diff --git a/client/src/main/kotlin/io/flamenet/client/Conversations.kt b/client/src/main/kotlin/io/flamenet/client/Conversations.kt
index 6da7c89..7e7df24 100644
--- a/client/src/main/kotlin/io/flamenet/client/Conversations.kt
+++ b/client/src/main/kotlin/io/flamenet/client/Conversations.kt
@@ -535,6 +535,9 @@ class Conversations(
      * client re-fetch and re-fail it on every poll, for ever.
      */
     fun poll(limit: Int = 100): List<Received> {
+        // S4: keep auditing the log we depend on. Self-throttled (one head an
+        // hour) and best-effort — it only acts on a proven fork.
+        trust.verifyLogConsistencyIfDue()
         val incoming = relay.pollInbox(identity.deviceId, vault.inboxWatermark, limit)
         val out = mutableListOf<Received>()
 
diff --git a/client/src/main/kotlin/io/flamenet/client/Trust.kt b/client/src/main/kotlin/io/flamenet/client/Trust.kt
index f90fbd0..81d6873 100644
--- a/client/src/main/kotlin/io/flamenet/client/Trust.kt
+++ b/client/src/main/kotlin/io/flamenet/client/Trust.kt
@@ -34,6 +34,28 @@ enum class IdentityTrust {
 }
 
 /** The relay showed two different logs, or a log that shrank. */
+/** Turn a key-log entry into a sentence — S4's "name what changed". Pure. */
+object KeyChangeDescription {
+    fun describe(kind: String, atIso: String?): String {
+        val what = when (kind) {
+            "publish" -> "A new device key was added"
+            "expire"  -> "A device key expired"
+            "revoke"  -> "A device key was revoked"
+            else      -> "Their key set changed"
+        }
+        val date = atIso?.let {
+            try {
+                val inst = java.time.Instant.parse(it)
+                java.time.format.DateTimeFormatter
+                    .ofLocalizedDateTime(java.time.format.FormatStyle.MEDIUM)
+                    .withZone(java.time.ZoneId.systemDefault())
+                    .format(inst)
+            } catch (_: Exception) { null }
+        }
+        return if (date != null) "$what on $date." else "$what."
+    }
+}
+
 class KeyLogForkException(detail: String) :
     RelayException("the relay's key log does not add up: $detail")
 
@@ -403,6 +425,37 @@ class Trust(
         if (!ok) forkDetected = true
     }
 
+    /** When the scheduled consistency sweep last succeeded, epoch millis. */
+    @Volatile private var lastLogCheck: Long = 0
+    private val logCheckIntervalMs = 3_600_000L
+
+    /**
+     * Verify the log is still an append-only extension of our pin, on a
+     * schedule rather than only when a session is being established (S4).
+     *
+     * This is what makes transparency act instead of merely record: a relay
+     * that quietly forked or rewound its log between two bundle fetches would
+     * otherwise go unseen — for an established conversation, possibly for ever.
+     * Best effort on the network (a failed fetch is retried next cycle); a
+     * proven fork raises [forkDetected], the standing alarm. Call from the
+     * poll loop; it self-throttles.
+     */
+    fun verifyLogConsistencyIfDue(now: Long = System.currentTimeMillis()) {
+        val pin = vault.loadKeyLogPin() ?: return       // nothing pinned to extend
+        if (now - lastLogCheck < logCheckIntervalMs) return
+        val head = try { head() } catch (_: Exception) { return }   // network, not evidence
+        val ok = try {
+            when {
+                pin.size > head.size -> false                        // shrank
+                pin.size == head.size -> pin.root == head.root       // same size, same root
+                else -> consistent(pin.size, head.size, pin.root, head.root)
+            }
+        } catch (_: Exception) { return }
+        if (!ok) { forkDetected = true; return }
+        vault.saveKeyLogPin(Vault.KeyLogPin(head.size, head.root))
+        lastLogCheck = now
+    }
+
     // -- Reading the relay's log ---------------------------------------------
 
     private data class Head(val size: Int, val root: String, val audience: String)
diff --git a/client/src/test/kotlin/io/flamenet/client/KeyChangeDescriptionTest.kt b/client/src/test/kotlin/io/flamenet/client/KeyChangeDescriptionTest.kt
new file mode 100644
index 0000000..fbd1fa9
--- /dev/null
+++ b/client/src/test/kotlin/io/flamenet/client/KeyChangeDescriptionTest.kt
@@ -0,0 +1,29 @@
+package io.flamenet.client
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+/** S4's "name what changed" formatter, pinned so the sentence can't drift. */
+class KeyChangeDescriptionTest {
+
+    @Test fun `each kind names itself`() {
+        assertTrue(KeyChangeDescription.describe("publish", null).startsWith("A new device key was added"))
+        assertTrue(KeyChangeDescription.describe("expire", null).startsWith("A device key expired"))
+        assertTrue(KeyChangeDescription.describe("revoke", null).startsWith("A device key was revoked"))
+    }
+
+    @Test fun `an unknown kind still reads as a change`() {
+        assertEquals("Their key set changed.", KeyChangeDescription.describe("wat", null))
+    }
+
+    @Test fun `a timestamp becomes a when clause`() {
+        val s = KeyChangeDescription.describe("publish", "2026-08-29T12:00:00Z")
+        assertTrue(s.contains(" on "), "expected a 'on <date>' clause, got: $s")
+        assertTrue(s.endsWith("."))
+    }
+
+    @Test fun `a bad timestamp degrades to no clause`() {
+        assertEquals("A new device key was added.", KeyChangeDescription.describe("publish", "not-a-date"))
+    }
+}