diff --git a/src/xbuild/__main__.py b/src/xbuild/__main__.py
index d378a0e..ad5b234 100644
--- a/src/xbuild/__main__.py
+++ b/src/xbuild/__main__.py
@@ -34,7 +34,8 @@ def _build(
     config_settings: ConfigSettings | None,
     skip_dependency_check: bool,
     installer: _env.Installer,
-    sysconfig_path: Path | None,
+    build_details_path: Path | None,
+    sysconfigdata_path: Path | None,
 ) -> str:
     if isolation:
         return _build_in_isolated_env(
@@ -43,7 +44,8 @@ def _build(
             distribution,
             config_settings,
             installer,
-            sysconfig_path=sysconfig_path,
+            build_details_path=build_details_path,
+            sysconfigdata_path=sysconfigdata_path,
         )
     else:
         return _build_in_current_env(
@@ -61,11 +63,13 @@ def _build_in_isolated_env(
     distribution: Distribution,
     config_settings: ConfigSettings | None,
     installer: _env.Installer,
-    sysconfig_path: Path | None,
+    build_details_path: Path | None,
+    sysconfigdata_path: Path | None,
 ) -> str:
     with XBuildIsolatedEnv(
         installer=installer,
-        sysconfig_path=sysconfig_path,
+        build_details_path=build_details_path,
+        sysconfigdata_path=sysconfigdata_path,
     ) as env:
         builder = ProjectXBuilder.from_isolated_env(env, srcdir)
 
@@ -210,15 +214,24 @@ def main_parser() -> argparse.ArgumentParser:
         help="Python package installer to use (defaults to pip)",
     )
 
-    # This is only a required argument if the current environment isn't cross-compiling.
-    # If/when this project is merged into `build`, the existence of `--sysconfig` as
-    # an argument will be the trigger for "this is a cross platform build".
-    parser.add_argument(
+    # This is only a required argument if the current environment isn't
+    # cross-compiling. If/when this project is merged into `build`, the
+    # existence of `--build-details/--sysconfig` as an argument will be the
+    # trigger for "this is a cross platform build". The two arguments are
+    # mutually exclusive.
+    config_group = parser.add_mutually_exclusive_group(required=True)
+    config_group.add_argument(
+        "--build-details",
+        dest="build_details_path",
+        type=Path,
+        help=("The path to a build-details.json file.",),
+    )
+    config_group.add_argument(
         "--sysconfig",
-        help="The path to a sysconfig_vars JSON file or sysconfigdata Python file",
-        required=not getattr(sys, "cross_compiling", False),
+        dest="sysconfigdata_path",
+        type=Path,
+        help=("The path to a sysconfigdata python file.",),
     )
-
     config_group = parser.add_mutually_exclusive_group()
     config_group.add_argument(
         "--config-setting",
@@ -290,8 +303,6 @@ def main(cli_args: Sequence[str], prog: str | None = None) -> None:
     outdir = os.path.join(args.srcdir, "dist") if args.outdir is None else args.outdir
 
     with _handle_build_error():
-        sysconfig_path = Path(args.sysconfig) if args.sysconfig else None
-
         built = [
             _build(
                 not args.no_isolation,
@@ -301,7 +312,8 @@ def main(cli_args: Sequence[str], prog: str | None = None) -> None:
                 config_settings,
                 args.skip_dependency_check,
                 args.installer,
-                sysconfig_path=sysconfig_path,
+                build_details_path=args.build_details_path,
+                sysconfigdata_path=args.sysconfigdata_path,
             )
         ]
         artifact_list = _natural_language_list(
diff --git a/src/xbuild/env.py b/src/xbuild/env.py
index a44f46c..b5e638a 100644
--- a/src/xbuild/env.py
+++ b/src/xbuild/env.py
@@ -56,12 +56,13 @@ def install_requirements(
 # and can handle both build and target dependency installs.
 ###########################################################################
 class XBuildIsolatedEnv(DefaultIsolatedEnv):
-    def __init__(self, *, installer, sysconfig_path):
+    def __init__(self, *, installer, build_details_path, sysconfigdata_path):
         if installer == "uv":
             raise RuntimeError("Can't support uv (for now)")
 
         super().__init__()
-        self.sysconfig_path = sysconfig_path
+        self.build_details_path = build_details_path
+        self.sysconfigdata_path = sysconfigdata_path
 
     def __enter__(self) -> XBuildIsolatedEnv:
         super().__enter__()
@@ -73,13 +74,17 @@ def __enter__(self) -> XBuildIsolatedEnv:
         if not getattr(sys, "cross_compiling", False):
             # We're in a local environment.
             # Make the isolated environment a cross environment.
-            if self.sysconfig_path is None:
+            if self.build_details_path is None and self.sysconfigdata_path is None:
                 raise RuntimeError(
-                    "Must specify the location of target platform sysconfig data "
-                    "with --sysconfig"
+                    "Must specify the location of target platform build_details.json "
+                    "with --build-details, or sysconfigdata with --sysconfig"
                 )
 
-            convert_venv(Path(self._path), self.sysconfig_path)
+            convert_venv(
+                Path(self._path),
+                build_details_path=self.build_details_path,
+                sysconfigdata_path=self.sysconfigdata_path,
+            )
         else:
             # We're already in a cross environment.
             # Copy any _cross_*.pth or _cross_*.py file, plus the cross-platform
diff --git a/src/xvenv/__main__.py b/src/xvenv/__main__.py
index 1c48c35..bb74fdc 100644
--- a/src/xvenv/__main__.py
+++ b/src/xvenv/__main__.py
@@ -36,11 +36,23 @@ def main_parser():
         default=0,
         help="increase verbosity",
     )
-    parser.add_argument(
-        "-s",
+
+    # Create mutually exclusive group for --build-details and --sysconfig
+    # One of these arguments must be provided
+    config_group = parser.add_mutually_exclusive_group(required=True)
+    config_group.add_argument(
+        "--build-details",
+        dest="build_details_path",
+        type=Path,
+        help=("The path to a build-details.json file.",),
+    )
+    config_group.add_argument(
         "--sysconfig",
-        help="The path to a sysconfig_vars JSON file or sysconfigdata Python file",
+        dest="sysconfigdata_path",
+        type=Path,
+        help=("The path to a sysconfigdata python file.",),
     )
+
     parser.add_argument(
         "venv",
         help="The location of a native virtual environment",
@@ -60,13 +72,22 @@ def main(cli_args: Sequence[str], prog: str | None = None) -> None:
     args = parser.parse_args(cli_args)
 
     venv_path = Path(args.venv).resolve()
-    sysconfig_path = Path(args.sysconfig).resolve()
+    build_details_path = (
+        Path(args.build_details_path).resolve() if args.build_details_path else None
+    )
+    sysconfigdata_path = (
+        Path(args.sysconfigdata_path).resolve() if args.sysconfigdata_path else None
+    )
 
     if not venv_path.exists():
         _error(f"Native virtual environment {venv_path} does not exist.")
     else:
         try:
-            description = convert_venv(venv_path, sysconfig_path)
+            description = convert_venv(
+                venv_path,
+                build_details_path=build_details_path,
+                sysconfigdata_path=sysconfigdata_path,
+            )
         except Exception as e:
             _error(e)
             sys.exit(1)
diff --git a/src/xvenv/convert.py b/src/xvenv/convert.py
index 4b4bdd6..3888f62 100644
--- a/src/xvenv/convert.py
+++ b/src/xvenv/convert.py
@@ -1,6 +1,5 @@
 import json
 import pprint
-import sys
 from importlib import import_module
 from importlib import util as importlib_util
 from pathlib import Path
@@ -81,12 +80,18 @@ def localize_sysconfig_vars(sysconfig_vars_path, venv_site_packages):
     return sysconfig_vars
 
 
-def convert_venv(venv_path: Path, sysconfig_path: Path):
+def convert_venv(
+    venv_path: Path,
+    build_details_path: Path | None,
+    sysconfigdata_path: Path | None,
+):
     """Convert a virtual environment into a cross-platform environment.
 
     :param venv_path: The path to the root of the venv.
-    :param sysconfig_path: The path to the sysconfig_vars JSON or sysconfigdata Python
-        file for the target platform.
+    :param build_details_path: The path to build-details.json file for the
+        target platform.
+    :param sysconfigdata_path: The path to the sysconfigdata python file
+        for the target platform.
     """
     if not venv_path.exists():
         raise ValueError(f"Virtual environment {venv_path} does not exist.")
@@ -102,42 +107,61 @@ def convert_venv(venv_path: Path, sysconfig_path: Path):
 
     venv_site_packages_path = platlibs[0]
 
-    if not sysconfig_path.is_file():
-        raise ValueError(f"Could not find sysconfig file {sysconfig_path}")
+    if build_details_path:
+        if not build_details_path.is_file():
+            raise ValueError(f"Could not find {build_details_path}")
 
-    match sysconfig_path.suffix:
-        case ".json":
-            sysconfig_vars_path = sysconfig_path
-            _, _, _, abiflags, platform, multiarch = sysconfig_vars_path.stem.split("_")
+        # If build_details.json exists, then so does sysconfig_vars.
+        with open(build_details_path) as fp:
+            build_details = json.load(fp)
 
-            sysconfigdata_path = (
-                sysconfig_vars_path.parent
+        # build_details platform is the full platform-min_version-multiarch
+        # format. We only need the platform part.
+        platform = build_details["platform"].split("-")[0]
+        version = build_details["language"]["version"]
+        abiflags = "".join(build_details["abi"]["flags"])
+        multiarch = build_details["implementation"]["_multiarch"]
+
+        localize_sysconfigdata(
+            (
+                build_details_path.parent
                 / f"_sysconfigdata_{abiflags}_{platform}_{multiarch}.py"
-            )
-        case ".py":
-            sysconfigdata_path = sysconfig_path
-            _, _, abiflags, platform, multiarch = sysconfigdata_path.stem.split("_")
+            ),
+            venv_site_packages_path,
+        )
+        localize_sysconfig_vars(
+            (
+                build_details_path.parent
+                / f"_sysconfig_vars_{abiflags}_{platform}_{multiarch}.json"
+            ),
+            venv_site_packages_path,
+        )
+    elif sysconfigdata_path:
+        if not sysconfigdata_path.is_file():
+            raise ValueError(f"Could not find {sysconfigdata_path}")
 
-            sysconfig_vars_path = (
-                sysconfigdata_path.parent
-                / f"_sysconfig_vars_{abiflags}_{platform}_{multiarch}.py"
-            )
-        case _:
-            raise ValueError(
-                "Don't know how to process sysconfig data "
-                f"of type {sysconfig_path.suffix}"
-            )
+        # If we've been given a sysconfigdata file, re
+        _, _, abiflags, platform, multiarch = sysconfigdata_path.stem.split("_", 4)
 
-    # Localize the sysconfig data. sysconfigdata *must* exist; sysconfig_vars
-    # will only exist on Python 3.14 or newer.
-    sysconfig = localize_sysconfigdata(sysconfigdata_path, venv_site_packages_path)
-    if sys.version_info[:2] >= (3, 14):
-        localize_sysconfig_vars(sysconfig_vars_path, venv_site_packages_path)
+        # Localize the sysconfig data.
+        sysconfigdata = localize_sysconfigdata(
+            sysconfigdata_path,
+            venv_site_packages_path,
+        )
+        version = sysconfigdata["VERSION"]
+
+        # We'll need to reconstruct build_details-like data once we have
+        # a platform module, as the keys in sysconfig data vary by platform.
+        build_details = None
+    else:
+        raise ValueError(
+            "Must provide path to either build_details.json or sysconfigdata"
+        )
 
-    if sysconfig["VERSION"] != venv_site_packages_path.parts[-2][6:]:
+    if version != venv_site_packages_path.parts[-2][6:]:
         raise ValueError(
             f"target venv is Python {venv_site_packages_path.parts[-2][6:]}; "
-            f"sysconfig file is for Python {sysconfig['VERSION']}"
+            f"build details file is for Python {version}"
         )
 
     # Generate the context for the templated cross-target file
@@ -153,7 +177,12 @@ def convert_venv(venv_path: Path, sysconfig_path: Path):
 
     try:
         platform_module = import_module(f"xvenv.platforms.{platform}")
-        platform_module.extend_context(context, sysconfig)
+        if build_details is None:
+            build_details = platform_module.build_details_from_sysconfigdata(
+                sysconfigdata
+            )
+
+        platform_module.extend_context(context, build_details)
     except ImportError:
         raise ValueError(
             f"Don't know how to build a cross-venv file for {platform}"
diff --git a/src/xvenv/platforms/android.py b/src/xvenv/platforms/android.py
index 358cba1..4ab5a4b 100644
--- a/src/xvenv/platforms/android.py
+++ b/src/xvenv/platforms/android.py
@@ -1,6 +1,16 @@
-def extend_context(context, sysconfig):
+def build_details_from_sysconfigdata(sysconfigdata):
+    # Reconstruct a build_details-alike structure from sysconfigdata.
+    arch, _, platform = sysconfigdata["MULTIARCH"].split("-")
+    min_api_level = sysconfigdata["ANDROID_API_LEVEL"]
+    build_details = {
+        "platform": f"{platform}-{min_api_level}-{arch}",
+    }
+    return build_details
+
+
+def extend_context(context, build_details):
     # Convert the API level into a release number
-    api_level = int(sysconfig["ANDROID_API_LEVEL"])
+    api_level = int(build_details["platform"].split("-")[1])
     if api_level >= 33:
         release = f"{api_level - 20}"
     elif api_level == 32:
diff --git a/src/xvenv/platforms/emscripten.py b/src/xvenv/platforms/emscripten.py
index 0efb8fb..9afeeb6 100644
--- a/src/xvenv/platforms/emscripten.py
+++ b/src/xvenv/platforms/emscripten.py
@@ -1,4 +1,12 @@
-def extend_context(context, sysconfig):
+def build_details_from_sysconfigdata(sysconfigdata):
+    # Reconstruct a build_details-alike structure from sysconfigdata.
+    build_details = {
+        "platform": "emscripten-4.0.12-wasm32",
+    }
+    return build_details
+
+
+def extend_context(context, build_details):
     emscripten_version = "4.0.12"
     context["release"] = emscripten_version
     context["platform_version"] = emscripten_version
diff --git a/src/xvenv/platforms/ios.py b/src/xvenv/platforms/ios.py
index c699f4b..71f9c29 100644
--- a/src/xvenv/platforms/ios.py
+++ b/src/xvenv/platforms/ios.py
@@ -1,5 +1,16 @@
-def extend_context(context, sysconfig):
-    release = sysconfig["IPHONEOS_DEPLOYMENT_TARGET"]
+def build_details_from_sysconfigdata(sysconfigdata):
+    # Reconstruct a build_details-alike structure from sysconfigdata.
+    platform = sysconfigdata["MACHDEP"]
+    multiarch = sysconfigdata["MULTIARCH"]
+    ios_target = sysconfigdata["IPHONEOS_DEPLOYMENT_TARGET"]
+    build_details = {
+        "platform": f"{platform}-{ios_target}-{multiarch}",
+    }
+    return build_details
+
+
+def extend_context(context, build_details):
+    release = build_details["platform"].split("-")[1]
     is_simulator = context["sdk"] == "iphonesimulator"
 
     ######################################################################