diff --git a/extension/download_tool.bzl b/extension/download_tool.bzl
index d7b3827..cf858c8 100644
--- a/extension/download_tool.bzl
+++ b/extension/download_tool.bzl
@@ -25,6 +25,17 @@ def _get_platform(rctx):
 
     return "{}-{}".format(os_key, arch_key)
 
+def _binary_path_for_platform(binary_subpath, platform):
+    """Return the binary path as packaged for the selected platform."""
+    if platform.startswith("windows-") and not binary_subpath.endswith(".exe"):
+        return binary_subpath + ".exe"
+    return binary_subpath
+
+# Exposed only so the path resolution can be covered without downloading LLVM.
+download_tool_test_utils = struct(
+    binary_path_for_platform = _binary_path_for_platform,
+)
+
 def _download_tool_impl(rctx):
     """Download and extract a tool for the current platform."""
     platform = _get_platform(rctx)
@@ -56,7 +67,7 @@ def _download_tool_impl(rctx):
     )
 
     # Create BUILD.bazel that exports the tool binary
-    binary_path = rctx.attr.binary_subpath
+    binary_path = _binary_path_for_platform(rctx.attr.binary_subpath, platform)
     rctx.file("BUILD.bazel", content = """
 package(default_visibility = ["//visibility:public"])
 
diff --git a/tests/BUILD.bazel b/tests/BUILD.bazel
index dd5bf71..b613744 100644
--- a/tests/BUILD.bazel
+++ b/tests/BUILD.bazel
@@ -1,6 +1,9 @@
 """Unit tests for the devtools runtime scripts."""
 
 load("@rules_python//python:py_test.bzl", "py_test")
+load(":download_tool_test.bzl", "download_tool_test_suite")
+
+download_tool_test_suite(name = "download_tool_tests")
 
 py_test(
     name = "compile_commands_test",
diff --git a/tests/download_tool_test.bzl b/tests/download_tool_test.bzl
new file mode 100644
index 0000000..6772637
--- /dev/null
+++ b/tests/download_tool_test.bzl
@@ -0,0 +1,34 @@
+"""Tests for platform-specific LLVM binary paths."""
+
+load("@bazel_skylib//lib:unittest.bzl", "asserts", "unittest")
+load("//extension:download_tool.bzl", "download_tool_test_utils")
+
+def _windows_binary_path_test_impl(ctx):
+    env = unittest.begin(ctx)
+    binary_path = download_tool_test_utils.binary_path_for_platform
+
+    asserts.equals(env, "bin/clang-format.exe", binary_path("bin/clang-format", "windows-x86_64"))
+    asserts.equals(env, "bin/clang-tidy.exe", binary_path("bin/clang-tidy", "windows-x86_64"))
+    asserts.equals(env, "bin/custom-tool.exe", binary_path("bin/custom-tool.exe", "windows-x86_64"))
+
+    return unittest.end(env)
+
+windows_binary_path_test = unittest.make(_windows_binary_path_test_impl)
+
+def _unix_binary_path_test_impl(ctx):
+    env = unittest.begin(ctx)
+    binary_path = download_tool_test_utils.binary_path_for_platform
+
+    asserts.equals(env, "bin/clang-format", binary_path("bin/clang-format", "linux-x86_64"))
+    asserts.equals(env, "bin/clang-tidy", binary_path("bin/clang-tidy", "darwin-aarch64"))
+
+    return unittest.end(env)
+
+unix_binary_path_test = unittest.make(_unix_binary_path_test_impl)
+
+def download_tool_test_suite(name):
+    unittest.suite(
+        name,
+        windows_binary_path_test,
+        unix_binary_path_test,
+    )