diff --git a/app/integrations/sms/__init__.py b/app/integrations/sms/__init__.py
index 7726bbd..a676c08 100644
--- a/app/integrations/sms/__init__.py
+++ b/app/integrations/sms/__init__.py
@@ -36,10 +36,23 @@ def _fallback():
 
 
 def send_code(phone: str) -> SendResult:
-    """发码：主成功即返回；失败抛 SmsError。返回 SendResult（cooldown + 实际渠道）。"""
+    """发码：主成功即返回；仅主「供应商不可用(503)」且配置了备时转备补发。
+
+    429（本地冷却/超频）、400（手机号无效）不转——不绕过防刷、不为无效号白烧。
+    备也失败则抛备的 SmsError。返回 SendResult（cooldown + 实际渠道 + 是否 fallback）。
+    """
     primary = _primary()
-    cooldown = primary.send_code(phone)
-    return SendResult(cooldown_sec=cooldown, provider=_NAME[primary], fallback=False)
+    fb = _fallback()
+    try:
+        cooldown = primary.send_code(phone)
+        return SendResult(cooldown_sec=cooldown, provider=_NAME[primary], fallback=False)
+    except SmsError as e:
+        if fb is not None and e.status_code == 503:
+            logger.warning("[SMS] primary=%s 不可用(%s)，fallback→%s",
+                           _NAME[primary], e, _NAME[fb])
+            cooldown = fb.send_code(phone)   # 备的冷却/错误码原样透出
+            return SendResult(cooldown_sec=cooldown, provider=_NAME[fb], fallback=True)
+        raise
 
 
 def verify_code(phone: str, code: str) -> bool:
diff --git a/tests/test_sms_fallback.py b/tests/test_sms_fallback.py
index 883f20b..1649400 100644
--- a/tests/test_sms_fallback.py
+++ b/tests/test_sms_fallback.py
@@ -5,12 +5,12 @@ monkeypatch 各 provider 的 send_code/verify_code 为标记函数，不发真
 """
 from __future__ import annotations
 
-import pytest  # noqa: F401
+import pytest
 
 from app.core.config import settings
 from app.integrations import sms
-from app.integrations.sms import aliyun, chuanglan, jiguang  # noqa: F401
-from app.integrations.sms.base import SendResult, SmsError  # noqa: F401  (SmsError 在 Task 4+ 使用)
+from app.integrations.sms import chuanglan, jiguang
+from app.integrations.sms.base import SendResult, SmsError
 
 PHONE = "13800138000"
 
@@ -36,3 +36,59 @@ def test_send_code_primary_success_returns_sendresult(monkeypatch):
     assert r.cooldown_sec == 60
     assert r.provider == "jiguang"
     assert r.fallback is False
+
+
+def test_send_code_503_falls_back_to_configured_backup(monkeypatch):
+    calls = []
+    monkeypatch.setattr(settings, "SMS_PROVIDER", "jiguang")
+    monkeypatch.setattr(settings, "SMS_FALLBACK_PROVIDER", "chuanglan")
+    monkeypatch.setattr(jiguang, "send_code",
+                        _raiser(SmsError("极光不可用", status_code=503)))
+    monkeypatch.setattr(chuanglan, "send_code",
+                        lambda phone: (calls.append("chuanglan"), 60)[1])
+
+    r = sms.send_code(PHONE)
+    assert r.provider == "chuanglan"
+    assert r.fallback is True
+    assert r.cooldown_sec == 60
+    assert calls == ["chuanglan"]
+
+
+@pytest.mark.parametrize("status", [429, 400])
+def test_send_code_non_503_does_not_fall_back(monkeypatch, status):
+    calls = []
+    monkeypatch.setattr(settings, "SMS_PROVIDER", "jiguang")
+    monkeypatch.setattr(settings, "SMS_FALLBACK_PROVIDER", "chuanglan")
+    monkeypatch.setattr(jiguang, "send_code",
+                        _raiser(SmsError("x", status_code=status)))
+    monkeypatch.setattr(chuanglan, "send_code",
+                        lambda phone: (calls.append("chuanglan"), 60)[1])
+
+    with pytest.raises(SmsError) as ei:
+        sms.send_code(PHONE)
+    assert ei.value.status_code == status
+    assert calls == []
+
+
+def test_send_code_503_without_backup_reraises(monkeypatch):
+    monkeypatch.setattr(settings, "SMS_PROVIDER", "jiguang")
+    monkeypatch.setattr(settings, "SMS_FALLBACK_PROVIDER", "")
+    monkeypatch.setattr(jiguang, "send_code",
+                        _raiser(SmsError("极光不可用", status_code=503)))
+
+    with pytest.raises(SmsError) as ei:
+        sms.send_code(PHONE)
+    assert ei.value.status_code == 503
+
+
+def test_send_code_backup_also_fails_raises_backup_error(monkeypatch):
+    monkeypatch.setattr(settings, "SMS_PROVIDER", "jiguang")
+    monkeypatch.setattr(settings, "SMS_FALLBACK_PROVIDER", "chuanglan")
+    monkeypatch.setattr(jiguang, "send_code",
+                        _raiser(SmsError("极光不可用", status_code=503)))
+    monkeypatch.setattr(chuanglan, "send_code",
+                        _raiser(SmsError("创蓝也不可用", status_code=503)))
+
+    with pytest.raises(SmsError) as ei:
+        sms.send_code(PHONE)
+    assert "创蓝" in str(ei.value)