diff --git a/app/src/test/java/de/jeanlucmakiola/clockula/testing/FakeRepositories.kt b/app/src/test/java/de/jeanlucmakiola/clockula/testing/FakeRepositories.kt
new file mode 100644
index 0000000..375b11d
--- /dev/null
+++ b/app/src/test/java/de/jeanlucmakiola/clockula/testing/FakeRepositories.kt
@@ -0,0 +1,175 @@
+package de.jeanlucmakiola.clockula.testing
+
+import de.jeanlucmakiola.clockula.data.stopwatch.StopwatchRepository
+import de.jeanlucmakiola.clockula.data.timers.TimerRepository
+import de.jeanlucmakiola.clockula.domain.Lap
+import de.jeanlucmakiola.clockula.domain.StopwatchRun
+import de.jeanlucmakiola.clockula.domain.StopwatchState
+import de.jeanlucmakiola.clockula.domain.Timer
+import de.jeanlucmakiola.clockula.domain.TimerDraft
+import de.jeanlucmakiola.clockula.domain.TimerState
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.update
+import kotlin.time.Duration
+
+/**
+ * A working [TimerRepository] over a [MutableStateFlow]: a write re-emits on the
+ * observing flow exactly as Room's would, and every mutating call is logged so
+ * "the pill stopped it rather than deleting it" is assertable.
+ *
+ * It has no clock, so the anchors it writes are deliberately absent — a test
+ * that cares about them hands the rows over with [set] instead.
+ */
+class FakeTimerRepository(initial: List<Timer> = emptyList()) : TimerRepository {
+
+    private val rows = MutableStateFlow(initial)
+    private val log = mutableListOf<String>()
+
+    /** Every mutating call, in order: "pause(3)", "start(3)", "reset(3)", … */
+    val calls: List<String> get() = log.toList()
+
+    fun set(timers: List<Timer>) {
+        rows.value = timers
+    }
+
+    override fun timers(): Flow<List<Timer>> = rows
+
+    override fun timer(id: Long): Flow<Timer?> = rows.map { all -> all.firstOrNull { it.id == id } }
+
+    override suspend fun find(id: Long): Timer? = rows.value.firstOrNull { it.id == id }
+
+    override suspend fun create(draft: TimerDraft): Long {
+        val id = (rows.value.maxOfOrNull { it.id } ?: 0L) + 1L
+        log += "create($id)"
+        rows.update {
+            it + runningTimer().copy(
+                id = id,
+                label = draft.label,
+                duration = draft.duration,
+                state = TimerState.IDLE,
+                remaining = draft.duration,
+                startedAtElapsedRealtime = null,
+                endsAtElapsedRealtime = null,
+                endsAtWallClock = null,
+                ringtoneUri = draft.ringtoneUri,
+                sortOrder = it.size,
+            )
+        }
+        return id
+    }
+
+    override suspend fun rename(id: Long, label: String) {
+        log += "rename($id)"
+        mutate(id) { it.copy(label = label) }
+    }
+
+    override suspend fun delete(id: Long) {
+        log += "delete($id)"
+        rows.update { all -> all.filterNot { it.id == id } }
+    }
+
+    override suspend fun start(id: Long) {
+        log += "start($id)"
+        mutate(id) { it.copy(state = TimerState.RUNNING) }
+    }
+
+    override suspend fun pause(id: Long) {
+        log += "pause($id)"
+        mutate(id) {
+            it.copy(
+                state = TimerState.PAUSED,
+                startedAtElapsedRealtime = null,
+                endsAtElapsedRealtime = null,
+                endsAtWallClock = null,
+            )
+        }
+    }
+
+    override suspend fun reset(id: Long) {
+        log += "reset($id)"
+        mutate(id) {
+            it.copy(
+                state = TimerState.IDLE,
+                remaining = it.duration,
+                startedAtElapsedRealtime = null,
+                endsAtElapsedRealtime = null,
+                endsAtWallClock = null,
+            )
+        }
+    }
+
+    override suspend fun addTime(id: Long, extra: Duration) {
+        log += "addTime($id)"
+        mutate(id) { it.copy(remaining = it.remaining + extra) }
+    }
+
+    override suspend fun markExpired(id: Long) {
+        log += "markExpired($id)"
+        mutate(id) { it.copy(state = TimerState.EXPIRED, remaining = Duration.ZERO) }
+    }
+
+    override suspend fun reorder(idsInOrder: List<Long>) {
+        log += "reorder(${idsInOrder.joinToString(",")})"
+        rows.update { all -> all.map { it.copy(sortOrder = idsInOrder.indexOf(it.id)) } }
+    }
+
+    override suspend fun repairAfterReboot() {
+        log += "repairAfterReboot()"
+    }
+
+    private fun mutate(id: Long, change: (Timer) -> Timer) {
+        rows.update { all -> all.map { if (it.id == id) change(it) else it } }
+    }
+}
+
+/** A working [StopwatchRepository] over a [MutableStateFlow], with the same call log. */
+class FakeStopwatchRepository(initial: StopwatchRun = StopwatchRun()) : StopwatchRepository {
+
+    private val state = MutableStateFlow(initial)
+    private val lapRows = MutableStateFlow(emptyList<Lap>())
+    private val log = mutableListOf<String>()
+
+    val calls: List<String> get() = log.toList()
+
+    fun set(run: StopwatchRun) {
+        state.value = run
+    }
+
+    override fun run(): Flow<StopwatchRun> = state
+
+    override fun laps(): Flow<List<Lap>> = lapRows
+
+    override suspend fun currentRun(): StopwatchRun = state.value
+
+    override suspend fun start() {
+        log += "start()"
+        state.update { it.copy(state = StopwatchState.RUNNING) }
+    }
+
+    override suspend fun pause() {
+        log += "pause()"
+        state.update {
+            it.copy(state = StopwatchState.PAUSED, startedAtElapsedRealtime = null)
+        }
+    }
+
+    override suspend fun reset() {
+        log += "reset()"
+        state.value = StopwatchRun()
+        lapRows.value = emptyList()
+    }
+
+    override suspend fun lap(): Lap? {
+        log += "lap()"
+        if (state.value.state != StopwatchState.RUNNING) return null
+        val lap = Lap(lapRows.value.size + 1, Duration.ZERO, state.value.accumulated)
+        lapRows.update { it + lap }
+        return lap
+    }
+
+    override suspend fun pauseAfterReboot() {
+        log += "pauseAfterReboot()"
+    }
+}
diff --git a/app/src/test/java/de/jeanlucmakiola/clockula/testing/FakeTicker.kt b/app/src/test/java/de/jeanlucmakiola/clockula/testing/FakeTicker.kt
new file mode 100644
index 0000000..cbc475b
--- /dev/null
+++ b/app/src/test/java/de/jeanlucmakiola/clockula/testing/FakeTicker.kt
@@ -0,0 +1,32 @@
+package de.jeanlucmakiola.clockula.testing
+
+import de.jeanlucmakiola.clockula.domain.time.Ticker
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.MutableSharedFlow
+import kotlin.time.Duration
+
+/**
+ * A [Ticker] the test drives by hand. Like `RealTicker` it replays one emission
+ * to a fresh collector, so a subscriber is never blank for a cadence.
+ */
+class FakeTicker : Ticker {
+
+    private val streams = mutableListOf<MutableSharedFlow<Unit>>()
+    private val periods = mutableListOf<Duration>()
+
+    /** Every period asked for, in order, so a test can pin the cadence. */
+    val requestedPeriods: List<Duration> get() = periods.toList()
+
+    override fun ticks(period: Duration): Flow<Unit> {
+        periods += period
+        val stream = MutableSharedFlow<Unit>(replay = 1, extraBufferCapacity = 64)
+        check(stream.tryEmit(Unit))
+        streams += stream
+        return stream
+    }
+
+    /** One tick to every live collector. */
+    suspend fun tick() {
+        streams.toList().forEach { it.emit(Unit) }
+    }
+}
diff --git a/app/src/test/java/de/jeanlucmakiola/clockula/testing/MainDispatcherExtension.kt b/app/src/test/java/de/jeanlucmakiola/clockula/testing/MainDispatcherExtension.kt
new file mode 100644
index 0000000..c1cd594
--- /dev/null
+++ b/app/src/test/java/de/jeanlucmakiola/clockula/testing/MainDispatcherExtension.kt
@@ -0,0 +1,29 @@
+package de.jeanlucmakiola.clockula.testing
+
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.test.TestDispatcher
+import kotlinx.coroutines.test.UnconfinedTestDispatcher
+import kotlinx.coroutines.test.resetMain
+import kotlinx.coroutines.test.setMain
+import org.junit.jupiter.api.extension.AfterEachCallback
+import org.junit.jupiter.api.extension.BeforeEachCallback
+import org.junit.jupiter.api.extension.ExtensionContext
+
+/**
+ * `Dispatchers.setMain` for JUnit 5 — twenty lines instead of a test framework
+ * (slice plan D17). A ViewModel's `viewModelScope` lives on Main, so the tests
+ * that exercise one hand it a [TestDispatcher] and share its scheduler with
+ * `runTest`.
+ */
+class MainDispatcherExtension(
+    val dispatcher: TestDispatcher = UnconfinedTestDispatcher(),
+) : BeforeEachCallback, AfterEachCallback {
+
+    override fun beforeEach(context: ExtensionContext) {
+        Dispatchers.setMain(dispatcher)
+    }
+
+    override fun afterEach(context: ExtensionContext) {
+        Dispatchers.resetMain()
+    }
+}