fix: stop handing exception text to remote A2A peers and trigger callers

a2a/src/main/java/com/google/adk/a2a/executor/AgentExecutor.java

@@ -66,6 +66,15 @@ public class AgentExecutor implements io.a2a.server.agentexecution.AgentExecutor
   private static final Logger logger = LoggerFactory.getLogger(AgentExecutor.class);
   private static final String USER_ID_PREFIX = "A2A_USER_";
   private static final String A2A_METADATA_KEY = "a2a_metadata";
+
+  /**
+   * Env var that puts exception text back into the failure message sent to the peer.
+   *
+   * <p>Off by default, and meant for local debugging only: enabling it on a network-reachable
+   * deployment restores the disclosure {@link #failedMessage} exists to prevent.
+   */
+  private static final String DEBUG_ERRORS_ENV_VAR = "ADK_DEBUG_ERRORS";
+
   private final Map<String, Disposable> activeTasks = new ConcurrentHashMap<>();
   private final Runner.Builder runnerBuilder;
   private final AgentExecutorConfig agentExecutorConfig;
@@ -233,11 +242,8 @@ public void execute(RequestContext ctx, EventQueue eventQueue) {
             .materialize()
             .flatMapCompletable(
                 notification -> {
-                  Throwable error = notification.getError();
-                  if (error != null) {
-                    logger.error("Runner failed to execute", error);
-                  }
-                  return handleExecutionEnd(ctx, error, eventQueue);
+                  // The failure is logged, with its correlation id, by failedMessage().
+                  return handleExecutionEnd(ctx, notification.getError(), eventQueue);
                 })
             .doFinally(() -> cleanupTask(ctx.getTaskId()))
             .subscribe(
@@ -304,16 +310,50 @@ private Maybe<Session> prepareSession(
                 }));
   }
 
+  /**
+   * Builds the failure message handed back to the remote peer.
+   *
+   * <p>The peer is not trusted with {@code e.getMessage()}: exception text routinely names absolute
+   * filesystem paths, class and module locations, configuration values and echoed request payloads,
+   * none of which the caller needs and all of which are useful reconnaissance. The throwable is
+   * logged here in full next to a short opaque id, and the peer gets a fixed summary plus that same
+   * id, so an operator handed the id can find the real stack trace. Set {@code ADK_DEBUG_ERRORS=1}
+   * to put the exception text back into the response while debugging locally.
+   */
   private static Message failedMessage(RequestContext context, Throwable e) {
+    String errorId = UUID.randomUUID().toString();
+    logger.error("Runner failed to execute [errorId={}]", errorId, e);
     return new Message.Builder()
         .messageId(UUID.randomUUID().toString())
         .contextId(context.getContextId())
         .taskId(context.getTaskId())
         .role(Message.Role.AGENT)
-        .parts(ImmutableList.of(new TextPart(e.getMessage())))
+        .parts(ImmutableList.of(new TextPart(failureText(e, errorId, debugErrorsEnabled()))))
         .build();
   }
 
+  /**
+   * Returns the failure text that is safe to hand to the remote peer.
+   *
+   * @param includeDetail whether to append the exception type and message; see {@link
+   *     #DEBUG_ERRORS_ENV_VAR}.
+   */
+  static String failureText(Throwable e, String errorId, boolean includeDetail) {
+    String text = "Agent execution failed. (error_id: " + errorId + ")";
+    if (includeDetail) {
+      text = text + ": " + e.getClass().getName() + ": " + e.getMessage();
+    }
+    return text;
+  }
+
+  private static boolean debugErrorsEnabled() {
+    String value = System.getenv(DEBUG_ERRORS_ENV_VAR);
+    if (value == null) {
+      return false;
+    }
+    return value.equals("1") || value.equalsIgnoreCase("true");
+  }
+
   // Processor that will process all events related to the one runner invocation.
   private static class EventProcessor {
     private final String runArtifactId;

a2a/src/test/java/com/google/adk/a2a/executor/AgentExecutorTest.java

@@ -63,6 +63,10 @@
 @RunWith(JUnit4.class)
 public final class AgentExecutorTest {
 
+  /** A throwable message shaped like the ones that leak host detail. */
+  private static final String SECRET_ERROR =
+      "Runner error: /home/victim/.config/adk/credentials.json (No such file)";
+
   private EventQueue eventQueue;
   private List<Object> enqueuedEvents;
   private TestAgent testAgent;
@@ -137,7 +141,7 @@ public void createAgentExecutor_noAgentExecutorConfig_throwsException() {
   public void execute_withBeforeExecuteCallback_cancelsExecutionOnError() {
     // If callback returns error, execution should stop/fail.
     Callbacks.BeforeExecuteCallback callback =
-        ctx -> Single.error(new RuntimeException("Cancelled"));
+        ctx -> Single.error(new RuntimeException(SECRET_ERROR));
 
     AgentExecutorConfig config =
         AgentExecutorConfig.builder().beforeExecuteCallback(callback).build();
@@ -162,7 +166,10 @@ public void execute_withBeforeExecuteCallback_cancelsExecutionOnError() {
     assertThat(statusEvent.getStatus().state().toString()).isEqualTo("FAILED");
     assertThat(statusEvent.getStatus().message().getParts().get(0)).isInstanceOf(TextPart.class);
     TextPart textPart = (TextPart) statusEvent.getStatus().message().getParts().get(0);
-    assertThat(textPart.getText()).contains("Cancelled");
+    // The remote peer gets a correlation id for the logged throwable, not its
+    // message -- see AgentExecutor#failedMessage.
+    assertThat(textPart.getText()).startsWith("Agent execution failed. (error_id: ");
+    assertThat(textPart.getText()).doesNotContain(SECRET_ERROR);
   }
 
   @Test
@@ -289,7 +296,7 @@ public void execute_withAfterExecuteCallback_modifiesStatus() {
 
   @Test
   public void execute_runnerFails_registersFailedEvent() {
-    testAgent.setEventsToEmit(Flowable.error(new RuntimeException("Runner error")));
+    testAgent.setEventsToEmit(Flowable.error(new RuntimeException(SECRET_ERROR)));
     AgentExecutor executor =
         new AgentExecutor.Builder()
             .agentExecutorConfig(AgentExecutorConfig.builder().build())
@@ -316,7 +323,28 @@ public void execute_runnerFails_registersFailedEvent() {
     assertThat(statusEvent.getStatus().state()).isEqualTo(TaskState.FAILED);
     assertThat(statusEvent.getStatus().message().getParts().get(0)).isInstanceOf(TextPart.class);
     TextPart textPart = (TextPart) statusEvent.getStatus().message().getParts().get(0);
-    assertThat(textPart.getText()).isEqualTo("Runner error");
+    // A runner failure is reported to the peer as a correlation id only: the
+    // throwable's message names host paths and is for the server log.
+    assertThat(textPart.getText()).startsWith("Agent execution failed. (error_id: ");
+    assertThat(textPart.getText()).doesNotContain(SECRET_ERROR);
+    assertThat(textPart.getText()).doesNotContain("/home/victim");
+  }
+
+  @Test
+  public void failureText_withoutDebug_carriesOnlyTheCorrelationId() {
+    String text = AgentExecutor.failureText(new RuntimeException(SECRET_ERROR), "abc-123", false);
+
+    assertThat(text).isEqualTo("Agent execution failed. (error_id: abc-123)");
+  }
+
+  @Test
+  public void failureText_withDebug_carriesTheThrowableDetail() {
+    // ADK_DEBUG_ERRORS=1 is the documented opt-in for local debugging.
+    String text = AgentExecutor.failureText(new RuntimeException(SECRET_ERROR), "abc-123", true);
+
+    assertThat(text).startsWith("Agent execution failed. (error_id: abc-123): ");
+    assertThat(text).contains("java.lang.RuntimeException");
+    assertThat(text).contains(SECRET_ERROR);
   }
 
   @Test