diff --git a/backend/internal/config/config.go b/backend/internal/config/config.go
index dd9a4e58..15592905 100644
--- a/backend/internal/config/config.go
+++ b/backend/internal/config/config.go
@@ -52,6 +52,11 @@ const (
 	ConnectionPoolIsolationAccountProxy = "account_proxy"
 )
 
+// DefaultUpstreamResponseReadMaxBytes 上游非流式响应体的默认读取上限。
+// 128 MB 可容纳 2-3 张 4K PNG（base64 膨胀 33%，单张 4K PNG 最坏约 67MB base64）。
+// 可通过 gateway.upstream_response_read_max_bytes 配置项覆盖。
+const DefaultUpstreamResponseReadMaxBytes int64 = 128 * 1024 * 1024
+
 type Config struct {
 	Server                  ServerConfig                  `mapstructure:"server"`
 	Log                     LogConfig                     `mapstructure:"log"`
@@ -1407,7 +1412,7 @@ func setDefaults() {
 	viper.SetDefault("gateway.antigravity_fallback_cooldown_minutes", 1)
 	viper.SetDefault("gateway.antigravity_extra_retries", 10)
 	viper.SetDefault("gateway.max_body_size", int64(256*1024*1024))
-	viper.SetDefault("gateway.upstream_response_read_max_bytes", int64(8*1024*1024))
+	viper.SetDefault("gateway.upstream_response_read_max_bytes", DefaultUpstreamResponseReadMaxBytes)
 	viper.SetDefault("gateway.proxy_probe_response_read_max_bytes", int64(1024*1024))
 	viper.SetDefault("gateway.gemini_debug_response_headers", false)
 	viper.SetDefault("gateway.connection_pool_isolation", ConnectionPoolIsolationAccountProxy)
diff --git a/backend/internal/service/upstream_response_limit.go b/backend/internal/service/upstream_response_limit.go
index a0444d52..ddf0e818 100644
--- a/backend/internal/service/upstream_response_limit.go
+++ b/backend/internal/service/upstream_response_limit.go
@@ -12,7 +12,9 @@ import (
 
 var ErrUpstreamResponseBodyTooLarge = errors.New("upstream response body too large")
 
-const defaultUpstreamResponseReadMaxBytes int64 = 8 * 1024 * 1024
+// defaultUpstreamResponseReadMaxBytes 源自 config.DefaultUpstreamResponseReadMaxBytes，
+// 仅在 cfg 为 nil 时作为兜底（测试或极端场景）。
+const defaultUpstreamResponseReadMaxBytes = config.DefaultUpstreamResponseReadMaxBytes
 
 func resolveUpstreamResponseReadLimit(cfg *config.Config) int64 {
 	if cfg != nil && cfg.Gateway.UpstreamResponseReadMaxBytes > 0 {