diff --git a/python/sglang/multimodal_gen/runtime/server_args.py b/python/sglang/multimodal_gen/runtime/server_args.py
index 4fc29be1c..8c3db7bf8 100644
--- a/python/sglang/multimodal_gen/runtime/server_args.py
+++ b/python/sglang/multimodal_gen/runtime/server_args.py
@@ -726,6 +726,8 @@ class ServerArgs(DisaggServerArgsMixin):
             if mode_explicit or not legacy_explicit:
                 self.warmup = self.warmup_mode != "off"
                 self.server_warmup = self.warmup_mode == "server"
+            elif self.warmup:
+                self.server_warmup = self.server_warmup or self.warmup_mode == "server"
 
         # Explicit resolutions imply warmup is on (request-based).
         if self.warmup_resolutions is not None:
diff --git a/python/sglang/multimodal_gen/test/unit/test_server_args.py b/python/sglang/multimodal_gen/test/unit/test_server_args.py
index d839bc141..c7c72337f 100644
--- a/python/sglang/multimodal_gen/test/unit/test_server_args.py
+++ b/python/sglang/multimodal_gen/test/unit/test_server_args.py
@@ -588,6 +588,29 @@ class TestWarmupModeNormalization(unittest.TestCase):
         self.assertFalse(sa.server_warmup)
         self.assertEqual(sa.warmup_mode, "request")
 
+    def test_legacy_warmup_on_uses_defaulted_server_mode(self):
+        # `serve --warmup` (legacy ON, mode defaulted to "server" but not
+        # explicit) must resolve to server-based warmup, not silently downgrade
+        # to request mode.
+        sa = self._resolve(warmup_mode="server", warmup=True, explicit=("warmup",))
+        self.assertEqual(sa.warmup_mode, "server")
+        self.assertTrue(sa.warmup)
+        self.assertTrue(sa.server_warmup)
+
+    def test_legacy_warmup_with_resolutions_runs_server_warmup(self):
+        # Dead-zone regression: `serve --warmup --warmup-resolutions X` must run
+        # server-based (synthetic) warmup, not end up with no warmup at all
+        # (request-based warmup bails out when warmup_resolutions is set).
+        sa = self._resolve(
+            warmup_mode="server",
+            warmup=True,
+            warmup_resolutions=["1024x1024"],
+            explicit=("warmup",),
+        )
+        self.assertTrue(sa.warmup)
+        self.assertTrue(sa.server_warmup)
+        self.assertEqual(sa.warmup_mode, "server")
+
     def test_disagg_role_disables_server_warmup(self):
         from sglang.multimodal_gen.runtime.disaggregation.roles import RoleType