Compose appended request messages

internal/usecase/runner.go

@@ -4,10 +4,13 @@ import (
 	"context"
 	"crypto/rand"
 	"crypto/sha256"
+	"encoding/binary"
 	"encoding/hex"
 	"encoding/json"
 	"errors"
 	"fmt"
+	"hash"
+	"io"
 	"os"
 	"strings"
 	"time"
@@ -444,9 +447,11 @@ func (r *Runner) completePreparationWithStructuredOutput(
 	if err != nil {
 		return nil, fmt.Errorf("%w: %w", ErrPromptRender, err)
 	}
+	effectivePrompt := *renderedPrompt
 	if state.directSessionID != "" {
-		renderedPrompt.SessionID = state.directSessionID
+		effectivePrompt.SessionID = state.directSessionID
 	}
+	effectivePrompt.Messages = domain.ConcatRenderedMessages(renderedPrompt.Messages, req.AppendedMessages)
 
 	end := time.Now().UTC()
 	effectiveModel := state.effectiveModel
@@ -462,9 +467,9 @@ func (r *Runner) completePreparationWithStructuredOutput(
 		OutputContract:       state.effectiveContract,
 		StructuredOutput:     structuredOutput,
 		InputHashes:          inputHashes,
-		SessionID:            renderedPrompt.SessionID,
-		RenderedPromptHash:   hashRenderedPrompt(*renderedPrompt),
-		Messages:             renderedPrompt.Messages,
+		SessionID:            effectivePrompt.SessionID,
+		RenderedPromptHash:   hashRenderedPrompt(effectivePrompt),
+		Messages:             effectivePrompt.Messages,
 		StartTime:            state.start,
 		EndTime:              end,
 		DurationMS:           end.Sub(state.start).Milliseconds(),
@@ -714,29 +719,38 @@ func resolveOutputContract(def *domain.PromptDefinition, override *domain.Output
 	return contract, nil
 }
 
+const renderedPromptHashVersion = "promptkit/rendered-prompt/v2\x00"
+
 func hashRenderedPrompt(p domain.RenderedPrompt) string {
-	var b strings.Builder
-	if p.SessionID != "" {
-		b.WriteString("session_id=")
-		b.WriteString(p.SessionID)
-		b.WriteString("\n---\n")
-	}
-	for _, msg := range p.Messages {
-		b.WriteString(msg.Role)
-		b.WriteByte('\n')
-		b.WriteString(msg.Content)
-		if msg.CacheControl != nil {
-			b.WriteString("\ncache_control.type=")
-			b.WriteString(string(msg.CacheControl.Type))
-			if msg.CacheControl.TTL != "" {
-				b.WriteString("\ncache_control.ttl=")
-				b.WriteString(msg.CacheControl.TTL)
-			}
+	hasher := sha256.New()
+	var lengthBuffer [8]byte
+	_, _ = io.WriteString(hasher, renderedPromptHashVersion)
+	writeRenderedPromptHashString(hasher, &lengthBuffer, p.SessionID)
+	writeRenderedPromptHashLength(hasher, &lengthBuffer, len(p.Messages))
+	for _, message := range p.Messages {
+		writeRenderedPromptHashString(hasher, &lengthBuffer, message.Role)
+		writeRenderedPromptHashString(hasher, &lengthBuffer, message.Content)
+		if message.CacheControl == nil {
+			lengthBuffer[0] = 0
+			_, _ = hasher.Write(lengthBuffer[:1])
+			continue
 		}
-		b.WriteString("\n---\n")
+		lengthBuffer[0] = 1
+		_, _ = hasher.Write(lengthBuffer[:1])
+		writeRenderedPromptHashString(hasher, &lengthBuffer, string(message.CacheControl.Type))
+		writeRenderedPromptHashString(hasher, &lengthBuffer, message.CacheControl.TTL)
 	}
-	h := sha256.Sum256([]byte(b.String()))
-	return hex.EncodeToString(h[:])
+	return hex.EncodeToString(hasher.Sum(nil))
+}
+
+func writeRenderedPromptHashLength(hasher hash.Hash, buffer *[8]byte, length int) {
+	binary.BigEndian.PutUint64(buffer[:], uint64(length))
+	_, _ = hasher.Write(buffer[:])
+}
+
+func writeRenderedPromptHashString(hasher hash.Hash, buffer *[8]byte, value string) {
+	writeRenderedPromptHashLength(hasher, buffer, len(value))
+	_, _ = io.WriteString(hasher, value)
 }
 
 func buildOutputArtifact(content string, format domain.OutputFormat) domain.Artifact {