diff --git a/internal/prompt/messages.go b/internal/prompt/messages.go
index daf5b84..d1c5745 100644
--- a/internal/prompt/messages.go
+++ b/internal/prompt/messages.go
@@ -9,6 +9,13 @@ import (
 
 var markdownImagePattern = regexp.MustCompile(`!\[(.*?)\]\((.*?)\)`)
 
+const (
+	systemMarker    = "<｜System｜>"
+	userMarker      = "<｜User｜>"
+	assistantMarker = "<｜Assistant｜>"
+	toolMarker      = "<｜Tool｜>"
+)
+
 func MessagesPrepare(messages []map[string]any) string {
 	type block struct {
 		Role string
@@ -35,21 +42,17 @@ func MessagesPrepare(messages []map[string]any) string {
 	for _, m := range merged {
 		switch m.Role {
 		case "assistant":
-			// Keep assistant turns on their own block so the model sees a clear
-			// boundary between prior answer text and the EOS marker.
-			parts = append(parts, "<｜Assistant｜>\n"+m.Text+"\n<｜end▁of▁sentence｜>")
+			parts = append(parts, formatRoleBlock(assistantMarker, m.Text))
 		case "tool":
 			if strings.TrimSpace(m.Text) != "" {
-				parts = append(parts, "<｜Tool｜>\n"+m.Text)
+				parts = append(parts, formatRoleBlock(toolMarker, m.Text))
 			}
 		case "system":
-			// Clear system boundary improves R1 and V3 context understanding significantly.
 			if text := strings.TrimSpace(m.Text); text != "" {
-				parts = append(parts, "<system_instructions>\n"+text+"\n</system_instructions>")
+				parts = append(parts, formatRoleBlock(systemMarker, text))
 			}
 		case "user":
-			// Put user turns on their own line so the role transition is explicit.
-			parts = append(parts, "<｜User｜>\n"+m.Text)
+			parts = append(parts, formatRoleBlock(userMarker, m.Text))
 		default:
 			if strings.TrimSpace(m.Text) != "" {
 				parts = append(parts, m.Text)
@@ -60,6 +63,10 @@ func MessagesPrepare(messages []map[string]any) string {
 	return markdownImagePattern.ReplaceAllString(out, `[${1}](${2})`)
 }
 
+func formatRoleBlock(marker, text string) string {
+	return marker + "\n" + text
+}
+
 func NormalizeContent(v any) string {
 	if v == nil {
 		return ""
diff --git a/internal/prompt/messages_test.go b/internal/prompt/messages_test.go
index 9114d39..fbeba54 100644
--- a/internal/prompt/messages_test.go
+++ b/internal/prompt/messages_test.go
@@ -1,6 +1,9 @@
 package prompt
 
-import "testing"
+import (
+	"strings"
+	"testing"
+)
 
 func TestNormalizeContentNilReturnsEmpty(t *testing.T) {
 	if got := NormalizeContent(nil); got != "" {
@@ -22,6 +25,20 @@ func TestMessagesPrepareNilContentNoNullLiteral(t *testing.T) {
 	}
 }
 
+func TestMessagesPrepareUsesUnifiedSystemMarkerAndNoEOSTag(t *testing.T) {
+	messages := []map[string]any{
+		{"role": "system", "content": "System rule"},
+		{"role": "assistant", "content": "Answer"},
+	}
+	got := MessagesPrepare(messages)
+	if !strings.Contains(got, "<｜System｜>\nSystem rule") {
+		t.Fatalf("expected unified system marker, got %q", got)
+	}
+	if strings.Contains(got, "<｜end▁of▁sentence｜>") {
+		t.Fatalf("did not expect EOS marker, got %q", got)
+	}
+}
+
 func TestNormalizeContentArrayFallsBackToContentWhenTextEmpty(t *testing.T) {
 	got := NormalizeContent([]any{
 		map[string]any{"type": "text", "text": "", "content": "from-content"},
diff --git a/internal/util/messages_test.go b/internal/util/messages_test.go
index 2ec3f50..e6e0cdc 100644
--- a/internal/util/messages_test.go
+++ b/internal/util/messages_test.go
@@ -25,18 +25,21 @@ func TestMessagesPrepareRoles(t *testing.T) {
 		{"role": "user", "content": "How are you"},
 	}
 	got := MessagesPrepare(messages)
-	if !contains(got, "<system_instructions>\nYou are helper\n</system_instructions>\n\n<｜User｜>\nHi") {
+	if !contains(got, "<｜System｜>\nYou are helper\n\n<｜User｜>\nHi") {
 		t.Fatalf("expected system/user separation in %q", got)
 	}
 	if !contains(got, "<｜User｜>\nHi\n\n<｜Assistant｜>\nHello") {
 		t.Fatalf("expected user/assistant separation in %q", got)
 	}
-	if !contains(got, "<｜Assistant｜>\nHello\n<｜end▁of▁sentence｜>\n\n<｜User｜>\nHow are you") {
+	if !contains(got, "<｜Assistant｜>\nHello\n\n<｜User｜>\nHow are you") {
 		t.Fatalf("expected assistant/user separation in %q", got)
 	}
 	if !contains(got, "<｜Assistant｜>") {
 		t.Fatalf("expected assistant marker in %q", got)
 	}
+	if !contains(got, "<｜System｜>") {
+		t.Fatalf("expected system marker in %q", got)
+	}
 	if !contains(got, "<｜User｜>") {
 		t.Fatalf("expected user marker in %q", got)
 	}
diff --git a/internal/util/util_edge_test.go b/internal/util/util_edge_test.go
index 9f91602..439b257 100644
--- a/internal/util/util_edge_test.go
+++ b/internal/util/util_edge_test.go
@@ -184,8 +184,11 @@ func TestMessagesPrepareAssistantMarkers(t *testing.T) {
 	if !strings.Contains(got, "<｜Assistant｜>") {
 		t.Fatalf("expected assistant marker, got %q", got)
 	}
-	if !strings.Contains(got, "<｜end▁of▁sentence｜>") {
-		t.Fatalf("expected end of sentence marker, got %q", got)
+	if strings.Contains(got, "<｜end▁of▁sentence｜>") {
+		t.Fatalf("did not expect end of sentence marker, got %q", got)
+	}
+	if strings.Contains(got, "<system_instructions>") {
+		t.Fatalf("did not expect legacy system marker, got %q", got)
 	}
 }