diff --git a/Library/Homebrew/test/os/mac/sdk_spec.rb b/Library/Homebrew/test/os/mac/sdk_spec.rb
new file mode 100644
index 0000000000..55c18aa221
--- /dev/null
+++ b/Library/Homebrew/test/os/mac/sdk_spec.rb
@@ -0,0 +1,97 @@
+# typed: false
+# frozen_string_literal: true
+
+describe OS::Mac::CLTSDKLocator do
+  subject(:locator) { described_class.new }
+
+  let(:big_sur_sdk) { OS::Mac::SDK.new(OS::Mac::Version.new("11"), "/some/path/MacOSX.sdk", :clt) }
+  let(:catalina_sdk) { OS::Mac::SDK.new(OS::Mac::Version.new("10.15"), "/some/path/MacOSX10.15.sdk", :clt) }
+
+  specify "#sdk_for" do
+    allow(locator).to receive(:all_sdks).and_return([big_sur_sdk, catalina_sdk])
+
+    expect(locator.sdk_for(OS::Mac::Version.new("11"))).to eq(big_sur_sdk)
+    expect(locator.sdk_for(OS::Mac::Version.new("10.15"))).to eq(catalina_sdk)
+    expect { locator.sdk_for(OS::Mac::Version.new("10.14")) }.to raise_error(described_class::NoSDKError)
+  end
+
+  describe "#sdk_if_applicable" do
+    before do
+      allow(locator).to receive(:all_sdks).and_return([big_sur_sdk, catalina_sdk])
+    end
+
+    it "returns the requested SDK" do
+      expect(locator.sdk_if_applicable(OS::Mac::Version.new("11"))).to eq(big_sur_sdk)
+      expect(locator.sdk_if_applicable(OS::Mac::Version.new("10.15"))).to eq(catalina_sdk)
+    end
+
+    it "returns the latest SDK if the requested version is not found" do
+      expect(locator.sdk_if_applicable(OS::Mac::Version.new("10.14"))).to eq(big_sur_sdk)
+      expect(locator.sdk_if_applicable(OS::Mac::Version.new("12"))).to eq(big_sur_sdk)
+    end
+
+    it "returns the SDK matching the OS version if no version is specified" do
+      allow(OS::Mac).to receive(:version).and_return(OS::Mac::Version.new("10.15"))
+      expect(locator.sdk_if_applicable).to eq(catalina_sdk)
+    end
+
+    it "returns the latest SDK on older OS versions when there's no matching SDK" do
+      allow(OS::Mac).to receive(:version).and_return(OS::Mac::Version.new("10.14"))
+      expect(locator.sdk_if_applicable).to eq(big_sur_sdk)
+    end
+
+    it "returns nil if the OS is newer than all SDKs" do
+      allow(OS::Mac).to receive(:version).and_return(OS::Mac::Version.new("12"))
+      expect(locator.sdk_if_applicable).to be_nil
+    end
+  end
+
+  describe "#all_sdks" do
+    let(:big_sur_sdk_prefix) { TEST_FIXTURE_DIR/"sdks/big_sur" }
+    let(:mojave_broken_sdk_prefix) { TEST_FIXTURE_DIR/"sdks/mojave_broken" }
+    let(:high_sierra_sdk_prefix) { TEST_FIXTURE_DIR/"sdks/high_sierra" }
+    let(:malformed_sdk_prefix) { TEST_FIXTURE_DIR/"sdks/malformed" }
+
+    it "reads the SDKSettings.json version of unversioned SDKs folders" do
+      allow(locator).to receive(:sdk_prefix).and_return(big_sur_sdk_prefix.to_s)
+
+      sdks = locator.all_sdks
+      expect(sdks.count).to eq(1)
+
+      sdk = sdks.first
+      expect(sdk.path).to eq(big_sur_sdk_prefix/"MacOSX.sdk")
+      expect(sdk.version).to eq(OS::Mac::Version.new("11"))
+      expect(sdk.source).to eq(:clt)
+    end
+
+    it "reads the SDKSettings.json version of versioned SDKs folders" do
+      allow(locator).to receive(:sdk_prefix).and_return(mojave_broken_sdk_prefix.to_s)
+
+      sdks = locator.all_sdks
+      expect(sdks.count).to eq(1)
+
+      sdk = sdks.first
+      expect(sdk.path).to eq(mojave_broken_sdk_prefix/"MacOSX10.14.sdk")
+      expect(sdk.version).to eq(OS::Mac::Version.new("10.15"))
+      expect(sdk.source).to eq(:clt)
+    end
+
+    it "reads the SDKSettings.plist version" do
+      allow(locator).to receive(:sdk_prefix).and_return(high_sierra_sdk_prefix.to_s)
+
+      sdks = locator.all_sdks
+      expect(sdks.count).to eq(1)
+
+      sdk = sdks.first
+      expect(sdk.path).to eq(high_sierra_sdk_prefix/"MacOSX10.13.sdk")
+      expect(sdk.version).to eq(OS::Mac::Version.new("10.13"))
+      expect(sdk.source).to eq(:clt)
+    end
+
+    it "rejects malformed sdks" do
+      allow(locator).to receive(:sdk_prefix).and_return(malformed_sdk_prefix.to_s)
+
+      expect(locator.all_sdks).to be_empty
+    end
+  end
+end
diff --git a/Library/Homebrew/test/support/fixtures/sdks/big_sur/MacOSX.sdk/SDKSettings.json b/Library/Homebrew/test/support/fixtures/sdks/big_sur/MacOSX.sdk/SDKSettings.json
new file mode 100644
index 0000000000..3e9fb27234
--- /dev/null
+++ b/Library/Homebrew/test/support/fixtures/sdks/big_sur/MacOSX.sdk/SDKSettings.json
@@ -0,0 +1 @@
+{"Version":"11.1"}
diff --git a/Library/Homebrew/test/support/fixtures/sdks/high_sierra/MacOSX10.13.sdk/SDKSettings.plist b/Library/Homebrew/test/support/fixtures/sdks/high_sierra/MacOSX10.13.sdk/SDKSettings.plist
new file mode 100644
index 0000000000..7e57e7b081
--- /dev/null
+++ b/Library/Homebrew/test/support/fixtures/sdks/high_sierra/MacOSX10.13.sdk/SDKSettings.plist
@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>Version</key>
+	<string>10.13</string>
+</dict>
+</plist>
diff --git a/Library/Homebrew/test/support/fixtures/sdks/malformed/MacOSX10.15.sdk/SDKSettings.json b/Library/Homebrew/test/support/fixtures/sdks/malformed/MacOSX10.15.sdk/SDKSettings.json
new file mode 100644
index 0000000000..8613898c19
--- /dev/null
+++ b/Library/Homebrew/test/support/fixtures/sdks/malformed/MacOSX10.15.sdk/SDKSettings.json
@@ -0,0 +1 @@
+{"Version":"broken"}
diff --git a/Library/Homebrew/test/support/fixtures/sdks/mojave_broken/MacOSX10.14.sdk/SDKSettings.json b/Library/Homebrew/test/support/fixtures/sdks/mojave_broken/MacOSX10.14.sdk/SDKSettings.json
new file mode 100644
index 0000000000..62938d457a
--- /dev/null
+++ b/Library/Homebrew/test/support/fixtures/sdks/mojave_broken/MacOSX10.14.sdk/SDKSettings.json
@@ -0,0 +1 @@
+{"Version":"10.15.6"}