diff --git a/backend/app/services/mcp/image_service_tools.py b/backend/app/services/mcp/image_service_tools.py
index 5929f98f..fb2b9615 100644
--- a/backend/app/services/mcp/image_service_tools.py
+++ b/backend/app/services/mcp/image_service_tools.py
@@ -30,8 +30,6 @@
 
 # image-service 所有 Gemini 模型都接受的比例；Seedream 接受任意比例。
 ASPECT_RATIOS = ("1:1", "2:3", "3:2", "3:4", "4:3", "4:5", "5:4", "9:16", "16:9", "21:9")
-# 主模型被内容审核拦截时，由 image-service 换这个模型重试一次（其他错误不切换）。
-FALLBACK_MODEL = "doubao-seedream-4-5"
 # image-service 结构化错误码（P0 设计 §2.1）；不认识的码按通用上游失败处理。
 KNOWN_ERROR_CODES = frozenset({
     "invalid_params",
@@ -447,10 +445,10 @@ def _normalized_arguments(args: Any) -> dict[str, Any]:
     regenerate = args.get("regenerate", False)
     if not isinstance(regenerate, bool):
         raise ImageGenerationError("invalid_arguments")
+    # 主力/备用模型由 image-service 的 DEFAULT_MODEL / DEFAULT_FALLBACK_MODEL 决定，便于切换。
     arguments: dict[str, Any] = {
         "prompt": prompt.strip(),
         "aspect_ratio": aspect_ratio,
-        "fallback_model": FALLBACK_MODEL,
         "grounding": True,
     }
     if regenerate:
diff --git a/backend/test/test_image_service_tools.py b/backend/test/test_image_service_tools.py
index c410ad1e..50b3e876 100644
--- a/backend/test/test_image_service_tools.py
+++ b/backend/test/test_image_service_tools.py
@@ -12,7 +12,6 @@
 )
 from app.services.mcp.image_service_tools import (  # noqa: E402
     EXECUTION_TIMEOUT_SECONDS,
-    FALLBACK_MODEL,
     MCP_CALL_TIMEOUT_SECONDS,
     ImageGenerationError,
     parse_generate_image_text,
@@ -150,7 +149,7 @@ async def test_success_downloads_from_endpoint_origin_and_stores_for_bound_conve
         self.assertEqual(client.calls[0][1], "generate_image")
         self.assertEqual(
             client.calls[0][2],
-            {"prompt": "a red fox", "aspect_ratio": "16:9", "fallback_model": FALLBACK_MODEL, "grounding": True},
+            {"prompt": "a red fox", "aspect_ratio": "16:9", "grounding": True},
         )
         self.assertEqual(client.min_timeouts, [MCP_CALL_TIMEOUT_SECONDS])
         self.assertEqual(server.requested_urls, [f"http://192.168.1.11:8090/static/images/{IMAGE_NAME}"])