refactor: add Role.USER constant for the "user" content role and event author

core/src/main/java/com/google/adk/agents/BaseAgent.java

@@ -135,7 +135,7 @@ private static void validateAgentName(String name) {
       throw new IllegalArgumentException(
           format("Agent name '%s' does not match regex '%s'.", name, IDENTIFIER_REGEX));
     }
-    if (name.equals("user")) {
+    if (name.equals(Role.USER)) {
       throw new IllegalArgumentException(
           "Agent name cannot be 'user'; reserved for end-user input.");
     }

core/src/main/java/com/google/adk/agents/Role.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.adk.agents;
+
+/** Standard role names for {@code Content} and event authors. */
+public final class Role {
+  /** The user interacting with the agent or model: a {@code Content} role and an event author. */
+  public static final String USER = "user";
+
+  private Role() {}
+}

core/src/main/java/com/google/adk/apps/App.java

@@ -18,6 +18,7 @@
 
 import com.google.adk.agents.BaseAgent;
 import com.google.adk.agents.ContextCacheConfig;
+import com.google.adk.agents.Role;
 import com.google.adk.plugins.Plugin;
 import com.google.adk.summarizer.EventsCompactionConfig;
 import com.google.common.collect.ImmutableList;
@@ -168,7 +169,7 @@ private static void validateAppName(String name) {
               + name
               + "': must be a valid identifier consisting of letters, digits, and underscores.");
     }
-    if (name.equals("user")) {
+    if (name.equals(Role.USER)) {
       throw new IllegalArgumentException("App name cannot be 'user'; reserved for end-user input.");
     }
   }

core/src/main/java/com/google/adk/codeexecutors/CodeExecutionUtils.java

@@ -22,6 +22,7 @@
 
 import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
 import com.google.adk.JsonBaseModel;
+import com.google.adk.agents.Role;
 import com.google.auto.value.AutoValue;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Iterables;
@@ -114,7 +115,7 @@ public static Content convertCodeExecutionParts(
                   + lastPart.codeExecutionResult().get().output()
                   + executionResultDelimiters.get(1));
       newParts.set(newParts.size() - 1, newPart);
-      return Content.builder().parts(newParts).role("user").build();
+      return Content.builder().parts(newParts).role(Role.USER).build();
     }
 
     return content;

core/src/main/java/com/google/adk/flows/llmflows/CodeExecution.java

@@ -22,6 +22,7 @@
 
 import com.google.adk.agents.InvocationContext;
 import com.google.adk.agents.LlmAgent;
+import com.google.adk.agents.Role;
 import com.google.adk.codeexecutors.BuiltInCodeExecutor;
 import com.google.adk.codeexecutors.CodeExecutionUtils;
 import com.google.adk.codeexecutors.CodeExecutionUtils.CodeExecutionInput;
@@ -347,7 +348,7 @@ private static List<File> extractAndReplaceInlineFiles(
     for (int i = 0; i < llmRequest.contents().size(); i++) {
       Content content = llmRequest.contents().get(i);
       if (content.role().isEmpty()
-          || !Objects.equals(content.role().get(), "user")
+          || !Objects.equals(content.role().get(), Role.USER)
           || content.parts().isEmpty()) {
         continue;
       }

core/src/main/java/com/google/adk/flows/llmflows/Contents.java

@@ -23,6 +23,7 @@
 import com.google.adk.JsonBaseModel;
 import com.google.adk.agents.InvocationContext;
 import com.google.adk.agents.LlmAgent;
+import com.google.adk.agents.Role;
 import com.google.adk.events.Event;
 import com.google.adk.events.EventCompaction;
 import com.google.adk.models.LlmRequest;
@@ -112,7 +113,7 @@ private ImmutableList<Content> getCurrentTurnContents(
     // Find the latest event that starts the current turn and process from there.
     for (int i = events.size() - 1; i >= 0; i--) {
       Event event = events.get(i);
-      if (event.author().equals("user") || isOtherAgentReply(agentName, event)) {
+      if (event.author().equals(Role.USER) || isOtherAgentReply(agentName, event)) {
         return getContents(
             currentBranch, events.subList(i, events.size()), agentName, groupFunctionResponses);
       }
@@ -404,7 +405,7 @@ private static Event createCompactionEvent(Event event) {
   private static boolean isOtherAgentReply(String agentName, Event event) {
     return !agentName.isEmpty()
         && !event.author().equals(agentName)
-        && !event.author().equals("user");
+        && !event.author().equals(Role.USER);
   }
 
   /**
@@ -482,8 +483,8 @@ private static boolean isOtherAgentReply(String agentName, Event event) {
       return null;
     }
 
-    Content content = Content.builder().role("user").parts(parts).build();
-    return event.toBuilder().author("user").content(content).build();
+    Content content = Content.builder().role(Role.USER).parts(parts).build();
+    return event.toBuilder().author(Role.USER).content(content).build();
   }
 
   private static String convertMapToJson(Map<String, Object> struct) {

core/src/main/java/com/google/adk/flows/llmflows/Functions.java

@@ -25,6 +25,7 @@
 import com.google.adk.agents.Callbacks.OnToolErrorCallback;
 import com.google.adk.agents.InvocationContext;
 import com.google.adk.agents.LlmAgent;
+import com.google.adk.agents.Role;
 import com.google.adk.agents.RunConfig.ToolExecutionMode;
 import com.google.adk.events.Event;
 import com.google.adk.events.EventActions;
@@ -379,7 +380,7 @@ private static Maybe<Map<String, Object>> processFunctionLive(
                   result -> {
                     String resultText = "Function " + tool.name() + " returned: " + result;
                     Content updateContent =
-                        Content.builder().role("user").parts(Part.fromText(resultText)).build();
+                        Content.builder().role(Role.USER).parts(Part.fromText(resultText)).build();
                     invocationContext.liveRequestQueue().get().content(updateContent);
                   },
                   error -> logger.error("Error in streaming tool " + tool.name(), error.getCause()),
@@ -588,7 +589,7 @@ private static Optional<Event> mergeParallelFunctionResponseEvents(
             .invocationId(baseEvent.invocationId())
             .author(baseEvent.author())
             .branch(baseEvent.branch().orElse(null))
-            .content(Content.builder().role("user").parts(mergedParts).build())
+            .content(Content.builder().role(Role.USER).parts(mergedParts).build())
             .actions(mergedActionsBuilder.build())
             .timestamp(baseEvent.timestamp())
             .build());
@@ -739,7 +740,7 @@ private static Event buildResponseEvent(
         .invocationId(invocationContext.invocationId())
         .author(invocationContext.agent().name())
         .branch(invocationContext.branch().orElse(null))
-        .content(Content.builder().role("user").parts(partFunctionResponse).build())
+        .content(Content.builder().role(Role.USER).parts(partFunctionResponse).build())
         .actions(toolContext.eventActions())
         .build();
   }

core/src/main/java/com/google/adk/flows/llmflows/RequestConfirmationLlmRequestProcessor.java

@@ -26,6 +26,7 @@
 import com.google.adk.JsonBaseModel;
 import com.google.adk.agents.InvocationContext;
 import com.google.adk.agents.LlmAgent;
+import com.google.adk.agents.Role;
 import com.google.adk.events.Event;
 import com.google.adk.events.ToolConfirmation;
 import com.google.adk.models.LlmRequest;
@@ -169,7 +170,7 @@ private static Optional<ConfirmationResult> findMostRecentConfirmations(
     // function responses.
     for (int i = events.size() - 1; i >= 0; i--) {
       Event event = events.get(i);
-      if (!Objects.equals(event.author(), "user") || event.functionResponses().isEmpty()) {
+      if (!Objects.equals(event.author(), Role.USER) || event.functionResponses().isEmpty()) {
         continue;
       }
 

core/src/main/java/com/google/adk/models/GeminiUtil.java

@@ -17,6 +17,7 @@
 
 import static com.google.common.collect.ImmutableList.toImmutableList;
 
+import com.google.adk.agents.Role;
 import com.google.common.base.Ascii;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
@@ -202,11 +203,11 @@ private static Part removeClientFunctionCallIdFromPart(Part part) {
   static List<Content> ensureModelResponse(List<Content> contents) {
     // Last content must be from the user, otherwise the model won't respond.
     if (contents.isEmpty()
-        || !Ascii.equalsIgnoreCase(Iterables.getLast(contents).role().orElse(""), "user")) {
+        || !Ascii.equalsIgnoreCase(Iterables.getLast(contents).role().orElse(""), Role.USER)) {
       Content userContent =
           Content.builder()
               .parts(ImmutableList.of(Part.fromText(CONTINUE_OUTPUT_MESSAGE)))
-              .role("user")
+              .role(Role.USER)
               .build();
       return Stream.concat(contents.stream(), Stream.of(userContent)).collect(toImmutableList());
     }

core/src/main/java/com/google/adk/models/LlmRequest.java

@@ -25,6 +25,7 @@
 import com.fasterxml.jackson.annotation.JsonProperty;
 import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
 import com.google.adk.JsonBaseModel;
+import com.google.adk.agents.Role;
 import com.google.adk.tools.BaseTool;
 import com.google.auto.value.AutoValue;
 import com.google.common.collect.ImmutableList;
@@ -195,7 +196,7 @@ private Content addInstructions(
                   .map(text -> text + "\n\n" + instructions)
                   .orElse(instructions));
 
-      String role = currentSystemInstruction.flatMap(Content::role).orElse("user");
+      String role = currentSystemInstruction.flatMap(Content::role).orElse(Role.USER);
 
       return Content.builder().parts(part).role(role).build();
     }

core/src/main/java/com/google/adk/models/chat/ChatCompletionsRequest.java

@@ -28,6 +28,7 @@
 import com.fasterxml.jackson.databind.SerializerProvider;
 import com.fasterxml.jackson.databind.module.SimpleModule;
 import com.google.adk.JsonBaseModel;
+import com.google.adk.agents.Role;
 import com.google.adk.models.LlmRequest;
 import com.google.common.base.Ascii;
 import com.google.common.collect.ImmutableList;
@@ -377,7 +378,7 @@ private static Optional<Message> processSystemInstruction(
    */
   private static List<Message> processContent(Content content) {
     Message msg = new Message();
-    String role = content.role().orElse("user");
+    String role = content.role().orElse(Role.USER);
     msg.role = role.equals("model") ? "assistant" : role;
 
     List<ContentPart> contentParts = new ArrayList<>();

core/src/main/java/com/google/adk/plugins/ContextFilterPlugin.java

@@ -18,6 +18,7 @@
 import static com.google.common.base.Preconditions.checkArgument;
 
 import com.google.adk.agents.CallbackContext;
+import com.google.adk.agents.Role;
 import com.google.adk.models.LlmRequest;
 import com.google.adk.models.LlmResponse;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
@@ -63,7 +64,6 @@
 public class ContextFilterPlugin extends BasePlugin {
   private static final Logger logger = LoggerFactory.getLogger(ContextFilterPlugin.class);
   private static final String MODEL_ROLE = "model";
-  private static final String USER_ROLE = "user";
 
   private final Optional<Integer> numInvocationsToKeep;
   private final Optional<UnaryOperator<List<Content>>> customFilter;
@@ -159,7 +159,7 @@ private List<Content> trimContentsByInvocations(int numInvocations, List<Content
     int finalSplitIndex = adjustIndexForToolCalls(candidateSplitIndex, contents);
     // The Nth model turn can be preceded by user turns; expand window to include them.
     while (finalSplitIndex > 0
-        && hasRole(contents.get(finalSplitIndex - 1), USER_ROLE)
+        && hasRole(contents.get(finalSplitIndex - 1), Role.USER)
         && !isFunctionResponse(contents.get(finalSplitIndex - 1))) {
       finalSplitIndex--;
     }
@@ -174,7 +174,7 @@ private int findNthModelTurnStartIndex(int numInvocations, List<Content> content
         if (modelTurnsToFind == 0) {
           int startIndex = i;
           // Include all preceding user messages in the same turn.
-          while (startIndex > 0 && hasRole(contents.get(startIndex - 1), USER_ROLE)) {
+          while (startIndex > 0 && hasRole(contents.get(startIndex - 1), Role.USER)) {
             startIndex--;
           }
           return startIndex;

core/src/main/java/com/google/adk/runner/Runner.java

@@ -24,6 +24,7 @@
 import com.google.adk.agents.InvocationContext;
 import com.google.adk.agents.LiveRequestQueue;
 import com.google.adk.agents.LlmAgent;
+import com.google.adk.agents.Role;
 import com.google.adk.agents.RunConfig;
 import com.google.adk.agents.SequentialAgent;
 import com.google.adk.apps.App;
@@ -391,7 +392,7 @@ private Single<Event> appendNewMessageToSession(
         Event.builder()
             .id(Event.generateEventId())
             .invocationId(invocationContext.invocationId())
-            .author("user")
+            .author(Role.USER)
             .content(messageToAppend);
 
     // Add state delta if provided
@@ -867,7 +868,7 @@ private BaseAgent findAgentToRun(Session session, BaseAgent rootAgent) {
       if (author == null) {
         continue;
       }
-      if (author.equals("user")) {
+      if (author.equals(Role.USER)) {
         continue;
       }
 

core/src/main/java/com/google/adk/summarizer/LlmEventSummarizer.java

@@ -20,6 +20,7 @@
 import static java.util.stream.Collectors.joining;
 
 import com.google.adk.JsonBaseModel;
+import com.google.adk.agents.Role;
 import com.google.adk.events.Event;
 import com.google.adk.events.EventActions;
 import com.google.adk.events.EventCompaction;
@@ -76,7 +77,7 @@ public Maybe<Event> summarizeEvents(List<Event> events) {
             .contents(
                 ImmutableList.of(
                     Content.builder()
-                        .role("user")
+                        .role(Role.USER)
                         .parts(ImmutableList.of(Part.fromText(prompt)))
                         .build()))
             .build();
@@ -101,7 +102,7 @@ public Maybe<Event> summarizeEvents(List<Event> events) {
                             compaction ->
                                 Event.builder()
                                     .id(Event.generateEventId())
-                                    .author("user")
+                                    .author(Role.USER)
                                     .actions(EventActions.builder().compaction(compaction).build())
                                     .invocationId(Event.generateEventId())
                                     .build())));