diff --git a/src/main/java/com/google/devtools/build/lib/analysis/starlark/FunctionTransitionUtil.java b/src/main/java/com/google/devtools/build/lib/analysis/starlark/FunctionTransitionUtil.java
index 4d463944612614..bb3b0b294ad913 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/starlark/FunctionTransitionUtil.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/starlark/FunctionTransitionUtil.java
@@ -77,7 +77,7 @@ static Map<String, BuildOptions> applyAndValidate(
       StructImpl attrObject,
       EventHandler eventHandler)
       throws EvalException, InterruptedException {
-    checkForBlacklistedOptions(starlarkTransition);
+    checkForDenylistedOptions(starlarkTransition);
 
     // TODO(waltl): consider building this once and use it across different split
     // transitions.
@@ -91,14 +91,48 @@ static Map<String, BuildOptions> applyAndValidate(
     validateFunctionOutputsMatchesDeclaredOutputs(transitions.values(), starlarkTransition);
 
     for (Map.Entry<String, Map<String, Object>> entry : transitions.entrySet()) {
+      Map<String, Object> newValues = handleImplicitPlatformChange(entry.getValue());
       BuildOptions transitionedOptions =
-          applyTransition(buildOptions, entry.getValue(), optionInfoMap, starlarkTransition);
+          applyTransition(buildOptions, newValues, optionInfoMap, starlarkTransition);
       splitBuildOptions.put(entry.getKey(), transitionedOptions);
     }
     return splitBuildOptions.build();
   }
 
-  private static void checkForBlacklistedOptions(StarlarkDefinedConfigTransition transition)
+  /**
+   * If the transition changes --cpu but not --platforms, clear out --platforms.
+   *
+   * <p>Purpose:
+   *
+   * <ol>
+   *   <li>A platform mapping sets --cpu=foo when --platforms=foo.
+   *   <li>A transition sets --cpu=bar.
+   *   <li>Because --platforms=foo, the platform mapping kicks in to set --cpu back to foo.
+   *   <li>Result: the mapping accidentally overrides the transition
+   * </ol>
+   *
+   * <p>Transitions can alo explicitly set --platforms to be clear what platform they set.
+   *
+   * <p>Platform mappings:
+   * https://docs.bazel.build/versions/master/platforms-intro.html#platform-mappings.
+   *
+   * <p>This doesn't check that the changed value is actually different than the source (i.e.
+   * setting {@code --cpu=foo} when {@code --cpu} is already {@code foo}). That could unnecessarily
+   * fork configurations that are really the same. That's a possible optimization TODO.
+   */
+  private static Map<String, Object> handleImplicitPlatformChange(
+      Map<String, Object> originalOutput) {
+    boolean changesCpu = originalOutput.containsKey(COMMAND_LINE_OPTION_PREFIX + "cpu");
+    boolean changesPlatforms = originalOutput.containsKey(COMMAND_LINE_OPTION_PREFIX + "platforms");
+    return changesCpu && !changesPlatforms
+        ? ImmutableMap.<String, Object>builder()
+            .putAll(originalOutput)
+            .put(COMMAND_LINE_OPTION_PREFIX + "platforms", ImmutableList.<Label>of())
+            .build()
+        : originalOutput;
+  }
+
+  private static void checkForDenylistedOptions(StarlarkDefinedConfigTransition transition)
       throws EvalException {
     if (transition.getOutputs().contains("//command_line_option:define")) {
       throw Starlark.errorf(
diff --git a/src/test/java/com/google/devtools/build/lib/analysis/StarlarkAttrTransitionProviderTest.java b/src/test/java/com/google/devtools/build/lib/analysis/StarlarkAttrTransitionProviderTest.java
index b6e19c2a9f0290..d22031d9fb02e2 100644
--- a/src/test/java/com/google/devtools/build/lib/analysis/StarlarkAttrTransitionProviderTest.java
+++ b/src/test/java/com/google/devtools/build/lib/analysis/StarlarkAttrTransitionProviderTest.java
@@ -40,6 +40,7 @@
 import com.google.devtools.build.lib.packages.util.BazelMockAndroidSupport;
 import com.google.devtools.build.lib.rules.cpp.CppConfiguration;
 import com.google.devtools.build.lib.skyframe.ConfiguredTargetAndData;
+import com.google.devtools.build.lib.testutil.TestConstants;
 import com.google.devtools.build.lib.testutil.TestRuleClassProvider;
 import java.util.Collection;
 import java.util.List;
@@ -91,7 +92,6 @@ private void writeAllowlistFile() throws Exception {
   }
 
   private void writeBasicTestFiles() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
     getAnalysisMock().ccSupport().setupCcToolchainConfigForCpu(mockToolsConfig, "armeabi-v7a");
     scratch.file(
@@ -128,7 +128,6 @@ private void writeBasicTestFiles() throws Exception {
 
   @Test
   public void testStarlarkSplitTransitionSplitAttr() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
     scratch.file(
         "test/starlark/rules.bzl",
@@ -186,7 +185,6 @@ public void testStarlarkSplitTransitionSplitAttr() throws Exception {
 
   @Test
   public void testStarlarkListSplitTransitionSplitAttr() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
     scratch.file(
         "test/starlark/rules.bzl",
@@ -238,7 +236,6 @@ public void testStarlarkListSplitTransitionSplitAttr() throws Exception {
 
   @Test
   public void testStarlarkPatchTransitionSplitAttr() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
     scratch.file(
         "test/starlark/rules.bzl",
@@ -289,7 +286,6 @@ public void testStarlarkPatchTransitionSplitAttr() throws Exception {
   public void testStarlarkConfigSplitAttr() throws Exception {
     // This is a customized test case for b/152078818, where a starlark transition that takes a
     // starlark config as input caused a failure when no custom values were provided for the config.
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
     scratch.file(
         "test/starlark/rules.bzl",
@@ -353,7 +349,6 @@ public void testFunctionSplitTransitionCheckAttrDep() throws Exception {
 
   @Test
   public void testTargetAndRuleNotInAllowlist() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
     getAnalysisMock().ccSupport().setupCcToolchainConfigForCpu(mockToolsConfig, "armeabi-v7a");
     scratch.file(
@@ -420,7 +415,6 @@ private void testSplitTransitionCheckAttrDep(ConfiguredTarget target) throws Exc
   }
 
   private void writeReadSettingsTestFiles() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
 
     scratch.file(
@@ -472,7 +466,6 @@ public void testReadSettingsSplitDepAttrDep() throws Exception {
   }
 
   private void writeOptionConversionTestFiles() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
 
     scratch.file(
@@ -521,7 +514,6 @@ public void testOptionConversionCpu() throws Exception {
   }
 
   private void writeReadAndPassthroughOptionsTestFiles() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
 
     scratch.file(
@@ -606,7 +598,6 @@ public void testCompilationModeReadableInStarlarkTransitions() throws Exception
 
   @Test
   public void testUndeclaredOptionKey() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
 
     scratch.file(
@@ -639,7 +630,6 @@ public void testUndeclaredOptionKey() throws Exception {
 
   @Test
   public void testDeclaredOutputNotReturned() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
 
     scratch.file(
@@ -676,7 +666,6 @@ public void testDeclaredOutputNotReturned() throws Exception {
 
   @Test
   public void testSettingsContainOnlyInputs() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
 
     scratch.file(
@@ -713,7 +702,6 @@ public void testSettingsContainOnlyInputs() throws Exception {
 
   @Test
   public void testInvalidInputKey() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
 
     scratch.file(
@@ -748,7 +736,6 @@ public void testInvalidInputKey() throws Exception {
 
   @Test
   public void testInvalidNativeOptionInput() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
 
     scratch.file(
@@ -784,7 +771,6 @@ public void testInvalidNativeOptionInput() throws Exception {
 
   @Test
   public void testInvalidNativeOptionOutput() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
 
     scratch.file(
@@ -821,7 +807,6 @@ public void testInvalidNativeOptionOutput() throws Exception {
   public void testBannedNativeOptionOutput() throws Exception {
     // Just picked an arbirtary incompatible_ flag; however, could be any flag
     // besides incompatible_enable_cc_toolchain_resolution (and might not even need to be real).
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
 
     scratch.file(
@@ -856,7 +841,6 @@ public void testBannedNativeOptionOutput() throws Exception {
 
   @Test
   public void testAllowIncompatibleEnableCcToolchainResolution() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
 
     scratch.file(
@@ -901,7 +885,6 @@ public void testAllowIncompatibleEnableCcToolchainResolution() throws Exception
 
   @Test
   public void testInvalidOutputKey() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
 
     scratch.file(
@@ -936,7 +919,6 @@ public void testInvalidOutputKey() throws Exception {
 
   @Test
   public void testInvalidOptionValue() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
 
     scratch.file(
@@ -969,7 +951,6 @@ public void testInvalidOptionValue() throws Exception {
 
   @Test
   public void testDuplicateOutputs() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
 
     scratch.file(
@@ -1913,7 +1894,6 @@ public void starlarkSplitTransitionRequiredFragments() throws Exception {
    */
   private void testNoOpTransitionLeavesSameConfig_native(boolean directRead) throws Exception {
     writeAllowlistFile();
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
 
     String outputValue = directRead ? "settings['//command_line_option:test_arg']" : "['frisbee']";
     String inputs = directRead ? "['//command_line_option:test_arg']" : "[]";
@@ -1974,7 +1954,6 @@ public void testNoOpTransitionLeavesSameConfig_native_setToSame() throws Excepti
   private void testNoOpTransitionLeavesSameConfig_starlark(boolean directRead, boolean setToDefault)
       throws Exception {
     writeAllowlistFile();
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
 
     String outputValue = directRead ? "settings['//test:flag']" : "'frisbee'";
     String inputs = directRead ? "['//test:flag']" : "[]";
@@ -2047,4 +2026,130 @@ public void testOptionConversionDynamicMode() throws Exception {
   public void testOptionConversionCrosstoolTop() throws Exception {
     // TODO(waltl): check that crosstool_top is parsed properly.
   }
+
+  /**
+   * Changing --cpu implicitly changes the target platform. Test that the old value of --platforms
+   * gets cleared out (platform mappings can then kick in to set --platforms correctly).
+   */
+  @Test
+  public void testImplicitPlatformsChange() throws Exception {
+    writeAllowlistFile();
+    getAnalysisMock().ccSupport().setupCcToolchainConfigForCpu(mockToolsConfig, "armeabi-v7a");
+    scratch.file("platforms/BUILD", "platform(name = 'my_platform', constraint_values = [])");
+    scratch.file(
+        "test/starlark/my_rule.bzl",
+        "def transition_func(settings, attr):",
+        "  return {'//command_line_option:cpu': 'armeabi-v7a'}",
+        "my_transition = transition(implementation = transition_func,",
+        "  inputs = [], outputs = ['//command_line_option:cpu'])",
+        "def impl(ctx): ",
+        "  return []",
+        "my_rule = rule(",
+        "  implementation = impl,",
+        "  attrs = {",
+        "    'dep':  attr.label(cfg = my_transition),",
+        "    '_allowlist_function_transition': attr.label(",
+        "        default = '//tools/allowlists/function_transition_allowlist',",
+        "    ),",
+        "  })");
+    scratch.file(
+        "test/starlark/BUILD",
+        "load('//test/starlark:my_rule.bzl', 'my_rule')",
+        "my_rule(name = 'test', dep = ':main1')",
+        "cc_binary(name = 'main1', srcs = ['main1.c'])");
+
+    useConfiguration("--platforms=//platforms:my_platform");
+    ConfiguredTarget dep =
+        getDirectPrerequisite(getConfiguredTarget("//test/starlark:test"), "//test/starlark:main1");
+    // When --platforms is empty and no platform mapping triggers, PlatformMappingValue sets
+    // --platforms to PlatformOptions.computeTargetPlatform(), which defaults to the host.
+    assertThat(getConfiguration(dep).getOptions().get(PlatformOptions.class).platforms)
+        .containsExactly(
+            Label.parseAbsoluteUnchecked(TestConstants.PLATFORM_PACKAGE_ROOT + ":default_host"));
+  }
+
+  @Test
+  public void testExplicitPlatformsChange() throws Exception {
+    writeAllowlistFile();
+    getAnalysisMock().ccSupport().setupCcToolchainConfigForCpu(mockToolsConfig, "armeabi-v7a");
+    scratch.file(
+        "platforms/BUILD",
+        "platform(name = 'my_platform', constraint_values = [])",
+        "platform(name = 'my_other_platform', constraint_values = [])");
+    scratch.file(
+        "test/starlark/my_rule.bzl",
+        "def transition_func(settings, attr):",
+        "  return {",
+        "    '//command_line_option:cpu': 'armeabi-v7a',",
+        "    '//command_line_option:platforms': ['//platforms:my_other_platform']",
+        "  }",
+        "my_transition = transition(implementation = transition_func,",
+        "  inputs = [],",
+        "  outputs = [",
+        "    '//command_line_option:cpu',",
+        "    '//command_line_option:platforms'",
+        "  ]",
+        ")",
+        "def impl(ctx): ",
+        "  return []",
+        "my_rule = rule(",
+        "  implementation = impl,",
+        "  attrs = {",
+        "    'dep':  attr.label(cfg = my_transition),",
+        "    '_allowlist_function_transition': attr.label(",
+        "        default = '//tools/allowlists/function_transition_allowlist',",
+        "    ),",
+        "  })");
+    scratch.file(
+        "test/starlark/BUILD",
+        "load('//test/starlark:my_rule.bzl', 'my_rule')",
+        "my_rule(name = 'test', dep = ':main1')",
+        "cc_binary(name = 'main1', srcs = ['main1.c'])");
+
+    useConfiguration("--platforms=//platforms:my_platform");
+    ConfiguredTarget dep =
+        getDirectPrerequisite(getConfiguredTarget("//test/starlark:test"), "//test/starlark:main1");
+    assertThat(getConfiguration(dep).getOptions().get(PlatformOptions.class).platforms)
+        .containsExactly(Label.parseAbsoluteUnchecked("//platforms:my_other_platform"));
+  }
+
+  /* If the transition doesn't change --cpu, it doesn't constitute a platform change. */
+  @Test
+  public void testNoPlatformChange() throws Exception {
+    writeAllowlistFile();
+    scratch.file("platforms/BUILD", "platform(name = 'my_platform', constraint_values = [])");
+    scratch.file(
+        "test/starlark/my_rule.bzl",
+        "def transition_func(settings, attr):",
+        "  return {",
+        "    '//command_line_option:test_arg': ['blah'],",
+        "  }",
+        "my_transition = transition(implementation = transition_func,",
+        "  inputs = [],",
+        "  outputs = [",
+        "    '//command_line_option:test_arg',",
+        "  ]",
+        ")",
+        "def impl(ctx): ",
+        "  return []",
+        "my_rule = rule(",
+        "  implementation = impl,",
+        "  attrs = {",
+        "    'dep':  attr.label(cfg = my_transition),",
+        "    '_allowlist_function_transition': attr.label(",
+        "        default = '//tools/allowlists/function_transition_allowlist',",
+        "    ),",
+        "  })");
+    scratch.file(
+        "test/starlark/BUILD",
+        "load('//test/starlark:my_rule.bzl', 'my_rule')",
+        "my_rule(name = 'test', dep = ':main1')",
+        "cc_binary(name = 'main1', srcs = ['main1.c'])");
+
+    useConfiguration("--platforms=//platforms:my_platform");
+    ConfiguredTarget dep =
+        getDirectPrerequisite(getConfiguredTarget("//test/starlark:test"), "//test/starlark:main1");
+    assertThat(getConfiguration(dep).getOptions().get(PlatformOptions.class).platforms)
+        .containsExactly(Label.parseAbsoluteUnchecked("//platforms:my_platform"));
+  }
 }
diff --git a/src/test/java/com/google/devtools/build/lib/analysis/StarlarkRuleTransitionProviderTest.java b/src/test/java/com/google/devtools/build/lib/analysis/StarlarkRuleTransitionProviderTest.java
index 73ee4bf1f5e8d3..bb66659cb14c21 100644
--- a/src/test/java/com/google/devtools/build/lib/analysis/StarlarkRuleTransitionProviderTest.java
+++ b/src/test/java/com/google/devtools/build/lib/analysis/StarlarkRuleTransitionProviderTest.java
@@ -29,6 +29,7 @@
 import com.google.devtools.build.lib.packages.Rule;
 import com.google.devtools.build.lib.rules.cpp.CppOptions;
 import com.google.devtools.build.lib.skyframe.ConfiguredTargetAndData;
+import com.google.devtools.build.lib.testutil.TestConstants;
 import com.google.devtools.build.lib.testutil.TestRuleClassProvider;
 import com.google.devtools.common.options.Option;
 import com.google.devtools.common.options.OptionDocumentationCategory;
@@ -442,7 +443,6 @@ public void testCannotTransitionOnBuildSettingWithoutFlag() throws Exception {
 
   @Test
   public void testTransitionOnBuildSetting_fromDefault() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -465,7 +465,6 @@ public void testTransitionOnBuildSetting_fromDefault() throws Exception {
 
   @Test
   public void testTransitionOnBuildSetting_fromCommandLine() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -490,7 +489,6 @@ public void testTransitionOnBuildSetting_fromCommandLine() throws Exception {
 
   @Test
   public void testTransitionOnBuildSetting_badValue() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -512,7 +510,6 @@ public void testTransitionOnBuildSetting_badValue() throws Exception {
 
   @Test
   public void testTransitionOnBuildSetting_noSuchTarget() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -533,7 +530,6 @@ public void testTransitionOnBuildSetting_noSuchTarget() throws Exception {
 
   @Test
   public void testTransitionOnBuildSetting_noSuchPackage() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -552,7 +548,6 @@ public void testTransitionOnBuildSetting_noSuchPackage() throws Exception {
 
   @Test
   public void testTransitionOnBuildSetting_notABuildSetting() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
     scratch.file(
         "test/transitions.bzl",
@@ -597,7 +592,6 @@ public void testTransitionOnBuildSetting_notABuildSetting() throws Exception {
 
   @Test
   public void testTransitionOnBuildSetting_dontStoreDefault() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -618,7 +612,6 @@ public void testTransitionOnBuildSetting_dontStoreDefault() throws Exception {
 
   @Test
   public void testTransitionReadsBuildSetting_fromDefault() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -641,7 +634,6 @@ public void testTransitionReadsBuildSetting_fromDefault() throws Exception {
 
   @Test
   public void testTransitionReadsBuildSetting_fromCommandLine() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -666,7 +658,6 @@ public void testTransitionReadsBuildSetting_fromCommandLine() throws Exception {
 
   @Test
   public void testTransitionReadsBuildSetting_notABuildSetting() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
     scratch.file(
         "test/transitions.bzl",
@@ -711,7 +702,6 @@ public void testTransitionReadsBuildSetting_notABuildSetting() throws Exception
 
   @Test
   public void testTransitionReadsBuildSetting_noSuchTarget() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -732,7 +722,6 @@ public void testTransitionReadsBuildSetting_noSuchTarget() throws Exception {
 
   @Test
   public void testAliasedBuildSetting() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -767,7 +756,6 @@ public void testAliasedBuildSetting() throws Exception {
 
   @Test
   public void testAliasedBuildSetting_chainedAliases() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -803,7 +791,6 @@ public void testAliasedBuildSetting_chainedAliases() throws Exception {
 
   @Test
   public void testAliasedBuildSetting_configuredActualValue() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -848,7 +835,6 @@ public void testAliasedBuildSetting_configuredActualValue() throws Exception {
 
   @Test
   public void testAliasedBuildSetting_cyclicalAliases() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -874,7 +860,6 @@ public void testAliasedBuildSetting_cyclicalAliases() throws Exception {
 
   @Test
   public void testAliasedBuildSetting_setAliasAndActual() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -909,7 +894,6 @@ public void testAliasedBuildSetting_setAliasAndActual() throws Exception {
 
   @Test
   public void testAliasedBuildSetting_outputReturnMismatch() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -942,7 +926,6 @@ public void testAliasedBuildSetting_outputReturnMismatch() throws Exception {
 
   @Test
   public void testOneParamTransitionFunctionApiFails() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     writeAllowlistFile();
     scratch.file(
         "test/transitions.bzl",
@@ -1496,7 +1479,6 @@ public void invalidMnemonicFromDepTransition() throws Exception {
 
   @Test
   public void testTransitionOnAllowMultiplesBuildSettingRequiresList() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -1546,7 +1528,6 @@ public void testTransitionOnAllowMultiplesBuildSettingRequiresList() throws Exce
 
   @Test
   public void testTransitionOnAllowMultiplesBuildSetting() throws Exception {
-    setBuildLanguageOptions("--experimental_starlark_config_transitions=true");
     scratch.file(
         "test/transitions.bzl",
         "def _transition_impl(settings, attr):",
@@ -1594,4 +1575,139 @@ public void testTransitionOnAllowMultiplesBuildSetting() throws Exception {
             (List<?>) starlarkOptions.get(Label.parseAbsoluteUnchecked("//test:cute-animal-fact")))
         .containsExactly("puffins mate for life");
   }
+
+  /**
+   * Changing --cpu implicitly changes the target platform. Test that the old value of --platforms
+   * gets cleared out (platform mappings can then kick in to set --platforms correctly).
+   */
+  @Test
+  public void testImplicitPlatformsChange() throws Exception {
+    scratch.file("platforms/BUILD", "platform(name = 'my_platform', constraint_values = [])");
+    scratch.file(
+        "test/transitions.bzl",
+        "def _transition_impl(settings, attr):",
+        "  return {'//command_line_option:cpu': 'ppc'}",
+        "my_transition = transition(",
+        "  implementation = _transition_impl,",
+        "  inputs = [],",
+        "  outputs = ['//command_line_option:cpu']",
+        ")");
+    writeAllowlistFile();
+    scratch.file(
+        "test/rules.bzl",
+        "load('//test:transitions.bzl', 'my_transition')",
+        "def _rule_impl(ctx):",
+        "  return []",
+        "my_rule = rule(",
+        "  implementation = _rule_impl,",
+        "  cfg = my_transition,",
+        "  attrs = {",
+        "    '_allowlist_function_transition': attr.label(",
+        "        default = '//tools/allowlists/function_transition_allowlist',",
+        "    ),",
+        "  },",
+        ")");
+    scratch.file("test/BUILD", "load('//test:rules.bzl', 'my_rule')", "my_rule(name = 'test')");
+
+    useConfiguration("--platforms=//platforms:my_platform");
+    // When --platforms is empty and no platform mapping triggers, PlatformMappingValue sets
+    // --platforms to PlatformOptions.computeTargetPlatform(), which defaults to the host.
+    assertThat(
+            getConfiguration(getConfiguredTarget("//test:test"))
+                .getOptions()
+                .get(PlatformOptions.class)
+                .platforms)
+        .containsExactly(
+            Label.parseAbsoluteUnchecked(TestConstants.PLATFORM_PACKAGE_ROOT + ":default_host"));
+  }
+
+  @Test
+  public void testExplicitPlatformsChange() throws Exception {
+    scratch.file(
+        "platforms/BUILD",
+        "platform(name = 'my_platform', constraint_values = [])",
+        "platform(name = 'my_other_platform', constraint_values = [])");
+    scratch.file(
+        "test/transitions.bzl",
+        "def _transition_impl(settings, attr):",
+        "  return {",
+        "    '//command_line_option:cpu': 'ppc',",
+        "    '//command_line_option:platforms': ['//platforms:my_other_platform']",
+        "  }",
+        "my_transition = transition(",
+        "  implementation = _transition_impl,",
+        "  inputs = [],",
+        "  outputs = [",
+        "    '//command_line_option:cpu',",
+        "    '//command_line_option:platforms'",
+        "  ]",
+        ")");
+    writeAllowlistFile();
+    scratch.file(
+        "test/rules.bzl",
+        "load('//test:transitions.bzl', 'my_transition')",
+        "def _rule_impl(ctx):",
+        "  return []",
+        "my_rule = rule(",
+        "  implementation = _rule_impl,",
+        "  cfg = my_transition,",
+        "  attrs = {",
+        "    '_allowlist_function_transition': attr.label(",
+        "        default = '//tools/allowlists/function_transition_allowlist',",
+        "    ),",
+        "  },",
+        ")");
+    scratch.file("test/BUILD", "load('//test:rules.bzl', 'my_rule')", "my_rule(name = 'test')");
+
+    useConfiguration("--platforms=//platforms:my_platform");
+    assertThat(
+            getConfiguration(getConfiguredTarget("//test:test"))
+                .getOptions()
+                .get(PlatformOptions.class)
+                .platforms)
+        .containsExactly(Label.parseAbsoluteUnchecked("//platforms:my_other_platform"));
+  }
+
+  /* If the transition doesn't change --cpu, it doesn't constitute a platform change. */
+  @Test
+  public void testNoPlatformChange() throws Exception {
+    scratch.file("platforms/BUILD", "platform(name = 'my_platform', constraint_values = [])");
+    scratch.file(
+        "test/transitions.bzl",
+        "def _transition_impl(settings, attr):",
+        "  return {",
+        "    '//command_line_option:test_arg': ['blah'],",
+        "  }",
+        "my_transition = transition(",
+        "  implementation = _transition_impl,",
+        "  inputs = [],",
+        "  outputs = [",
+        "    '//command_line_option:test_arg',",
+        "  ]",
+        ")");
+    writeAllowlistFile();
+    scratch.file(
+        "test/rules.bzl",
+        "load('//test:transitions.bzl', 'my_transition')",
+        "def _rule_impl(ctx):",
+        "  return []",
+        "my_rule = rule(",
+        "  implementation = _rule_impl,",
+        "  cfg = my_transition,",
+        "  attrs = {",
+        "    '_allowlist_function_transition': attr.label(",
+        "        default = '//tools/allowlists/function_transition_allowlist',",
+        "    ),",
+        "  },",
+        ")");
+    scratch.file("test/BUILD", "load('//test:rules.bzl', 'my_rule')", "my_rule(name = 'test')");
+
+    useConfiguration("--platforms=//platforms:my_platform");
+    assertThat(
+            getConfiguration(getConfiguredTarget("//test:test"))
+                .getOptions()
+                .get(PlatformOptions.class)
+                .platforms)
+        .containsExactly(Label.parseAbsoluteUnchecked("//platforms:my_platform"));
+  }
 }