[Dev UX] Add auto-detection for VLLM_PRECOMPILED_WHEEL_VARIANT during install (#32948)

Signed-off-by: mgoin <mgoin64@gmail.com>
Signed-off-by: Michael Goin <mgoin64@gmail.com>
Co-authored-by: Shengqi Chen <i@harrychen.xyz>

docs/getting_started/installation/gpu.cuda.inc.md

@@ -118,7 +118,7 @@ There are more environment variables to control the behavior of Python-only buil
 
 * `VLLM_PRECOMPILED_WHEEL_LOCATION`: specify the exact wheel URL or local file path of a pre-compiled wheel to use. All other logic to find the wheel will be skipped.
 * `VLLM_PRECOMPILED_WHEEL_COMMIT`: override the commit hash to download the pre-compiled wheel. It can be `nightly` to use the last **already built** commit on the main branch.
-* `VLLM_PRECOMPILED_WHEEL_VARIANT`: specify the variant subdirectory to use on the nightly index, e.g., `cu129`, `cpu`. If not specified, the CUDA variant with `VLLM_MAIN_CUDA_VERSION` will be tried, then fallback to the default variant on the remote index.
+* `VLLM_PRECOMPILED_WHEEL_VARIANT`: specify the variant subdirectory to use on the nightly index, e.g., `cu129`, `cu130`, `cpu`. If not specified, the variant is auto-detected based on your system's CUDA version (from PyTorch or nvidia-smi). You can also set `VLLM_MAIN_CUDA_VERSION` to override auto-detection.
 
 You can find more information about vLLM's wheels in [Install the latest code](#install-the-latest-code).
 

setup.py

@@ -438,6 +438,49 @@ class precompiled_wheel_utils:
         except ImportError:
             return False
 
+    @staticmethod
+    def detect_system_cuda_variant() -> str:
+        """Auto-detect CUDA variant from torch, nvidia-smi, or env default."""
+
+        # Map CUDA major version to hosted wheel variants on wheels.vllm.ai
+        supported = {12: "cu129", 13: "cu130"}
+
+        # Respect explicitly set VLLM_MAIN_CUDA_VERSION
+        if envs.is_set("VLLM_MAIN_CUDA_VERSION"):
+            v = envs.VLLM_MAIN_CUDA_VERSION
+            print(f"Using VLLM_MAIN_CUDA_VERSION={v}")
+            return "cu" + v.replace(".", "")[:3]
+
+        # Try torch.version.cuda
+        cuda_version = None
+        try:
+            import torch
+
+            cuda_version = torch.version.cuda
+        except Exception:
+            pass
+
+        # Try nvidia-smi
+        if not cuda_version:
+            try:
+                out = subprocess.run(
+                    ["nvidia-smi"], capture_output=True, text=True, timeout=10
+                )
+                if m := re.search(r"CUDA Version:\s*(\d+\.\d+)", out.stdout):
+                    cuda_version = m.group(1)
+            except Exception:
+                pass
+
+        # Fall back to default
+        if not cuda_version:
+            cuda_version = envs.VLLM_MAIN_CUDA_VERSION
+
+        # Map to supported variant
+        major = int(cuda_version.split(".")[0])
+        variant = supported.get(major, supported[max(supported)])
+        print(f"Detected CUDA {cuda_version}, using variant {variant}")
+        return variant
+
     @staticmethod
     def find_local_rocm_wheel() -> str | None:
         """Search for a local vllm wheel in common locations."""
@@ -513,8 +556,8 @@ class precompiled_wheel_utils:
         1. user-specified wheel location (can be either local or remote, via
            VLLM_PRECOMPILED_WHEEL_LOCATION)
         2. user-specified variant (VLLM_PRECOMPILED_WHEEL_VARIANT) from nightly repo
-        3. the variant corresponding to VLLM_MAIN_CUDA_VERSION from nightly repo
-        4. the default variant from nightly repo
+           or auto-detected CUDA variant based on system (torch, nvidia-smi)
+        3. the default variant from nightly repo
 
         If downloading from the nightly repo, the commit can be specified via
         VLLM_PRECOMPILED_WHEEL_COMMIT; otherwise, the head commit in the main branch
@@ -533,9 +576,11 @@ class precompiled_wheel_utils:
             import platform
 
             arch = platform.machine()
-            # try to fetch the wheel metadata from the nightly wheel repo
-            main_variant = "cu" + envs.VLLM_MAIN_CUDA_VERSION.replace(".", "")
-            variant = os.getenv("VLLM_PRECOMPILED_WHEEL_VARIANT", main_variant)
+            # try to fetch the wheel metadata from the nightly wheel repo,
+            # detecting CUDA variant from system if not specified
+            variant = os.getenv("VLLM_PRECOMPILED_WHEEL_VARIANT", None)
+            if variant is None:
+                variant = precompiled_wheel_utils.detect_system_cuda_variant()
             commit = os.getenv("VLLM_PRECOMPILED_WHEEL_COMMIT", "").lower()
             if not commit or len(commit) != 40:
                 print(