Convert SamplingParams to msgspec Struct (#29198)

Co-authored-by: Rain Jiang <96632942+rainj-me@users.noreply.github.com>

test/registered/unit/sampling/test_sampling_params.py

@@ -6,9 +6,12 @@ register_cpu_ci(est_time=7, suite="base-a-test-cpu")
 register_cpu_ci(est_time=7, suite="base-c-test-cpu")
 register_xpu_ci(est_time=10, suite="stage-a-test-1-gpu-xpu")
 
+import copy
 import unittest
 from unittest.mock import MagicMock
 
+import msgspec
+
 from sglang.srt.sampling.sampling_params import (
     MAX_LEN,
     TOP_K_ALL,
@@ -363,6 +366,84 @@ class TestSamplingParamsNormalize(CustomTestCase):
         self.assertEqual(sp.stop_regex_max_len, 3)
 
 
+class TestSamplingParamsMsgspecStruct(CustomTestCase):
+
+    def test_copy_remains_mutable_and_independent(self):
+        sp = SamplingParams(max_new_tokens=8, custom_params={"a": 1})
+
+        copied = copy.copy(sp)
+        copied.max_new_tokens = 16
+        copied.custom_params = {"b": 2}
+
+        self.assertEqual(sp.max_new_tokens, 8)
+        self.assertEqual(sp.custom_params, {"a": 1})
+        self.assertEqual(copied.max_new_tokens, 16)
+        self.assertEqual(copied.custom_params, {"b": 2})
+
+    def test_none_values_still_use_constructor_defaults(self):
+        sp = SamplingParams(
+            temperature=None,
+            top_p=None,
+            top_k=None,
+            min_p=None,
+            frequency_penalty=None,
+            presence_penalty=None,
+            repetition_penalty=None,
+            min_new_tokens=None,
+            n=None,
+            ignore_eos=None,
+            skip_special_tokens=None,
+            spaces_between_special_tokens=None,
+            no_stop_trim=None,
+        )
+
+        self.assertEqual(sp.temperature, 1.0)
+        self.assertEqual(sp.top_p, 1.0)
+        self.assertEqual(sp.top_k, TOP_K_ALL)
+        self.assertEqual(sp.min_p, 0.0)
+        self.assertEqual(sp.frequency_penalty, 0.0)
+        self.assertEqual(sp.presence_penalty, 0.0)
+        self.assertEqual(sp.repetition_penalty, 1.0)
+        self.assertEqual(sp.min_new_tokens, 0)
+        self.assertEqual(sp.n, 1)
+        self.assertFalse(sp.ignore_eos)
+        self.assertTrue(sp.skip_special_tokens)
+        self.assertTrue(sp.spaces_between_special_tokens)
+        self.assertFalse(sp.no_stop_trim)
+
+    def test_msgpack_omits_default_fields(self):
+        encoded = msgspec.msgpack.encode(SamplingParams())
+
+        self.assertEqual(msgspec.msgpack.decode(encoded), {})
+
+    def test_msgpack_round_trip_preserves_normalized_state(self):
+        tokenizer = MagicMock()
+        tokenizer.encode.side_effect = lambda s, add_special_tokens=False: {
+            "hello": [101, 102],
+            "world": [201],
+        }[s]
+        sp = SamplingParams(
+            stop=["hello", "world"],
+            stop_regex=r"[a-z]{3}",
+            stop_token_ids=[1, 2],
+            temperature=0.5,
+        )
+        sp.normalize(tokenizer)
+
+        encoder = msgspec.msgpack.Encoder()
+        decoder = msgspec.msgpack.Decoder(SamplingParams)
+        rebuilt = decoder.decode(encoder.encode(sp))
+
+        self.assertIsInstance(rebuilt, SamplingParams)
+        self.assertTrue(rebuilt.is_normalized)
+        self.assertEqual(rebuilt.stop_strs, ["hello", "world"])
+        self.assertEqual(rebuilt.stop_str_max_len, 2)
+        self.assertEqual(rebuilt.stop_regex_strs, [r"[a-z]{3}"])
+        self.assertEqual(rebuilt.stop_regex_max_len, 3)
+        self.assertEqual(rebuilt.stop_token_ids, {1, 2})
+        self.assertEqual(rebuilt.temperature, 0.5)
+
+
 class TestRegexMaxLength(CustomTestCase):
 
     def test_literal_string(self):