feat: better reporting of the variant being requested.

kernels/src/kernels/cli/__init__.py

@@ -65,7 +65,8 @@ def main():
     versions_parser.set_defaults(func=kernel_versions)
 
     upload_parser = subparsers.add_parser(
-        "upload", help="(Deprecated) Upload kernels to the Hub. Use `kernel-builder upload` instead."
+        "upload",
+        help="(Deprecated) Upload kernels to the Hub. Use `kernel-builder upload` instead.",
     )
     upload_parser.add_argument(
         "kernel_dir",

kernels/src/kernels/utils.py

@@ -25,6 +25,7 @@
 from kernels.status import resolve_status
 from kernels.variants import (
     Variant,
+    describe_system_variant,
     get_variants,
     get_variants_local,
     resolve_variant,
@@ -138,8 +139,9 @@ def install_kernel(
     variant = resolve_variant(variants, backend)
 
     if variant is None:
+        requested = describe_system_variant(backend)
         raise FileNotFoundError(
-            f"Cannot find a build variant for this system in {repo_id} (revision: {revision}). Available variants: {', '.join([variant.variant_str for variant in variants])}"
+            f"Cannot find a build variant for this system in {repo_id} (revision: {revision}). Requested variant: {requested}. Available variants: {', '.join([variant.variant_str for variant in variants])}"
         )
 
     allow_patterns = [f"build/{variant.variant_str}/*"]
@@ -407,8 +409,9 @@ def load_kernel(
     variant = resolve_variant(variants, backend)
 
     if variant is None:
+        requested = describe_system_variant(backend)
         raise FileNotFoundError(
-            f"Cannot find a build variant for this system in {repo_id} (revision: {locked_sha}). Available variants: {', '.join([variant.variant_str for variant in variants])}"
+            f"Cannot find a build variant for this system in {repo_id} (revision: {locked_sha}). Requested variant: {requested}. Available variants: {', '.join([variant.variant_str for variant in variants])}"
         )
 
     allow_patterns = [f"build/{variant.variant_str}/*"]

kernels/src/kernels/variants.py

@@ -198,11 +198,8 @@ def resolve_variant(
     return resolved[0] if resolved else None
 
 
-def resolve_variants(
-    variants: list[Variant], backend: str | None = None
-) -> list[Variant]:
-    """Return the matching variants for the current system, sorted
-    by decreasing order of preference."""
+def _get_system_info(backend: str | None = None):
+    """Gather system information used for variant resolution."""
     selected_backend = _select_backend(backend)
 
     cpu = platform.machine()
@@ -232,6 +229,59 @@ def resolve_variants(
         tvm_ffi_version = parse(tvm_ffi.__version__)
         tvm_ffi_version = Version(f"{tvm_ffi_version.major}.{tvm_ffi_version.minor}")
 
+    return selected_backend, cpu, os, torch_version, torch_cxx11_abi, tvm_ffi_version
+
+
+def describe_system_variant(backend: str | None = None) -> str:
+    """Return a human-readable description of the variant the current system requires."""
+    selected_backend, cpu, os, torch_version, torch_cxx11_abi, tvm_ffi_version = (
+        _get_system_info(backend)
+    )
+    return _describe_system_variant(
+        selected_backend=selected_backend,
+        cpu=cpu,
+        os=os,
+        torch_version=torch_version,
+        torch_cxx11_abi=torch_cxx11_abi,
+        tvm_ffi_version=tvm_ffi_version,
+    )
+
+
+def _describe_system_variant(
+    selected_backend: Backend,
+    cpu: str,
+    os: str,
+    torch_version: Version | None,
+    torch_cxx11_abi: bool | None,
+    tvm_ffi_version: Version | None,
+) -> str:
+    """Return a human-readable description of the requested variant given explicit system parameters."""
+    parts = []
+
+    if torch_version is not None:
+        parts.append(f"torch{torch_version.major}{torch_version.minor}")
+
+    if tvm_ffi_version is not None:
+        parts.append(f"tvm-ffi{tvm_ffi_version.major}{tvm_ffi_version.minor}")
+
+    if torch_cxx11_abi is not None:
+        parts.append("cxx11" if torch_cxx11_abi else "cxx98")
+
+    parts.append(selected_backend.variant_str)
+    parts.append(f"{cpu}-{os}")
+
+    return "-".join(parts)
+
+
+def resolve_variants(
+    variants: list[Variant], backend: str | None = None
+) -> list[Variant]:
+    """Return the matching variants for the current system, sorted
+    by decreasing order of preference."""
+    selected_backend, cpu, os, torch_version, torch_cxx11_abi, tvm_ffi_version = (
+        _get_system_info(backend)
+    )
+
     return _resolve_variant_for_system(
         variants=variants,
         selected_backend=selected_backend,

kernels/tests/test_variants.py

@@ -3,7 +3,12 @@
 from packaging.version import Version
 
 from kernels.backends import CPU, CUDA, ROCm
-from kernels.variants import Variant, _resolve_variant_for_system, get_variants
+from kernels.variants import (
+    Variant,
+    _describe_system_variant,
+    _resolve_variant_for_system,
+    get_variants,
+)
 
 VARIANT_STRINGS = [
     "torch25-cxx98-cu118-aarch64-linux",
@@ -359,3 +364,37 @@ def test_resolve_cuda_no_different_major_no_noarch():
         tvm_ffi_version=None,
     )
     assert result == []
+
+
+def test_error_message_includes_requested_variant():
+    # Simulate a no-match scenario: ROCm 7.0 system with torch 2.9, but only
+    # CUDA variants available.
+    variants = RESOLVE_VARIANTS_NO_NOARCH
+    selected_backend = ROCm(Version("7.0"))
+    result = _resolve_variant_for_system(
+        variants=variants,
+        selected_backend=selected_backend,
+        cpu="x86_64",
+        os="linux",
+        torch_version=Version("2.9"),
+        torch_cxx11_abi=True,
+        tvm_ffi_version=None,
+    )
+    assert result == []
+
+    # Build the error message the same way utils.py does.
+    requested = _describe_system_variant(
+        selected_backend=selected_backend,
+        cpu="x86_64",
+        os="linux",
+        torch_version=Version("2.9"),
+        torch_cxx11_abi=True,
+        tvm_ffi_version=None,
+    )
+    error_msg = (
+        f"Cannot find a build variant for this system in test/repo (revision: abc123). "
+        f"Requested variant: {requested}. "
+        f"Available variants: {', '.join([v.variant_str for v in variants])}"
+    )
+    assert "Requested variant: torch29-cxx11-rocm70-x86_64-linux" in error_msg
+    assert "Available variants:" in error_msg