diff --git a/backend/internal/config/config.go b/backend/internal/config/config.go
index dd9a4e588f..3d9438ce30 100644
--- a/backend/internal/config/config.go
+++ b/backend/internal/config/config.go
@@ -1407,7 +1407,7 @@ 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", int64(64*1024*1024))
 	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 a0444d5209..bc3bfdaf77 100644
--- a/backend/internal/service/upstream_response_limit.go
+++ b/backend/internal/service/upstream_response_limit.go
@@ -12,7 +12,10 @@ import (
 
 var ErrUpstreamResponseBodyTooLarge = errors.New("upstream response body too large")
 
-const defaultUpstreamResponseReadMaxBytes int64 = 8 * 1024 * 1024
+// defaultUpstreamResponseReadMaxBytes 上游非流式响应体的默认读取上限。
+// 64 MB 足以容纳多张高清图（base64 膨胀 33%，64MB ≈ 48MB 原始字节）。
+// 可通过 gateway.upstream_response_read_max_bytes 配置项覆盖。
+const defaultUpstreamResponseReadMaxBytes int64 = 64 * 1024 * 1024
 
 func resolveUpstreamResponseReadLimit(cfg *config.Config) int64 {
 	if cfg != nil && cfg.Gateway.UpstreamResponseReadMaxBytes > 0 {