diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3c249f497..3948aab36 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,7 +2,7 @@
 
 ## Unreleased
 
-- 火山方舟 Seedream 图片生成显式关闭“AI 生成”水印，并由后端下载方舟 CDN 结果后写入服务器或对象存储，恢复素材库自动入库。
+- 火山方舟 Seedream 图片生成显式关闭“AI 生成”水印，并要求生成响应直接返回 Base64 后写入服务器或对象存储，避免为持久化结果再次请求方舟 CDN；中转服务忽略返回格式时仍保留 URL 下载兼容。
 - 项目开源协议由 AGPL-3.0 调整为 MIT，保留上游及项目贡献者版权声明。
 - 修复任务中心等资源入口仍由应用服务器转发对象存储媒体的问题：配置 CDN 加速域名后，登录资源、公开画布分享和管理员媒体预览统一重定向到 CDN；内部 Blob 读取保留同源代理兼容。
 
diff --git a/backend/internal/service/provider.go b/backend/internal/service/provider.go
index 747e3f65e..fe43e9ff9 100644
--- a/backend/internal/service/provider.go
+++ b/backend/internal/service/provider.go
@@ -1067,10 +1067,11 @@ func volcengineArkImageDataURLs(ctx context.Context, config providerConfig, payl
 
 func volcengineArkImageBody(input canvasGenerationInput) (map[string]interface{}, error) {
 	body := map[string]interface{}{
-		"model":     input.Config.Model,
-		"prompt":    withSystemPrompt(input.Config, input.Prompt),
-		"n":         1,
-		"watermark": false,
+		"model":           input.Config.Model,
+		"prompt":          withSystemPrompt(input.Config, input.Prompt),
+		"n":               1,
+		"response_format": "b64_json",
+		"watermark":       false,
 	}
 	if key, value := imageSizeParameter(input.ImageCapability, input.Config.Size); value != "" {
 		if key == "size" {
diff --git a/backend/internal/service/provider_test.go b/backend/internal/service/provider_test.go
index a8e30b920..13190b4ab 100644
--- a/backend/internal/service/provider_test.go
+++ b/backend/internal/service/provider_test.go
@@ -180,6 +180,9 @@ func TestVolcengineArkImageBodyUsesJSONReferencesAndDownscalesSize(t *testing.T)
 	if watermark, ok := body["watermark"].(bool); !ok || watermark {
 		t.Fatalf("watermark = %#v, want false", body["watermark"])
 	}
+	if responseFormat, _ := body["response_format"].(string); responseFormat != "b64_json" {
+		t.Fatalf("response_format = %#v, want b64_json", body["response_format"])
+	}
 	size, _ := body["size"].(string)
 	parts := strings.Split(size, "x")
 	if len(parts) != 2 {
diff --git a/web/src/services/api/image.ts b/web/src/services/api/image.ts
index 4f83e98c0..fede3cce3 100644
--- a/web/src/services/api/image.ts
+++ b/web/src/services/api/image.ts
@@ -919,6 +919,7 @@ export async function requestGeneration(config: AiConfig, prompt: string, option
                   model: requestConfig.model,
                   prompt: withSystemPrompt(requestConfig, prompt),
                   n,
+                  response_format: "b64_json",
                   watermark: false,
                   ...(normalizedRequestSize ? { [normalizedRequestSize.parameter]: normalizedRequestSize.value } : {}),
               }
@@ -1014,6 +1015,7 @@ export async function requestEdit(config: AiConfig, prompt: string, references:
                     model: requestConfig.model,
                     prompt: withSystemPrompt(requestConfig, requestPrompt),
                     n,
+                    response_format: "b64_json",
                     watermark: false,
                     ...(requestSize ? { [requestSize.parameter]: requestSize.value } : {}),
                     ...(images.length === 1 ? { image: images[0] } : images.length > 1 ? { image: images } : {}),