diff --git a/agent/compaction/compaction_test.go b/agent/compaction/compaction_test.go
index d1525ae0..bb7abf59 100644
--- a/agent/compaction/compaction_test.go
+++ b/agent/compaction/compaction_test.go
@@ -696,3 +696,40 @@ func messageTexts(messages []*message.Message) []string {
 	}
 	return texts
 }
+
+func TestNewProvider_KeepsRetainedHistorySourceAcrossTurns(t *testing.T) {
+	// A compaction provider generates only summary messages; genuine prior-turn
+	// history returned from the persisted index must keep its original Source.
+	// Across turns the index is rebuilt from persisted groups whose message
+	// pointers differ from this turn's input, so identity-based attribution
+	// wrongly stamps real history as context-provider generated.
+	provider := compaction.NewContextProvider(compaction.ContextProviderConfig{
+		Strategy: &compaction.TruncationStrategy{Trigger: compaction.Never()},
+		SourceID: "compaction-test",
+	})
+	session := agenttest.CreateSession()
+
+	if _, _, err := invokeProvider(provider, t.Context(), []*message.Message{
+		textMessage(message.RoleUser, "u1"),
+		textMessage(message.RoleAssistant, "a1"),
+	}, agent.WithSession(session)); err != nil {
+		t.Fatalf("turn 1: %v", err)
+	}
+
+	out, _, err := invokeProvider(provider, t.Context(), []*message.Message{
+		textMessage(message.RoleUser, "u1"),
+		textMessage(message.RoleAssistant, "a1"),
+		textMessage(message.RoleUser, "u2"),
+		textMessage(message.RoleAssistant, "a2"),
+	}, agent.WithSession(session))
+	if err != nil {
+		t.Fatalf("turn 2: %v", err)
+	}
+
+	cp := message.Source{Type: agent.SourceTypeContextProvider, ID: "compaction-test"}
+	for i, msg := range out {
+		if msg.Source == cp {
+			t.Errorf("message %d (%q) mislabeled as context-provider generated; genuine history must keep its original Source", i, msg.String())
+		}
+	}
+}
diff --git a/agent/compaction/provider.go b/agent/compaction/provider.go
index 4e03c1f8..51825443 100644
--- a/agent/compaction/provider.go
+++ b/agent/compaction/provider.go
@@ -128,16 +128,17 @@ func (p *contextProvider) markGeneratedMessages(messages, inputMessages []*messa
 	if len(messages) == 0 {
 		return messages
 	}
-	originals := make(map[*message.Message]struct{}, len(inputMessages))
-	for _, msg := range inputMessages {
-		originals[msg] = struct{}{}
-	}
 	source := message.Source{Type: agent.SourceTypeContextProvider, ID: p.sourceID}
 	for i, msg := range messages {
-		if _, ok := originals[msg]; ok {
+		if msg == nil || msg.Source == source {
 			continue
 		}
-		if msg == nil || msg.Source == source {
+		// A message is provider-generated only when it is not one of this turn's
+		// input messages. Compare by content, not pointer identity: with a session
+		// the index is rebuilt from persisted groups whose message pointers differ
+		// from the incoming messages, so an identity check would wrongly stamp
+		// genuine prior-turn history as context-provider generated.
+		if containsMessageByContent(inputMessages, msg) {
 			continue
 		}
 		marked := msg.Clone()
@@ -146,3 +147,12 @@ func (p *contextProvider) markGeneratedMessages(messages, inputMessages []*messa
 	}
 	return messages
 }
+
+func containsMessageByContent(messages []*message.Message, target *message.Message) bool {
+	for _, candidate := range messages {
+		if messageContentEqual(candidate, target) {
+			return true
+		}
+	}
+	return false
+}