Locate compaction prefix boundary positionally to avoid dropping messages

agent/compaction/index.go

@@ -78,16 +78,23 @@ func (index *MessageIndex) Update(messages []*message.Message) {
 		return
 	}
 	processedMessageCount := index.includedRawMessageCount()
-	if index.lastProcessedMessage != nil && len(messages) >= processedMessageCount && messageContentEqual(messages[len(messages)-1], index.lastProcessedMessage) {
-		return
-	}
-
+	// Locate where the already-processed prefix ends. lastProcessedMessage sits
+	// at index processedMessageCount-1 when nothing earlier was trimmed, so check
+	// that expected position first. Matching only the last occurrence of its
+	// content would latch the boundary onto a later duplicate when a message is
+	// repeated (e.g. a user re-sending "continue"), silently dropping the turns
+	// appended since. Fall back to a search only when the prefix was trimmed.
 	foundIndex := -1
 	if index.lastProcessedMessage != nil {
-		for i := len(messages) - 1; i >= 0; i-- {
-			if messageContentEqual(messages[i], index.lastProcessedMessage) {
-				foundIndex = i
-				break
+		if expected := processedMessageCount - 1; expected >= 0 && expected < len(messages) &&
+			messageContentEqual(messages[expected], index.lastProcessedMessage) {
+			foundIndex = expected
+		} else {
+			for i := len(messages) - 1; i >= 0; i-- {
+				if messageContentEqual(messages[i], index.lastProcessedMessage) {
+					foundIndex = i
+					break
+				}
 			}
 		}
 	}

agent/compaction/index_test.go

@@ -343,3 +343,30 @@ func splitWords(text string) func(func(string) bool) {
 		}
 	}
 }
+
+// A user re-sending an identical short message (or any repeated content) must
+// not cause Update to drop the turns appended since the previous update. The
+// processed-prefix boundary must be located positionally, not by matching the
+// last occurrence of the previous message's content.
+func TestMessageIndex_Update_RepeatedContentDoesNotDropMessages(t *testing.T) {
+	turn1 := []*message.Message{
+		textMessage(message.RoleUser, "hi"),
+		textMessage(message.RoleAssistant, "hello"),
+		textMessage(message.RoleUser, "continue"),
+	}
+	index := &compaction.MessageIndex{}
+	index.Update(turn1)
+	if got, want := messageTexts(index.AllMessages()), []string{"hi", "hello", "continue"}; !slices.Equal(got, want) {
+		t.Fatalf("after turn 1: got %v, want %v", got, want)
+	}
+
+	// Turn 2: the assistant replies, then the user repeats "continue" verbatim.
+	turn2 := append(slices.Clone(turn1),
+		textMessage(message.RoleAssistant, "sure"),
+		textMessage(message.RoleUser, "continue"),
+	)
+	index.Update(turn2)
+	if got, want := messageTexts(index.AllMessages()), []string{"hi", "hello", "continue", "sure", "continue"}; !slices.Equal(got, want) {
+		t.Fatalf("after turn 2 (repeated \"continue\"): got %v, want %v — appended turns were dropped", got, want)
+	}
+}