unprefixed
+    #endif
+
+    /// Keychain access group shared across apps from this team. Requires
+    /// `keychain-access-groups` in the entitlements of every app that uses it.
+    public static let sharedKeychainGroup = "\(teamID).com.scarriffleservices.calendarr.shared"
+}
diff --git a/CalendarrKit/Sources/CalendarrCore/CalendarKey.swift b/CalendarrKit/Sources/CalendarrCore/CalendarKey.swift
new file mode 100644
index 0000000..8b66ffa
--- /dev/null
+++ b/CalendarrKit/Sources/CalendarrCore/CalendarKey.swift
@@ -0,0 +1,30 @@
+import Foundation
+
+/// Stable identifier for one calendar, used to tie a snapshot event back to the
+/// calendar it came from.
+///
+/// This exists because the server reports `calendar_id` inconsistently: a raw
+/// numeric for CalDAV sources, but `"<source>-<id>"` for local, iCal, Google and
+/// Home Assistant (e.g. `"local-3"`). Any consumer that filters by calendar has
+/// to normalise the two forms identically, or filtering silently does nothing
+/// for the prefixed sources — which is exactly the bug this rule was written to
+/// fix. Keeping the rule in one shared place is why it lives here rather than
+/// being reimplemented per app.
+public enum CalendarKey {
+    /// Build a key, stripping a redundant `"<source>-"` prefix from the id.
+    public static func make(source: String, calendarId: String) -> String {
+        var id = calendarId
+        let prefix = "\(source)-"
+        if id.hasPrefix(prefix) { id = String(id.dropFirst(prefix.count)) }
+        return "\(source):\(id)"
+    }
+
+    /// Split a key back into its source and numeric id.
+    /// Returns nil for malformed keys or non-numeric ids.
+    public static func parse(_ key: String) -> (source: String, id: Int)? {
+        guard let colon = key.firstIndex(of: ":") else { return nil }
+        let source = String(key[..<colon])
+        guard let id = Int(key[key.index(after: colon)...]) else { return nil }
+        return (source, id)
+    }
+}
diff --git a/CalendarrKit/Sources/CalendarrCore/Complications/ComplicationTimeline.swift b/CalendarrKit/Sources/CalendarrCore/Complications/ComplicationTimeline.swift
new file mode 100644
index 0000000..c04aa1e
--- /dev/null
+++ b/CalendarrKit/Sources/CalendarrCore/Complications/ComplicationTimeline.swift
@@ -0,0 +1,61 @@
+import Foundation
+
+/// When a complication needs to be redrawn.
+///
+/// A watch-face complication has a small refresh budget, so the timeline is
+/// pre-computed from data the watch already holds instead of being refreshed on
+/// a schedule: an entry on every event boundary means the complication changes
+/// in the exact minute the next appointment does, without a network call.
+public enum ComplicationTimeline {
+    /// Dates at which a complication should re-render.
+    ///
+    /// Always begins with `now` so the first entry renders immediately. Event
+    /// starts and ends within `horizon` become entries.
+    ///
+    /// The first `fineGrainedWindow` then gets a mark every minute, and the rest
+    /// of the horizon one every hour. The fine-grained part is what lets a
+    /// complication show a *compact* countdown — "23m" rather than "1:23:45" —
+    /// which is the only kind that fits an accessoryCorner or a circular gauge.
+    /// A live `Text(style: .timer)` needs no entries at all but is eight
+    /// characters wide, and overlaps the curved label it sits next to.
+    ///
+    /// The result is sorted, deduplicated to the second, and capped at
+    /// `maxEntries`, keeping the earliest.
+    public static func entryDates(from now: Date,
+                                  in snapshot: CalendarrSnapshot,
+                                  horizon: TimeInterval = 24 * 3600,
+                                  fineGrainedWindow: TimeInterval = 3600,
+                                  maxEntries: Int = 150) -> [Date] {
+        let end = now.addingTimeInterval(horizon)
+        var seen: Set<Int> = []
+        var dates: [Date] = []
+
+        func add(_ date: Date) {
+            guard date >= now, date <= end else { return }
+            // Deduplicate to whole seconds: two events that meet exactly would
+            // otherwise produce two entries a float apart.
+            let key = Int(date.timeIntervalSince1970.rounded())
+            guard seen.insert(key).inserted else { return }
+            dates.append(date)
+        }
+
+        add(now)
+        for event in snapshot.events {
+            add(event.start)
+            add(event.end)
+        }
+        let fineEnd = min(end, now.addingTimeInterval(fineGrainedWindow))
+        var minute = now.addingTimeInterval(60)
+        while minute <= fineEnd {
+            add(minute)
+            minute = minute.addingTimeInterval(60)
+        }
+        var hourly = now.addingTimeInterval(3600)
+        while hourly <= end {
+            add(hourly)
+            hourly = hourly.addingTimeInterval(3600)
+        }
+
+        return Array(dates.sorted().prefix(maxEntries))
+    }
+}
diff --git a/CalendarrKit/Sources/CalendarrCore/Session/SharedSession.swift b/CalendarrKit/Sources/CalendarrCore/Session/SharedSession.swift
new file mode 100644
index 0000000..7f07545
--- /dev/null
+++ b/CalendarrKit/Sources/CalendarrCore/Session/SharedSession.swift
@@ -0,0 +1,85 @@
+import Foundation
+
+/// Non-secret session facts published alongside the snapshot.
+///
+/// This exists so a consumer can answer "which server, and is anyone signed in?"
+/// without reading Calendarr's own `UserDefaults`, which lives in a different
+/// sandbox. The auth token is deliberately *not* here — it belongs in the shared
+/// keychain group, not in a plain file in the container.
+public struct SharedSession: Codable, Sendable {
+    public static let currentSchema = 1
+    public static let maxSupportedSchema = 1
+
+    public let schemaVersion: Int
+    public let baseURL: String
+    public let username: String
+    public let isLoggedIn: Bool
+    public let writtenAt: Date
+
+    public init(baseURL: String, username: String, isLoggedIn: Bool, writtenAt: Date) {
+        self.schemaVersion = Self.currentSchema
+        self.baseURL = baseURL
+        self.username = username
+        self.isLoggedIn = isLoggedIn
+        self.writtenAt = writtenAt
+    }
+
+    public init(from decoder: Decoder) throws {
+        let c = try decoder.container(keyedBy: CodingKeys.self)
+        schemaVersion = try c.decodeIfPresent(Int.self, forKey: .schemaVersion) ?? 1
+        baseURL       = try c.decode(String.self, forKey: .baseURL)
+        username      = try c.decodeIfPresent(String.self, forKey: .username) ?? ""
+        isLoggedIn    = try c.decodeIfPresent(Bool.self, forKey: .isLoggedIn) ?? true
+        writtenAt     = try c.decodeIfPresent(Date.self, forKey: .writtenAt) ?? Date(timeIntervalSince1970: 0)
+    }
+
+    enum CodingKeys: String, CodingKey {
+        case schemaVersion, baseURL, username, isLoggedIn, writtenAt
+    }
+
+    /// The server URL as a `URL`, or nil when unset or malformed.
+    public var url: URL? { baseURL.isEmpty ? nil : URL(string: baseURL) }
+}
+
+public struct SharedSessionStore: Sendable {
+    private static let filename = "session.json"
+
+    public let appGroupID: String
+
+    public init(appGroupID: String = CalendarrAppGroup.current) {
+        self.appGroupID = appGroupID
+    }
+
+    private var fileURL: URL? {
+        FileManager.default
+            .containerURL(forSecurityApplicationGroupIdentifier: appGroupID)?
+            .appendingPathComponent(Self.filename)
+    }
+
+    /// Nil when Calendarr has never published, or the file is unreadable or newer
+    /// than this reader understands.
+    public func read() -> SharedSession? {
+        guard let url = fileURL, let data = try? Data(contentsOf: url) else { return nil }
+        let decoder = JSONDecoder()
+        decoder.dateDecodingStrategy = .iso8601
+        guard let session = try? decoder.decode(SharedSession.self, from: data),
+              session.schemaVersion <= SharedSession.maxSupportedSchema else { return nil }
+        return session
+    }
+
+    @_spi(Writer)
+    public func write(_ session: SharedSession) throws {
+        guard let url = fileURL else {
+            throw SnapshotStoreError.containerUnavailable(appGroupID: appGroupID)
+        }
+        let encoder = JSONEncoder()
+        encoder.dateEncodingStrategy = .iso8601
+        try encoder.encode(session).write(to: url, options: .atomic)
+    }
+
+    @_spi(Writer)
+    public func clear() {
+        guard let url = fileURL else { return }
+        try? FileManager.default.removeItem(at: url)
+    }
+}
diff --git a/CalendarrKit/Sources/CalendarrCore/Snapshot/SnapshotChangeNotifier.swift b/CalendarrKit/Sources/CalendarrCore/Snapshot/SnapshotChangeNotifier.swift
new file mode 100644
index 0000000..1005e9c
--- /dev/null
+++ b/CalendarrKit/Sources/CalendarrCore/Snapshot/SnapshotChangeNotifier.swift
@@ -0,0 +1,62 @@
+import Foundation
+
+/// Cross-process "the snapshot changed" broadcast.
+///
+/// Two sandboxed apps cannot see each other's `NotificationCenter`, and neither
+/// can poll the container cheaply. Darwin notifications are the one signal that
+/// crosses the process boundary without an extra entitlement, a running
+/// connection, or a shared framework.
+///
+/// The payload is always empty — Darwin notifications carry no data. The signal
+/// means only "read again". If it is ever missed, a consumer that also reloads
+/// on window activation degrades to a slightly stale view rather than a wrong one.
+public enum SnapshotChangeNotifier {
+    public static let name = "com.scarriffleservices.calendarr.snapshot-changed"
+
+    /// Announce that the snapshot was rewritten. Called by `SnapshotStore`.
+    public static func post() {
+        CFNotificationCenterPostNotification(
+            CFNotificationCenterGetDarwinNotifyCenter(),
+            CFNotificationName(name as CFString),
+            nil, nil, true)
+    }
+
+    /// Start listening. The handler runs on an arbitrary thread, so hop to
+    /// whatever actor your UI needs before touching state.
+    ///
+    /// Returns a token you must keep alive; deinit removes the observer.
+    public static func observe(_ handler: @escaping @Sendable () -> Void) -> ObservationToken {
+        ObservationToken(handler: handler)
+    }
+
+    public final class ObservationToken: @unchecked Sendable {
+        private let handler: @Sendable () -> Void
+
+        fileprivate init(handler: @escaping @Sendable () -> Void) {
+            self.handler = handler
+            // Darwin notifications hand back no context pointer we can rely on
+            // across process boundaries, so route through an unretained pointer
+            // to this token and look it up in the callback.
+            let observer = Unmanaged.passUnretained(self).toOpaque()
+            CFNotificationCenterAddObserver(
+                CFNotificationCenterGetDarwinNotifyCenter(),
+                observer,
+                { _, observer, _, _, _ in
+                    guard let observer else { return }
+                    Unmanaged<ObservationToken>.fromOpaque(observer)
+                        .takeUnretainedValue().handler()
+                },
+                SnapshotChangeNotifier.name as CFString,
+                nil,
+                .deliverImmediately)
+        }
+
+        deinit {
+            CFNotificationCenterRemoveObserver(
+                CFNotificationCenterGetDarwinNotifyCenter(),
+                Unmanaged.passUnretained(self).toOpaque(),
+                CFNotificationName(SnapshotChangeNotifier.name as CFString),
+                nil)
+        }
+    }
+}
diff --git a/CalendarrKit/Sources/CalendarrCore/Snapshot/SnapshotFilter.swift b/CalendarrKit/Sources/CalendarrCore/Snapshot/SnapshotFilter.swift
new file mode 100644
index 0000000..a7c8afe
--- /dev/null
+++ b/CalendarrKit/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/CalendarrKit/Sources/CalendarrCore/Snapshot/SnapshotModels.swift b/CalendarrKit/Sources/CalendarrCore/Snapshot/SnapshotModels.swift
new file mode 100644
index 0000000..47762cc
--- /dev/null
+++ b/CalendarrKit/Sources/CalendarrCore/Snapshot/SnapshotModels.swift
@@ -0,0 +1,223 @@
+import Foundation
+
+// MARK: - Coverage policy
+
+/// How much of the calendar the writer publishes. Shared so the writer and the
+/// v1-file reconstruction below can never disagree about it.
+public enum SnapshotCoverage {
+    /// Days before today. The current week must stay complete for widgets that
+    /// render Monday–today.
+    public static let daysBehind = 7
+    /// Days after today.
+    public static let daysAhead = 42
+    /// Hard cap on events written, applied after sorting by start date.
+    public static let maxEvents = 500
+}
+
+// MARK: - Event
+
+/// One event as published to the shared container.
+///
+/// Deliberately lossy: notes, URL, source, creator, privacy, read-only status,
+/// reminders and recurrence rules are all dropped. This is enough to *display* a
+/// calendar and deliberately not enough to edit one or reason about permissions.
+public struct SnapshotEvent: Codable, Sendable, Identifiable, Hashable {
+    public let id: String
+    public let title: String
+    public let start: Date
+    public let end: Date
+    public let isAllDay: Bool
+    public let colorHex: String
+    public let location: String
+    /// Ties the event to a calendar. See `CalendarKey`.
+    public let calendarKey: String
+
+    public init(id: String, title: String, start: Date, end: Date,
+                isAllDay: Bool, colorHex: String, location: String, calendarKey: String) {
+        self.id = id; self.title = title; self.start = start; self.end = end
+        self.isAllDay = isAllDay; self.colorHex = colorHex
+        self.location = location; self.calendarKey = calendarKey
+    }
+
+    // Lenient: caches written before calendarKey existed simply lack the field.
+    public init(from decoder: Decoder) throws {
+        let c = try decoder.container(keyedBy: CodingKeys.self)
+        id          = try c.decode(String.self, forKey: .id)
+        title       = try c.decode(String.self, forKey: .title)
+        start       = try c.decode(Date.self,   forKey: .start)
+        end         = try c.decode(Date.self,   forKey: .end)
+        isAllDay    = try c.decode(Bool.self,   forKey: .isAllDay)
+        colorHex    = try c.decode(String.self, forKey: .colorHex)
+        location    = try c.decode(String.self, forKey: .location)
+        calendarKey = try c.decodeIfPresent(String.self, forKey: .calendarKey) ?? ""
+    }
+}
+
+// MARK: - Calendar
+
+/// A calendar available for filtering, published alongside the events so a
+/// consumer can build a picker without a network call.
+public struct SnapshotCalendar: Codable, Sendable, Identifiable, Hashable {
+    /// A `CalendarKey`, matching `SnapshotEvent.calendarKey`.
+    public let id: String
+    public let name: String
+    public let colorHex: String
+
+    public init(id: String, name: String, colorHex: String) {
+        self.id = id; self.name = name; self.colorHex = colorHex
+    }
+}
+
+// MARK: - Theme
+
+/// The colours the app is currently rendering with, so a consumer can match it
+/// without reading the app's own preferences.
+public struct SnapshotTheme: Sendable, Hashable {
+    public let today: String
+    public let text: String
+    public let background: String
+    public let line: String
+    public let primary: String
+    public let accent: String
+
+    public init(today: String, text: String, background: String,
+                line: String, primary: String, accent: String) {
+        self.today = today; self.text = text; self.background = background
+        self.line = line; self.primary = primary; self.accent = accent
+    }
+}
+
+// MARK: - Snapshot
+
+/// The published calendar snapshot.
+///
+/// ## Wire compatibility
+///
+/// The six colour values are stored **flat** at the top level rather than nested
+/// under `theme`, even though `theme` is the nicer thing to read in code. That is
+/// deliberate: v1 files carry them flat and a v1 reader requires them flat, so
+/// nesting would break older readers on the very first v2 write. Within a major
+/// version this format only ever gains optional fields — old readers ignore what
+/// they do not know, new readers default what is missing. `theme` is a computed
+/// view over the stored values, so callers get the clean shape anyway.
+public struct CalendarrSnapshot: Codable, Sendable {
+    /// Bump only for a breaking change, and see the rules in the README.
+    public static let currentSchema = 2
+    /// Files claiming a higher schema are refused rather than mis-decoded.
+    public static let maxSupportedSchema = 2
+
+    public let schemaVersion: Int
+    public let writtenAt: Date
+    /// Start of the published window. Outside `coverageStart..<coverageEnd` the
+    /// snapshot carries *no information* — which is not the same as "no events".
+    public let coverageStart: Date
+    public let coverageEnd: Date
+    /// False once the user signs out. Distinguishes "signed out" from "never ran".
+    public let isLoggedIn: Bool
+    /// CFBundleShortVersionString of whoever wrote the file. Diagnostics only.
+    public let writerVersion: String
+    public let events: [SnapshotEvent]
+    public let language: String
+
+    // Stored flat for wire compatibility — read them via `theme`.
+    let todayColorHex: String
+    let textColorHex: String
+    let backgroundColorHex: String
+    let lineColorHex: String
+    let primaryColorHex: String
+    let accentColorHex: String
+
+    public var theme: SnapshotTheme {
+        SnapshotTheme(today: todayColorHex, text: textColorHex,
+                      background: backgroundColorHex, line: lineColorHex,
+                      primary: primaryColorHex, accent: accentColorHex)
+    }
+
+    /// True when `date` falls inside the published window, i.e. when an absence
+    /// of events for that date genuinely means "nothing scheduled".
+    public func covers(_ date: Date) -> Bool {
+        date >= coverageStart && date < coverageEnd
+    }
+
+    public init(writtenAt: Date,
+                coverageStart: Date,
+                coverageEnd: Date,
+                isLoggedIn: Bool,
+                writerVersion: String,
+                events: [SnapshotEvent],
+                theme: SnapshotTheme,
+                language: String) {
+        self.schemaVersion = Self.currentSchema
+        self.writtenAt = writtenAt
+        self.coverageStart = coverageStart
+        self.coverageEnd = coverageEnd
+        self.isLoggedIn = isLoggedIn
+        self.writerVersion = writerVersion
+        self.events = events
+        self.language = language
+        self.todayColorHex = theme.today
+        self.textColorHex = theme.text
+        self.backgroundColorHex = theme.background
+        self.lineColorHex = theme.line
+        self.primaryColorHex = theme.primary
+        self.accentColorHex = theme.accent
+    }
+
+    /// Lenient decoder. Everything added after v1 is optional here, and a v1
+    /// file's coverage window is reconstructed from the policy that produced it.
+    public init(from decoder: Decoder) throws {
+        let c = try decoder.container(keyedBy: CodingKeys.self)
+        schemaVersion = try c.decodeIfPresent(Int.self, forKey: .schemaVersion) ?? 1
+        writtenAt     = try c.decode(Date.self, forKey: .writtenAt)
+        events        = try c.decode([SnapshotEvent].self, forKey: .events)
+        language      = try c.decode(String.self, forKey: .language)
+
+        todayColorHex      = try c.decode(String.self, forKey: .todayColorHex)
+        textColorHex       = try c.decode(String.self, forKey: .textColorHex)
+        backgroundColorHex = try c.decode(String.self, forKey: .backgroundColorHex)
+        lineColorHex       = try c.decode(String.self, forKey: .lineColorHex)
+        primaryColorHex    = try c.decodeIfPresent(String.self, forKey: .primaryColorHex) ?? "#4285f4"
+        accentColorHex     = try c.decodeIfPresent(String.self, forKey: .accentColorHex)  ?? "#ea4335"
+
+        isLoggedIn    = try c.decodeIfPresent(Bool.self, forKey: .isLoggedIn) ?? true
+        writerVersion = try c.decodeIfPresent(String.self, forKey: .writerVersion) ?? "unknown"
+
+        // v1 had no explicit window; it always wrote the same one, measured from
+        // the start of the day it was written.
+        let cal = Calendar(identifier: .gregorian)
+        let dayStart = cal.startOfDay(for: writtenAt)
+        coverageStart = try c.decodeIfPresent(Date.self, forKey: .coverageStart)
+            ?? cal.date(byAdding: .day, value: -SnapshotCoverage.daysBehind, to: dayStart) ?? dayStart
+        coverageEnd = try c.decodeIfPresent(Date.self, forKey: .coverageEnd)
+            ?? cal.date(byAdding: .day, value: SnapshotCoverage.daysAhead, to: dayStart) ?? dayStart
+    }
+
+    enum CodingKeys: String, CodingKey {
+        case schemaVersion, writtenAt, coverageStart, coverageEnd, isLoggedIn, writerVersion
+        case events, language
+        case todayColorHex, textColorHex, backgroundColorHex, lineColorHex
+        case primaryColorHex, accentColorHex
+    }
+}
+
+// MARK: - Read result
+
+/// Why a read produced no usable snapshot. The distinction matters for the UI:
+/// "never ran", "signed out" and "genuinely empty" call for different messages.
+public enum SnapshotReadResult: Sendable {
+    case ok(CalendarrSnapshot)
+    /// The user signed out of Calendarr. Show a sign-in prompt, not stale events.
+    case loggedOut(writtenAt: Date)
+    /// Calendarr has never published on this machine. Ask the user to open it once.
+    case neverWritten
+    /// Written by a newer Calendarr than this reader understands. Refuse rather
+    /// than guess; ask the user to update.
+    case incompatible(foundSchema: Int, maxSupported: Int)
+    /// Corrupt file, or — much more likely — a misconfigured App Group.
+    case unreadable(String)
+
+    public var snapshot: CalendarrSnapshot? {
+        if case .ok(let s) = self { return s }
+        return nil
+    }
+}
diff --git a/CalendarrKit/Sources/CalendarrCore/Snapshot/SnapshotStore.swift b/CalendarrKit/Sources/CalendarrCore/Snapshot/SnapshotStore.swift
new file mode 100644
index 0000000..9c59763
--- /dev/null
+++ b/CalendarrKit/Sources/CalendarrCore/Snapshot/SnapshotStore.swift
@@ -0,0 +1,139 @@
+import Foundation
+
+public enum SnapshotStoreError: Error, LocalizedError {
+    case containerUnavailable(appGroupID: String)
+
+    public var errorDescription: String? {
+        switch self {
+        case .containerUnavailable(let id):
+            return "App Group container unavailable for \(id). Check that "
+                 + "com.apple.security.application-groups in this target's "
+                 + "entitlements matches exactly, and that macOS builds use the "
+                 + "Team ID prefix."
+        }
+    }
+}
+
+/// Reads — and, for Calendarr itself, writes — the shared calendar snapshot.
+///
+/// Writing is gated behind `@_spi(Writer)` so a read-only consumer cannot
+/// corrupt the shared cache even by accident. Calendarr imports the package as
+/// `@_spi(Writer) import CalendarrCore`; everyone else gets read access only.
+public struct SnapshotStore: Sendable {
+    private static let cacheFilename     = "widget-cache.json"
+    private static let calendarsFilename = "widget-calendars.json"
+
+    public let appGroupID: String
+
+    public init(appGroupID: String = CalendarrAppGroup.current) {
+        self.appGroupID = appGroupID
+    }
+
+    private var containerURL: URL? {
+        FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: appGroupID)
+    }
+
+    private static func encoder() -> JSONEncoder {
+        let e = JSONEncoder()
+        e.dateEncodingStrategy = .iso8601
+        return e
+    }
+
+    private static func decoder() -> JSONDecoder {
+        let d = JSONDecoder()
+        d.dateDecodingStrategy = .iso8601
+        return d
+    }
+
+    // MARK: - Reading
+
+    /// Load the published snapshot.
+    ///
+    /// Never returns a bare nil: every failure mode is named, because they call
+    /// for different things in the UI and because a silent nil is precisely how
+    /// a misconfigured App Group used to hide itself.
+    public func read() -> SnapshotReadResult {
+        guard let container = containerURL else {
+            return .unreadable(SnapshotStoreError
+                .containerUnavailable(appGroupID: appGroupID).localizedDescription)
+        }
+        let url = container.appendingPathComponent(Self.cacheFilename)
+
+        guard FileManager.default.fileExists(atPath: url.path) else {
+            // No cache. Signing out deletes the events — keeping them would leak
+            // the previous user's calendar — so the session file is what tells
+            // "signed out" apart from "never ran here". Answering that in one
+            // call keeps every consumer from reimplementing the distinction.
+            if let session = SharedSessionStore(appGroupID: appGroupID).read(),
+               !session.isLoggedIn {
+                return .loggedOut(writtenAt: session.writtenAt)
+            }
+            return .neverWritten
+        }
+
+        let data: Data
+        do { data = try Data(contentsOf: url) }
+        catch { return .unreadable("Could not read \(Self.cacheFilename): \(error.localizedDescription)") }
+
+        // Check the version before decoding: a newer writer may have changed the
+        // meaning of a field we think we understand, so refusing beats guessing.
+        if let probe = try? Self.decoder().decode(SchemaProbe.self, from: data) {
+            let found = probe.schemaVersion ?? 1
+            if found > CalendarrSnapshot.maxSupportedSchema {
+                return .incompatible(foundSchema: found,
+                                     maxSupported: CalendarrSnapshot.maxSupportedSchema)
+            }
+        }
+
+        do {
+            let snapshot = try Self.decoder().decode(CalendarrSnapshot.self, from: data)
+            guard snapshot.isLoggedIn else { return .loggedOut(writtenAt: snapshot.writtenAt) }
+            return .ok(snapshot)
+        } catch {
+            return .unreadable("Could not decode \(Self.cacheFilename): \(error)")
+        }
+    }
+
+    /// The calendars available for filtering. Empty when nothing is published.
+    public func readCalendars() -> [SnapshotCalendar] {
+        guard let url = containerURL?.appendingPathComponent(Self.calendarsFilename),
+              let data = try? Data(contentsOf: url) else { return [] }
+        return (try? Self.decoder().decode([SnapshotCalendar].self, from: data)) ?? []
+    }
+
+    /// Reads only the version field, so an unsupported file is never fully decoded.
+    private struct SchemaProbe: Decodable { let schemaVersion: Int? }
+
+    // MARK: - Writing (Calendarr only)
+
+    @_spi(Writer)
+    public func write(_ snapshot: CalendarrSnapshot) throws {
+        guard let container = containerURL else {
+            throw SnapshotStoreError.containerUnavailable(appGroupID: appGroupID)
+        }
+        let data = try Self.encoder().encode(snapshot)
+        try data.write(to: container.appendingPathComponent(Self.cacheFilename), options: .atomic)
+        SnapshotChangeNotifier.post()
+    }
+
+    @_spi(Writer)
+    public func writeCalendars(_ calendars: [SnapshotCalendar]) throws {
+        guard let container = containerURL else {
+            throw SnapshotStoreError.containerUnavailable(appGroupID: appGroupID)
+        }
+        let data = try Self.encoder().encode(calendars)
+        try data.write(to: container.appendingPathComponent(Self.calendarsFilename), options: .atomic)
+    }
+
+    /// Remove everything published. Call this on sign-out and on server reset —
+    /// the container outlives the session, so without it the next reader keeps
+    /// rendering the previous user's calendar.
+    @_spi(Writer)
+    public func clear() {
+        guard let container = containerURL else { return }
+        for name in [Self.cacheFilename, Self.calendarsFilename] {
+            try? FileManager.default.removeItem(at: container.appendingPathComponent(name))
+        }
+        SnapshotChangeNotifier.post()
+    }
+}
diff --git a/CalendarrKit/Sources/CalendarrCore/Snapshot/WatchCoverage.swift b/CalendarrKit/Sources/CalendarrCore/Snapshot/WatchCoverage.swift
new file mode 100644
index 0000000..19b25d7
--- /dev/null
+++ b/CalendarrKit/Sources/CalendarrCore/Snapshot/WatchCoverage.swift
@@ -0,0 +1,67 @@
+import Foundation
+
+/// How much of the calendar travels to the watch.
+///
+/// Deliberately narrower than `SnapshotCoverage`: the payload crosses
+/// WatchConnectivity on every publish, and a watch complication never needs six
+/// weeks of history to answer "what is next".
+public enum WatchCoverage {
+    /// Days before today. One is enough for "still running" all-day events.
+    public static let daysBehind = 1
+    /// Days after today.
+    public static let daysAhead = 14
+    /// Hard cap on events sent, applied after sorting by start date.
+    public static let maxEvents = 120
+}
+
+extension CalendarrSnapshot {
+    /// A narrower copy of this snapshot, suitable for sending to the watch.
+    ///
+    /// The coverage window shrinks together with the events, and never widens
+    /// past what the writer already claimed. If `WatchCoverage.maxEvents`
+    /// truncates the list, `coverageEnd` is pulled back to the first event we
+    /// had to drop: everything before that point is provably complete, and
+    /// beyond it the watch must not pretend to know anything.
+    public func trimmedForWatch(now: Date = Date(),
+                                calendar: Calendar = Calendar(identifier: .gregorian)) -> CalendarrSnapshot {
+        let dayStart = calendar.startOfDay(for: now)
+        let windowStart = max(coverageStart,
+                              calendar.date(byAdding: .day, value: -WatchCoverage.daysBehind, to: dayStart) ?? dayStart)
+        let windowEnd = min(coverageEnd,
+                            calendar.date(byAdding: .day, value: WatchCoverage.daysAhead, to: dayStart) ?? dayStart)
+
+        guard windowEnd > windowStart else {
+            return Self.rebuilt(from: self, start: windowStart, end: windowStart, events: [])
+        }
+
+        let inWindow = events
+            .filter { $0.start < windowEnd && $0.end > windowStart }
+            .sorted { $0.start < $1.start }
+
+        guard inWindow.count > WatchCoverage.maxEvents else {
+            return Self.rebuilt(from: self, start: windowStart, end: windowEnd, events: inWindow)
+        }
+
+        let kept = Array(inWindow.prefix(WatchCoverage.maxEvents))
+        // The first dropped event marks the edge of what we can prove. Its start
+        // becomes the new end of coverage; the last kept event is dropped too
+        // when it starts at that same instant, which keeps the invariant
+        // "every event we carry starts before coverageEnd" exactly true.
+        let provenEnd = max(windowStart, min(windowEnd, inWindow[WatchCoverage.maxEvents].start))
+        return Self.rebuilt(from: self, start: windowStart, end: provenEnd,
+                            events: kept.filter { $0.start < provenEnd && $0.end > windowStart })
+    }
+
+    private static func rebuilt(from source: CalendarrSnapshot,
+                                start: Date, end: Date,
+                                events: [SnapshotEvent]) -> CalendarrSnapshot {
+        CalendarrSnapshot(writtenAt: source.writtenAt,
+                          coverageStart: start,
+                          coverageEnd: end,
+                          isLoggedIn: source.isLoggedIn,
+                          writerVersion: source.writerVersion,
+                          events: events,
+                          theme: source.theme,
+                          language: source.language)
+    }
+}
diff --git a/CalendarrKit/Sources/CalendarrCore/Transport/WatchPushPolicy.swift b/CalendarrKit/Sources/CalendarrCore/Transport/WatchPushPolicy.swift
new file mode 100644
index 0000000..b95ed91
--- /dev/null
+++ b/CalendarrKit/Sources/CalendarrCore/Transport/WatchPushPolicy.swift
@@ -0,0 +1,35 @@
+import Foundation
+
+/// Identifies the appointment a complication is currently showing.
+///
+/// The start date is part of the identity on purpose: a moved event keeps its
+/// id, and the complication is wrong until it is told.
+public struct NextEventIdentity: Equatable, Sendable, Codable {
+    public let id: String
+    public let start: Date
+
+    public init(id: String, start: Date) {
+        self.id = id
+        self.start = start
+    }
+}
+
+public enum WatchPushPolicy {
+    /// The first event that has not ended yet — which is what every "next
+    /// appointment" complication renders, including one that is running now.
+    public static func nextEventIdentity(in snapshot: CalendarrSnapshot,
+                                         at now: Date) -> NextEventIdentity? {
+        snapshot.events
+            .filter { $0.end > now }
+            .min { $0.start < $1.start }
+            .map { NextEventIdentity(id: $0.id, start: $0.start) }
+    }
+
+    /// Whether this change is worth one of the day's limited complication
+    /// pushes. Only a changed next appointment is — everything else reaches the
+    /// watch on the next application-context delivery.
+    public static func shouldPushComplicationUpdate(previous: NextEventIdentity?,
+                                                    current: NextEventIdentity?) -> Bool {
+        previous != current
+    }
+}
diff --git a/CalendarrKit/Sources/CalendarrCore/Transport/WatchTransportPayload.swift b/CalendarrKit/Sources/CalendarrCore/Transport/WatchTransportPayload.swift
new file mode 100644
index 0000000..a1be9a5
--- /dev/null
+++ b/CalendarrKit/Sources/CalendarrCore/Transport/WatchTransportPayload.swift
@@ -0,0 +1,101 @@
+import Foundation
+
+/// Dictionary keys used on the WatchConnectivity wire. Stable within a major
+/// version, like the snapshot filenames.
+public enum WatchTransportKey {
+    public static let schema    = "schema"
+    public static let snapshot  = "snapshot"
+    public static let calendars = "calendars"
+    public static let session   = "session"
+}
+
+public enum WatchTransportError: Error, Equatable, LocalizedError {
+    case missingSnapshot
+    case unsupportedSchema(found: Int, maxSupported: Int)
+    case malformed(String)
+
+    public var errorDescription: String? {
+        switch self {
+        case .missingSnapshot:
+            return "Watch payload carried no snapshot."
+        case .unsupportedSchema(let found, let max):
+            return "Watch payload schema \(found) is newer than this build understands (\(max))."
+        case .malformed(let why):
+            return "Watch payload could not be decoded: \(why)"
+        }
+    }
+}
+
+/// What the phone sends to the watch: a trimmed snapshot, the calendar list for
+/// a filter UI, and the non-secret session facts so the watch can tell "signed
+/// out" from "never received anything".
+///
+/// The auth token is deliberately absent. The watch never talks to the server,
+/// so there is nothing for it to authenticate with and nothing to leak.
+public struct WatchTransportPayload: Sendable {
+    /// Bump only for a breaking change to the dictionary shape.
+    public static let currentSchema = 1
+    /// Payloads claiming a higher schema are refused rather than mis-decoded.
+    public static let maxSupportedSchema = 1
+
+    public let snapshot: CalendarrSnapshot
+    public let calendars: [SnapshotCalendar]
+    public let session: SharedSession?
+
+    public init(snapshot: CalendarrSnapshot,
+                calendars: [SnapshotCalendar],
+                session: SharedSession?) {
+        self.snapshot = snapshot
+        self.calendars = calendars
+        self.session = session
+    }
+
+    private static func encoder() -> JSONEncoder {
+        let e = JSONEncoder(); e.dateEncodingStrategy = .iso8601; return e
+    }
+
+    private static func decoder() -> JSONDecoder {
+        let d = JSONDecoder(); d.dateDecodingStrategy = .iso8601; return d
+    }
+
+    /// Encode for `WCSession`. Values are `Data` and `Int` only, because
+    /// WatchConnectivity accepts property-list types and rejects everything
+    /// else at runtime rather than at compile time.
+    public func encoded() throws -> [String: Any] {
+        let e = Self.encoder()
+        var dict: [String: Any] = [
+            WatchTransportKey.schema: Self.currentSchema,
+            WatchTransportKey.snapshot: try e.encode(snapshot),
+            WatchTransportKey.calendars: try e.encode(calendars),
+        ]
+        if let session { dict[WatchTransportKey.session] = try e.encode(session) }
+        return dict
+    }
+
+    public static func decode(_ dict: [String: Any]) throws -> WatchTransportPayload {
+        let found = dict[WatchTransportKey.schema] as? Int ?? 1
+        guard found <= maxSupportedSchema else {
+            throw WatchTransportError.unsupportedSchema(found: found, maxSupported: maxSupportedSchema)
+        }
+        guard let snapshotData = dict[WatchTransportKey.snapshot] as? Data else {
+            throw WatchTransportError.missingSnapshot
+        }
+
+        let d = decoder()
+        let snapshot: CalendarrSnapshot
+        do { snapshot = try d.decode(CalendarrSnapshot.self, from: snapshotData) }
+        catch { throw WatchTransportError.malformed("snapshot: \(error)") }
+
+        var calendars: [SnapshotCalendar] = []
+        if let data = dict[WatchTransportKey.calendars] as? Data {
+            calendars = (try? d.decode([SnapshotCalendar].self, from: data)) ?? []
+        }
+
+        var session: SharedSession?
+        if let data = dict[WatchTransportKey.session] as? Data {
+            session = try? d.decode(SharedSession.self, from: data)
+        }
+
+        return WatchTransportPayload(snapshot: snapshot, calendars: calendars, session: session)
+    }
+}
diff --git a/CalendarrKit/Tests/CalendarrCoreTests/CalendarKeyTests.swift b/CalendarrKit/Tests/CalendarrCoreTests/CalendarKeyTests.swift
new file mode 100644
index 0000000..715dd03
--- /dev/null
+++ b/CalendarrKit/Tests/CalendarrCoreTests/CalendarKeyTests.swift
@@ -0,0 +1,51 @@
+import Testing
+@testable import CalendarrCore
+
+/// The server reports `calendar_id` two different ways, and normalising them
+/// inconsistently is what made per-calendar filtering silently do nothing for
+/// some sources. These pin the rule down.
+struct CalendarKeyTests {
+
+    @Test("CalDAV ids arrive bare and are used as-is")
+    func bareNumericId() {
+        #expect(CalendarKey.make(source: "caldav", calendarId: "12") == "caldav:12")
+    }
+
+    @Test("Prefixed ids from the other sources get the prefix stripped")
+    func strippedPrefix() {
+        #expect(CalendarKey.make(source: "local", calendarId: "local-3") == "local:3")
+        #expect(CalendarKey.make(source: "google", calendarId: "google-5") == "google:5")
+        #expect(CalendarKey.make(source: "ical", calendarId: "ical-7") == "ical:7")
+        #expect(CalendarKey.make(source: "homeassistant",
+                                 calendarId: "homeassistant-9") == "homeassistant:9")
+    }
+
+    @Test("Both spellings of the same calendar produce the same key")
+    func bothFormsAgree() {
+        #expect(CalendarKey.make(source: "local", calendarId: "local-3")
+             == CalendarKey.make(source: "local", calendarId: "3"))
+    }
+
+    @Test("Only a leading prefix is stripped, and only once")
+    func stripsOnlyLeadingPrefix() {
+        // A prefix appearing later in the id must survive untouched.
+        #expect(CalendarKey.make(source: "local", calendarId: "3-local") == "local:3-local")
+        // Two prefixes: only the first goes.
+        #expect(CalendarKey.make(source: "local", calendarId: "local-local-3") == "local:local-3")
+    }
+
+    @Test("parse round-trips make")
+    func roundTrip() {
+        let key = CalendarKey.make(source: "local", calendarId: "local-42")
+        let parsed = CalendarKey.parse(key)
+        #expect(parsed?.source == "local")
+        #expect(parsed?.id == 42)
+    }
+
+    @Test("parse rejects what it cannot represent")
+    func rejectsMalformed() {
+        #expect(CalendarKey.parse("nocolon") == nil)
+        #expect(CalendarKey.parse("local:abc") == nil)   // non-numeric id
+        #expect(CalendarKey.parse("local:") == nil)
+    }
+}
diff --git a/CalendarrKit/Tests/CalendarrCoreTests/ComplicationTimelineTests.swift b/CalendarrKit/Tests/CalendarrCoreTests/ComplicationTimelineTests.swift
new file mode 100644
index 0000000..eea5e88
--- /dev/null
+++ b/CalendarrKit/Tests/CalendarrCoreTests/ComplicationTimelineTests.swift
@@ -0,0 +1,110 @@
+import Foundation
+import Testing
+@testable import CalendarrCore
+
+struct ComplicationTimelineTests {
+
+    private static let now = Date(timeIntervalSince1970: 1_789_000_000)
+
+    private static func event(_ id: String, startOffset: TimeInterval,
+                              duration: TimeInterval) -> SnapshotEvent {
+        SnapshotEvent(id: id, title: id,
+                      start: now.addingTimeInterval(startOffset),
+                      end: now.addingTimeInterval(startOffset + duration),
+                      isAllDay: false, colorHex: "#4285f4", location: "", calendarKey: "local:1")
+    }
+
+    private static func snapshot(_ events: [SnapshotEvent]) -> CalendarrSnapshot {
+        CalendarrSnapshot(writtenAt: now,
+                          coverageStart: now.addingTimeInterval(-86400),
+                          coverageEnd: now.addingTimeInterval(14 * 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("The first entry is always now, so the complication renders immediately")
+    func startsAtNow() {
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: Self.snapshot([]))
+        #expect(dates.first == Self.now)
+    }
+
+    @Test("Entries are sorted and free of duplicates")
+    func sortedAndDeduplicated() {
+        let snap = Self.snapshot([Self.event("a", startOffset: 3600, duration: 1800),
+                                  Self.event("b", startOffset: 5400, duration: 1800)])
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: snap)
+
+        #expect(dates == dates.sorted())
+        #expect(Set(dates).count == dates.count)
+    }
+
+    @Test("Event starts and ends inside the horizon become entries")
+    func includesEventBoundaries() {
+        let snap = Self.snapshot([Self.event("a", startOffset: 3600, duration: 1800)])
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: snap)
+
+        #expect(dates.contains(Self.now.addingTimeInterval(3600)))
+        #expect(dates.contains(Self.now.addingTimeInterval(5400)))
+    }
+
+    @Test("Boundaries outside the horizon are ignored")
+    func ignoresBoundariesBeyondHorizon() {
+        let snap = Self.snapshot([Self.event("far", startOffset: 40 * 3600, duration: 1800)])
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: snap, horizon: 24 * 3600)
+
+        #expect(dates.contains(Self.now.addingTimeInterval(40 * 3600)) == false)
+        #expect(dates.allSatisfy { $0 <= Self.now.addingTimeInterval(24 * 3600) })
+    }
+
+    @Test("Boundaries already in the past are ignored")
+    func ignoresPastBoundaries() {
+        let snap = Self.snapshot([Self.event("past", startOffset: -7200, duration: 1800)])
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: snap)
+
+        #expect(dates.allSatisfy { $0 >= Self.now })
+    }
+
+    @Test("An empty calendar still gets hourly entries across the horizon")
+    func fillsHourlyWhenEmpty() {
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: Self.snapshot([]),
+                                                    horizon: 24 * 3600)
+        #expect(dates.count >= 24)
+    }
+
+    @Test("The first hour gets minute-granular entries, so a compact countdown stays honest")
+    func minuteGranularFirstHour() {
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: Self.snapshot([]))
+        let firstHour = dates.filter { $0 <= Self.now.addingTimeInterval(3600) }
+
+        // 60 minute marks plus `now` itself.
+        #expect(firstHour.count >= 60)
+        #expect(firstHour.contains(Self.now.addingTimeInterval(60)))
+        #expect(firstHour.contains(Self.now.addingTimeInterval(1800)))
+    }
+
+    @Test("Past the first hour the entries thin out to hourly")
+    func hourlyAfterFirstHour() {
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: Self.snapshot([]))
+        let later = dates.filter { $0 > Self.now.addingTimeInterval(3600) }
+
+        // 23 further hours, not 23 * 60 minutes.
+        #expect(later.count < 40)
+        #expect(later.contains(Self.now.addingTimeInterval(2 * 3600)))
+    }
+
+    @Test("The entry count is capped, keeping the earliest entries")
+    func capsEntries() {
+        var events: [SnapshotEvent] = []
+        for i in 0..<300 {
+            events.append(Self.event("e\(i)", startOffset: Double(i) * 60, duration: 30))
+        }
+        let dates = ComplicationTimeline.entryDates(from: Self.now, in: Self.snapshot(events),
+                                                    maxEntries: 50)
+
+        #expect(dates.count == 50)
+        #expect(dates.first == Self.now)
+        #expect(dates == dates.sorted())
+    }
+}
diff --git a/CalendarrKit/Tests/CalendarrCoreTests/SnapshotFilterTests.swift b/CalendarrKit/Tests/CalendarrCoreTests/SnapshotFilterTests.swift
new file mode 100644
index 0000000..10edc7a
--- /dev/null
+++ b/CalendarrKit/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)
+    }
+}
diff --git a/CalendarrKit/Tests/CalendarrCoreTests/SnapshotSchemaTests.swift b/CalendarrKit/Tests/CalendarrCoreTests/SnapshotSchemaTests.swift
new file mode 100644
index 0000000..af71473
--- /dev/null
+++ b/CalendarrKit/Tests/CalendarrCoreTests/SnapshotSchemaTests.swift
@@ -0,0 +1,172 @@
+import Foundation
+import Testing
+@testable import CalendarrCore
+
+/// The wire format is a contract between apps that ship independently, so both
+/// directions of compatibility need a test: a new reader must understand an old
+/// file, and an old reader must survive a new one.
+struct SnapshotSchemaTests {
+
+    private static func decoder() -> JSONDecoder {
+        let d = JSONDecoder(); d.dateDecodingStrategy = .iso8601; return d
+    }
+    private static func encoder() -> JSONEncoder {
+        let e = JSONEncoder(); e.dateEncodingStrategy = .iso8601; return e
+    }
+
+    /// A real v1 file: no schemaVersion, no coverage window, no isLoggedIn,
+    /// colours flat at the top level.
+    private static let v1Fixture = """
+    {
+      "writtenAt": "2026-08-10T12:00:00Z",
+      "events": [
+        {
+          "id": "abc",
+          "title": "Zahnarzt",
+          "start": "2026-08-11T09:00:00Z",
+          "end": "2026-08-11T10:00:00Z",
+          "isAllDay": false,
+          "colorHex": "#58B900",
+          "location": "Bern",
+          "calendarKey": "local:3"
+        }
+      ],
+      "todayColorHex": "#6FB669",
+      "textColorHex": "#FFFFFF",
+      "backgroundColorHex": "#000000",
+      "lineColorHex": "#3B3B3D",
+      "primaryColorHex": "#58B900",
+      "accentColorHex": "#45A148",
+      "language": "de"
+    }
+    """
+
+    @Test("A v1 file decodes, and its missing fields get sane defaults")
+    func decodesV1() throws {
+        let snap = try Self.decoder().decode(
+            CalendarrSnapshot.self, from: Data(Self.v1Fixture.utf8))
+
+        #expect(snap.schemaVersion == 1)
+        #expect(snap.events.count == 1)
+        #expect(snap.events[0].calendarKey == "local:3")
+        #expect(snap.language == "de")
+        #expect(snap.theme.today == "#6FB669")
+        #expect(snap.theme.accent == "#45A148")
+
+        // v1 never said whether anyone was signed in; assuming yes preserves the
+        // old behaviour of simply rendering whatever was cached.
+        #expect(snap.isLoggedIn)
+        #expect(snap.writerVersion == "unknown")
+
+        // The window is reconstructed from the policy v1 always used.
+        let cal = Calendar(identifier: .gregorian)
+        let dayStart = cal.startOfDay(for: snap.writtenAt)
+        #expect(snap.coverageStart == cal.date(byAdding: .day, value: -7, to: dayStart))
+        #expect(snap.coverageEnd == cal.date(byAdding: .day, value: 42, to: dayStart))
+    }
+
+    /// Older still: written before per-calendar filtering existed, so the events
+    /// carry no calendarKey at all.
+    private static let v1FixtureNoCalendarKey = """
+    {
+      "writtenAt": "2026-08-10T12:00:00Z",
+      "events": [
+        {
+          "id": "abc",
+          "title": "Zahnarzt",
+          "start": "2026-08-11T09:00:00Z",
+          "end": "2026-08-11T10:00:00Z",
+          "isAllDay": false,
+          "colorHex": "#58B900",
+          "location": "Bern"
+        }
+      ],
+      "todayColorHex": "#6FB669",
+      "textColorHex": "#FFFFFF",
+      "backgroundColorHex": "#000000",
+      "lineColorHex": "#3B3B3D",
+      "language": "de"
+    }
+    """
+
+    @Test("A v1 file that predates calendarKey and the newer colours still decodes")
+    func decodesV1WithoutCalendarKey() throws {
+        let snap = try Self.decoder().decode(
+            CalendarrSnapshot.self, from: Data(Self.v1FixtureNoCalendarKey.utf8))
+        #expect(snap.events[0].calendarKey == "")
+        // primary/accent were added later too, so they fall back to defaults.
+        #expect(snap.theme.primary == "#4285f4")
+        #expect(snap.theme.accent == "#ea4335")
+    }
+
+    @Test("v2 round-trips without losing anything")
+    func roundTripsV2() throws {
+        let start = Date(timeIntervalSince1970: 1_780_000_000)
+        let original = CalendarrSnapshot(
+            writtenAt: start,
+            coverageStart: start.addingTimeInterval(-7 * 86_400),
+            coverageEnd: start.addingTimeInterval(42 * 86_400),
+            isLoggedIn: true,
+            writerVersion: "3.5",
+            events: [SnapshotEvent(id: "x", title: "Termin", start: start,
+                                   end: start.addingTimeInterval(3600),
+                                   isAllDay: false, colorHex: "#58B900",
+                                   location: "", calendarKey: "local:1")],
+            theme: SnapshotTheme(today: "#1", text: "#2", background: "#3",
+                                 line: "#4", primary: "#5", accent: "#6"),
+            language: "ch")
+
+        let decoded = try Self.decoder().decode(
+            CalendarrSnapshot.self, from: Self.encoder().encode(original))
+
+        #expect(decoded.schemaVersion == CalendarrSnapshot.currentSchema)
+        #expect(decoded.writerVersion == "3.5")
+        #expect(decoded.language == "ch")
+        #expect(decoded.coverageStart == original.coverageStart)
+        #expect(decoded.coverageEnd == original.coverageEnd)
+        #expect(decoded.theme == original.theme)
+        #expect(decoded.events == original.events)
+    }
+
+    /// The reason the colours are stored flat rather than nested under `theme`:
+    /// a v1 reader requires them at the top level, so nesting them would have
+    /// broken every older build on the first v2 write.
+    @Test("v2 keeps the colour keys where a v1 reader expects them")
+    func v2StaysReadableByV1() throws {
+        let start = Date(timeIntervalSince1970: 1_780_000_000)
+        let snap = CalendarrSnapshot(
+            writtenAt: start, coverageStart: start, coverageEnd: start,
+            isLoggedIn: true, writerVersion: "3.5", events: [],
+            theme: SnapshotTheme(today: "#a", text: "#b", background: "#c",
+                                 line: "#d", primary: "#e", accent: "#f"),
+            language: "de")
+
+        let object = try JSONSerialization.jsonObject(
+            with: Self.encoder().encode(snap)) as? [String: Any]
+
+        for key in ["todayColorHex", "textColorHex", "backgroundColorHex",
+                    "lineColorHex", "primaryColorHex", "accentColorHex",
+                    "writtenAt", "events", "language"] {
+            #expect(object?[key] != nil, "v1 readers require \(key) at the top level")
+        }
+        #expect(object?["theme"] == nil, "theme must stay a computed view, not a stored key")
+    }
+
+    @Test("covers() separates 'no events' from 'no information'")
+    func coverageWindow() {
+        let now = Date(timeIntervalSince1970: 1_780_000_000)
+        let snap = CalendarrSnapshot(
+            writtenAt: now,
+            coverageStart: now.addingTimeInterval(-86_400),
+            coverageEnd: now.addingTimeInterval(86_400),
+            isLoggedIn: true, writerVersion: "3.5", events: [],
+            theme: SnapshotTheme(today: "", text: "", background: "",
+                                 line: "", primary: "", accent: ""),
+            language: "de")
+
+        #expect(snap.covers(now))
+        #expect(snap.covers(now.addingTimeInterval(-86_399)))
+        #expect(!snap.covers(now.addingTimeInterval(-90_000)))
+        #expect(!snap.covers(snap.coverageEnd))   // half-open: end is excluded
+    }
+}
diff --git a/CalendarrKit/Tests/CalendarrCoreTests/WatchPushPolicyTests.swift b/CalendarrKit/Tests/CalendarrCoreTests/WatchPushPolicyTests.swift
new file mode 100644
index 0000000..a703009
--- /dev/null
+++ b/CalendarrKit/Tests/CalendarrCoreTests/WatchPushPolicyTests.swift
@@ -0,0 +1,76 @@
+import Foundation
+import Testing
+@testable import CalendarrCore
+
+struct WatchPushPolicyTests {
+
+    private static let now = Date(timeIntervalSince1970: 1_789_000_000)
+
+    private static func event(_ id: String, startOffset: TimeInterval,
+                              duration: TimeInterval = 3600) -> SnapshotEvent {
+        SnapshotEvent(id: id, title: id,
+                      start: now.addingTimeInterval(startOffset),
+                      end: now.addingTimeInterval(startOffset + duration),
+                      isAllDay: false, colorHex: "#4285f4", location: "", calendarKey: "local:1")
+    }
+
+    private static func snapshot(_ events: [SnapshotEvent]) -> CalendarrSnapshot {
+        CalendarrSnapshot(writtenAt: now,
+                          coverageStart: now.addingTimeInterval(-86400),
+                          coverageEnd: now.addingTimeInterval(14 * 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("The next event is the first one that has not ended yet")
+    func picksFirstUnfinished() {
+        let snap = Self.snapshot([Self.event("past", startOffset: -7200),
+                                  Self.event("running", startOffset: -600),
+                                  Self.event("later", startOffset: 3600)])
+
+        #expect(WatchPushPolicy.nextEventIdentity(in: snap, at: Self.now)?.id == "running")
+    }
+
+    @Test("An empty calendar has no next event")
+    func emptyHasNoIdentity() {
+        #expect(WatchPushPolicy.nextEventIdentity(in: Self.snapshot([]), at: Self.now) == nil)
+    }
+
+    @Test("Events are considered in start order regardless of input order")
+    func sortsBeforePicking() {
+        let snap = Self.snapshot([Self.event("late", startOffset: 7200),
+                                  Self.event("soon", startOffset: 1800)])
+
+        #expect(WatchPushPolicy.nextEventIdentity(in: snap, at: Self.now)?.id == "soon")
+    }
+
+    @Test("An unchanged next event does not spend a push")
+    func unchangedDoesNotPush() {
+        let identity = NextEventIdentity(id: "a", start: Self.now)
+        #expect(WatchPushPolicy.shouldPushComplicationUpdate(previous: identity, current: identity) == false)
+    }
+
+    @Test("A different event, a moved event, or a first event all spend a push")
+    func changesPush() {
+        let a = NextEventIdentity(id: "a", start: Self.now)
+        let b = NextEventIdentity(id: "b", start: Self.now)
+        let moved = NextEventIdentity(id: "a", start: Self.now.addingTimeInterval(600))
+
+        #expect(WatchPushPolicy.shouldPushComplicationUpdate(previous: a, current: b))
+        #expect(WatchPushPolicy.shouldPushComplicationUpdate(previous: a, current: moved))
+        #expect(WatchPushPolicy.shouldPushComplicationUpdate(previous: nil, current: a))
+    }
+
+    @Test("Losing the last event spends a push, so the watch stops showing it")
+    func clearingPushes() {
+        let a = NextEventIdentity(id: "a", start: Self.now)
+        #expect(WatchPushPolicy.shouldPushComplicationUpdate(previous: a, current: nil))
+    }
+
+    @Test("Two empty calendars in a row do not push")
+    func stayingEmptyDoesNotPush() {
+        #expect(WatchPushPolicy.shouldPushComplicationUpdate(previous: nil, current: nil) == false)
+    }
+}
diff --git a/CalendarrKit/Tests/CalendarrCoreTests/WatchTransportTests.swift b/CalendarrKit/Tests/CalendarrCoreTests/WatchTransportTests.swift
new file mode 100644
index 0000000..af0c86f
--- /dev/null
+++ b/CalendarrKit/Tests/CalendarrCoreTests/WatchTransportTests.swift
@@ -0,0 +1,81 @@
+import Foundation
+import Testing
+@testable import CalendarrCore
+
+/// The phone and the watch ship on independent schedules, so the transport gets
+/// the same treatment as the file format: refuse a newer schema rather than
+/// mis-decode it.
+struct WatchTransportTests {
+
+    private static let now = Date(timeIntervalSince1970: 1_789_000_000)
+
+    private static func snapshot() -> CalendarrSnapshot {
+        CalendarrSnapshot(writtenAt: now,
+                          coverageStart: now.addingTimeInterval(-86400),
+                          coverageEnd: now.addingTimeInterval(14 * 86400),
+                          isLoggedIn: true, writerVersion: "1.2",
+                          events: [SnapshotEvent(id: "a", title: "Zahnarzt",
+                                                 start: now.addingTimeInterval(3600),
+                                                 end: now.addingTimeInterval(7200),
+                                                 isAllDay: false, colorHex: "#4285f4",
+                                                 location: "Bern", calendarKey: "local:3")],
+                          theme: SnapshotTheme(today: "#1", text: "#2", background: "#3",
+                                               line: "#4", primary: "#5", accent: "#6"),
+                          language: "de")
+    }
+
+    @Test("A payload survives a round trip")
+    func roundTrips() throws {
+        let payload = WatchTransportPayload(
+            snapshot: Self.snapshot(),
+            calendars: [SnapshotCalendar(id: "local:3", name: "Privat", colorHex: "#4285f4")],
+            session: SharedSession(baseURL: "https://cal.example.com", username: "guido",
+                                   isLoggedIn: true, writtenAt: Self.now))
+
+        let decoded = try WatchTransportPayload.decode(try payload.encoded())
+
+        #expect(decoded.snapshot.events.map(\.id) == ["a"])
+        #expect(decoded.snapshot.language == "de")
+        #expect(decoded.calendars.map(\.name) == ["Privat"])
+        #expect(decoded.session?.username == "guido")
+    }
+
+    @Test("A payload with no session record still decodes")
+    func sessionIsOptional() throws {
+        let payload = WatchTransportPayload(snapshot: Self.snapshot(), calendars: [], session: nil)
+        let decoded = try WatchTransportPayload.decode(try payload.encoded())
+
+        #expect(decoded.session == nil)
+        #expect(decoded.calendars.isEmpty)
+    }
+
+    @Test("The encoded form only contains property-list types")
+    func encodesToPlistTypes() throws {
+        let dict = try WatchTransportPayload(snapshot: Self.snapshot(), calendars: [], session: nil).encoded()
+
+        #expect(dict[WatchTransportKey.schema] is Int)
+        #expect(dict[WatchTransportKey.snapshot] is Data)
+        #expect(PropertyListSerialization.propertyList(dict, isValidFor: .binary))
+    }
+
+    @Test("A newer schema is refused rather than guessed at")
+    func refusesNewerSchema() throws {
+        var dict = try WatchTransportPayload(snapshot: Self.snapshot(), calendars: [], session: nil).encoded()
+        dict[WatchTransportKey.schema] = WatchTransportPayload.maxSupportedSchema + 1
+
+        #expect(throws: WatchTransportError.unsupportedSchema(
+            found: WatchTransportPayload.maxSupportedSchema + 1,
+            maxSupported: WatchTransportPayload.maxSupportedSchema)) {
+            try WatchTransportPayload.decode(dict)
+        }
+    }
+
+    @Test("A payload without a snapshot is rejected")
+    func rejectsMissingSnapshot() {
+        let dict: [String: Any] = [WatchTransportKey.schema: WatchTransportPayload.currentSchema]
+
+        #expect(throws: WatchTransportError.missingSnapshot) {
+            try WatchTransportPayload.decode(dict)
+        }
+    }
+}
diff --git a/CalendarrKit/Tests/CalendarrCoreTests/WatchTrimmingTests.swift b/CalendarrKit/Tests/CalendarrCoreTests/WatchTrimmingTests.swift
new file mode 100644
index 0000000..711a493
--- /dev/null
+++ b/CalendarrKit/Tests/CalendarrCoreTests/WatchTrimmingTests.swift
@@ -0,0 +1,118 @@
+import Foundation
+import Testing
+@testable import CalendarrCore
+
+/// The watch payload is a narrower snapshot, not a different one. These tests
+/// pin down the part that is easy to get wrong: the coverage window has to
+/// shrink with the events, or the watch renders an empty week it knows nothing
+/// about.
+struct WatchTrimmingTests {
+
+    private static let cal = Calendar(identifier: .gregorian)
+    private static let now = Date(timeIntervalSince1970: 1_789_000_000)
+
+    private static func event(_ id: String, dayOffset: Int, hour: Int = 9) -> SnapshotEvent {
+        let dayStart = cal.startOfDay(for: now)
+        let day = cal.date(byAdding: .day, value: dayOffset, to: dayStart)!
+        let start = cal.date(byAdding: .hour, value: hour, to: day)!
+        return SnapshotEvent(id: id, title: "Event \(id)",
+                             start: start, end: start.addingTimeInterval(3600),
+                             isAllDay: false, colorHex: "#4285f4",
+                             location: "", calendarKey: "local:1")
+    }
+
+    private static func snapshot(_ events: [SnapshotEvent]) -> CalendarrSnapshot {
+        let dayStart = cal.startOfDay(for: now)
+        return CalendarrSnapshot(
+            writtenAt: now,
+            coverageStart: cal.date(byAdding: .day, value: -SnapshotCoverage.daysBehind, to: dayStart)!,
+            coverageEnd: cal.date(byAdding: .day, value: SnapshotCoverage.daysAhead, to: dayStart)!,
+            isLoggedIn: true,
+            writerVersion: "1.0",
+            events: events,
+            theme: SnapshotTheme(today: "#1", text: "#2", background: "#3",
+                                 line: "#4", primary: "#5", accent: "#6"),
+            language: "de")
+    }
+
+    @Test("The window narrows to the watch policy, not just the events")
+    func narrowsWindow() {
+        let trimmed = Self.snapshot([]).trimmedForWatch(now: Self.now, calendar: Self.cal)
+        let dayStart = Self.cal.startOfDay(for: Self.now)
+
+        #expect(trimmed.coverageStart == Self.cal.date(byAdding: .day, value: -WatchCoverage.daysBehind, to: dayStart))
+        #expect(trimmed.coverageEnd == Self.cal.date(byAdding: .day, value: WatchCoverage.daysAhead, to: dayStart))
+    }
+
+    @Test("Events outside the watch window are dropped")
+    func dropsEventsOutsideWindow() {
+        let inside = Self.event("in", dayOffset: 3)
+        let outside = Self.event("out", dayOffset: 30)
+        let trimmed = Self.snapshot([inside, outside]).trimmedForWatch(now: Self.now, calendar: Self.cal)
+
+        #expect(trimmed.events.map(\.id) == ["in"])
+    }
+
+    @Test("An event still running at the window start is kept")
+    func keepsRunningEvent() {
+        let dayStart = Self.cal.startOfDay(for: Self.now)
+        let start = Self.cal.date(byAdding: .day, value: -5, to: dayStart)!
+        let running = SnapshotEvent(id: "running", title: "Urlaub",
+                                    start: start,
+                                    end: Self.cal.date(byAdding: .day, value: 5, to: dayStart)!,
+                                    isAllDay: true, colorHex: "#4285f4",
+                                    location: "", calendarKey: "local:1")
+        let trimmed = Self.snapshot([running]).trimmedForWatch(now: Self.now, calendar: Self.cal)
+
+        #expect(trimmed.events.map(\.id) == ["running"])
+    }
+
+    @Test("Metadata survives trimming unchanged")
+    func preservesMetadata() {
+        let original = Self.snapshot([Self.event("a", dayOffset: 1)])
+        let trimmed = original.trimmedForWatch(now: Self.now, calendar: Self.cal)
+
+        #expect(trimmed.language == "de")
+        #expect(trimmed.isLoggedIn == true)
+        #expect(trimmed.writerVersion == "1.0")
+        #expect(trimmed.theme.accent == "#6")
+        #expect(trimmed.writtenAt == original.writtenAt)
+        #expect(trimmed.schemaVersion == CalendarrSnapshot.currentSchema)
+    }
+
+    @Test("When the event cap truncates, coverageEnd pulls back to what we can prove")
+    func capPullsBackCoverage() {
+        var many: [SnapshotEvent] = []
+        for day in 0..<14 {
+            for hour in 8..<18 { many.append(Self.event("d\(day)h\(hour)", dayOffset: day, hour: hour)) }
+        }
+        #expect(many.count > WatchCoverage.maxEvents)
+
+        let trimmed = Self.snapshot(many).trimmedForWatch(now: Self.now, calendar: Self.cal)
+
+        #expect(trimmed.events.count <= WatchCoverage.maxEvents)
+        #expect(trimmed.events.allSatisfy { $0.start < trimmed.coverageEnd })
+        let dayStart = Self.cal.startOfDay(for: Self.now)
+        let fullEnd = Self.cal.date(byAdding: .day, value: WatchCoverage.daysAhead, to: dayStart)!
+        #expect(trimmed.coverageEnd < fullEnd)
+        #expect(trimmed.coverageEnd > trimmed.coverageStart)
+    }
+
+    @Test("Trimming never widens a window the writer already narrowed")
+    func neverWidens() {
+        let dayStart = Self.cal.startOfDay(for: Self.now)
+        let narrow = CalendarrSnapshot(
+            writtenAt: Self.now,
+            coverageStart: dayStart,
+            coverageEnd: Self.cal.date(byAdding: .day, value: 2, to: dayStart)!,
+            isLoggedIn: true, writerVersion: "1.0", events: [],
+            theme: SnapshotTheme(today: "#1", text: "#2", background: "#3",
+                                 line: "#4", primary: "#5", accent: "#6"),
+            language: "de")
+
+        let trimmed = narrow.trimmedForWatch(now: Self.now, calendar: Self.cal)
+
+        #expect(trimmed.coverageStart == dayStart)
+        #expect(trimmed.coverageEnd == Self.cal.date(byAdding: .day, value: 2, to: dayStart))
+    }
+}