Fix the chunked-prefix-cache gate writing config the backends never read (#33168)

The load-time gate (maybe_disable_chunked_prefix_cache) wrote its
ServerArgs instance while every reader has moved to the published
config: the attention backends assert / branch on
get_schedule().disable_chunked_prefix_cache when they initialize, so the
flip never reached them and a backend outside
CHUNKED_PREFIX_CACHE_SUPPORTED_ATTENTION_BACKENDS kept chunked prefix
enabled.

Reroute the writer through get_context().override (which writes the
published bags) and flip the two remaining instance reads — the gate's
own log check and the prefill cuda-graph runner's capture flag — to the
bag. A regression test pins the three contracts: the gate lands on the
bag, the pristine ServerArgs instance stays untouched, and the
draft-worker guard never writes.

The ServerArgs.override call-site ratchet drops 39 -> 38.

python/sglang/srt/model_executor/model_runner.py

@@ -310,17 +310,16 @@ class ModelRunner:
         if server_args.show_time_cost:
             enable_show_time_cost()
 
-        misc_utils.maybe_disable_chunked_prefix_cache(
-            server_args=server_args,
-            use_mla_backend=self.use_mla_backend,
-            is_draft_worker=self.is_draft_worker,
-        )
-
         # Set the global server_args in the scheduler process (target worker
         # only, so a draft init cannot clobber target-derived global state).
         if not self.is_draft_worker:
             set_global_server_args_for_scheduler(server_args)
 
+        misc_utils.maybe_disable_chunked_prefix_cache(
+            use_mla_backend=self.use_mla_backend,
+            is_draft_worker=self.is_draft_worker,
+        )
+
         # Init OpenMP threads binding for CPU
         if self.device == "cpu":
             self.init_threads_binding()

python/sglang/srt/model_executor/model_runner_components/misc_utils.py

@@ -4,6 +4,7 @@ import logging
 from typing import TYPE_CHECKING, Any, Optional
 
 from sglang.srt.configs.model_config import dsa_layer_skips_topk, is_deepseek_dsa
+from sglang.srt.runtime_context import get_context, get_exec, get_schedule
 from sglang.srt.server_args import CHUNKED_PREFIX_CACHE_SUPPORTED_ATTENTION_BACKENDS
 
 if TYPE_CHECKING:
@@ -14,7 +15,7 @@ logger = logging.getLogger(__name__)
 
 
 def maybe_disable_chunked_prefix_cache(
-    *, server_args: ServerArgs, use_mla_backend: bool, is_draft_worker: bool
+    *, use_mla_backend: bool, is_draft_worker: bool
 ) -> None:
     # Chunked prefix caching requires an MLA model on a backend whose
     # kernels read that layout. This is a load-time gate, not a
@@ -26,15 +27,15 @@ def maybe_disable_chunked_prefix_cache(
         return
     if (
         not use_mla_backend
-        or server_args.attention_backend
+        or get_exec().kernel.attention_backend
         not in CHUNKED_PREFIX_CACHE_SUPPORTED_ATTENTION_BACKENDS
     ):
-        if not server_args.disable_chunked_prefix_cache:
+        if not get_schedule().disable_chunked_prefix_cache:
-            server_args.override(
+            get_context().override(
                 "model_runner.chunked_prefix_cache_gate",
                 disable_chunked_prefix_cache=True,
             )
-    if not server_args.disable_chunked_prefix_cache:
+    if not get_schedule().disable_chunked_prefix_cache:
         logger.info("Chunked prefix cache is turned on.")
 
 

python/sglang/srt/model_executor/runner/prefill_cuda_graph_runner.py

@@ -103,7 +103,7 @@ from sglang.srt.model_executor.runner_utils.buffers import (
     PrefillInputBuffers,
 )
 from sglang.srt.model_loader.utils import resolve_language_model
-from sglang.srt.runtime_context import get_parallel
+from sglang.srt.runtime_context import get_parallel, get_schedule
 from sglang.srt.speculative.eagle_utils import get_draft_input_from_target_hidden_dim
 from sglang.srt.utils import (
     get_available_gpu_memory,
@@ -403,8 +403,7 @@ class PrefillCudaGraphRunner(BaseCudaGraphRunner):
         # This flag controls whether the model dispatches through the distinct
         # chunked-prefix topology; backend capability is validated separately.
         self._capture_chunked_prefix = (
-            self._is_full_backend
+            self._is_full_backend and not get_schedule().disable_chunked_prefix_cache
-            and not model_runner.server_args.disable_chunked_prefix_cache
         )
         self._prefix_chunk_len = 0
         self._prefix_chunk_capacity = 0

python/sglang/srt/speculative/draft_worker_common.py

@@ -10,7 +10,7 @@ import torch
 from sglang.srt.layers.logits_processor import LogitsProcessorOutput
 from sglang.srt.managers.tp_worker import TpModelWorker
 from sglang.srt.model_executor.forward_batch_info import CaptureHiddenMode
-from sglang.srt.runtime_context import get_context
+from sglang.srt.runtime_context import get_context, get_schedule
 from sglang.srt.server_args import ServerArgs
 from sglang.srt.speculative.dflash_info import DFlashVerifyInput
 from sglang.srt.speculative.dflash_info_v2 import DFlashDraftInputV2
@@ -61,6 +61,26 @@ def _resolve_draft_attention_backend_fallback(
     return draft_backend
 
 
+def draft_server_args_overrides(target_model_config, draft_backend) -> dict:
+    """Pre-publish field adjustments for a draft ``ServerArgs`` copy.
+
+    Downstream draft-worker logic keys on ``speculative_draft_attention_backend``
+    (backend selection in ``_get_attention_backend``, the fa4-draft KV dtype
+    override in ``configure_kv_cache_dtype``); ``context_length`` keeps the
+    draft aligned with the target; ``disable_chunked_prefix_cache`` is the
+    target's resolved gate (bag-only, absent from the pristine copy).
+    """
+    return dict(
+        skip_tokenizer_init=True,
+        speculative_draft_attention_backend=draft_backend,
+        prefill_attention_backend=None,
+        decode_attention_backend=None,
+        attention_backend=draft_backend,
+        context_length=target_model_config.context_len,
+        disable_chunked_prefix_cache=get_schedule().disable_chunked_prefix_cache,
+    )
+
+
 def build_draft_tp_worker(
     *,
     server_args: ServerArgs,
@@ -81,20 +101,10 @@ def build_draft_tp_worker(
         )
     )
     # Post-resolution ServerArgs rejects bare assignment; route the draft-copy
-    # adjustments through the audited mutation point. Keep the resolved value
+    # adjustments through the audited mutation point.
-    # on speculative_draft_attention_backend: downstream draft-worker logic
-    # keys on that field (backend selection in _get_attention_backend and the
-    # fa4-draft KV dtype override in configure_kv_cache_dtype), so nulling it
-    # would silently skip those paths. context_length keeps the draft aligned
-    # with the target.
     draft_server_args.override(
         "draft_worker.build",
-        skip_tokenizer_init=True,
+        **draft_server_args_overrides(target_model_config, draft_backend),
-        speculative_draft_attention_backend=draft_backend,
-        prefill_attention_backend=None,
-        decode_attention_backend=None,
-        attention_backend=draft_backend,
-        context_length=target_model_config.context_len,
     )
 
     # The draft's layers must resolve config from the draft's own bags.

test/registered/unit/model_executor/test_chunked_prefix_cache_gate.py

@@ -0,0 +1,72 @@
+"""The load-time chunked-prefix gate must land on the published config bag.
+
+Regression: the gate wrote the ServerArgs instance while every attention
+backend reads ``get_schedule().disable_chunked_prefix_cache`` — the bag never
+saw the flip, so an unsupported backend kept chunked prefix enabled.
+"""
+
+from sglang.test.ci.ci_register import register_cpu_ci
+
+register_cpu_ci(est_time=4, suite="base-a-test-cpu")
+
+import unittest
+
+from sglang.srt.model_executor.model_runner_components.misc_utils import (
+    maybe_disable_chunked_prefix_cache,
+)
+from sglang.srt.runtime_context import get_context, get_schedule, get_server_args
+from sglang.test.test_utils import CustomTestCase
+
+
+class TestChunkedPrefixCacheGate(CustomTestCase):
+    def _seed(self, **fields):
+        override = get_context().override_server_args(**fields)
+        override.install()
+        self.addCleanup(override.restore)
+
+    def test_unsupported_backend_disables_on_the_bag(self):
+        self._seed(attention_backend="triton")
+        maybe_disable_chunked_prefix_cache(use_mla_backend=True, is_draft_worker=False)
+        self.assertTrue(get_schedule().disable_chunked_prefix_cache)
+        self.assertFalse(get_server_args().disable_chunked_prefix_cache)
+
+    def test_supported_backend_keeps_chunked_prefix(self):
+        self._seed(attention_backend="fa3")
+        maybe_disable_chunked_prefix_cache(use_mla_backend=True, is_draft_worker=False)
+        self.assertFalse(get_schedule().disable_chunked_prefix_cache)
+
+    def test_draft_worker_never_writes(self):
+        self._seed(attention_backend="triton")
+        maybe_disable_chunked_prefix_cache(use_mla_backend=False, is_draft_worker=True)
+        self.assertFalse(get_schedule().disable_chunked_prefix_cache)
+
+    def test_republish_discards_the_gate_so_it_must_run_after_publish(self):
+        # Pins the ordering contract in ModelRunner.__init__: publishing
+        # rebuilds the bags from the pristine instance, so the gate runs after
+        # the target-worker publish.
+        self._seed(attention_backend="triton")
+        sa = get_server_args()
+        maybe_disable_chunked_prefix_cache(use_mla_backend=True, is_draft_worker=False)
+        self.assertTrue(get_schedule().disable_chunked_prefix_cache)
+        get_context().set_server_args(sa)  # what a later republish would do
+        self.assertFalse(get_schedule().disable_chunked_prefix_cache)
+
+    def test_draft_copy_overrides_carry_the_gate(self):
+        # The draft copy comes from the pristine instance, which never sees
+        # the bag-only gate; the copy's pre-publish overrides carry it.
+        from types import SimpleNamespace
+
+        from sglang.srt.speculative.draft_worker_common import (
+            draft_server_args_overrides,
+        )
+
+        self._seed(attention_backend="triton")
+        maybe_disable_chunked_prefix_cache(use_mla_backend=True, is_draft_worker=False)
+        fields = draft_server_args_overrides(
+            SimpleNamespace(context_len=64), draft_backend="fa3"
+        )
+        self.assertTrue(fields["disable_chunked_prefix_cache"])
+
+
+if __name__ == "__main__":
+    unittest.main()

test/registered/unit/test_server_args_writer_ratchet.py

@@ -49,7 +49,7 @@ _EXCLUDED = (
     "multimodal_gen",
 )
 
-_BASELINE = 39
+_BASELINE = 38
 
 
 class TestServerArgsWriterRatchet(CustomTestCase):