Avoid intermediate conversation history copies

internal/domain/exchange.go

@@ -51,10 +51,42 @@ func (e *Exchange) Messages() []RenderedMessage {
 	if e == nil {
 		return nil
 	}
-	messages := make([]RenderedMessage, e.count)
-	for current := e; current != nil; current = current.parent {
-		start := current.count - len(current.messages)
-		copyRenderedMessages(messages[start:current.count], current.messages)
+	return e.MessagesWithSuffix()
+}
+
+// MessagesWithSuffix returns independently owned history followed by each part.
+// A nil exchange supplies no history.
+func (e *Exchange) MessagesWithSuffix(parts ...[]RenderedMessage) []RenderedMessage {
+	count := e.MessageCount()
+	for _, part := range parts {
+		count += len(part)
+	}
+	messages := make([]RenderedMessage, count)
+	e.visitChunks(func(start int, chunk []RenderedMessage) {
+		copyRenderedMessages(messages[start:], chunk)
+	})
+	offset := e.MessageCount()
+	for _, part := range parts {
+		copyRenderedMessages(messages[offset:], part)
+		offset += len(part)
 	}
 	return messages
 }
+
+// VisitMessages visits indexed history entries without materializing a slice.
+// Entries and their cache metadata are borrowed read-only values. Visits need
+// not be in index order; callers can fill a destination using the supplied index.
+func (e *Exchange) VisitMessages(visit func(int, RenderedMessage)) {
+	e.visitChunks(func(start int, chunk []RenderedMessage) {
+		for index, message := range chunk {
+			visit(start+index, message)
+		}
+	})
+}
+
+func (e *Exchange) visitChunks(visit func(int, []RenderedMessage)) {
+	for current := e; current != nil; current = current.parent {
+		start := current.count - len(current.messages)
+		visit(start, current.messages)
+	}
+}

internal/domain/exchange_test.go

@@ -51,3 +51,25 @@ func TestExchangeManyTurns(t *testing.T) {
 		}
 	}
 }
+
+func TestExchangeMessagesWithSuffix(t *testing.T) {
+	for _, history := range []*Exchange{nil, CaptureExchange(nil, "source", nil, "answer")} {
+		control := &CacheControl{Type: CacheControlEphemeral, TTL: "1h"}
+		first := []RenderedMessage{{Role: RoleUser, Content: "new", CacheControl: control}}
+		last := []RenderedMessage{{Role: RoleDeveloper, Content: "suffix"}}
+		got := history.MessagesWithSuffix(first, nil, last)
+		want := append(history.Messages(), first...)
+		want = append(want, last...)
+		if !reflect.DeepEqual(got, want) {
+			t.Fatalf("messages = %#v, want %#v", got, want)
+		}
+		got[history.MessageCount()].CacheControl.TTL = "changed"
+		got[history.MessageCount()].Content = "changed"
+		if first[0].Content != "new" || control.TTL != "1h" {
+			t.Fatal("assembled messages alias the caller suffix")
+		}
+		if history != nil && history.Messages()[0].Content != "answer" {
+			t.Fatal("assembly changed retained history")
+		}
+	}
+}

internal/usecase/runner.go

@@ -482,10 +482,7 @@ func (r *Runner) completePreparationWithStructuredOutput(
 	if state.directSessionID != "" {
 		effectivePrompt.SessionID = state.directSessionID
 	}
-	effectivePrompt.Messages = domain.ConcatRenderedMessages(renderedPrompt.Messages, req.AppendedMessages)
-	if req.ContinueFrom != nil {
-		effectivePrompt.Messages = domain.ConcatRenderedMessages(req.ContinueFrom.Messages(), effectivePrompt.Messages)
-	}
+	effectivePrompt.Messages = req.ContinueFrom.MessagesWithSuffix(renderedPrompt.Messages, req.AppendedMessages)
 
 	end := time.Now().UTC()
 	effectiveModel := state.effectiveModel