Use local builders for rendered templates

internal/prompt/go_renderer.go

@@ -1,7 +1,6 @@
 package prompt
 
 import (
-	"bytes"
 	"context"
 	"errors"
 	"fmt"
@@ -126,7 +125,7 @@ func (r *goRenderer) Render(ctx context.Context, definition *domain.PromptDefini
 		if err := ctx.Err(); err != nil {
 			return nil, err
 		}
-		var buf bytes.Buffer
+		var buf strings.Builder
 		executeErr := tmpl.Execute(&buf, vars)
 		if err := ctx.Err(); err != nil {
 			return nil, err
@@ -235,7 +234,7 @@ func executeSessionTemplate(ctx context.Context, tmpl *template.Template, vars m
 	if err := ctx.Err(); err != nil {
 		return "", err
 	}
-	var buf bytes.Buffer
+	var buf strings.Builder
 	executeErr := tmpl.Execute(&buf, vars)
 	if err := ctx.Err(); err != nil {
 		return "", err

internal/prompt/renderer_test.go

@@ -4,8 +4,11 @@ import (
 	"bytes"
 	"context"
 	"errors"
+	"fmt"
 	"strings"
+	"sync"
 	"testing"
+	"text/template"
 
 	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
 )
@@ -501,6 +504,75 @@ func TestGoRendererArtifactTextLifecycle(t *testing.T) {
 	}
 }
 
+func TestCompiledRendererKeepsConcurrentRequestsIndependent(t *testing.T) {
+	const (
+		promptID = "concurrent-render"
+		version  = "v1"
+	)
+	sessionTemplate, err := CompileTemplate("session", "session-{{.request}}")
+	if err != nil {
+		t.Fatalf("compile session template: %v", err)
+	}
+	messageTemplate, err := CompileTemplate("message", `request={{.request}};document={{input "document"}}`)
+	if err != nil {
+		t.Fatalf("compile message template: %v", err)
+	}
+	renderer := NewCompiledRenderer(compiledRendererTestLookup{
+		id:      promptID,
+		version: version,
+		templates: CompiledTemplates{
+			Session:  sessionTemplate,
+			Messages: []*template.Template{messageTemplate},
+		},
+	})
+	definition := &domain.PromptDefinition{
+		ID: promptID, Version: version, SessionID: "session-{{.request}}",
+		Inputs:    []domain.PromptInput{{Name: "document"}},
+		Templates: []domain.PromptMessageTemplate{{Role: "user", Content: `request={{.request}};document={{input "document"}}`}},
+	}
+
+	const requestCount = 24
+	errs := make(chan error, requestCount)
+	var wait sync.WaitGroup
+	wait.Add(requestCount)
+	for i := 0; i < requestCount; i++ {
+		go func(i int) {
+			defer wait.Done()
+			request := fmt.Sprintf("request-%d", i)
+			document := fmt.Sprintf("document-%d", i)
+			rendered, err := renderer.Render(context.Background(), definition,
+				map[string]*domain.Artifact{"document": {Body: []byte(document)}},
+				map[string]string{"request": request},
+			)
+			if err != nil {
+				errs <- fmt.Errorf("render %s: %w", request, err)
+				return
+			}
+			if rendered.SessionID != "session-"+request || len(rendered.Messages) != 1 ||
+				rendered.Messages[0].Content != "request="+request+";document="+document {
+				errs <- fmt.Errorf("render %s returned session %q and messages %#v", request, rendered.SessionID, rendered.Messages)
+			}
+		}(i)
+	}
+	wait.Wait()
+	close(errs)
+	for err := range errs {
+		t.Error(err)
+	}
+}
+
+type compiledRendererTestLookup struct {
+	id, version string
+	templates   CompiledTemplates
+}
+
+func (lookup compiledRendererTestLookup) GetCompiledTemplates(id, version string) (CompiledTemplates, bool) {
+	if id != lookup.id || version != lookup.version {
+		return CompiledTemplates{}, false
+	}
+	return lookup.templates, true
+}
+
 // BenchmarkGoRendererArtifactReferences measures NewGoRenderer's per-call
 // template-parsing fallback rather than the captured compiled path.
 func BenchmarkGoRendererArtifactReferences(b *testing.B) {