[diffusion] fix: fix gated repo failing the generate cmd (#22040)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

python/sglang/cli/utils.py

@@ -4,6 +4,8 @@ import os
 import subprocess
 from functools import lru_cache
 
+from huggingface_hub import HfApi
+
 from sglang.srt.environ import envs
 from sglang.utils import (
     has_diffusion_overlay_registry_match,
@@ -23,6 +25,16 @@ def _is_overlay_diffusion_model(model_path: str) -> bool:
     return has_diffusion_overlay_registry_match(model_path, _load_overlay_registry())
 
 
+def _is_registered_diffusion_model(model_path: str) -> bool:
+    try:
+        # if diffusion dependencies are not installed
+        from sglang.multimodal_gen.registry import get_model_info
+    except ImportError:
+        return False
+
+    return get_model_info(model_path, backend="sglang") is not None
+
+
 def _is_diffusers_model_dir(model_dir: str) -> bool:
     """Check if a local directory contains a valid diffusers model_index.json."""
     config_path = os.path.join(model_dir, "model_index.json")
@@ -35,11 +47,22 @@ def _is_diffusers_model_dir(model_dir: str) -> bool:
     return "_diffusers_version" in config
 
 
+def _is_gated_diffusion_repo(repo_id: str) -> bool:
+    """Query HF model card metadata to check if a gated repo is a diffusers model."""
+    try:
+        info = HfApi().model_info(repo_id)
+        return getattr(info, "library_name", None) == "diffusers"
+    except Exception:
+        return False
+
+
 def get_is_diffusion_model(model_path: str) -> bool:
     """Detect whether model_path points to a diffusion model.
 
     For local directories, checks the filesystem directly.
     For HF/ModelScope model IDs, attempts to fetch only model_index.json.
+    For gated repos where file download fails, falls back to HF model card
+    metadata (library_name == "diffusers").
     Returns False on any failure (network error, 404, offline mode, etc.)
     so that the caller falls through to the standard LLM server path.
     """
@@ -70,7 +93,9 @@ def get_is_diffusion_model(model_path: str) -> bool:
         return _is_diffusers_model_dir(os.path.dirname(file_path))
     except Exception as e:
         logger.debug("Failed to auto-detect diffusion model for %s: %s", model_path, e)
-        return False
+        # For gated repos, file download fails but model card is still accessible.
+        # Check library_name from HF metadata as a fallback.
+        return _is_gated_diffusion_repo(model_path)
 
 
 def get_model_path(extra_argv):