diff --git a/internal/adapter/openai/message_normalize.go b/internal/adapter/openai/message_normalize.go
index bbc5800..724cb9f 100644
--- a/internal/adapter/openai/message_normalize.go
+++ b/internal/adapter/openai/message_normalize.go
@@ -6,6 +6,7 @@ import (
 	"strings"
 
 	"ds2api/internal/config"
+	"ds2api/internal/prompt"
 )
 
 func normalizeOpenAIMessagesForPrompt(raw []any, traceID string) []map[string]any {
@@ -132,35 +133,7 @@ func formatToolResultForPrompt(msg map[string]any) string {
 }
 
 func normalizeOpenAIContentForPrompt(v any) string {
-	if v == nil {
-		return ""
-	}
-	switch x := v.(type) {
-	case string:
-		return x
-	case []any:
-		parts := make([]string, 0, len(x))
-		for _, item := range x {
-			m, ok := item.(map[string]any)
-			if !ok {
-				continue
-			}
-			t := strings.ToLower(strings.TrimSpace(asString(m["type"])))
-			if t != "text" && t != "output_text" && t != "input_text" {
-				continue
-			}
-			if text := asString(m["text"]); text != "" {
-				parts = append(parts, text)
-				continue
-			}
-			if text := asString(m["content"]); text != "" {
-				parts = append(parts, text)
-			}
-		}
-		return strings.Join(parts, "\n")
-	default:
-		return marshalToPromptString(v)
-	}
+	return prompt.NormalizeContent(v)
 }
 
 func normalizeOpenAIArgumentsForPrompt(v any) string {
diff --git a/internal/prompt/messages.go b/internal/prompt/messages.go
index 69cfe5a..5e124b4 100644
--- a/internal/prompt/messages.go
+++ b/internal/prompt/messages.go
@@ -51,6 +51,9 @@ func MessagesPrepare(messages []map[string]any) string {
 }
 
 func NormalizeContent(v any) string {
+	if v == nil {
+		return ""
+	}
 	switch x := v.(type) {
 	case string:
 		return x
diff --git a/internal/prompt/messages_test.go b/internal/prompt/messages_test.go
new file mode 100644
index 0000000..0407552
--- /dev/null
+++ b/internal/prompt/messages_test.go
@@ -0,0 +1,23 @@
+package prompt
+
+import "testing"
+
+func TestNormalizeContentNilReturnsEmpty(t *testing.T) {
+	if got := NormalizeContent(nil); got != "" {
+		t.Fatalf("expected empty string for nil content, got %q", got)
+	}
+}
+
+func TestMessagesPrepareNilContentNoNullLiteral(t *testing.T) {
+	messages := []map[string]any{
+		{"role": "assistant", "content": nil},
+		{"role": "user", "content": "ok"},
+	}
+	got := MessagesPrepare(messages)
+	if got == "" {
+		t.Fatalf("expected non-empty output")
+	}
+	if got == "null" {
+		t.Fatalf("expected no null literal output, got %q", got)
+	}
+}