diff --git a/java/dagger/internal/codegen/DelegateComponentProcessor.java b/java/dagger/internal/codegen/DelegateComponentProcessor.java
index 96e5b731592..4c87febdfc8 100644
--- a/java/dagger/internal/codegen/DelegateComponentProcessor.java
+++ b/java/dagger/internal/codegen/DelegateComponentProcessor.java
@@ -73,8 +73,7 @@ public void initialize(
           legacyPlugin ->
               env.getMessager()
                   .printMessage(
-                      // TODO(b/250036454): Use ERROR here instead.
-                      Kind.WARNING,
+                      Kind.ERROR,
                       "Cannot use legacy dagger.spi.BindingGraphPlugin while compiling with KSP: "
                           + legacyPlugin.pluginName()
                           + ". Either compile with KAPT or migrate the plugin to implement "
diff --git a/java/dagger/spi/model/DaggerProcessingEnv.java b/java/dagger/spi/model/DaggerProcessingEnv.java
index b9ef8a12bf0..27c04933249 100644
--- a/java/dagger/spi/model/DaggerProcessingEnv.java
+++ b/java/dagger/spi/model/DaggerProcessingEnv.java
@@ -25,12 +25,20 @@
 /** Wrapper type for an element. */
 @AutoValue
 public abstract class DaggerProcessingEnv {
+  /** Represents a type of backend used for compilation. */
+  public enum Backend { JAVAC, KSP }
+
   public static DaggerProcessingEnv from(XProcessingEnv processingEnv) {
     return new AutoValue_DaggerProcessingEnv(processingEnv);
   }
 
   public abstract XProcessingEnv xprocessing();
 
+  /** Returns the backend used in this compilation. */
+  public Backend getBackend() {
+    return Backend.valueOf(xprocessing().getBackend().name());
+  }
+
   public ProcessingEnvironment java() {
     return toJavac(xprocessing());
   }