[diffusion] BCG: auto-capture the default warmup resolution instead of hard-requiring --warmup-resolutions (H200 SANA denoise 0.73->0.457 s with a single flag) (#34174)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

python/sglang/multimodal_gen/runtime/server_args/server_args.py

@@ -549,11 +549,16 @@ class ServerArgs(DisaggServerArgsMixin):
         # latent shape, so the user must declare the resolutions up front. We
         # capture every one of them at warmup; serving then never re-captures.
         if not self.warmup_resolutions:
-            raise ValueError(
-                "--enable-breakable-cuda-graph requires --warmup-resolutions: "
-                "diffusion CUDA graphs only replay for a fixed resolution, so "
-                "every served resolution must be declared and captured at "
-                "warmup, e.g. --warmup-resolutions 1024x1024 1328x1328."
+            # No explicit resolutions: capture the model's default warmup
+            # resolution (derived by build_warmup_reqs) so
+            # --enable-breakable-cuda-graph works standalone. BCG graphs are
+            # resolution-specific; a request at any other resolution simply
+            # falls back to eager (the runner never re-captures at serving
+            # time). Pass --warmup-resolutions to capture additional shapes.
+            logger.info(
+                "[Diffusion BCG] --warmup-resolutions unset; capturing the "
+                "model default warmup resolution. Requests at other "
+                "resolutions run eager."
             )
         if self.bcg_text_buckets is not None and not any(
             int(b) > 0 for b in self.bcg_text_buckets
@@ -572,6 +577,8 @@ class ServerArgs(DisaggServerArgsMixin):
             pipeline_config_name in BREAKABLE_CUDA_GRAPH_SUPPORTED_PIPELINE_CONFIGS
             and self._is_breakable_cuda_graph_supported_model()
         ):
+            if not self.warmup_resolutions:
+                self._default_bcg_warmup_resolution()
             return
 
         logger.warning(
@@ -588,6 +595,39 @@ class ServerArgs(DisaggServerArgsMixin):
         refs.update(_normalized_bcg_model_refs(self.model_path))
         return bool(refs & BREAKABLE_CUDA_GRAPH_SUPPORTED_MODEL_IDS)
 
+    def _default_bcg_warmup_resolution(self) -> None:
+        """Seed --warmup-resolutions with the model default for BCG.
+
+        BCG graphs are resolution-specific and captured at warmup. When the
+        user does not pre-declare resolutions we capture the model's default
+        warmup resolution so --enable-breakable-cuda-graph works standalone;
+        requests at any other resolution fall back to eager.
+        """
+        from sglang.multimodal_gen.runtime.warmup_request_builder import (
+            _resolve_default_warmup_resolution,
+            get_model_sampling_defaults,
+        )
+
+        try:
+            sampling_defaults = get_model_sampling_defaults(self)
+            width, height = _resolve_default_warmup_resolution(
+                self, sampling_defaults, server_based_warmup=True
+            )
+        except Exception as exc:  # pragma: no cover - defensive
+            logger.warning(
+                "[Diffusion BCG] could not derive a default warmup resolution "
+                "(%s); no graph will be captured and serving runs eager.",
+                exc,
+            )
+            return
+        self.warmup_resolutions = [f"{width}x{height}"]
+        logger.info(
+            "[Diffusion BCG] --warmup-resolutions unset; capturing the model "
+            "default %dx%d. Requests at other resolutions run eager.",
+            width,
+            height,
+        )
+
     def _adjust_save_paths(self):
         """Normalize empty-string save paths to None (disabled)."""
         if self.output_path is not None and self.output_path.strip() == "":

python/sglang/multimodal_gen/test/unit/test_server_args.py

@@ -588,6 +588,15 @@ class TestWarmupModeNormalization(unittest.TestCase):
         sa = self._resolve(enable_breakable_cuda_graph=True)
         self.assertEqual(sa.warmup_mode, "server")
 
+    def test_breakable_cuda_graph_allows_unset_resolutions(self):
+        # BCG no longer hard-requires --warmup-resolutions; the model
+        # default warmup resolution is captured at warmup instead.
+        sa = ServerArgs.__new__(ServerArgs)
+        sa.enable_breakable_cuda_graph = True
+        sa.warmup_resolutions = None
+        sa.bcg_text_buckets = None
+        sa._validate_breakable_cuda_graph()  # must not raise
+
     def test_disagg_role_disables_server_warmup(self):
         from sglang.multimodal_gen.runtime.disaggregation.roles import RoleType