[Config] msgspec.Struct for the config tier (#38753)

test/registered/cpu/test_server_args_backend.py

@@ -17,7 +17,7 @@ register_cpu_ci(est_time=10, suite="base-b-test-cpu-arm64")
 
 class TestServerArgsCPUBackend(CustomTestCase):
     def _make_server_args(self, attention_backend=None):
-        server_args = ServerArgs.__new__(ServerArgs)
+        server_args = ServerArgs(model_path="dummy")
         server_args.device = "cpu"
         server_args.attention_backend = attention_backend
         server_args.sampling_backend = None

test/registered/prefill_only/test_multi_item_scoring.py

@@ -21,6 +21,7 @@ import torch
 from transformers import AutoConfig, AutoTokenizer
 
 from sglang.srt.entrypoints.engine import Engine
+from sglang.srt.server_args import _declared_default
 from sglang.test.ci.ci_register import register_cuda_ci
 from sglang.test.test_utils import (
     DEFAULT_SMALL_MODEL_NAME_FOR_TEST,
@@ -56,9 +57,8 @@ class TestMISServerArgsValidation(unittest.TestCase):
 
     def test_enable_mis_default(self):
         """Test that enable_mis defaults to False."""
-        from sglang.srt.server_args import ServerArgs
 
-        self.assertEqual(ServerArgs.enable_mis, False)
+        self.assertEqual(_declared_default("enable_mis"), False)
 
 
 class TestMultiItemScoringOptimization(CustomTestCase):

test/registered/unit/entrypoints/test_server_info.py

@@ -21,11 +21,13 @@ Current coverage:
 """
 
 import asyncio
-import dataclasses
 import json
 import unittest
 from types import SimpleNamespace
 
+import msgspec
+import msgspec.structs
+
 from sglang.srt.arg_groups.validation_hook import check_load_publish_args
 from sglang.srt.entrypoints import http_server
 from sglang.srt.lora.lora_registry import LoRARef
@@ -457,7 +459,7 @@ class TestServerInfoExistingFieldsPreserved(CustomTestCase):
 
         info = _call_server_info_with(args)
 
-        for field in dataclasses.fields(ServerArgs):
+        for field in msgspec.structs.fields(ServerArgs):
             self.assertIn(
                 field.name,
                 info,

test/registered/unit/managers/test_tokenizer_config_updates.py

@@ -10,6 +10,9 @@ import re
 import unittest
 from pathlib import Path
 
+import msgspec
+import msgspec.structs
+
 import sglang
 from sglang.srt.managers.tokenizer_manager import TokenizerManager
 from sglang.srt.runtime_context import get_context, publish, reset_context
@@ -85,14 +88,15 @@ class TestTokenizerConfigUpdates(CustomTestCase):
         )
 
     def test_the_dump_snapshot_identifies_the_running_checkpoint(self):
-        import dataclasses
 
         manager = _manager(self, load_format="auto")
         manager.model_path = "at-startup"
         manager.served_model_name = "at-startup"
         manager._update_model_path_info("after-reload", "dummy")
 
-        snapshot = manager.resolved_config_dict(dataclasses.asdict(manager.server_args))
+        snapshot = manager.resolved_config_dict(
+            msgspec.structs.asdict(manager.server_args)
+        )
         self.assertEqual(snapshot["model_path"], "after-reload")
         self.assertEqual(snapshot["served_model_name"], "after-reload")
         self.assertEqual(snapshot["load_format"], "dummy")
@@ -116,7 +120,6 @@ class TestTokenizerConfigUpdates(CustomTestCase):
         self.assertIsNone(manager._dump_config_snapshot())
 
     def test_an_unpickleable_field_does_not_lose_the_dump(self):
-        import dataclasses
         import pickle
 
         # What --custom-sigquit-handler leaves on a real ServerArgs.
@@ -128,7 +131,7 @@ class TestTokenizerConfigUpdates(CustomTestCase):
             "server_args": manager.server_args,
             "config_updates": get_context().overrides_log(),
             "resolved_config": manager.resolved_config_dict(
-                dataclasses.asdict(manager.server_args)
+                msgspec.structs.asdict(manager.server_args)
             ),
             "requests": [],
         }

test/registered/unit/parser/test_template_manager.py

@@ -4,6 +4,8 @@ import unittest
 from types import ModuleType, SimpleNamespace
 from unittest.mock import Mock, patch
 
+import msgspec
+
 from sglang.srt.parser.template_detection import (
     REASONING_PARSER_RULES,
     TOOL_CALL_PARSER_RULES,
@@ -892,7 +894,9 @@ class TestResolveAutoParsers(unittest.TestCase):
             reasoning_parser="auto",
             tool_call_parser="auto",
         )
-        object.__setattr__(args, "model_path", "nonexistent/model-does-not-exist-xyz")
+        msgspec.Struct.__setattr__(
+            args, "model_path", "nonexistent/model-does-not-exist-xyz"
+        )
         with _patch_hf_transformers_utils(
             Mock(side_effect=RuntimeError("tokenizer unavailable")),
             Mock(side_effect=RuntimeError("config unavailable")),

test/registered/unit/server_args/test_page_major_backend_allowlist.py

@@ -31,6 +31,8 @@ the unified pool today.
 import unittest
 from types import SimpleNamespace
 
+import msgspec
+
 from sglang.srt.arg_groups.kv_cache_hook import handle_page_major_kv_layout
 from sglang.srt.configs.model_config import AttentionArch
 from sglang.srt.server_args import ServerArgs
@@ -50,7 +52,7 @@ def _accepts(
 ) -> bool:
     """Run just `handle_page_major_kv_layout` against a minimal stand-in, since
     ServerArgs' real constructor pulls in a model config."""
-    sa = ServerArgs.__new__(ServerArgs)
+    sa = ServerArgs(model_path="dummy")
     for name, value in {
         "enable_unified_memory": unified,
         # The unified pool sets this itself; without it the flag must be explicit
@@ -64,8 +66,8 @@ def _accepts(
         "linear_attn_prefill_backend": linear_prefill,
         "mamba_backend": "triton",
     }.items():
-        object.__setattr__(sa, name, value)
-    object.__setattr__(
+        msgspec.Struct.__setattr__(sa, name, value)
+    setattr(
         sa,
         "_model_config",
         SimpleNamespace(

test/registered/unit/server_args/test_record_holds_the_raw_input.py

@@ -8,13 +8,15 @@ value the caller still holds and the snapshot cannot see it.
 """
 
 import copy
-import dataclasses
 import json
 import os
 import shutil
 import tempfile
 import unittest
 
+import msgspec
+import msgspec.structs
+
 from sglang.srt.server_args import ServerArgs
 from sglang.test.ci.ci_register import register_cpu_ci
 from sglang.test.test_utils import CustomTestCase
@@ -108,7 +110,7 @@ class TestRecordHoldsTheRawInput(CustomTestCase):
 
                 moved = {
                     field.name: (raw[field.name], getattr(server_args, field.name))
-                    for field in dataclasses.fields(server_args)
+                    for field in msgspec.structs.fields(server_args)
                     if _moved(getattr(server_args, field.name), raw[field.name])
                 }
                 self.assertEqual(

test/registered/unit/server_args/test_resolution_declarations.py

@@ -9,7 +9,6 @@ or is projected into the wrong namespace therefore fails on observed state.
 
 import ast
 import copy
-import dataclasses
 import json
 import os
 import pathlib
@@ -18,6 +17,9 @@ import tempfile
 import unittest
 import unittest.mock
 
+import msgspec
+import msgspec.structs
+
 import sglang
 from sglang.srt.arg_groups.overrides import resolution_result
 from sglang.srt.server_args import ServerArgs
@@ -30,7 +32,7 @@ _SRT = pathlib.Path(sglang.__file__).resolve().parent / "srt"
 
 # Every field of the record: resolution has no bare-assignment writer left, so
 # the scan states that as a whole rather than a converted-so-far list.
-_RESOLVED_FIELDS = frozenset(field.name for field in dataclasses.fields(ServerArgs))
+_RESOLVED_FIELDS = frozenset(field.name for field in msgspec.structs.fields(ServerArgs))
 
 # Shapes the agreement check runs on. Each needs a real config.json:
 # `model_path="dummy"` takes the pipeline's early return.
@@ -201,14 +203,14 @@ class TestResolutionDeclarations(CustomTestCase):
             supplied = {"random_seed": 42, **shape}
             server_args = self._resolve(shape)
             overlay = _stash_overlay(server_args)
-            for field in dataclasses.fields(server_args):
+            for field in msgspec.structs.fields(server_args):
                 if field.name in ("model_path", "device") or field.name in overlay:
                     continue
                 if field.name in supplied:
                     before = supplied[field.name]
-                elif field.default is not dataclasses.MISSING:
+                elif field.default is not msgspec.NODEFAULT:
                     before = field.default
-                elif field.default_factory is not dataclasses.MISSING:
+                elif field.default_factory is not msgspec.NODEFAULT:
                     before = field.default_factory()
                 else:
                     continue
@@ -278,7 +280,7 @@ class TestResolutionDeclarations(CustomTestCase):
         dump = server_args.resolved_dict()
         self.assertEqual(
             sorted(dump),
-            sorted(field.name for field in dataclasses.fields(server_args)),
+            sorted(field.name for field in msgspec.structs.fields(server_args)),
             "the readback dump is no longer exactly the fields",
         )
         leaked = sorted(
@@ -532,7 +534,7 @@ class TestResolutionDeclarations(CustomTestCase):
             overlay = _stash_overlay(server_args)
             raw_input = getattr(server_args, "_raw_input", None)
             self.assertTrue(raw_input, f"{shape}: the record kept no raw snapshot")
-            for field in dataclasses.fields(server_args):
+            for field in msgspec.structs.fields(server_args):
                 name = field.name
                 if name in overlay or name not in raw_input:
                     continue

test/registered/unit/server_args/test_resolution_is_reproducible.py

@@ -33,6 +33,8 @@ import tempfile
 import unittest
 import unittest.mock
 
+import msgspec
+import msgspec.structs
 import torch
 
 from sglang.srt.arg_groups.overrides import (
@@ -270,7 +272,7 @@ class TestResolutionIsReproducible(_RestoresProcessState, CustomTestCase):
         the one of those that a shared mutable could corrupt.
         """
         out = {}
-        for field in dataclasses.fields(server_args):
+        for field in msgspec.structs.fields(server_args):
             if field.name in _NOT_COMPARABLE:
                 continue
             # The resolution result, not the field: a declaration-only resolver
@@ -517,7 +519,7 @@ class TestALateDeclarationKeepsTheResolution(_RestoresProcessState, CustomTestCa
         record arrives resolved, and a copy would throw that away.
         """
         parent = self._resolved()
-        bare = dataclasses.replace(parent, dist_init_addr="1.2.3.4:5000")
+        bare = msgspec.structs.replace(parent, dist_init_addr="1.2.3.4:5000")
         self.assertFalse(
             getattr(bare, "_resolution_finished", False),
             "a bare replace carried the flag; then this test proves nothing",
@@ -528,7 +530,7 @@ class TestALateDeclarationKeepsTheResolution(_RestoresProcessState, CustomTestCa
                 resolution_result(parent, field.name),
                 resolution_result(bare, field.name),
             )
-            for field in dataclasses.fields(parent)
+            for field in msgspec.structs.fields(parent)
             if field.name not in ("dist_init_addr", "random_seed")
             and repr(resolution_result(parent, field.name))
             != repr(resolution_result(bare, field.name))

test/registered/unit/server_args/test_resolution_reads_the_declarations.py

@@ -22,10 +22,12 @@ the two scopes it can derive exactly.
 """
 
 import ast
-import dataclasses
 import pathlib
 import re
 
+import msgspec
+import msgspec.structs
+
 import sglang
 from sglang.srt.server_args import ServerArgs
 from sglang.test.ci.ci_register import register_cpu_ci
@@ -34,7 +36,7 @@ from sglang.test.test_utils import CustomTestCase
 register_cpu_ci(est_time=45, suite="base-a-test-cpu")
 
 _SRT = pathlib.Path(sglang.__file__).resolve().parent / "srt"
-_FIELDS = frozenset(field.name for field in dataclasses.fields(ServerArgs))
+_FIELDS = frozenset(field.name for field in msgspec.structs.fields(ServerArgs))
 
 # Names a config travels under. `args` is included because the platform hooks
 # use it; a false positive would be a function taking an argparse Namespace and

test/registered/unit/server_args/test_server_args.py

@@ -1,5 +1,4 @@
 import argparse
-import dataclasses
 import json
 import os
 import pickle
@@ -10,6 +9,9 @@ import unittest
 from types import SimpleNamespace
 from unittest.mock import MagicMock, patch
 
+import msgspec
+import msgspec.structs
+
 import sglang.srt.server_args as server_args_module
 from sglang.srt.arg_groups import parallel_hook, pd_disaggregation_hook, serving_hook
 from sglang.srt.arg_groups.attention_hook import (
@@ -267,7 +269,7 @@ class TestPrepareServerArgs(CustomTestCase):
         # And across the hop that matters: the scheduler and the draft worker
         # rebuild the record from its fields and resolve again, so the bit has
         # to survive `asdict` and come back the same the second time.
-        reconstructed = ServerArgs(**dataclasses.asdict(inherited))
+        reconstructed = ServerArgs(**msgspec.structs.asdict(inherited))
         handle_missing_default_values(reconstructed)
 
         self.assertFalse(
@@ -1046,7 +1048,9 @@ class TestContextParallelServerArgs(CustomTestCase):
         ServerArgs.add_cli_args(self.parser)
 
     def _new_cp_args(self, **overrides):
-        server_args = object.__new__(ServerArgs)
+        # Constructed, not conjured: a Struct has no uninitialized form, and
+        # every field this case does not name wants its declared default
+        # anyway.
         defaults = dict(
             enable_prefill_cp=False,
             cp_strategy=None,
@@ -1061,9 +1065,7 @@ class TestContextParallelServerArgs(CustomTestCase):
             enable_aiter_allreduce_fusion=False,
         )
         defaults.update(overrides)
-        for key, value in defaults.items():
-            setattr(server_args, key, value)
-        return server_args
+        return ServerArgs(**defaults)
 
     def test_canonical_prefill_cp_requires_strategy(self):
         args = self.parser.parse_args(["--model", "dummy", "--enable-prefill-cp"])
@@ -2170,7 +2172,7 @@ class TestDeepEPv2Args(CustomTestCase):
             prefill=PhaseConfig(backend=Backend.FULL, max_bs=512),
         )
         server_args._resolved_overrides = []
-        valid = {f.name for f in dataclasses.fields(ServerArgs)}
+        valid = {f.name for f in msgspec.structs.fields(ServerArgs)}
         for key, value in overrides.items():
             # Reject stale field names before setattr silently accepts them.
             assert key in valid, f"{key} is not a ServerArgs field"
@@ -2489,7 +2491,7 @@ class TestHandleCrashDumpEnv(CustomTestCase):
     )
 
     def _run_handler(self, crash_dump_folder, preset_env=None):
-        server_args = ServerArgs.__new__(ServerArgs)
+        server_args = ServerArgs(model_path="dummy")
         server_args.crash_dump_folder = crash_dump_folder
         with patch.dict(os.environ, preset_env or {}):
             for key in self._COREDUMP_ENV_KEYS:
@@ -2868,7 +2870,7 @@ class TestTheInputIsSealedDuringResolution(CustomTestCase):
         server_args = ServerArgs(model_path="dummy", device="cuda")
         # The seal is what the pipeline runs under; drive it directly rather
         # than injecting a violation into a real handler.
-        object.__setattr__(server_args, "_input_frozen", True)
+        msgspec.Struct.__setattr__(server_args, "_input_frozen", True)
         with self.assertRaisesRegex(AttributeError, "during resolution"):
             server_args.tp_size = 4
         # and the message says what to do instead
@@ -2901,7 +2903,7 @@ class TestTheInputIsSealedDuringResolution(CustomTestCase):
         )
 
         server_args = ServerArgs(model_path="dummy", device="cuda")
-        object.__setattr__(server_args, "_input_frozen", True)
+        msgspec.Struct.__setattr__(server_args, "_input_frozen", True)
 
         def foreign(config):
             # What a plugin does: read what is decided, assign a default.
@@ -2961,7 +2963,7 @@ class TestLaunchCommand(CustomTestCase):
         """It describes how the configuration was asked for, so it is not part
         of the configuration: no CLI flag, no namespace, not in the bags."""
         self.assertNotIn(
-            "launch_command", {f.name for f in dataclasses.fields(ServerArgs)}
+            "launch_command", {f.name for f in msgspec.structs.fields(ServerArgs)}
         )
         self.assertNotIn(
             "launch_command", ServerArgs(model_path="/tmp/x").resolved_dict()

test/registered/unit/server_args/test_unified_prefill_cuda_graph_gate.py

@@ -32,6 +32,8 @@ user's stated intent), and decode capture is untouched either way.
 import unittest
 from types import SimpleNamespace
 
+import msgspec
+
 from sglang.srt.arg_groups.kv_cache_hook import handle_unified_memory_pool
 from sglang.srt.model_executor.cuda_graph_config import Backend
 from sglang.srt.server_args import ServerArgs
@@ -42,7 +44,7 @@ register_cpu_ci(est_time=8, suite="base-a-test-cpu")
 
 def _run_handler(*, prefill_backend, explicit):
     """Run just `handle_unified_memory_pool` over a minimal stand-in."""
-    sa = ServerArgs.__new__(ServerArgs)
+    sa = ServerArgs(model_path="dummy")
     cg = SimpleNamespace(
         prefill=SimpleNamespace(backend=prefill_backend),
         decode=SimpleNamespace(backend=Backend.FULL),
@@ -59,7 +61,7 @@ def _run_handler(*, prefill_backend, explicit):
         "cuda_graph_config": cg,
         "cuda_graph_backend_prefill": prefill_backend if explicit else None,
     }.items():
-        object.__setattr__(sa, name, value)
+        msgspec.Struct.__setattr__(sa, name, value)
     handle_unified_memory_pool(sa)
     return cg
 

test/registered/unit/server_args/test_unified_tbo_gate.py

@@ -23,6 +23,8 @@ pair, so without this gate a running server crashes mid-serving.
 import unittest
 from types import SimpleNamespace
 
+import msgspec
+
 from sglang.srt.arg_groups.kv_cache_hook import handle_unified_memory_pool
 from sglang.srt.model_executor.cuda_graph_config import Backend
 from sglang.srt.server_args import ServerArgs
@@ -33,7 +35,7 @@ register_cpu_ci(est_time=5, suite="base-a-test-cpu")
 
 def _run_handler(*, unified, tbo):
     """Run just `handle_unified_memory_pool` over a minimal stand-in."""
-    sa = ServerArgs.__new__(ServerArgs)
+    sa = ServerArgs(model_path="dummy")
     for name, value in {
         "enable_unified_memory": unified,
         "enable_two_batch_overlap": tbo,
@@ -49,7 +51,7 @@ def _run_handler(*, unified, tbo):
         ),
         "cuda_graph_backend_prefill": Backend.DISABLED,
     }.items():
-        object.__setattr__(sa, name, value)
+        msgspec.Struct.__setattr__(sa, name, value)
     handle_unified_memory_pool(sa)
 
 

test/registered/unit/test_model_overrides.py

@@ -36,6 +36,7 @@ from sglang.srt.runtime_context import (
     override_platform,
     reset_context,
 )
+from sglang.srt.server_args import _declared_default
 from sglang.test.test_utils import CustomTestCase
 
 
@@ -1613,14 +1614,13 @@ class TestGoldenModelOverrides(_IsolatedPublish):
         from sglang.srt.arg_groups.model_overrides.deepseek_v4 import (
             _deepseek_v4_overrides,
         )
-        from sglang.srt.server_args import ServerArgs
 
         hf = SimpleNamespace(architectures=["DeepseekV4ForCausalLM"])
 
         def _args(**kw):
             defaults = dict(
                 device="cuda",
-                swa_full_tokens_ratio=ServerArgs.swa_full_tokens_ratio,
+                swa_full_tokens_ratio=_declared_default("swa_full_tokens_ratio"),
                 moe_a2a_backend="none",
                 moe_runner_backend="auto",
                 _model_config=SimpleNamespace(is_fp4_experts=True, nvfp4_moe_meta=None),

test/registered/unit/test_runtime_context.py

@@ -14,6 +14,9 @@ import unittest
 import warnings
 from unittest.mock import patch
 
+import msgspec
+import msgspec.structs
+
 import sglang as _sglang
 import sglang.srt.server_args as server_args_module
 from sglang.srt.arg_groups.arg_utils import NS, A, Arg
@@ -438,7 +441,7 @@ class TestServerArgsScopedOverride(_IsolatedServerArgs):
         from sglang.srt.runtime_context import get_spec
 
         name = "_speculative_draft_quantization_explicitly_set"
-        self.assertIn(name, ServerArgs.__dataclass_fields__)
+        self.assertIn(name, ServerArgs.__struct_fields__)
 
         published = get_context().override_server_args(**{name: True}).install()
         # The record keeps the operator's input, as it does for every other
@@ -473,7 +476,6 @@ class TestServerArgsScopedOverride(_IsolatedServerArgs):
             override.install()
 
 
-@dataclasses.dataclass
 class _FakeCaptureGroup(_FlagGroupBase):
     gamma: int = 0
 
@@ -1639,13 +1641,12 @@ class TestTheDerivedHalfIsDeclared(CustomTestCase):
     def test_a_declared_quotient_is_not_a_record_field(self):
         """It has no operator input to preserve, and the record is what crosses
         a process boundary."""
-        import dataclasses
 
         from sglang.srt.arg_groups.arg_utils import Derived
         from sglang.srt.arg_groups.fields.parallel import Parallel
         from sglang.srt.server_args import ServerArgs
 
-        fields = {f.name for f in dataclasses.fields(ServerArgs)}
+        fields = {f.name for f in msgspec.structs.fields(ServerArgs)}
         for name, value in vars(Parallel).items():
             if isinstance(value, Derived):
                 self.assertNotIn(name, fields)

test/registered/unit/test_runtime_context_config_bags.py

@@ -9,6 +9,9 @@ import dataclasses
 import unittest
 from unittest import mock
 
+import msgspec
+import msgspec.structs
+
 from sglang.srt import runtime_context as rc
 from sglang.srt.arg_groups.arg_utils import NS, A
 from sglang.srt.arg_groups.overrides import resolution_result
@@ -83,7 +86,7 @@ class TestConfigBags(CustomTestCase):
         import dataclasses
 
         sa, reference = self._resolve_published_and_sibling()
-        defaults = {f.name: f.default for f in dataclasses.fields(ServerArgs)}
+        defaults = {f.name: f.default for f in msgspec.structs.fields(ServerArgs)}
         # Leaves resolution writes on this input on both CI device shapes
         # (CUDA host and CPU-only runner): each starts at a None default.
         sampled = (

test/registered/unit/test_runtime_context_override.py

@@ -8,6 +8,8 @@ field aborts before any write; provenance is recorded.
 
 import unittest
 
+import msgspec
+
 from sglang.srt import runtime_context as rc
 from sglang.srt.server_args import ServerArgs
 from sglang.test.ci.ci_register import register_cpu_ci
@@ -110,7 +112,7 @@ class TestContextOverride(CustomTestCase):
         # server_args is read-only after resolution: resolved config changes go
         # to the bags, a per-runner config to a derived variant.
         sa = ServerArgs(model_path="dummy")
-        object.__setattr__(sa, "_resolution_finished", True)
+        msgspec.Struct.__setattr__(sa, "_resolution_finished", True)
         with self.assertRaises(AttributeError):
             sa.page_size = 999
 

test/registered/unit/test_server_args_cli_metadata.py

@@ -3,7 +3,7 @@
 import argparse
 import unittest
 
-from sglang.srt.server_args import ServerArgs
+from sglang.srt.server_args import ServerArgs, _declared_default
 from sglang.srt.utils.common import human_readable_int
 from sglang.test.ci.ci_register import register_cpu_ci
 from sglang.test.test_utils import CustomTestCase
@@ -23,7 +23,9 @@ class TestServerArgsMigratedCliMetadata(CustomTestCase):
         }
 
     def test_argparse_shape_is_preserved_for_representative_migrated_options(self):
-        self.assertEqual(self.actions_by_option["--dtype"].default, ServerArgs.dtype)
+        self.assertEqual(
+            self.actions_by_option["--dtype"].default, _declared_default("dtype")
+        )
         self.assertEqual(
             self.actions_by_option["--dtype"].choices,
             ["auto", "half", "float16", "bfloat16", "float", "float32"],

test/registered/unit/test_server_args_namespaces.py

@@ -11,9 +11,11 @@ This is the guardrail that fails when an upstream PR adds a field to a namespace
 class that has no ``_NS_PATH``, or adds one outside the taxonomy below.
 """
 
-import dataclasses
 import unittest
 
+import msgspec
+import msgspec.structs
+
 from sglang.srt.arg_groups.arg_utils import namespace_of
 from sglang.srt.server_args import ServerArgs
 from sglang.test.ci.ci_register import register_cpu_ci
@@ -48,7 +50,7 @@ VALID_NAMESPACES = {
 
 
 def _field_names():
-    return {f.name for f in dataclasses.fields(ServerArgs)}
+    return {f.name for f in msgspec.structs.fields(ServerArgs)}
 
 
 class TestServerArgsNamespaces(CustomTestCase):

test/registered/unit/test_split_attention_backend_decisions.py

@@ -153,13 +153,13 @@ class TestSplitBackendsReachTheDecisions(CustomTestCase):
     def test_the_flashinfer_version_guard_sees_a_split_launch(self):
         # The launcher runs before any publish, so it asks the record; the
         # member and the accessor answer the same pair.
-        args = ServerArgs.__new__(ServerArgs)
+        args = ServerArgs(model_path="dummy")
         for name, value in (
             ("attention_backend", None),
             ("prefill_attention_backend", None),
             ("decode_attention_backend", "flashinfer"),
         ):
-            object.__setattr__(args, name, value)
+            setattr(args, name, value)
         self.assertIn("flashinfer", attention_backends_of(resolved_view(args)))
 
     def test_support_triton_is_the_regression_being_guarded(self):

test/registered/unit/test_supplied_instance_exposure_ratchet.py

@@ -43,7 +43,6 @@ process-wide at all, so neither the read nor the field is on this axis.
 """
 
 import ast
-import dataclasses
 import json
 import os
 import shutil
@@ -51,6 +50,9 @@ import tempfile
 import unittest
 from pathlib import Path
 
+import msgspec
+import msgspec.structs
+
 import sglang
 from sglang.srt.server_args import ServerArgs
 from sglang.test.ci.ci_register import register_cpu_ci, register_cuda_ci
@@ -367,10 +369,10 @@ class TestSuppliedInstanceExposure(CustomTestCase):
                     "would drift"
                 )
             defaults = {}
-            for field in dataclasses.fields(resolved):
-                if field.default is not dataclasses.MISSING:
+            for field in msgspec.structs.fields(resolved):
+                if field.default is not msgspec.NODEFAULT:
                     defaults[field.name] = field.default
-                elif field.default_factory is not dataclasses.MISSING:
+                elif field.default_factory is not msgspec.NODEFAULT:
                     defaults[field.name] = field.default_factory()
             for field_name, default in defaults.items():
                 if field_name in _PASSED or field_name in extra: