Avoid intermediate conversation history copies

docs/internal/runner.md

@@ -32,7 +32,8 @@ The runner has no durable run or session store.
 
 Captured exchanges are immutable parent-linked domain values. Each holds only
 its new rendered suffix, final assistant message, and run identities; flattening
-iteratively makes a detached history for preparation or inspection. Preparation
+iteratively fills a single detached message buffer for preparation or projects
+directly into public inspection values. Preparation
 prepends this history to newly rendered definition messages and appended caller
 messages before computing the rendered hash. Follow-up selection and credentials
 remain those of the new request. Prepared execution retains capture and parent

docs/roadmap/remediation.md

@@ -91,7 +91,12 @@ does not require changing runtime behavior or duplicating existing tests.
 
 ## 2. Avoid Intermediate History Copies During Assembly And Inspection
 
-**Priority:** P3. **Status:** For consideration. **Plan stages:** 2 and 5.
+**Priority:** P3. **Status:** Complete. **Plan stages:** 2 and 5.
+
+**Completion:** Preparation now fills one history-plus-suffix buffer. Inspection
+projects indexed history directly into public messages, cloning exposed cache
+metadata once. The parent-linked representation and public types are unchanged.
+Focused race-enabled history, continuation, preparation, and ownership tests passed.
 
 **Locations:** `completePreparationWithStructuredOutput` in
 [the runner](../../internal/usecase/runner.go), `Exchange.Messages` in

exchange.go

@@ -33,7 +33,11 @@ func (e *Exchange) Details() ExchangeDetails {
 	if e == nil || e.internal == nil {
 		return ExchangeDetails{}
 	}
-	return ExchangeDetails{RunID: e.internal.RunID(), ParentRunID: e.internal.ParentRunID(), Messages: fromDomainRenderedMessages(e.internal.Messages())}
+	messages := make([]RenderedMessage, e.internal.MessageCount())
+	e.internal.VisitMessages(func(index int, message domain.RenderedMessage) {
+		messages[index] = RenderedMessage{Role: message.Role, Content: message.Content, CacheControl: fromDomainCacheControl(message.CacheControl)}
+	})
+	return ExchangeDetails{RunID: e.internal.RunID(), ParentRunID: e.internal.ParentRunID(), Messages: messages}
 }
 
 // String returns an opaque representation without conversation content.

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