diff --git a/src/main/java/com/google/devtools/build/lib/analysis/ToolchainResolver.java b/src/main/java/com/google/devtools/build/lib/analysis/ToolchainResolver.java
index ead2d52..04e548a 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/ToolchainResolver.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/ToolchainResolver.java
@@ -66,6 +66,7 @@
   // Optional data.
   private ImmutableSet<Label> requiredToolchainTypeLabels = ImmutableSet.of();
   private ImmutableSet<Label> execConstraintLabels = ImmutableSet.of();
+  private boolean shouldSanityCheckConfiguration = false;
 
   // Determined during execution.
   private boolean debug = false;
@@ -101,6 +102,17 @@
   }
 
   /**
+   * Sets whether the experimental retroactive trimming mode is in use. This determines whether
+   * sanity checks regarding the fragments in use for the configurations of platforms and toolchains
+   * are used - specifically, whether platforms use only the PlatformConfiguration, and toolchains
+   * do not use any configuration at all.
+   */
+  public ToolchainResolver setShouldSanityCheckConfiguration(boolean useSanityChecks) {
+    this.shouldSanityCheckConfiguration = useSanityChecks;
+    return this;
+  }
+
+  /**
    * Determines the specific toolchains that are required, given the requested toolchain types,
    * target platform, and configuration.
    *
@@ -187,7 +199,9 @@
 
     // Load the host and target platforms early, to check for errors.
     PlatformLookupUtil.getPlatformInfo(
-        ImmutableList.of(hostPlatformKey, targetPlatformKey), environment);
+        ImmutableList.of(hostPlatformKey, targetPlatformKey),
+        environment,
+        shouldSanityCheckConfiguration);
     if (environment.valuesMissing()) {
       throw new ValueMissingException();
     }
@@ -245,7 +259,8 @@
     // platform is the host platform), Skyframe will return the correct results immediately without
     // need of a restart.
     Map<ConfiguredTargetKey, PlatformInfo> platformInfoMap =
-        PlatformLookupUtil.getPlatformInfo(platformKeys, environment);
+        PlatformLookupUtil.getPlatformInfo(
+            platformKeys, environment, shouldSanityCheckConfiguration);
     if (platformInfoMap == null) {
       throw new ValueMissingException();
     }
@@ -367,7 +382,8 @@
     Map<ConfiguredTargetKey, PlatformInfo> platforms =
         PlatformLookupUtil.getPlatformInfo(
             ImmutableList.of(selectedExecutionPlatformKey.get(), platformKeys.targetPlatformKey()),
-            environment);
+            environment,
+            shouldSanityCheckConfiguration);
     if (platforms == null) {
       throw new ValueMissingException();
     }
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/ConfiguredTargetFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/ConfiguredTargetFunction.java
index 854ffe3..5c6481b 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/ConfiguredTargetFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/ConfiguredTargetFunction.java
@@ -313,6 +313,8 @@
               new ToolchainResolver(env, configuredTargetKey.getConfigurationKey())
                   .setRequiredToolchainTypes(requiredToolchains)
                   .setExecConstraintLabels(execConstraintLabels)
+                  .setShouldSanityCheckConfiguration(
+                      configuration.trimConfigurationsRetroactively())
                   .resolve();
           if (env.valuesMissing()) {
             return null;
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/PlatformLookupUtil.java b/src/main/java/com/google/devtools/build/lib/skyframe/PlatformLookupUtil.java
index d9f998b..4c1c306 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/PlatformLookupUtil.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/PlatformLookupUtil.java
@@ -14,8 +14,12 @@
 
 package com.google.devtools.build.lib.skyframe;
 
+import static com.google.common.base.Predicates.equalTo;
+import static com.google.common.base.Predicates.not;
+
 import com.google.devtools.build.lib.actions.MutableActionGraph.ActionConflictException;
 import com.google.devtools.build.lib.analysis.ConfiguredTarget;
+import com.google.devtools.build.lib.analysis.PlatformConfiguration;
 import com.google.devtools.build.lib.analysis.platform.PlatformInfo;
 import com.google.devtools.build.lib.analysis.platform.PlatformProviderUtils;
 import com.google.devtools.build.lib.cmdline.Label;
@@ -33,7 +37,7 @@
 
   @Nullable
   public static Map<ConfiguredTargetKey, PlatformInfo> getPlatformInfo(
-      Iterable<ConfiguredTargetKey> platformKeys, Environment env)
+      Iterable<ConfiguredTargetKey> platformKeys, Environment env, boolean sanityCheckConfiguration)
       throws InterruptedException, InvalidPlatformException {
 
     Map<
@@ -49,7 +53,7 @@
     boolean valuesMissing = env.valuesMissing();
     Map<ConfiguredTargetKey, PlatformInfo> platforms = valuesMissing ? null : new HashMap<>();
     for (ConfiguredTargetKey key : platformKeys) {
-      PlatformInfo platformInfo = findPlatformInfo(key, values.get(key));
+      PlatformInfo platformInfo = findPlatformInfo(key, values.get(key), sanityCheckConfiguration);
       if (!valuesMissing && platformInfo != null) {
         platforms.put(key, platformInfo);
       }
@@ -72,7 +76,8 @@
       ConfiguredTargetKey key,
       ValueOrException3<
               ConfiguredValueCreationException, NoSuchThingException, ActionConflictException>
-          valueOrException)
+          valueOrException,
+      boolean sanityCheckConfiguration)
       throws InvalidPlatformException {
 
     try {
@@ -82,6 +87,21 @@
       }
 
       ConfiguredTarget configuredTarget = ctv.getConfiguredTarget();
+      BuildConfigurationValue.Key configurationKey = configuredTarget.getConfigurationKey();
+      // This check is necessary because trimming for other rules assumes that platform resolution
+      // uses the platform fragment and _only_ the platform fragment. Without this check, it's
+      // possible another fragment could slip in without us realizing, and thus break this
+      // assumption.
+      if (sanityCheckConfiguration
+          && configurationKey.getFragments().stream()
+              .anyMatch(not(equalTo(PlatformConfiguration.class)))) {
+        // Only the PlatformConfiguration fragment may be present on a platform rule in retroactive
+        // trimming mode.
+        throw new InvalidPlatformException(
+            configuredTarget.getLabel(),
+            "has fragments other than PlatformConfiguration, "
+                + "which is forbidden in retroactive trimming mode");
+      }
       PlatformInfo platformInfo = PlatformProviderUtils.platform(configuredTarget);
       if (platformInfo == null) {
         throw new InvalidPlatformException(configuredTarget.getLabel());
@@ -99,12 +119,14 @@
 
   /** Exception used when a platform label is not a valid platform. */
   public static final class InvalidPlatformException extends ToolchainException {
+    private static final String DEFAULT_ERROR = "does not provide PlatformInfo";
+
     InvalidPlatformException(Label label) {
-      super(formatError(label));
+      super(formatError(label, DEFAULT_ERROR));
     }
 
     InvalidPlatformException(Label label, ConfiguredValueCreationException e) {
-      super(formatError(label), e);
+      super(formatError(label, DEFAULT_ERROR), e);
     }
 
     public InvalidPlatformException(Label label, NoSuchThingException e) {
@@ -113,12 +135,15 @@
     }
 
     public InvalidPlatformException(Label label, ActionConflictException e) {
-      super(formatError(label), e);
+      super(formatError(label, DEFAULT_ERROR), e);
     }
 
-    private static String formatError(Label label) {
-      return String.format(
-          "Target %s was referenced as a platform, but does not provide PlatformInfo", label);
+    InvalidPlatformException(Label label, String error) {
+      super(formatError(label, error));
+    }
+
+    private static String formatError(Label label, String error) {
+      return String.format("Target %s was referenced as a platform, but %s", label, error);
     }
   }
 }
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/RegisteredExecutionPlatformsFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/RegisteredExecutionPlatformsFunction.java
index 9348cec..255fe0e 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/RegisteredExecutionPlatformsFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/RegisteredExecutionPlatformsFunction.java
@@ -14,6 +14,8 @@
 
 package com.google.devtools.build.lib.skyframe;
 
+import static com.google.common.base.Predicates.equalTo;
+import static com.google.common.base.Predicates.not;
 import static com.google.common.collect.ImmutableList.toImmutableList;
 
 import com.google.auto.value.AutoValue;
@@ -92,7 +94,8 @@
 
     // Load the configured target for each, and get the declared execution platforms providers.
     ImmutableList<ConfiguredTargetKey> registeredExecutionPlatformKeys =
-        configureRegisteredExecutionPlatforms(env, configuration, platformLabels);
+        configureRegisteredExecutionPlatforms(
+            env, configuration, configuration.trimConfigurationsRetroactively(), platformLabels);
     if (env.valuesMissing()) {
       return null;
     }
@@ -122,6 +125,7 @@
   private ImmutableList<ConfiguredTargetKey> configureRegisteredExecutionPlatforms(
       Environment env,
       BuildConfiguration configuration,
+      boolean sanityCheckConfiguration,
       List<Label> labels)
       throws InterruptedException, RegisteredExecutionPlatformsFunctionException {
 
@@ -145,6 +149,22 @@
         }
         ConfiguredTarget target =
             ((ConfiguredTargetValue) valueOrException.get()).getConfiguredTarget();
+        // This check is necessary because trimming for other rules assumes that platform resolution
+        // uses the platform fragment and _only_ the platform fragment. Without this check, it's
+        // possible another fragment could slip in without us realizing, and thus break this
+        // assumption.
+        if (sanityCheckConfiguration
+            && target.getConfigurationKey().getFragments().stream()
+                .anyMatch(not(equalTo(PlatformConfiguration.class)))) {
+          // Only the PlatformConfiguration fragment may be present on a platform rule in
+          // retroactive trimming mode.
+          throw new RegisteredExecutionPlatformsFunctionException(
+              new InvalidPlatformException(
+                  target.getLabel(),
+                  "has fragments other than PlatformConfiguration, "
+                      + "which is forbidden in retroactive trimming mode"),
+              Transience.PERSISTENT);
+        }
         PlatformInfo platformInfo = PlatformProviderUtils.platform(target);
         if (platformInfo == null) {
           throw new RegisteredExecutionPlatformsFunctionException(
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/RegisteredToolchainsFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/RegisteredToolchainsFunction.java
index 69ac1b7..7d7b054 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/RegisteredToolchainsFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/RegisteredToolchainsFunction.java
@@ -145,8 +145,23 @@
           valuesMissing = true;
           continue;
         }
+
         ConfiguredTarget target =
             ((ConfiguredTargetValue) valueOrException.get()).getConfiguredTarget();
+        if (configuration.trimConfigurationsRetroactively()
+            && !target.getConfigurationKey().getFragments().isEmpty()) {
+          // No fragment may be present on a toolchain rule in retroactive trimming mode.
+          // This is because trimming expects that platform and toolchain resolution uses only
+          // the platform configuration. In theory, this means toolchains could use platforms, but
+          // the current expectation is that toolchains should not use anything at all, so better
+          // to go with the stricter expectation for now.
+          throw new RegisteredToolchainsFunctionException(
+              new InvalidToolchainLabelException(
+                  toolchainLabel,
+                  "this toolchain uses configuration, which is forbidden in retroactive trimming "
+                      + "mode"),
+              Transience.PERSISTENT);
+        }
         DeclaredToolchainInfo toolchainInfo = target.getProvider(DeclaredToolchainInfo.class);
 
         if (toolchainInfo == null) {
@@ -185,6 +200,10 @@
               "target does not provide the DeclaredToolchainInfo provider"));
     }
 
+    public InvalidToolchainLabelException(Label invalidLabel, String reason) {
+      super(formatMessage(invalidLabel.getCanonicalForm(), reason));
+    }
+
     public InvalidToolchainLabelException(TargetPatternUtil.InvalidTargetPatternException e) {
       this(e.getInvalidPattern(), e.getTpe());
     }
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/SingleToolchainResolutionFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/SingleToolchainResolutionFunction.java
index 2313d70..1e37fe4 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/SingleToolchainResolutionFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/SingleToolchainResolutionFunction.java
@@ -82,6 +82,7 @@
         key.toolchainTypeLabel(),
         key.availableExecutionPlatformKeys(),
         key.targetPlatformKey(),
+        configuration.trimConfigurationsRetroactively(),
         toolchains.registeredToolchains(),
         env,
         debug ? env.getListener() : null);
@@ -97,6 +98,7 @@
       Label toolchainTypeLabel,
       List<ConfiguredTargetKey> availableExecutionPlatformKeys,
       ConfiguredTargetKey targetPlatformKey,
+      boolean sanityCheckConfigurations,
       ImmutableList<DeclaredToolchainInfo> toolchains,
       Environment env,
       @Nullable EventHandler eventHandler)
@@ -112,7 +114,8 @@
                   .add(targetPlatformKey)
                   .addAll(availableExecutionPlatformKeys)
                   .build(),
-              env);
+              env,
+              sanityCheckConfigurations);
       if (env.valuesMissing()) {
         return null;
       }
diff --git a/src/test/java/com/google/devtools/build/lib/skyframe/PlatformLookupUtilTest.java b/src/test/java/com/google/devtools/build/lib/skyframe/PlatformLookupUtilTest.java
index cf3c590..a851e8b 100644
--- a/src/test/java/com/google/devtools/build/lib/skyframe/PlatformLookupUtilTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skyframe/PlatformLookupUtilTest.java
@@ -175,7 +175,7 @@
       GetPlatformInfoKey key = (GetPlatformInfoKey) skyKey;
       try {
         Map<ConfiguredTargetKey, PlatformInfo> platforms =
-            PlatformLookupUtil.getPlatformInfo(key.platformKeys(), env);
+            PlatformLookupUtil.getPlatformInfo(key.platformKeys(), env, false);
         if (env.valuesMissing()) {
           return null;
         }