diff --git a/internal/adapter/openai/message_normalize.go b/internal/adapter/openai/message_normalize.go
index 299f908..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 {
@@ -132,32 +133,7 @@ func formatToolResultForPrompt(msg map[string]any) string {
 }
 
 func normalizeOpenAIContentForPrompt(v any) string {
-	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/adapter/openai/message_normalize_test.go b/internal/adapter/openai/message_normalize_test.go
index ba17a52..ecb3bbd 100644
--- a/internal/adapter/openai/message_normalize_test.go
+++ b/internal/adapter/openai/message_normalize_test.go
@@ -194,6 +194,36 @@ func TestNormalizeOpenAIMessagesForPrompt_PreservesConcatenatedToolArguments(t *
 	}
 }
 
+func TestNormalizeOpenAIMessagesForPrompt_AssistantNilContentDoesNotInjectNullLiteral(t *testing.T) {
+	raw := []any{
+		map[string]any{
+			"role":    "assistant",
+			"content": nil,
+			"tool_calls": []any{
+				map[string]any{
+					"id": "call_screenshot",
+					"function": map[string]any{
+						"name":      "send_file_to_user",
+						"arguments": `{"file_path":"/tmp/a.png"}`,
+					},
+				},
+			},
+		},
+	}
+
+	normalized := normalizeOpenAIMessagesForPrompt(raw, "")
+	if len(normalized) != 1 {
+		t.Fatalf("expected one normalized message, got %d", len(normalized))
+	}
+	content, _ := normalized[0]["content"].(string)
+	if strings.Contains(content, "<｜Assistant｜>null") || strings.HasPrefix(strings.TrimSpace(content), "null") {
+		t.Fatalf("unexpected null literal injected into assistant tool history: %q", content)
+	}
+	if !strings.Contains(content, "function.name: send_file_to_user") {
+		t.Fatalf("expected tool history block preserved, got %q", content)
+	}
+}
+
 func TestNormalizeOpenAIMessagesForPrompt_DeveloperRoleMapsToSystem(t *testing.T) {
 	raw := []any{
 		map[string]any{"role": "developer", "content": "必须先走工具调用"},
@@ -207,3 +237,23 @@ func TestNormalizeOpenAIMessagesForPrompt_DeveloperRoleMapsToSystem(t *testing.T
 		t.Fatalf("expected developer role converted to system, got %#v", normalized[0]["role"])
 	}
 }
+
+func TestNormalizeOpenAIMessagesForPrompt_AssistantArrayContentFallbackWhenTextEmpty(t *testing.T) {
+	raw := []any{
+		map[string]any{
+			"role": "assistant",
+			"content": []any{
+				map[string]any{"type": "text", "text": "", "content": "工具说明文本"},
+			},
+		},
+	}
+
+	normalized := normalizeOpenAIMessagesForPrompt(raw, "")
+	if len(normalized) != 1 {
+		t.Fatalf("expected one normalized message, got %d", len(normalized))
+	}
+	content, _ := normalized[0]["content"].(string)
+	if content != "工具说明文本" {
+		t.Fatalf("expected content fallback text preserved, got %q", content)
+	}
+}
diff --git a/internal/prompt/messages.go b/internal/prompt/messages.go
index 69cfe5a..e86c391 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
@@ -64,11 +67,11 @@ func NormalizeContent(v any) string {
 			typeStr, _ := m["type"].(string)
 			typeStr = strings.ToLower(strings.TrimSpace(typeStr))
 			if typeStr == "text" || typeStr == "output_text" || typeStr == "input_text" {
-				if txt, ok := m["text"].(string); ok {
+				if txt, ok := m["text"].(string); ok && txt != "" {
 					parts = append(parts, txt)
 					continue
 				}
-				if txt, ok := m["content"].(string); ok {
+				if txt, ok := m["content"].(string); ok && txt != "" {
 					parts = append(parts, txt)
 				}
 			}
diff --git a/internal/prompt/messages_test.go b/internal/prompt/messages_test.go
new file mode 100644
index 0000000..9114d39
--- /dev/null
+++ b/internal/prompt/messages_test.go
@@ -0,0 +1,32 @@
+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)
+	}
+}
+
+func TestNormalizeContentArrayFallsBackToContentWhenTextEmpty(t *testing.T) {
+	got := NormalizeContent([]any{
+		map[string]any{"type": "text", "text": "", "content": "from-content"},
+	})
+	if got != "from-content" {
+		t.Fatalf("expected fallback to content when text is empty, got %q", got)
+	}
+}