[diffusion] refactor: resolve indexed component weight sets (#36883)

python/sglang/multimodal_gen/runtime/loader/component_loaders/transformer_loader.py

@@ -25,9 +25,9 @@ from sglang.multimodal_gen.runtime.loader.minimax_h3_weights import (
 )
 from sglang.multimodal_gen.runtime.loader.transformer_load_utils import (
     TransformerQuantLoadSpec,
+    resolve_transformer_checkpoint_files,
     resolve_transformer_gguf_to_load,
     resolve_transformer_quant_load_spec,
-    resolve_transformer_safetensors_to_load,
 )
 from sglang.multimodal_gen.runtime.loader.utils import _normalize_component_type
 from sglang.multimodal_gen.runtime.loader.weight_load_plan import WeightLoadPlan
@@ -214,10 +214,13 @@ class TransformerLoader(ComponentLoader):
             # A GGUF file holds the whole transformer; the remaining components
             # still load from the base model path.
             safetensors_list = []
+            transformer_override_config_path = None
         else:
-            safetensors_list = resolve_transformer_safetensors_to_load(
+            checkpoint_files = resolve_transformer_checkpoint_files(
                 component_server_args, component_model_path
             )
+            safetensors_list = list(checkpoint_files.safetensors)
+            transformer_override_config_path = checkpoint_files.config_path
 
         # 2. dit config
         # Config from Diffusers supersedes sgl_diffusion's model config
@@ -286,6 +289,7 @@ class TransformerLoader(ComponentLoader):
             component_name=component_name,
             gguf_file=gguf_file,
             checkpoint_quant_config=checkpoint_quant_config,
+            transformer_override_config_path=transformer_override_config_path,
         )
         if quant_spec.gguf_file is not None and is_minimax_h3:
             assert quant_spec.quant_config is not None

python/sglang/multimodal_gen/runtime/loader/transformer_load_utils.py

@@ -45,11 +45,6 @@ from sglang.multimodal_gen.runtime.managers.memory_managers.component_residency
 )
 from sglang.multimodal_gen.runtime.platforms import current_platform
 from sglang.multimodal_gen.runtime.server_args import ServerArgs
-from sglang.multimodal_gen.runtime.utils.hf_diffusers_utils import (
-    hf_hub_download,
-    maybe_download_model,
-    snapshot_download,
-)
 from sglang.multimodal_gen.runtime.utils.logging_utils import init_logger
 from sglang.multimodal_gen.runtime.utils.precision import resolve_precision
 from sglang.multimodal_gen.runtime.utils.quantization_utils import (
@@ -58,6 +53,11 @@ from sglang.multimodal_gen.runtime.utils.quantization_utils import (
     get_quant_config,
     get_quant_config_from_safetensors_metadata,
 )
+from sglang.multimodal_gen.runtime.weights.source import (
+    materialize_weight_set,
+    materialize_weight_set_config,
+    resolve_safetensors_weight_set,
+)
 from sglang.srt.utils.hf_transformers import (
     check_gguf_file,
     resolve_hf_gguf_reference,
@@ -71,11 +71,6 @@ _PRECISION_VARIANT_SUFFIX_RE = re.compile(
     r"^(?P<stem>.+?)(?P<precision>\.(?:fp16|bf16|fp32))(?P<shard>-\d+-of-\d+)?(?P<ext>\.safetensors)$"
 )
 _MIXED_SAFETENSORS_RE = re.compile(r".*-mixed(?:-\d+-of-\d+)?\.safetensors$")
-_HF_SAFETENSORS_URL_RE = re.compile(
-    r"https?://huggingface\.co/(?P<repo>[^/]+/[^/]+)/"
-    r"(?:blob|resolve)/(?P<revision>[^/]+)/(?P<filename>.+\.safetensors)$",
-    re.IGNORECASE,
-)
 
 
 def _get_quant_config_name(config: Optional[QuantizationConfig]) -> Optional[str]:
@@ -213,6 +208,14 @@ class TransformerQuantLoadSpec:
         )
 
 
+@dataclass(frozen=True)
+class TransformerCheckpointFiles:
+    """Files from one checkpoint revision needed during transformer loading."""
+
+    safetensors: tuple[str, ...]
+    config_path: str | None
+
+
 class _TransformerQuantAdapter:
     def prepare(self) -> None:
         """initialize"""
@@ -612,86 +615,55 @@ def resolve_transformer_gguf_to_load(
     return resolved
 
 
-def resolve_transformer_safetensors_to_load(
+def resolve_transformer_checkpoint_files(
     server_args: ServerArgs, component_model_path: str
-) -> list[str]:
+) -> TransformerCheckpointFiles:
     """Resolve transformer weights from the base component path or an override."""
     quantized_path = server_args.transformer_weights_path
 
     if quantized_path:
-        original_quantized_path = quantized_path
+        resolved_set = resolve_safetensors_weight_set(
-        direct_url = _HF_SAFETENSORS_URL_RE.fullmatch(original_quantized_path)
-        if direct_url is not None:
-            quantized_path = hf_hub_download(
-                repo_id=direct_url.group("repo"),
-                filename=direct_url.group("filename"),
-                revision=direct_url.group("revision"),
-            )
-        else:
-            parts = original_quantized_path.strip("/").split("/")
-            is_hub_file = (
-                not os.path.exists(original_quantized_path)
-                and not os.path.isabs(original_quantized_path)
-                and not original_quantized_path.startswith((".", "~"))
-                and len(parts) > 2
-                and original_quantized_path.endswith(".safetensors")
-            )
-            quantized_path = (
-                hf_hub_download(
-                    repo_id="/".join(parts[:2]),
-                    filename="/".join(parts[2:]),
-                    revision=server_args.revision,
-                )
-                if is_hub_file
-                else maybe_download_model(original_quantized_path)
-            )
-        logger.info("using quantized transformer weights from: %s", quantized_path)
-        if os.path.isfile(quantized_path) and quantized_path.endswith(".safetensors"):
-            safetensors_list = [quantized_path]
-        else:
-            safetensors_list = _list_safetensors_files(quantized_path)
-            if not safetensors_list and not os.path.exists(original_quantized_path):
-                logger.warning(
-                    "No safetensors files found in cached transformer weights path "
-                    "%s; refreshing snapshot for %s",
             quantized_path,
-                    original_quantized_path,
+            revision=server_args.revision,
+            select_unindexed_weight=_select_single_mixed_safetensors_file,
         )
-                quantized_path = snapshot_download(
+        safetensors_list = materialize_weight_set(resolved_set)
-                    repo_id=original_quantized_path,
+        logger.info(
-                    ignore_patterns=["*.onnx", "*.msgpack"],
+            "using transformer weight set from %s: %s",
-                    allow_patterns=[
+            quantized_path,
-                        "*.json",
+            safetensors_list,
-                        "*.safetensors",
+        )
-                        "*.safetensors.index.json",
+        return TransformerCheckpointFiles(
-                    ],
+            safetensors=safetensors_list,
-                    max_workers=8,
+            config_path=materialize_weight_set_config(resolved_set),
         )
-                safetensors_list = _list_safetensors_files(quantized_path)
-    else:
-        safetensors_list = _list_safetensors_files(component_model_path)
 
+    safetensors_list = _list_safetensors_files(component_model_path)
     if safetensors_list:
-        # Diffusers repos occasionally ship more than one shard split for the
+        # Preserve legacy cleanup for the base component. Explicit overrides
-        # same checkpoint (e.g. a 4-way and an 8-way split side by side). The
+        # are resolved above, where an index is already the final authority.
-        # index file is the authoritative source for which files belong to
-        # the checkpoint that was actually exported; anything else is a
-        # leftover sibling variant.
         safetensors_list = filter_duplicate_safetensors_files(
             safetensors_list,
             os.path.dirname(safetensors_list[0]),
             SAFE_WEIGHTS_INDEX_NAME,
         )
-
         safetensors_list = _prefer_mixed_safetensors_files(safetensors_list)
-    safetensors_list = _filter_duplicate_precision_variant_safetensors(safetensors_list)
+        safetensors_list = _filter_duplicate_precision_variant_safetensors(
-
+            safetensors_list
-    if not safetensors_list:
-        raise ValueError(
-            f"no safetensors files found in {quantized_path or component_model_path}"
         )
 
-    return safetensors_list
+    if not safetensors_list:
+        raise ValueError(f"no safetensors files found in {component_model_path}")
+
+    return TransformerCheckpointFiles(tuple(safetensors_list), None)
+
+
+def _select_single_mixed_safetensors_file(
+    candidates: tuple[str, ...],
+) -> str | None:
+    """Preserve the transformer's established mixed-export preference."""
+    mixed = tuple(path for path in candidates if _MIXED_SAFETENSORS_RE.fullmatch(path))
+    return mixed[0] if len(mixed) == 1 else None
 
 
 def _prefer_mixed_safetensors_files(safetensors_list: list[str]) -> list[str]:
@@ -772,6 +744,7 @@ def resolve_transformer_quant_load_spec(
     component_name: str | None = None,
     gguf_file: str | None = None,
     checkpoint_quant_config: QuantizationConfig | None = None,
+    transformer_override_config_path: str | None = None,
 ) -> TransformerQuantLoadSpec:
     if gguf_file is not None:
         if checkpoint_quant_config is not None:
@@ -803,6 +776,7 @@ def resolve_transformer_quant_load_spec(
             server_args=server_args,
             safetensors_list=safetensors_list,
             component_model_path=component_model_path,
+            transformer_override_config_path=transformer_override_config_path,
         )
 
     if quant_config is not None:
@@ -945,37 +919,15 @@ def _build_transformer_quant_adapters(
 
 
 def _resolve_quant_config_from_transformer_override(
-    transformer_weights_path: str,
+    override_config_path: str,
 ) -> Optional[QuantizationConfig]:
     """Resolve quant config from an override transformer repo or directory."""
-    expanded_path = os.path.expanduser(transformer_weights_path)
-    if os.path.isfile(expanded_path):
-        return None
-
-    # A single local safetensors file does not carry a directory-level config.json.
-    # Let downstream metadata probing handle it instead of misrouting it through HF.
-    if expanded_path.endswith(".safetensors") and (
-        os.path.isabs(expanded_path)
-        or expanded_path.startswith(".")
-        or os.sep in expanded_path
-        or (os.path.altsep and os.path.altsep in expanded_path)
-    ):
-        return None
-
-    override_quantized_path = maybe_download_model(transformer_weights_path)
-    if not os.path.isdir(override_quantized_path):
-        return None
-
-    override_config_path = os.path.join(override_quantized_path, "config.json")
-    if not os.path.isfile(override_config_path):
-        return None
-
     with open(override_config_path, encoding="utf-8") as f:
         override_hf_config = json.load(f)
 
     return get_quant_config(
         override_hf_config,
-        override_quantized_path,
+        os.path.dirname(override_config_path),
     )
 
 
@@ -985,6 +937,7 @@ def _resolve_quant_config(
     server_args: ServerArgs,
     safetensors_list: list[str],
     component_model_path: str,
+    transformer_override_config_path: str | None = None,
 ) -> Optional[QuantizationConfig]:
     """
     resolve quant config from checkpoints' metadata
@@ -1055,11 +1008,11 @@ def _resolve_quant_config(
             fallback_group_size,
         )
     quant_config = _merge_modelopt_fp4_configs(quant_config, inferred_nvfp4_config)
-    if quant_config is not None or not server_args.transformer_weights_path:
+    if quant_config is not None or transformer_override_config_path is None:
         return quant_config
 
     quant_config = _resolve_quant_config_from_transformer_override(
-        server_args.transformer_weights_path
+        transformer_override_config_path,
     )
     quant_config = _merge_modelopt_fp4_configs(quant_config, inferred_nvfp4_config)
     if quant_config is not None:

python/sglang/multimodal_gen/runtime/weights/source.py

@@ -2,8 +2,10 @@
 
 from __future__ import annotations
 
+import json
 import os
-from dataclasses import dataclass
+from collections.abc import Callable, Mapping
+from dataclasses import dataclass, replace
 from pathlib import Path, PurePosixPath
 from typing import Literal
 from urllib.parse import unquote, urlparse
@@ -14,6 +16,8 @@ from huggingface_hub.utils import validate_repo_id
 WeightSourceKind = Literal["local", "huggingface"]
 
 _WEIGHT_SUFFIXES = (".safetensors", ".gguf", ".bin", ".pt", ".pth", ".ckpt")
+_SAFETENSORS_INDEX_SUFFIX = ".safetensors.index.json"
+_WEIGHT_REFERENCE_SUFFIXES = _WEIGHT_SUFFIXES + (_SAFETENSORS_INDEX_SUFFIX,)
 
 
 @dataclass(frozen=True)
@@ -40,6 +44,17 @@ class ResolvedWeight:
     selected_file: str
 
 
+@dataclass(frozen=True)
+class ResolvedWeightSet:
+    inventory: WeightInventory
+    selected_files: tuple[str, ...]
+    index_file: str | None = None
+
+
+class NoSafetensorsWeightsError(FileNotFoundError):
+    """The source has no safetensors payload to resolve as a weight set."""
+
+
 def is_explicit_weight_file_reference(source: str) -> bool:
     """Whether a component override names one weight file, not a component root."""
     expanded = os.path.expanduser(source)
@@ -65,6 +80,23 @@ def _merge_revision(url_revision: str | None, revision: str | None) -> str | Non
     return url_revision or revision
 
 
+def _local_index_inventory(index_path: Path) -> tuple[str, ...]:
+    """List only an exact local index and the shards it declares."""
+    with index_path.open(encoding="utf-8") as index_stream:
+        index = json.load(index_stream)
+    weight_map = index.get("weight_map") if isinstance(index, Mapping) else None
+    shard_names = weight_map.values() if isinstance(weight_map, Mapping) else ()
+    files = {index_path.name}
+    for shard_name in shard_names:
+        if not isinstance(shard_name, str):
+            continue
+        shard_name = _validate_relative_hub_path(shard_name, "index shard")
+        shard_path = index_path.parent / shard_name
+        if shard_path.is_file():
+            files.add(shard_path.relative_to(index_path.parent).as_posix())
+    return tuple(sorted(files))
+
+
 def _parse_huggingface_url(source: str, revision: str | None) -> WeightSource:
     parsed = urlparse(source)
     if parsed.netloc.lower() not in ("huggingface.co", "www.huggingface.co"):
@@ -149,7 +181,7 @@ def parse_weight_source(
     tail = "/".join(parts[2:]) or None
     filename = (
         _validate_relative_hub_path(tail, "filename")
-        if tail is not None and tail.lower().endswith(_WEIGHT_SUFFIXES)
+        if tail is not None and tail.lower().endswith(_WEIGHT_REFERENCE_SUFFIXES)
         else None
     )
     subfolder = tail if filename is None else None
@@ -169,12 +201,11 @@ def _filter_inventory_files(
     files: tuple[str, ...], source: WeightSource
 ) -> tuple[str, ...]:
     if source.filename is not None:
-        selected = tuple(path for path in files if path == source.filename)
+        if source.filename not in files:
-        if not selected:
             raise FileNotFoundError(
                 f"Weight file {source.filename!r} was not found in {source.repo_id}"
             )
-        return selected
+        return (source.filename,)
     if source.subfolder is None:
         return files
     prefix = source.subfolder.rstrip("/") + "/"
@@ -245,7 +276,7 @@ def select_weight_file(
         path for path in inventory.files if path.lower().endswith(_WEIGHT_SUFFIXES)
     )
     if inventory.source.filename is not None:
-        return inventory.files[0]
+        return _select_named_file(candidates, inventory.source.filename)
     if weight_name is not None:
         return _select_named_file(candidates, weight_name)
     if len(candidates) == 1:
@@ -274,18 +305,210 @@ def resolve_weight(
     )
 
 
-def materialize_weight(resolved: ResolvedWeight) -> str:
+def _materialize_inventory_file(inventory: WeightInventory, filename: str) -> str:
-    """Return the selected local file, downloading one pinned Hub file if needed."""
+    source = inventory.source
-    source = resolved.inventory.source
     if source.kind == "local":
         assert source.local_path is not None
         if os.path.isfile(source.local_path):
+            local_path = Path(source.local_path)
+            if filename == local_path.name:
                 return source.local_path
-        return os.path.join(source.local_path, resolved.selected_file)
+            return str(local_path.parent / filename)
+        return os.path.join(source.local_path, filename)
 
     assert source.repo_id is not None
     return hf_hub_download(
         repo_id=source.repo_id,
-        filename=resolved.selected_file,
+        filename=filename,
-        revision=resolved.inventory.resolved_revision,
+        revision=inventory.resolved_revision,
     )
+
+
+def _resolve_index_shard(
+    inventory: WeightInventory, index_file: str, shard_name: str
+) -> str:
+    shard_name = _validate_relative_hub_path(shard_name, "index shard")
+    if not shard_name.lower().endswith(".safetensors"):
+        raise ValueError(
+            f"Safetensors index {index_file!r} references a non-safetensors "
+            f"shard: {shard_name!r}"
+        )
+    index_parent = PurePosixPath(index_file).parent
+    candidates = (shard_name, (index_parent / shard_name).as_posix())
+    matches = tuple(
+        dict.fromkeys(path for path in candidates if path in inventory.files)
+    )
+    if len(matches) == 1:
+        return matches[0]
+    if not matches:
+        raise FileNotFoundError(
+            f"Safetensors index {index_file!r} references missing shard {shard_name!r}"
+        )
+    raise ValueError(
+        f"Safetensors index shard {shard_name!r} is ambiguous: {list(matches)}"
+    )
+
+
+def _read_safetensors_index(
+    inventory: WeightInventory, index_file: str
+) -> tuple[str, ...]:
+    with open(
+        _materialize_inventory_file(inventory, index_file), encoding="utf-8"
+    ) as index_stream:
+        index = json.load(index_stream)
+    weight_map = index.get("weight_map") if isinstance(index, Mapping) else None
+    if not isinstance(weight_map, Mapping) or not weight_map:
+        raise ValueError(
+            f"Safetensors index {index_file!r} must contain a non-empty weight_map"
+        )
+    shard_names = tuple(weight_map.values())
+    if not all(isinstance(name, str) for name in shard_names):
+        raise ValueError(
+            f"Safetensors index {index_file!r} contains a non-string shard name"
+        )
+    return tuple(
+        sorted(
+            {
+                _resolve_index_shard(inventory, index_file, shard_name)
+                for shard_name in shard_names
+            }
+        )
+    )
+
+
+def resolve_safetensors_weight_set(
+    source: str,
+    *,
+    revision: str | None = None,
+    weight_name: str | None = None,
+    select_unindexed_weight: Callable[[tuple[str, ...]], str | None] | None = None,
+) -> ResolvedWeightSet:
+    """Resolve one safetensors checkpoint, using its index as shard authority."""
+    parsed_source = parse_weight_source(source, revision=revision)
+    if (
+        parsed_source.kind == "local"
+        and parsed_source.local_path is not None
+        and os.path.isfile(parsed_source.local_path)
+        and parsed_source.local_path.lower().endswith(_SAFETENSORS_INDEX_SUFFIX)
+    ):
+        inventory = WeightInventory(
+            parsed_source,
+            None,
+            _local_index_inventory(Path(parsed_source.local_path)),
+        )
+    elif parsed_source.kind == "huggingface" and parsed_source.filename is not None:
+        parent = PurePosixPath(parsed_source.filename).parent
+        scoped_inventory = resolve_weight_inventory(
+            replace(
+                parsed_source,
+                filename=None,
+                subfolder=None if parent == PurePosixPath(".") else parent.as_posix(),
+            )
+        )
+        inventory = WeightInventory(
+            parsed_source,
+            scoped_inventory.resolved_revision,
+            scoped_inventory.files,
+        )
+    else:
+        inventory = resolve_weight_inventory(parsed_source)
+    weights = tuple(
+        path for path in inventory.files if path.lower().endswith(".safetensors")
+    )
+    indexes = tuple(
+        path
+        for path in inventory.files
+        if path.lower().endswith(_SAFETENSORS_INDEX_SUFFIX)
+    )
+    selected_name = inventory.source.filename or weight_name
+    if (
+        selected_name is None
+        and inventory.source.kind == "local"
+        and inventory.source.local_path is not None
+        and os.path.isfile(inventory.source.local_path)
+    ):
+        selected_name = Path(inventory.source.local_path).name
+    if selected_name is not None:
+        if not selected_name.lower().endswith(
+            (".safetensors", _SAFETENSORS_INDEX_SUFFIX)
+        ):
+            raise NoSafetensorsWeightsError(
+                f"Selected file is not safetensors: {selected_name!r}"
+            )
+        selected = _select_named_file(weights + indexes, selected_name)
+        if selected in weights:
+            return ResolvedWeightSet(inventory, (selected,))
+        return ResolvedWeightSet(
+            inventory,
+            _read_safetensors_index(inventory, selected),
+            index_file=selected,
+        )
+    if len(indexes) == 1:
+        return ResolvedWeightSet(
+            inventory,
+            _read_safetensors_index(inventory, indexes[0]),
+            index_file=indexes[0],
+        )
+    if len(indexes) > 1:
+        raise ValueError(
+            "Source contains multiple safetensors indexes; select one with an "
+            f"exact index name. Candidates: {list(indexes)}"
+        )
+    if len(weights) == 1:
+        return ResolvedWeightSet(inventory, weights)
+    if not weights:
+        raise NoSafetensorsWeightsError("Source contains no safetensors weights")
+    if select_unindexed_weight is not None:
+        selected = select_unindexed_weight(weights)
+        if selected is not None:
+            if selected not in weights:
+                raise ValueError(
+                    "Unindexed weight selector returned a file outside the source: "
+                    f"{selected!r}"
+                )
+            return ResolvedWeightSet(inventory, (selected,))
+    raise ValueError(
+        "Source contains multiple safetensors files without an index; they may "
+        "be independent variants. Select one exact file or provide a standard "
+        f"safetensors index. Candidates: {list(weights)}"
+    )
+
+
+def materialize_weight_set(resolved: ResolvedWeightSet) -> tuple[str, ...]:
+    """Materialize every selected file from one pinned checkpoint revision."""
+    return tuple(
+        _materialize_inventory_file(resolved.inventory, filename)
+        for filename in resolved.selected_files
+    )
+
+
+def materialize_weight_set_config(resolved: ResolvedWeightSet) -> str | None:
+    """Materialize adjacent runtime configuration from the pinned revision."""
+    source = resolved.inventory.source
+    if source.kind == "local" and source.local_path is not None:
+        local_path = Path(source.local_path)
+        if local_path.is_file():
+            config_path = local_path.with_name("config.json")
+            return str(config_path) if config_path.is_file() else None
+
+    anchor = resolved.index_file or resolved.selected_files[0]
+    config_file = PurePosixPath(anchor).with_name("config.json").as_posix()
+    if config_file not in resolved.inventory.files:
+        return None
+    config_parent = PurePosixPath(config_file).parent
+    metadata_files = tuple(
+        path
+        for path in resolved.inventory.files
+        if PurePosixPath(path).parent == config_parent
+        and PurePosixPath(path).name.startswith("quant_model_description")
+        and path.lower().endswith(".json")
+    )
+    config_path = _materialize_inventory_file(resolved.inventory, config_file)
+    for metadata_file in metadata_files:
+        _materialize_inventory_file(resolved.inventory, metadata_file)
+    return config_path
+
+
+def materialize_weight(resolved: ResolvedWeight) -> str:
+    """Return the selected local file, downloading one pinned Hub file if needed."""
+    return _materialize_inventory_file(resolved.inventory, resolved.selected_file)

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

@@ -3,7 +3,6 @@ This unittest is introduced in #22360, preventing duplicate transformer safetens
 """
 
 import json
-import os
 import sys
 import tempfile
 import types
@@ -100,8 +99,8 @@ from sglang.multimodal_gen.runtime.loader.transformer_load_utils import (
     _Flux2Nvfp4FallbackAdapter,
     _needs_device_weight_postprocess,
     _resolve_quant_config,
+    resolve_transformer_checkpoint_files,
     resolve_transformer_quant_load_spec,
-    resolve_transformer_safetensors_to_load,
 )
 from sglang.multimodal_gen.runtime.loader.weight_load_plan import WeightLoadPlan
 from sglang.multimodal_gen.runtime.models.dits.flux import FluxSingleTransformerBlock
@@ -262,50 +261,79 @@ class TestTransformerQuantHelpers(unittest.TestCase):
 
         self.assertIsNone(backend)
 
-    def test_resolve_transformer_safetensors_to_load_uses_single_override_file(self):
+    def test_resolve_transformer_checkpoint_files_uses_single_override_file(self):
         with tempfile.NamedTemporaryFile(suffix=".safetensors") as f:
             server_args = self._make_server_args(transformer_weights_path=f.name)
-            resolved = resolve_transformer_safetensors_to_load(
+            with (
+                patch(
+                    "sglang.multimodal_gen.runtime.weights.source.HfApi.model_info"
+                ) as model_info,
+                patch(
+                    "sglang.multimodal_gen.runtime.weights.source.hf_hub_download"
+                ) as download,
+            ):
+                resolved = resolve_transformer_checkpoint_files(
                     server_args, "/unused/component/path"
                 )
 
-        self.assertEqual(resolved, [f.name])
+        self.assertEqual(resolved.safetensors, (f.name,))
+        self.assertIsNone(resolved.config_path)
+        model_info.assert_not_called()
+        download.assert_not_called()
 
-    @patch(
+    def test_resolve_transformer_checkpoint_files_uses_one_hf_revision(self):
-        "sglang.multimodal_gen.runtime.loader.transformer_load_utils.hf_hub_download",
+        filename = "weights/model.safetensors"
-        return_value="/cache/model.safetensors",
-    )
-    def test_resolve_transformer_safetensors_to_load_uses_hf_file_reference(
-        self, mock_download
-    ):
-        filename = "diffusion_models/minimax_h3_fl2va_pruned_bf16.safetensors"
         references = (
             (
-                f"https://huggingface.co/Comfy-Org/MiniMax-H3/resolve/main/{filename}",
+                f"https://huggingface.co/owner/repo/resolve/main/{filename}",
                 "main",
             ),
-            (f"Comfy-Org/MiniMax-H3/{filename}", "test-revision"),
+            (f"owner/repo/{filename}", "test-revision"),
         )
 
         for reference, revision in references:
             with self.subTest(reference=reference):
-                server_args = self._make_server_args(transformer_weights_path=reference)
+                server_args = self._make_server_args(
-                with patch(
+                    transformer_weights_path=reference,
-                    "os.path.isfile",
-                    side_effect=lambda path: path == "/cache/model.safetensors",
-                ):
-                    self.assertEqual(
-                        resolve_transformer_safetensors_to_load(
-                            server_args, "/unused/component/path"
-                        ),
-                        ["/cache/model.safetensors"],
-                    )
-                mock_download.assert_called_once_with(
-                    repo_id="Comfy-Org/MiniMax-H3",
-                    filename=filename,
                     revision=revision,
                 )
-                mock_download.reset_mock()
+                model_info = SimpleNamespace(
+                    sha="immutable-sha",
+                    siblings=[
+                        SimpleNamespace(rfilename=filename),
+                        SimpleNamespace(rfilename="weights/config.json"),
+                    ],
+                )
+
+                def download(*, filename, **_kwargs):
+                    return f"/cache/{filename.rsplit('/', 1)[-1]}"
+
+                with (
+                    patch(
+                        "sglang.multimodal_gen.runtime.weights.source.HfApi.model_info",
+                        return_value=model_info,
+                    ) as model_info_call,
+                    patch(
+                        "sglang.multimodal_gen.runtime.weights.source.hf_hub_download",
+                        side_effect=download,
+                    ) as download,
+                ):
+                    resolved = resolve_transformer_checkpoint_files(
+                        server_args, "/unused/component/path"
+                    )
+                self.assertEqual(resolved.safetensors, ("/cache/model.safetensors",))
+                self.assertEqual(resolved.config_path, "/cache/config.json")
+                model_info_call.assert_called_once_with("owner/repo", revision=revision)
+                self.assertEqual(
+                    {call.kwargs["filename"] for call in download.call_args_list},
+                    {filename, "weights/config.json"},
+                )
+                self.assertTrue(
+                    all(
+                        call.kwargs["revision"] == "immutable-sha"
+                        for call in download.call_args_list
+                    )
+                )
 
     def test_inspect_minimax_h3_safetensors_detects_curve_and_comfy_format(self):
         marker = json.dumps(
@@ -739,13 +767,7 @@ class TestTransformerQuantHelpers(unittest.TestCase):
                 checkpoint_quant_config=ComfyFp8Config({}),
             )
 
-    @patch(
+    def test_resolve_transformer_override_prefers_single_mixed_export(self):
-        "sglang.multimodal_gen.runtime.loader.transformer_load_utils.maybe_download_model",
-        side_effect=lambda path, **kw: path,
-    )
-    def test_resolve_transformer_safetensors_to_load_prefers_mixed_export(
-        self, _mock_download
-    ):
         with tempfile.TemporaryDirectory() as tmpdir:
             mixed = f"{tmpdir}/flux2-dev-nvfp4-mixed.safetensors"
             full = f"{tmpdir}/flux2-dev-nvfp4.safetensors"
@@ -753,39 +775,11 @@ class TestTransformerQuantHelpers(unittest.TestCase):
             open(full, "a").close()
 
             server_args = self._make_server_args(transformer_weights_path=tmpdir)
-            resolved = resolve_transformer_safetensors_to_load(
+            resolved = resolve_transformer_checkpoint_files(
                 server_args, "/unused/component/path"
             )
 
-        self.assertEqual(resolved, [mixed])
+        self.assertEqual(resolved.safetensors, (mixed,))
-
-    @patch(
-        "sglang.multimodal_gen.runtime.loader.transformer_load_utils.snapshot_download",
-    )
-    @patch(
-        "sglang.multimodal_gen.runtime.loader.transformer_load_utils.maybe_download_model",
-    )
-    def test_resolve_transformer_safetensors_to_load_refreshes_empty_cached_repo(
-        self, mock_download_model, mock_snapshot_download
-    ):
-        with tempfile.TemporaryDirectory() as cached_dir:
-            repo_id = "black-forest-labs/FLUX.2-dev-NVFP4"
-            mixed = os.path.join(cached_dir, "flux2-dev-nvfp4-mixed.safetensors")
-            mock_download_model.return_value = cached_dir
-
-            def _snapshot_download(**_kwargs):
-                open(mixed, "a").close()
-                return cached_dir
-
-            mock_snapshot_download.side_effect = _snapshot_download
-
-            server_args = self._make_server_args(transformer_weights_path=repo_id)
-            resolved = resolve_transformer_safetensors_to_load(
-                server_args, "/unused/component/path"
-            )
-
-        self.assertEqual(resolved, [mixed])
-        mock_snapshot_download.assert_called_once()
 
     def test_filter_transformer_precision_variants_prefers_canonical_file(self):
         files = [
@@ -957,9 +951,6 @@ class TestTransformerQuantHelpers(unittest.TestCase):
         "sglang.multimodal_gen.runtime.loader.transformer_load_utils.build_nvfp4_config_from_safetensors_list",
         return_value=None,
     )
-    @patch(
-        "sglang.multimodal_gen.runtime.loader.transformer_load_utils.maybe_download_model"
-    )
     @patch(
         "sglang.multimodal_gen.runtime.loader.transformer_load_utils.get_quant_config_from_safetensors_metadata",
         return_value=None,
@@ -967,21 +958,12 @@ class TestTransformerQuantHelpers(unittest.TestCase):
     @patch(
         "sglang.multimodal_gen.runtime.loader.transformer_load_utils.get_metadata_from_safetensors_file"
     )
-    @patch(
-        "sglang.multimodal_gen.runtime.loader.transformer_load_utils.maybe_download_model",
-        side_effect=lambda path, **kw: path,
-    )
     def test_resolve_transformer_quant_load_spec_keeps_nunchaku_hook(
         self,
-        _mock_download,
         mock_metadata,
         _mock_quant_metadata,
-        mock_maybe_download,
         _mock_nvfp4,
     ):
-        mock_maybe_download.side_effect = AssertionError(
-            "local safetensors path should not trigger maybe_download_model"
-        )
         mock_metadata.return_value = {
             "config": json.dumps({"_class_name": _FakeFluxTransformer.__name__})
         }
@@ -1006,7 +988,6 @@ class TestTransformerQuantHelpers(unittest.TestCase):
         self.assertIsNone(spec.param_dtype)
         self.assertEqual(len(spec.post_load_hooks), 1)
         self.assertIs(nunchaku_config.model_cls, _FakeFluxTransformer)
-        mock_maybe_download.assert_not_called()
 
     def test_flux2_mixed_nvfp4_fallback_disables_conflicting_offloads(self):
         server_args = self._make_server_args(

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

@@ -4,9 +4,13 @@ from unittest.mock import patch
 import pytest
 
 from sglang.multimodal_gen.runtime.weights.source import (
+    NoSafetensorsWeightsError,
     is_explicit_weight_file_reference,
     materialize_weight,
+    materialize_weight_set,
+    materialize_weight_set_config,
     parse_weight_source,
+    resolve_safetensors_weight_set,
     resolve_weight,
     resolve_weight_inventory,
 )
@@ -18,6 +22,9 @@ def test_explicit_weight_file_reference_does_not_claim_directories(tmp_path):
 
     assert not is_explicit_weight_file_reference(str(component))
     assert is_explicit_weight_file_reference("owner/repo/model.safetensors")
+    assert not is_explicit_weight_file_reference(
+        "owner/repo/model.safetensors.index.json"
+    )
     assert is_explicit_weight_file_reference(
         "https://huggingface.co/owner/repo/resolve/main/model.gguf?download=true"
     )
@@ -95,6 +102,137 @@ def test_weight_source_rejects_ambiguous_files(tmp_path):
         resolve_weight(str(tmp_path))
 
 
+def test_safetensors_index_selects_only_declared_shards(tmp_path):
+    (tmp_path / "model-00001-of-00002.safetensors").write_bytes(b"one")
+    (tmp_path / "model-00002-of-00002.safetensors").write_bytes(b"two")
+    (tmp_path / "alternate.safetensors").write_bytes(b"other variant")
+    (tmp_path / "model.safetensors.index.json").write_text(
+        '{"weight_map":{"a":"model-00001-of-00002.safetensors",'
+        '"b":"model-00002-of-00002.safetensors"}}'
+    )
+
+    resolved = resolve_safetensors_weight_set(str(tmp_path))
+
+    assert resolved.index_file == "model.safetensors.index.json"
+    assert resolved.selected_files == (
+        "model-00001-of-00002.safetensors",
+        "model-00002-of-00002.safetensors",
+    )
+    assert materialize_weight_set(resolved) == tuple(
+        str(tmp_path / filename) for filename in resolved.selected_files
+    )
+
+
+def test_exact_local_safetensors_index_resolves_adjacent_shards(tmp_path):
+    shard = tmp_path / "model-00001-of-00001.safetensors"
+    shard.write_bytes(b"one")
+    index = tmp_path / "model.safetensors.index.json"
+    index.write_text('{"weight_map":{"a":"model-00001-of-00001.safetensors"}}')
+
+    resolved = resolve_safetensors_weight_set(str(index))
+
+    assert resolved.index_file == index.name
+    assert materialize_weight_set(resolved) == (str(shard),)
+
+
+def test_safetensors_weight_set_rejects_unindexed_variants(tmp_path):
+    (tmp_path / "base.safetensors").write_bytes(b"base")
+    (tmp_path / "distilled.safetensors").write_bytes(b"distilled")
+
+    with pytest.raises(ValueError, match="without an index"):
+        resolve_safetensors_weight_set(str(tmp_path))
+
+
+def test_safetensors_index_rejects_non_weight_shard(tmp_path):
+    (tmp_path / "config.json").write_text("{}")
+    (tmp_path / "model.safetensors.index.json").write_text(
+        '{"weight_map":{"a":"config.json"}}'
+    )
+
+    with pytest.raises(ValueError, match="non-safetensors shard"):
+        resolve_safetensors_weight_set(str(tmp_path))
+
+
+def test_safetensors_index_missing_shard_is_not_no_weights_fallback(tmp_path):
+    (tmp_path / "model.safetensors.index.json").write_text(
+        '{"weight_map":{"a":"missing.safetensors"}}'
+    )
+
+    with pytest.raises(FileNotFoundError, match="missing shard") as error:
+        resolve_safetensors_weight_set(str(tmp_path))
+
+    assert not isinstance(error.value, NoSafetensorsWeightsError)
+
+
+def test_source_without_safetensors_has_distinct_error(tmp_path):
+    (tmp_path / "pytorch_model.bin").write_bytes(b"bin")
+
+    with pytest.raises(NoSafetensorsWeightsError):
+        resolve_safetensors_weight_set(str(tmp_path))
+
+
+def test_exact_non_safetensors_file_has_distinct_error(tmp_path):
+    weights = tmp_path / "pytorch_model.bin"
+    weights.write_bytes(b"bin")
+
+    with pytest.raises(NoSafetensorsWeightsError):
+        resolve_safetensors_weight_set(str(weights))
+
+
+def test_remote_safetensors_shards_use_one_pinned_revision(tmp_path):
+    index_path = tmp_path / "model.safetensors.index.json"
+    index_path.write_text(
+        '{"weight_map":{"a":"model-00001-of-00002.safetensors",'
+        '"b":"model-00002-of-00002.safetensors"}}'
+    )
+    config_path = tmp_path / "config.json"
+    config_path.write_text("{}")
+    model_info = SimpleNamespace(
+        sha="immutable-sha",
+        siblings=[
+            SimpleNamespace(rfilename=f"transformer/{filename}")
+            for filename in (
+                "model.safetensors.index.json",
+                "model-00001-of-00002.safetensors",
+                "model-00002-of-00002.safetensors",
+                "config.json",
+                "quant_model_description_w8a8.json",
+            )
+        ],
+    )
+
+    def download(*, filename, **_kwargs):
+        if filename.endswith("index.json"):
+            return str(index_path)
+        if filename.endswith("config.json"):
+            return str(config_path)
+        if "quant_model_description" in filename:
+            return str(tmp_path / "quant_model_description_w8a8.json")
+        return f"/{filename}"
+
+    with (
+        patch(
+            "sglang.multimodal_gen.runtime.weights.source.HfApi.model_info",
+            return_value=model_info,
+        ),
+        patch(
+            "sglang.multimodal_gen.runtime.weights.source.hf_hub_download",
+            side_effect=download,
+        ) as hub_download,
+    ):
+        resolved = resolve_safetensors_weight_set("owner/repo/transformer")
+        materialize_weight_set(resolved)
+        assert materialize_weight_set_config(resolved) == str(config_path)
+
+    assert all(
+        call.kwargs["revision"] == "immutable-sha"
+        for call in hub_download.call_args_list
+    )
+    assert "transformer/quant_model_description_w8a8.json" in {
+        call.kwargs["filename"] for call in hub_download.call_args_list
+    }
+
+
 def test_materialize_local_weight_returns_selected_file(tmp_path):
     checkpoint = tmp_path / "model.safetensors"
     checkpoint.write_bytes(b"fixture")