diff --git a/testing/fixtures/build.gradle b/testing/fixtures/build.gradle
index 57f8ed25..5dc6c86e 100644
--- a/testing/fixtures/build.gradle
+++ b/testing/fixtures/build.gradle
@@ -14,5 +14,6 @@ dependencies {
     implementation(project(":core:context"))
     implementation(project(":core:kernel"))
     implementation(project(":core:validation"))
+    implementation(project(":infrastructure:persistence"))
 }
 tasks.named("koverVerify").configure { enabled = false }
diff --git a/testing/fixtures/src/main/kotlin/com/correx/testing/fixtures/DeterministicHarness.kt b/testing/fixtures/src/main/kotlin/com/correx/testing/fixtures/DeterministicHarness.kt
new file mode 100644
index 00000000..0b5ea673
--- /dev/null
+++ b/testing/fixtures/src/main/kotlin/com/correx/testing/fixtures/DeterministicHarness.kt
@@ -0,0 +1,41 @@
+package com.correx.testing.fixtures
+
+import com.correx.core.events.events.EventMetadata
+import com.correx.core.events.events.EventPayload
+import com.correx.core.events.events.NewEvent
+import com.correx.core.events.types.EventId
+import com.correx.core.events.types.SessionId
+import com.correx.infrastructure.persistence.InMemoryEventStore
+import com.correx.core.sessions.projections.Projection
+import com.correx.core.sessions.projections.replay.DefaultEventReplayer
+import kotlinx.datetime.Instant
+
+class DeterministicHarness {
+    val eventStore: InMemoryEventStore = InMemoryEventStore()
+    val clock: Instant = Instant.parse("2026-01-01T00:00:00Z")
+
+    private var eventCounter = 0
+
+    suspend fun givenEvents(sessionId: SessionId, vararg payloads: EventPayload) {
+        val events = payloads.map { payload ->
+            val eventId = EventId("event-${eventCounter++}")
+            NewEvent(
+                metadata = EventMetadata(
+                    eventId = eventId,
+                    sessionId = sessionId,
+                    timestamp = clock,
+                    schemaVersion = 1,
+                    causationId = null,
+                    correlationId = null
+                ),
+                payload = payload
+            )
+        }
+        eventStore.appendAll(events)
+    }
+
+    fun <S> rebuild(sessionId: SessionId, projection: Projection<S>): S {
+        val replayer = DefaultEventReplayer(eventStore, projection)
+        return replayer.rebuild(sessionId)
+    }
+}
diff --git a/testing/replay/src/test/kotlin/DeterministicHarnessTest.kt b/testing/replay/src/test/kotlin/DeterministicHarnessTest.kt
new file mode 100644
index 00000000..58cfeb38
--- /dev/null
+++ b/testing/replay/src/test/kotlin/DeterministicHarnessTest.kt
@@ -0,0 +1,77 @@
+import com.correx.core.events.events.ChatSessionStartedEvent
+import com.correx.core.events.events.StoredEvent
+import com.correx.core.events.events.ToolInvokedEvent
+import com.correx.core.events.serialization.eventJson
+import com.correx.core.events.types.SessionId
+import com.correx.core.sessions.projections.Projection
+import com.correx.testing.fixtures.DeterministicHarness
+import kotlinx.coroutines.runBlocking
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+
+class DeterministicHarnessTest {
+
+    private data class SimpleState(val eventCount: Int = 0)
+
+    private val simpleProjection = object : Projection<SimpleState> {
+        override fun initial() = SimpleState()
+        override fun apply(state: SimpleState, event: StoredEvent) =
+            state.copy(eventCount = state.eventCount + 1)
+    }
+
+    @Test
+    fun `two harnesses with same payloads yield identical events`() = runBlocking {
+        val harness1 = DeterministicHarness()
+        val harness2 = DeterministicHarness()
+
+        val sessionId = SessionId("test-session")
+        val payload1 = ChatSessionStartedEvent(sessionId)
+        val payload2 = ToolInvokedEvent("read_file")
+
+        harness1.givenEvents(sessionId, payload1, payload2)
+        harness2.givenEvents(sessionId, payload1, payload2)
+
+        val events1 = harness1.eventStore.read(sessionId)
+        val events2 = harness2.eventStore.read(sessionId)
+
+        assertEquals(events1.size, events2.size)
+        events1.zip(events2).forEach { (e1, e2) ->
+            assertEquals(e1.metadata.eventId, e2.metadata.eventId, "eventIds must match")
+            assertEquals(e1.metadata.timestamp, e2.metadata.timestamp, "timestamps must match")
+            assertEquals(e1.sequence, e2.sequence, "sequences must match")
+            assertEquals(e1.sessionSequence, e2.sessionSequence, "sessionSequences must match")
+            assertEquals(
+                eventJson.encodeToString(StoredEvent.serializer(), e1),
+                eventJson.encodeToString(StoredEvent.serializer(), e2),
+                "stored event JSON must be byte-identical",
+            )
+        }
+    }
+
+    @Test
+    fun `rebuild applies events deterministically`() = runBlocking {
+        val harness = DeterministicHarness()
+        val sessionId = SessionId("rebuild-test")
+
+        harness.givenEvents(
+            sessionId,
+            ChatSessionStartedEvent(sessionId),
+            ToolInvokedEvent("read_file"),
+            ToolInvokedEvent("write_file")
+        )
+
+        val state = harness.rebuild(sessionId, simpleProjection)
+        assertEquals(3, state.eventCount)
+    }
+
+    @Test
+    fun `clock is fixed for deterministic timestamps`(): Unit = runBlocking {
+        val harness = DeterministicHarness()
+        val sessionId = SessionId("clock-test")
+
+        harness.givenEvents(sessionId, ChatSessionStartedEvent(sessionId))
+
+        val event = harness.eventStore.read(sessionId).first()
+        assertEquals("2026-01-01T00:00:00Z", event.metadata.timestamp.toString())
+    }
+}