diff --git a/python/private/python.bzl b/python/private/python.bzl
index c187904..c87beef 100644
--- a/python/private/python.bzl
+++ b/python/private/python.bzl
@@ -34,12 +34,13 @@
 
     Returns:
         A struct with the following attributes:
-            * `toolchains`: The list of toolchains to register. The last
-              element is special and is treated as the default toolchain.
-            * `defaults`: The default `kwargs` passed to
-              {bzl:obj}`python_register_toolchains`.
-            * `debug_info`: {type}`None | dict` extra information to be passed
-              to the debug repo.
+        * `toolchains`: The list of toolchains to register. The last
+          element is special and is treated as the default toolchain.
+        * `config`: Various toolchain config, see `_get_toolchain_config`.
+        * `debug_info`: {type}`None | dict` extra information to be passed
+          to the debug repo.
+        * `platforms`: {type}`dict[str, platform_info]` of the base set of
+          platforms toolchains should be created for, if possible.
     """
     if module_ctx.os.environ.get("RULES_PYTHON_BZLMOD_DEBUG", "0") == "1":
         debug_info = {
@@ -285,11 +286,12 @@
         kwargs.update(py.config.kwargs.get(toolchain_info.python_version, {}))
         kwargs.update(py.config.kwargs.get(full_python_version, {}))
         kwargs.update(py.config.default)
-        loaded_platforms[full_python_version] = python_register_toolchains(
+        toolchain_registered_platforms = python_register_toolchains(
             name = toolchain_info.name,
             _internal_bzlmod_toolchain_call = True,
             **kwargs
         )
+        loaded_platforms[full_python_version] = toolchain_registered_platforms
 
     # List of the base names ("python_3_10") for the toolchain repos
     base_toolchain_repo_names = []
@@ -332,20 +334,19 @@
         base_name = t.name
         base_toolchain_repo_names.append(base_name)
         fv = full_version(version = t.python_version, minor_mapping = py.config.minor_mapping)
-        for platform in loaded_platforms[fv]:
-            if platform not in PLATFORMS:
-                continue
+        platforms = loaded_platforms[fv]
+        for platform_name, platform_info in platforms.items():
             key = str(len(toolchain_names))
 
-            full_name = "{}_{}".format(base_name, platform)
+            full_name = "{}_{}".format(base_name, platform_name)
             toolchain_names.append(full_name)
             toolchain_repo_names[key] = full_name
-            toolchain_tcw_map[key] = PLATFORMS[platform].compatible_with
+            toolchain_tcw_map[key] = platform_info.compatible_with
 
             # The target_settings attribute may not be present for users
             # patching python/versions.bzl.
-            toolchain_ts_map[key] = getattr(PLATFORMS[platform], "target_settings", [])
-            toolchain_platform_keys[key] = platform
+            toolchain_ts_map[key] = getattr(platform_info, "target_settings", [])
+            toolchain_platform_keys[key] = platform_name
             toolchain_python_versions[key] = fv
 
             # The last toolchain is the default; it can't have version constraints
@@ -483,9 +484,9 @@
             return
 
         for platform in (tag.sha256 or []):
-            if platform not in PLATFORMS:
+            if platform not in default["platforms"]:
                 _fail("The platform must be one of {allowed} but got '{got}'".format(
-                    allowed = sorted(PLATFORMS),
+                    allowed = sorted(default["platforms"]),
                     got = platform,
                 ))
                 return
@@ -602,6 +603,26 @@
             override.fn(tag = tag, _fail = _fail, default = default)
 
 def _get_toolchain_config(*, modules, _fail = fail):
+    """Computes the configs for toolchains.
+
+    Args:
+        modules: The modules from module_ctx
+        _fail: Function to call for failing; only used for testing.
+
+    Returns:
+        A struct with the following:
+        * `kwargs`: {type}`dict[str, dict[str, object]` custom kwargs to pass to
+          `python_register_toolchains`, keyed by python version.
+          The first key is either a Major.Minor or Major.Minor.Patch
+          string.
+        * `minor_mapping`: {type}`dict[str, str]` the mapping of Major.Minor
+          to Major.Minor.Patch.
+        * `default`: {type}`dict[str, object]` of kwargs passed along to
+          `python_register_toolchains`. These keys take final precedence.
+        * `register_all_versions`: {type}`bool` whether all known versions
+          should be registered.
+    """
+
     # Items that can be overridden
     available_versions = {
         version: {
@@ -621,6 +642,7 @@
     }
     default = {
         "base_url": DEFAULT_RELEASE_BASE_URL,
+        "platforms": dict(PLATFORMS),  # Copy so it's mutable.
         "tool_versions": available_versions,
     }
 
diff --git a/python/private/python_register_toolchains.bzl b/python/private/python_register_toolchains.bzl
index cd3e9cb..6a4c0c3 100644
--- a/python/private/python_register_toolchains.bzl
+++ b/python/private/python_register_toolchains.bzl
@@ -41,6 +41,7 @@
         register_coverage_tool = False,
         set_python_version_constraint = False,
         tool_versions = None,
+        platforms = PLATFORMS,
         minor_mapping = None,
         **kwargs):
     """Convenience macro for users which does typical setup.
@@ -70,12 +71,18 @@
         tool_versions: {type}`dict` contains a mapping of version with SHASUM
             and platform info. If not supplied, the defaults in
             python/versions.bzl will be used.
+        platforms: {type}`dict[str, platform_info]` platforms to create toolchain
+            repositories for. Note that only a subset is created, depending
+            on what's available in `tool_versions`.
         minor_mapping: {type}`dict[str, str]` contains a mapping from `X.Y` to `X.Y.Z`
             version.
         **kwargs: passed to each {obj}`python_repository` call.
 
     Returns:
-        On bzlmod this returns the loaded platform labels. Otherwise None.
+        On workspace, returns None.
+
+        On bzlmod, returns a `dict[str, platform_info]`, which is the
+        subset of `platforms` that it created repositories for.
     """
     bzlmod_toolchain_call = kwargs.pop("_internal_bzlmod_toolchain_call", False)
     if bzlmod_toolchain_call:
@@ -104,13 +111,13 @@
                 ))
             register_coverage_tool = False
 
-    loaded_platforms = []
-    for platform in PLATFORMS.keys():
+    loaded_platforms = {}
+    for platform in platforms.keys():
         sha256 = tool_versions[python_version]["sha256"].get(platform, None)
         if not sha256:
             continue
 
-        loaded_platforms.append(platform)
+        loaded_platforms[platform] = platforms[platform]
         (release_filename, urls, strip_prefix, patches, patch_strip) = get_release_info(platform, python_version, base_url, tool_versions)
 
         # allow passing in a tool version
@@ -162,7 +169,7 @@
 
     host_toolchain(
         name = name + "_host",
-        platforms = loaded_platforms,
+        platforms = loaded_platforms.keys(),
         python_version = python_version,
     )
 
@@ -170,7 +177,7 @@
         name = name,
         python_version = python_version,
         user_repository_name = name,
-        platforms = loaded_platforms,
+        platforms = loaded_platforms.keys(),
     )
 
     # in bzlmod we write out our own toolchain repos
@@ -182,6 +189,6 @@
         python_version = python_version,
         set_python_version_constraint = set_python_version_constraint,
         user_repository_name = name,
-        platforms = loaded_platforms,
+        platforms = loaded_platforms.keys(),
     )
     return None
diff --git a/tests/python/python_tests.bzl b/tests/python/python_tests.bzl
index 443174c..19be1c4 100644
--- a/tests/python/python_tests.bzl
+++ b/tests/python/python_tests.bzl
@@ -149,6 +149,7 @@
         "base_url",
         "ignore_root_user_error",
         "tool_versions",
+        "platforms",
     ])
     env.expect.that_bool(py.config.default["ignore_root_user_error"]).equals(True)
     env.expect.that_str(py.default_python_version).equals("3.11")