diff --git a/backend/python/sglang/backend.py b/backend/python/sglang/backend.py
index 76d99a726aaf..12a5c6dfb45e 100644
--- a/backend/python/sglang/backend.py
+++ b/backend/python/sglang/backend.py
@@ -90,6 +90,14 @@
 
 
 _ONE_DAY_IN_SECONDS = 60 * 60 * 24
+
+# proto3 has no field presence, so an explicit 0 is indistinguishable from
+# "unset" and the zero-filter below would drop it. These two fields have a
+# meaningful zero a caller can actually intend: temperature 0 is greedy
+# decoding, and 0 is a valid seed. Silently substituting a default for either
+# turns a reproducible request into a random one.
+_EXPLICIT_ZERO_FIELDS = ("Temperature", "Seed")
+
 MAX_WORKERS = int(os.environ.get('PYTHON_GRPC_MAX_WORKERS', '1'))
 
 
@@ -323,7 +331,7 @@ def _build_sampling_params(self, request) -> dict:
             if not hasattr(request, proto_field):
                 continue
             value = getattr(request, proto_field)
-            if proto_field != "Temperature" and value in (None, 0, 0.0, [], False, ""):
+            if proto_field not in _EXPLICIT_ZERO_FIELDS and value in (None, 0, 0.0, [], False, ""):
                 continue
             # repeated fields come back as RepeatedScalarContainer — convert
             if hasattr(value, "__iter__") and not isinstance(value, (str, bytes)):
diff --git a/backend/python/sglang/test.py b/backend/python/sglang/test.py
index c50ed577f941..52b2d9b1f476 100644
--- a/backend/python/sglang/test.py
+++ b/backend/python/sglang/test.py
@@ -128,11 +128,14 @@ def kwargs_for(metadata):
         self.assertNotIn("enable_thinking", kwargs_for({}))
         self.assertIs(kwargs_for({"enable_thinking": "FALSE"})["enable_thinking"], False)
 
-    def test_explicit_zero_temperature_is_preserved(self):
-        """Temperature=0 is valid greedy decoding, not an unset value."""
+    def test_explicit_zero_temperature_and_seed_are_preserved(self):
+        """Temperature=0 is greedy decoding and 0 is a valid seed — neither is
+        an unset value. A dropped seed turns a reproducible request random."""
         from types import SimpleNamespace
 
         servicer = self._servicer()
+        import sys as _sys
+        _SEED_KEY_FOR_TEST = _sys.modules["backend"]._SEED_KEY
         request = SimpleNamespace(
             Temperature=0,
             N=0,
@@ -154,8 +157,12 @@ def test_explicit_zero_temperature_is_preserved(self):
 
         params = servicer._build_sampling_params(request)
         self.assertEqual(params["temperature"], 0)
-        # Other protobuf-default scalar fields must remain filtered.
+        self.assertEqual(params[_SEED_KEY_FOR_TEST], 0)
+        # Other protobuf-default scalar fields must remain filtered. top_k=0 in
+        # particular is not a value sglang accepts (-1 disables it), so it must
+        # keep falling through to the engine default.
         self.assertNotIn("top_p", params)
+        self.assertNotIn("top_k", params)
 
 
 if __name__ == "__main__":
diff --git a/backend/python/vllm/backend.py b/backend/python/vllm/backend.py
index 7235c8e0737a..af072b49b1be 100644
--- a/backend/python/vllm/backend.py
+++ b/backend/python/vllm/backend.py
@@ -60,6 +60,12 @@
 
 _ONE_DAY_IN_SECONDS = 60 * 60 * 24
 
+# proto3 has no field presence, so an explicit 0 is indistinguishable from
+# "unset". These two fields have a meaningful zero a caller can intend:
+# temperature 0 is greedy decoding, and 0 is a valid seed.
+_EXPLICIT_ZERO_FIELDS = ("Temperature", "Seed")
+
+
 # If MAX_WORKERS are specified in the environment use it, otherwise default to 1
 MAX_WORKERS = int(os.environ.get('PYTHON_GRPC_MAX_WORKERS', '1'))
 
@@ -553,7 +559,9 @@ def _build_sampling_params(self, request):
         for request_field, param_field in request_to_sampling_params.items():
             if hasattr(request, request_field):
                 value = getattr(request, request_field)
-                if request_field == "Temperature" or value not in (None, 0, [], False):
+                # See _EXPLICIT_ZERO_FIELDS: temperature 0 is greedy decoding
+                # and 0 is a valid seed, so neither may be filtered out.
+                if request_field in _EXPLICIT_ZERO_FIELDS or value not in (None, 0, [], False):
                     setattr(sampling_params, param_field, value)
 
         return sampling_params
diff --git a/backend/python/vllm/test.py b/backend/python/vllm/test.py
index a0679d4ff8e5..d2932648e255 100644
--- a/backend/python/vllm/test.py
+++ b/backend/python/vllm/test.py
@@ -121,16 +121,18 @@ def test_sampling_params(self):
         finally:
             self.tearDown()
 
-    def test_explicit_zero_temperature_is_preserved(self):
-        """Temperature=0 is valid greedy decoding, not an unset value."""
+    def test_explicit_zero_temperature_and_seed_are_preserved(self):
+        """Temperature=0 is greedy decoding and 0 is a valid seed — neither is
+        an unset value. A dropped seed turns a reproducible request random."""
         import sys, os
         sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
         from backend import BackendServicer
 
         servicer = BackendServicer()
-        request = backend_pb2.PredictOptions(Prompt="hello", Temperature=0)
+        request = backend_pb2.PredictOptions(Prompt="hello", Temperature=0, Seed=0)
         sampling_params = servicer._build_sampling_params(request)
         self.assertEqual(sampling_params.temperature, 0)
+        self.assertEqual(sampling_params.seed, 0)
         # Other protobuf-default scalar fields must remain filtered.
         self.assertEqual(sampling_params.top_p, 0.9)