diff --git a/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java b/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java
index 4a90723..9ed0d21 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java
@@ -30,6 +30,7 @@
 import com.google.devtools.build.lib.events.ExtendedEventHandler;
 import com.google.devtools.build.lib.events.ExtendedEventHandler.Postable;
 import com.google.devtools.build.lib.events.Location;
+import com.google.devtools.build.lib.events.Reporter;
 import com.google.devtools.build.lib.events.StoredEventHandler;
 import com.google.devtools.build.lib.packages.Globber.BadGlobException;
 import com.google.devtools.build.lib.packages.PackageValidator.InvalidPackageException;
@@ -136,6 +137,7 @@
     protected Iterable<EnvironmentExtension> environmentExtensions = ImmutableList.of();
     protected PackageValidator packageValidator = PackageValidator.NOOP_VALIDATOR;
     protected boolean doChecksForTesting = true;
+    protected Reporter reporter;
 
     public BuilderForTesting setEnvironmentExtensions(
         Iterable<EnvironmentExtension> environmentExtensions) {
@@ -153,6 +155,11 @@
       return this;
     }
 
+    public BuilderForTesting setReporter(Reporter reporter) {
+      this.reporter = reporter;
+      return this;
+    }
+
     public abstract PackageFactory build(RuleClassProvider ruleClassProvider, FileSystem fs);
   }
 
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..336849b 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;
@@ -30,11 +31,19 @@
   @Override
   public void setOptions(CommonQueryOptions options, AspectResolver aspectResolver) {}
 
+  /** Sets a handler for reporting status output / errors. */
+  public void setEventHandler(EventHandler eventHandler) {}
+
   @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, InterruptedException {
     setOptions(options, aspectResolver);
+    setEventHandler(eventHandler);
     OutputFormatterCallback.processAllTargets(
         createPostFactoStreamCallback(out, options), getOrderedTargets(result, options));
   }
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..a3e8314 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;
@@ -54,6 +55,10 @@
    * 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,
+      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 23db2a6..e0b580b 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;
@@ -98,6 +101,8 @@
   private boolean includeLocations = true;
   private boolean includeRuleInputsAndOutputs = true;
 
+  private EventHandler eventHandler;
+
   @Override
   public String getName() {
     return "proto";
@@ -116,6 +121,11 @@
     this.includeRuleInputsAndOutputs = options.protoIncludeRuleInputsAndOutputs;
   }
 
+  @Override
+  public void setEventHandler(EventHandler eventHandler) {
+    this.eventHandler = eventHandler;
+  }
+
   private static Predicate<String> newAttributePredicate(List<String> outputAttributes) {
     if (outputAttributes.equals(ImmutableList.of("all"))) {
       return Predicates.alwaysTrue();
@@ -198,7 +208,7 @@
           if (!includeAspectAttribute(attribute, labels)) {
             continue;
           }
-          Object attributeValue = getAspectAttributeValue(attribute, labels);
+          Object attributeValue = getAspectAttributeValue(target, attribute, labels);
           Build.Attribute serializedAttribute =
               AttributeFormatter.getAttributeProto(
                   attribute,
@@ -369,11 +379,29 @@
         && 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.
+      eventHandler.handle(
+          Event.error(
+              String.format(
+                  "Target \"%s\", aspect attribute \"%s\": type \"%s\" not yet supported with"
+                      + " --output=proto. See b/149982967",
+                  target.getLabel(), attribute.getName(), BuildType.LABEL_KEYED_STRING_DICT)));
+      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..5e6506f 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;
@@ -37,9 +38,14 @@
         && 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,
+      EventHandler eventHandler)
       throws IOException, InterruptedException {
     /*
      * This is not really streaming, but we are using the streaming interface for writing into the
@@ -63,7 +69,7 @@
       }
 
       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/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..66964a7 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
@@ -158,7 +158,8 @@
             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();
   }
 
diff --git a/src/test/java/com/google/devtools/build/lib/packages/util/PackageLoadingTestCase.java b/src/test/java/com/google/devtools/build/lib/packages/util/PackageLoadingTestCase.java
index 7fea1b1..56e9db9 100644
--- a/src/test/java/com/google/devtools/build/lib/packages/util/PackageLoadingTestCase.java
+++ b/src/test/java/com/google/devtools/build/lib/packages/util/PackageLoadingTestCase.java
@@ -100,6 +100,7 @@
         loadingMock
             .getPackageFactoryBuilderForTesting(directories)
             .setEnvironmentExtensions(getEnvironmentExtensions())
+            .setReporter(reporter)
             .build(ruleClassProvider, fileSystem);
     skyframeExecutor = createSkyframeExecutor();
     setUpSkyframe();