diff --git a/src/main/java/com/google/devtools/build/lib/query2/engine/OutputFormatterCallback.java b/src/main/java/com/google/devtools/build/lib/query2/engine/OutputFormatterCallback.java
index 50708d6..c66eefe 100644
--- a/src/main/java/com/google/devtools/build/lib/query2/engine/OutputFormatterCallback.java
+++ b/src/main/java/com/google/devtools/build/lib/query2/engine/OutputFormatterCallback.java
@@ -67,11 +67,11 @@
    * Use an {@code OutputFormatterCallback} with an already computed set of targets. Note that this
    * does not work in stream mode, as the {@code targets} would already be computed.
    *
-   * <p>The intended usage of this method is to use {@code StreamedFormatter} formaters in non
+   * <p>The intended usage of this method is to use {@code StreamedFormatter} formatters in non
    * streaming contexts.
    */
-  public static <T> void processAllTargets(OutputFormatterCallback<T> callback,
-      Iterable<T> targets) throws IOException, InterruptedException {
+  public static <T> void processAllTargets(OutputFormatterCallback<T> callback, Iterable<T> targets)
+      throws IOException, InterruptedException {
     boolean failFast = true;
     try {
       callback.start();
diff --git a/src/main/java/com/google/devtools/build/lib/query2/query/output/AbstractUnorderedFormatter.java b/src/main/java/com/google/devtools/build/lib/query2/query/output/AbstractUnorderedFormatter.java
index 6a6917a..b00ad6a 100644
--- a/src/main/java/com/google/devtools/build/lib/query2/query/output/AbstractUnorderedFormatter.java
+++ b/src/main/java/com/google/devtools/build/lib/query2/query/output/AbstractUnorderedFormatter.java
@@ -15,6 +15,7 @@
 package com.google.devtools.build.lib.query2.query.output;
 
 import com.google.common.collect.Iterables;
+import com.google.devtools.build.lib.events.EventHandler;
 import com.google.devtools.build.lib.graph.Digraph;
 import com.google.devtools.build.lib.graph.Node;
 import com.google.devtools.build.lib.packages.Target;
@@ -24,17 +25,27 @@
 import com.google.devtools.build.lib.query2.query.output.QueryOptions.OrderOutput;
 import java.io.IOException;
 import java.io.OutputStream;
+import javax.annotation.Nullable;
 
 abstract class AbstractUnorderedFormatter extends OutputFormatter implements StreamedFormatter {
 
   @Override
   public void setOptions(CommonQueryOptions options, AspectResolver aspectResolver) {}
 
+  /** Optionally sets a handler for reporting status output / errors. */
+  @Override
+  public void setEventHandler(@Nullable EventHandler eventHandler) {}
+
   @Override
   public void output(
-      QueryOptions options, Digraph<Target> result, OutputStream out, AspectResolver aspectResolver)
+      QueryOptions options,
+      Digraph<Target> result,
+      OutputStream out,
+      AspectResolver aspectResolver,
+      @Nullable EventHandler eventHandler)
       throws IOException, InterruptedException {
     setOptions(options, aspectResolver);
+    setEventHandler(eventHandler);
     OutputFormatterCallback.processAllTargets(
         createPostFactoStreamCallback(out, options), getOrderedTargets(result, options));
   }
@@ -46,4 +57,4 @@
             : result.getTopologicalOrder(new FormatUtils.TargetOrdering());
     return Iterables.transform(orderedResult, Node::getLabel);
   }
-}
\ No newline at end of file
+}
diff --git a/src/main/java/com/google/devtools/build/lib/query2/query/output/GraphOutputFormatter.java b/src/main/java/com/google/devtools/build/lib/query2/query/output/GraphOutputFormatter.java
index 7e9f377..3d71a0f 100644
--- a/src/main/java/com/google/devtools/build/lib/query2/query/output/GraphOutputFormatter.java
+++ b/src/main/java/com/google/devtools/build/lib/query2/query/output/GraphOutputFormatter.java
@@ -19,6 +19,7 @@
 import com.google.devtools.build.lib.cmdline.Label;
 import com.google.devtools.build.lib.collect.CollectionUtils;
 import com.google.devtools.build.lib.collect.EquivalenceRelation;
+import com.google.devtools.build.lib.events.EventHandler;
 import com.google.devtools.build.lib.graph.Digraph;
 import com.google.devtools.build.lib.graph.DotOutputVisitor;
 import com.google.devtools.build.lib.graph.LabelSerializer;
@@ -58,7 +59,8 @@
       QueryOptions options,
       Digraph<Target> result,
       OutputStream out,
-      AspectResolver aspectProvider) {
+      AspectResolver aspectProvider,
+      EventHandler eventHandler) {
     this.graphNodeStringLimit = options.graphNodeStringLimit;
     this.graphConditionalEdgesLimit = options.graphConditionalEdgesLimit;
 
diff --git a/src/main/java/com/google/devtools/build/lib/query2/query/output/MaxrankOutputFormatter.java b/src/main/java/com/google/devtools/build/lib/query2/query/output/MaxrankOutputFormatter.java
index 83dba1a..777cc3d 100644
--- a/src/main/java/com/google/devtools/build/lib/query2/query/output/MaxrankOutputFormatter.java
+++ b/src/main/java/com/google/devtools/build/lib/query2/query/output/MaxrankOutputFormatter.java
@@ -16,6 +16,7 @@
 
 import static java.util.Comparator.comparingInt;
 
+import com.google.devtools.build.lib.events.EventHandler;
 import com.google.devtools.build.lib.graph.Digraph;
 import com.google.devtools.build.lib.graph.Node;
 import com.google.devtools.build.lib.packages.Target;
@@ -51,7 +52,11 @@
 
   @Override
   public void output(
-      QueryOptions options, Digraph<Target> result, OutputStream out, AspectResolver aspectResolver)
+      QueryOptions options,
+      Digraph<Target> result,
+      OutputStream out,
+      AspectResolver aspectResolver,
+      EventHandler eventHandler)
       throws IOException {
     // In order to handle cycles correctly, we need work on the strong
     // component graph, as cycles should be treated a "clump" of nodes all on
diff --git a/src/main/java/com/google/devtools/build/lib/query2/query/output/MinrankOutputFormatter.java b/src/main/java/com/google/devtools/build/lib/query2/query/output/MinrankOutputFormatter.java
index efc5cb6..8e020cd 100644
--- a/src/main/java/com/google/devtools/build/lib/query2/query/output/MinrankOutputFormatter.java
+++ b/src/main/java/com/google/devtools/build/lib/query2/query/output/MinrankOutputFormatter.java
@@ -15,6 +15,7 @@
 package com.google.devtools.build.lib.query2.query.output;
 
 import com.google.devtools.build.lib.cmdline.Label;
+import com.google.devtools.build.lib.events.EventHandler;
 import com.google.devtools.build.lib.graph.Digraph;
 import com.google.devtools.build.lib.graph.Node;
 import com.google.devtools.build.lib.packages.Target;
@@ -63,7 +64,11 @@
 
   @Override
   public void output(
-      QueryOptions options, Digraph<Target> result, OutputStream out, AspectResolver aspectResolver)
+      QueryOptions options,
+      Digraph<Target> result,
+      OutputStream out,
+      AspectResolver aspectResolver,
+      EventHandler eventHandler)
       throws IOException {
     PrintStream printStream = new PrintStream(out);
     // getRoots() isn't defined for cyclic graphs, so in order to handle
diff --git a/src/main/java/com/google/devtools/build/lib/query2/query/output/OutputFormatter.java b/src/main/java/com/google/devtools/build/lib/query2/query/output/OutputFormatter.java
index 77770fc..da5c252 100644
--- a/src/main/java/com/google/devtools/build/lib/query2/query/output/OutputFormatter.java
+++ b/src/main/java/com/google/devtools/build/lib/query2/query/output/OutputFormatter.java
@@ -13,6 +13,7 @@
 // limitations under the License.
 package com.google.devtools.build.lib.query2.query.output;
 
+import com.google.devtools.build.lib.events.EventHandler;
 import com.google.devtools.build.lib.graph.Digraph;
 import com.google.devtools.build.lib.packages.Target;
 import com.google.devtools.build.lib.query2.engine.QueryEnvironment;
@@ -22,6 +23,7 @@
 import java.io.IOException;
 import java.io.OutputStream;
 import java.io.Serializable;
+import javax.annotation.Nullable;
 
 /**
  * Interface for classes which order, format and print the result of a Blaze
@@ -54,6 +56,10 @@
    * by the QueryEnvironment), and print it to "out".
    */
   public abstract void output(
-      QueryOptions options, Digraph<Target> result, OutputStream out, AspectResolver aspectProvider)
+      QueryOptions options,
+      Digraph<Target> result,
+      OutputStream out,
+      AspectResolver aspectProvider,
+      @Nullable EventHandler eventHandler)
       throws IOException, InterruptedException;
 }
diff --git a/src/main/java/com/google/devtools/build/lib/query2/query/output/ProtoOutputFormatter.java b/src/main/java/com/google/devtools/build/lib/query2/query/output/ProtoOutputFormatter.java
index 0a78721..5b4387a 100644
--- a/src/main/java/com/google/devtools/build/lib/query2/query/output/ProtoOutputFormatter.java
+++ b/src/main/java/com/google/devtools/build/lib/query2/query/output/ProtoOutputFormatter.java
@@ -24,11 +24,14 @@
 import com.google.common.base.Predicate;
 import com.google.common.base.Predicates;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableMultimap;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Iterables;
 import com.google.common.collect.Maps;
 import com.google.devtools.build.lib.cmdline.Label;
+import com.google.devtools.build.lib.events.Event;
+import com.google.devtools.build.lib.events.EventHandler;
 import com.google.devtools.build.lib.graph.Digraph;
 import com.google.devtools.build.lib.graph.Node;
 import com.google.devtools.build.lib.packages.AggregatingAttributeMapper;
@@ -100,6 +103,8 @@
   private boolean includeRuleInputsAndOutputs = true;
   private boolean includeSyntheticAttributeHash = false;
 
+  @Nullable private EventHandler eventHandler;
+
   @Override
   public String getName() {
     return "proto";
@@ -119,6 +124,11 @@
     this.includeSyntheticAttributeHash = options.protoIncludeSyntheticAttributeHash;
   }
 
+  @Override
+  public void setEventHandler(@Nullable EventHandler eventHandler) {
+    this.eventHandler = eventHandler;
+  }
+
   private static Predicate<String> newAttributePredicate(List<String> outputAttributes) {
     if (outputAttributes.equals(ImmutableList.of("all"))) {
       return Predicates.alwaysTrue();
@@ -201,7 +211,7 @@
           if (!includeAspectAttribute(attribute, labels)) {
             continue;
           }
-          Object attributeValue = getAspectAttributeValue(attribute, labels);
+          Object attributeValue = getAspectAttributeValue(target, attribute, labels);
           Build.Attribute serializedAttribute =
               AttributeFormatter.getAttributeProto(
                   attribute,
@@ -380,11 +390,35 @@
         && ruleAttributePredicate.apply(attr.getName());
   }
 
-  private static Object getAspectAttributeValue(Attribute attribute, Collection<Label> labels) {
+  private Object getAspectAttributeValue(
+      Target target, Attribute attribute, Collection<Label> labels) {
     Type<?> attributeType = attribute.getType();
     if (attributeType.equals(BuildType.LABEL)) {
       Preconditions.checkState(labels.size() == 1, "attribute=%s, labels=%s", attribute, labels);
       return Iterables.getOnlyElement(labels);
+    } else if (attributeType.equals(BuildType.LABEL_KEYED_STRING_DICT)) {
+      // Ideally we'd support LABEL_KEYED_STRING_DICT by getting the value directly from the aspect
+      // definition vs. trying to reverse-construct it from the flattened labels as this method
+      // does. Unfortunately any proper support surfaces a latent bug between --output=proto and
+      // aspect attributes: "{@code labels} isn't the set of labels for a single attribute value but
+      // for all values of all attributes with the same name. We can have multiple attributes with
+      // the same name because multiple aspects may attach to a rule, and nothing is stopping them
+      // from defining the same attribute names. That means the "Attribute" proto message doesn't
+      // really represent a single attribute, in spite of its documented purpose. This all calls for
+      // an API design upgrade to properly consider these relationships. Details at b/149982967.
+      if (eventHandler != null) {
+        eventHandler.handle(
+            Event.error(
+                String.format(
+                    "Target \"%s\", aspect attribute \"%s\": type \"%s\" not yet supported with"
+                        + " --output=proto.",
+                    target.getLabel(), attribute.getName(), BuildType.LABEL_KEYED_STRING_DICT)));
+      }
+      // This return value is misleading when the above error isn't get triggered: it implies an
+      // empty result with no signal that that result isn't accurate.
+      // TODO(bazel-team): either make the result accurate or trigger an error universally. Letting
+      // OutputFormatter.output() throw a QueryException is a promising approach.
+      return ImmutableMap.of();
     } else {
       Preconditions.checkState(
           attributeType.equals(BuildType.LABEL_LIST),
diff --git a/src/main/java/com/google/devtools/build/lib/query2/query/output/QueryOutputUtils.java b/src/main/java/com/google/devtools/build/lib/query2/query/output/QueryOutputUtils.java
index 89bf13f..e638a0d 100644
--- a/src/main/java/com/google/devtools/build/lib/query2/query/output/QueryOutputUtils.java
+++ b/src/main/java/com/google/devtools/build/lib/query2/query/output/QueryOutputUtils.java
@@ -13,6 +13,7 @@
 // limitations under the License.
 package com.google.devtools.build.lib.query2.query.output;
 
+import com.google.devtools.build.lib.events.EventHandler;
 import com.google.devtools.build.lib.graph.Digraph;
 import com.google.devtools.build.lib.packages.Target;
 import com.google.devtools.build.lib.profiler.Profiler;
@@ -25,6 +26,7 @@
 import java.io.IOException;
 import java.io.OutputStream;
 import java.util.Set;
+import javax.annotation.Nullable;
 
 /** Static utility methods for outputting a query. */
 public class QueryOutputUtils {
@@ -37,9 +39,14 @@
         && formatter instanceof StreamedFormatter;
   }
 
-  public static void output(QueryOptions queryOptions, QueryEvalResult result,
-      Set<Target> targetsResult, OutputFormatter formatter, OutputStream outputStream,
-      AspectResolver aspectResolver)
+  public static void output(
+      QueryOptions queryOptions,
+      QueryEvalResult result,
+      Set<Target> targetsResult,
+      OutputFormatter formatter,
+      OutputStream outputStream,
+      AspectResolver aspectResolver,
+      @Nullable EventHandler eventHandler)
       throws IOException, InterruptedException {
     /*
      * This is not really streaming, but we are using the streaming interface for writing into the
@@ -49,6 +56,7 @@
     if (shouldStreamResults(queryOptions, formatter)) {
       StreamedFormatter streamedFormatter = (StreamedFormatter) formatter;
       streamedFormatter.setOptions(queryOptions, aspectResolver);
+      streamedFormatter.setEventHandler(eventHandler);
       OutputFormatterCallback.processAllTargets(
           streamedFormatter.createPostFactoStreamCallback(outputStream, queryOptions),
           targetsResult);
@@ -63,7 +71,7 @@
       }
 
       try (SilentCloseable closeable = Profiler.instance().profile("formatter.output")) {
-        formatter.output(queryOptions, subgraph, outputStream, aspectResolver);
+        formatter.output(queryOptions, subgraph, outputStream, aspectResolver, eventHandler);
       }
     }
   }
diff --git a/src/main/java/com/google/devtools/build/lib/query2/query/output/StreamedFormatter.java b/src/main/java/com/google/devtools/build/lib/query2/query/output/StreamedFormatter.java
index bbe9697..44f951f 100644
--- a/src/main/java/com/google/devtools/build/lib/query2/query/output/StreamedFormatter.java
+++ b/src/main/java/com/google/devtools/build/lib/query2/query/output/StreamedFormatter.java
@@ -14,6 +14,7 @@
 
 package com.google.devtools.build.lib.query2.query.output;
 
+import com.google.devtools.build.lib.events.EventHandler;
 import com.google.devtools.build.lib.packages.Target;
 import com.google.devtools.build.lib.query2.common.CommonQueryOptions;
 import com.google.devtools.build.lib.query2.engine.OutputFormatterCallback;
@@ -21,6 +22,7 @@
 import com.google.devtools.build.lib.query2.engine.ThreadSafeOutputFormatterCallback;
 import com.google.devtools.build.lib.query2.query.aspectresolvers.AspectResolver;
 import java.io.OutputStream;
+import javax.annotation.Nullable;
 
 /**
  * Unordered streamed output formatter (wrt. dependency ordering).
@@ -37,6 +39,9 @@
   /** Specifies options to be used by subsequent calls to {@link #createStreamCallback}. */
   void setOptions(CommonQueryOptions options, AspectResolver aspectResolver);
 
+  /** Sets an optional handler for reporting status output / errors. */
+  void setEventHandler(@Nullable EventHandler eventHandler);
+
   /**
    * Returns a {@link ThreadSafeOutputFormatterCallback} whose
    * {@link OutputFormatterCallback#process} outputs formatted {@link Target}s to the given
diff --git a/src/main/java/com/google/devtools/build/lib/rules/genquery/GenQuery.java b/src/main/java/com/google/devtools/build/lib/rules/genquery/GenQuery.java
index fbbc6bb..252e1e2 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/genquery/GenQuery.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/genquery/GenQuery.java
@@ -404,7 +404,8 @@
           result,
           formatter,
           outputStream,
-          queryOptions.aspectDeps.createResolver(packageProvider, getEventHandler(ruleContext)));
+          queryOptions.aspectDeps.createResolver(packageProvider, getEventHandler(ruleContext)),
+          getEventHandler(ruleContext));
       outputStream.close();
     } catch (ClosedByInterruptException e) {
       throw new InterruptedException(e.getMessage());
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/commands/QueryCommand.java b/src/main/java/com/google/devtools/build/lib/runtime/commands/QueryCommand.java
index c0ced79..19f3151 100644
--- a/src/main/java/com/google/devtools/build/lib/runtime/commands/QueryCommand.java
+++ b/src/main/java/com/google/devtools/build/lib/runtime/commands/QueryCommand.java
@@ -104,6 +104,7 @@
       streamedFormatter.setOptions(
           queryOptions,
           queryOptions.aspectDeps.createResolver(env.getPackageManager(), env.getReporter()));
+      streamedFormatter.setEventHandler(env.getReporter());
       callback = streamedFormatter.createStreamCallback(out, queryOptions, queryEnv);
     } else {
       callback = QueryUtil.newOrderedAggregateAllOutputFormatterCallback(queryEnv);
@@ -158,7 +159,8 @@
             targets,
             formatter,
             out,
-            queryOptions.aspectDeps.createResolver(env.getPackageManager(), env.getReporter()));
+            queryOptions.aspectDeps.createResolver(env.getPackageManager(), env.getReporter()),
+            env.getReporter());
       } catch (ClosedByInterruptException | InterruptedException e) {
         env.getReporter().handle(Event.error("query interrupted"));
         return Either.ofLeft(BlazeCommandResult.exitCode(ExitCode.INTERRUPTED));
diff --git a/src/test/java/com/google/devtools/build/lib/buildtool/QueryIntegrationTest.java b/src/test/java/com/google/devtools/build/lib/buildtool/QueryIntegrationTest.java
index f3840da..8d4952a 100644
--- a/src/test/java/com/google/devtools/build/lib/buildtool/QueryIntegrationTest.java
+++ b/src/test/java/com/google/devtools/build/lib/buildtool/QueryIntegrationTest.java
@@ -303,13 +303,15 @@
 
     ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
 
+    Reporter reporter = new Reporter(new EventBus(), events.collector());
     QueryOutputUtils.output(
-        queryOptions, result,
-        callback.getResult(), formatter,
+        queryOptions,
+        result,
+        callback.getResult(),
+        formatter,
         outputStream,
-        queryOptions.aspectDeps.createResolver(
-            env.getPackageManager(),
-            new Reporter(new EventBus(), events.collector())));
+        queryOptions.aspectDeps.createResolver(env.getPackageManager(), reporter),
+        reporter);
     return outputStream.toByteArray();
   }