feat: run child nodes from a workflow node with Context.runNode

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

@@ -22,8 +22,12 @@
 import com.google.adk.events.ToolConfirmation;
 import com.google.adk.memory.SearchMemoryResponse;
 import com.google.adk.sessions.State;
+import com.google.adk.workflow.DynamicNodeFailedException;
+import com.google.adk.workflow.Node;
 import com.google.adk.workflow.NodeExecutionState;
+import com.google.adk.workflow.NodeRunner;
 import com.google.adk.workflow.Route;
+import com.google.adk.workflow.RunNodeConfig;
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
 import com.google.genai.types.Part;
@@ -285,7 +289,8 @@ public final void eventAuthor(String eventAuthor) {
   }
 
   /**
-   * Returns this workflow node's output, or empty if it has produced none.
+   * Returns this workflow node's output, which is the output of its {@code useAsOutput} child once
+   * that child emits one, or empty if it has none.
    *
    * @throws IllegalStateException if this is not the context of a workflow node.
    */
@@ -315,4 +320,46 @@ public final Optional<ImmutableList<Route>> routes() {
   public final void routes(List<? extends Route> routes) {
     requireNodeState().selectRoutes(routes);
   }
+
+  /**
+   * Runs {@code node} as a child of this workflow node without an input, as {@link #runNode(Node,
+   * Object, RunNodeConfig)} does with the default {@link RunNodeConfig}.
+   */
+  @Experimental
+  public final Maybe<Object> runNode(Node node) {
+    return runNode(node, /* nodeInput= */ null);
+  }
+
+  /**
+   * Runs {@code node} as a child of this workflow node, as {@link #runNode(Node, Object,
+   * RunNodeConfig)} does with the default {@link RunNodeConfig}.
+   *
+   * @throws IllegalArgumentException if {@code nodeInput} is a {@link RunNodeConfig}, which belongs
+   *     in the three-argument overload.
+   */
+  @Experimental
+  public final Maybe<Object> runNode(Node node, @Nullable Object nodeInput) {
+    Preconditions.checkArgument(
+        !(nodeInput instanceof RunNodeConfig),
+        "Pass the RunNodeConfig as the third argument of runNode, not as the node input.");
+    return runNode(node, nodeInput, RunNodeConfig.builder().build());
+  }
+
+  /**
+   * Returns a {@link Maybe} that, when subscribed, runs {@code node} as a child of this workflow
+   * node and emits the child's output, or completes empty if it has none. Include it in this node's
+   * run and catch only failures you handle, such as {@link DynamicNodeFailedException}: when the
+   * child waits, it fails with an internal {@link RuntimeException} that must propagate to end this
+   * node's run, so that it waits too.
+   *
+   * <p>It fails with {@link DynamicNodeFailedException} if the child fails; with {@link
+   * IllegalStateException} if this is not a workflow node's context, if this node's {@link
+   * Node#rerunOnResume} is false, or if {@code config} delegates an output that this node already
+   * has or has delegated; and with {@link IllegalArgumentException} if the child's name cannot form
+   * a node path.
+   */
+  @Experimental
+  public final Maybe<Object> runNode(Node node, @Nullable Object nodeInput, RunNodeConfig config) {
+    return NodeRunner.runChild(this, node, nodeInput, config);
+  }
 }

core/src/main/java/com/google/adk/workflow/DynamicNodeFailedException.java

@@ -19,12 +19,12 @@
 import com.google.adk.annotations.Experimental;
 
 /**
- * An exception that {@link NodeRunner#runChild} signals when the child node fails. If the caller
- * does not handle this exception, the caller fails with the child's error without emitting an error
- * event of its own.
+ * An exception that {@link com.google.adk.agents.Context#runNode} signals when the child node
+ * fails. If the caller does not handle this exception, the caller fails with the child's error
+ * without emitting an error event of its own.
  */
 @Experimental
-final class DynamicNodeFailedException extends RuntimeException {
+public final class DynamicNodeFailedException extends RuntimeException {
 
   private final String errorNodePath;
 

core/src/main/java/com/google/adk/workflow/NodeRunner.java

@@ -49,7 +49,7 @@
  * The runner for one workflow node. It stamps every emitted event with its author, node path, and
  * branch before forwarding it, and records the node's output, routes, interrupts, and failure on
  * the node state of the returned context. {@link #runRoot} runs a node as the root of an
- * invocation, and {@code runChild} runs one as a child that another node starts itself.
+ * invocation, and {@link #runChild} runs one as a child that another node starts itself.
  */
 @Experimental
 public final class NodeRunner {
@@ -179,19 +179,12 @@ public static Flowable<Event> runRoot(
   }
 
   /**
-   * Returns a {@link Maybe} that, when subscribed, runs {@code node} as a child of the node whose
-   * context is {@code parent} and emits the child's output, or completes empty if it has none. The
-   * caller includes it in its run and catches only failures it handles, such as {@link
-   * DynamicNodeFailedException}: when the child waits, it fails with an internal {@link
-   * RuntimeException} that must propagate to end the caller's run, so that the caller waits too.
-   *
-   * <p>It fails with {@link DynamicNodeFailedException} if the child fails; with {@link
-   * IllegalStateException} if {@code parent} is not a workflow node's context, if the caller's
-   * {@link Node#rerunOnResume} is false, or if {@code config} delegates an output that the caller
-   * already has or has delegated; and with {@link IllegalArgumentException} if the child's name
-   * cannot form a node path.
+   * Runs {@code node} as a child of the node whose context is {@code parent}, as {@link
+   * Context#runNode(Node, Object, RunNodeConfig)} describes. Nodes call that method; this one is
+   * public only so {@link Context} can call it from another package.
    */
-  static Maybe<Object> runChild(
+  @Experimental
+  public static Maybe<Object> runChild(
       Context parent, Node node, @Nullable Object nodeInput, RunNodeConfig config) {
     checkNotNull(parent);
     checkNotNull(node);

core/src/main/java/com/google/adk/workflow/RunNodeConfig.java

@@ -24,10 +24,10 @@
 import java.util.Optional;
 import org.jspecify.annotations.Nullable;
 
-/** Configuration for running a child node with {@link NodeRunner#runChild}. */
+/** Configuration for running a child node with {@link com.google.adk.agents.Context#runNode}. */
 @Experimental
 @AutoValue
-abstract class RunNodeConfig {
+public abstract class RunNodeConfig {
 
   /**
    * Returns the ID of the child's run, which names the run in node paths and branches. When empty,

core/src/test/java/com/google/adk/agents/ContextTest.java

@@ -31,8 +31,11 @@
 import com.google.adk.runner.Runner;
 import com.google.adk.sessions.Session;
 import com.google.adk.sessions.State;
+import com.google.adk.testing.TestNode;
 import com.google.adk.tools.FunctionTool;
 import com.google.adk.tools.ToolContext;
+import com.google.adk.workflow.Node;
+import com.google.adk.workflow.RunNodeConfig;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.genai.types.Content;
@@ -94,6 +97,25 @@ public void nodeMethods_onCallbackContext_throwIllegalStateException() {
     assertThrows(IllegalStateException.class, () -> context.routes(ImmutableList.of()));
   }
 
+  @Test
+  public void runNode_onCallbackContext_failsWithIllegalStateException() {
+    CallbackContext context = new CallbackContext(newInvocationContext(), null);
+
+    context.runNode(TestNode.of("child")).test().assertError(IllegalStateException.class);
+  }
+
+  @Test
+  public void runNode_configPassedAsTheInput_throwsIllegalArgumentException() {
+    CallbackContext context = new CallbackContext(newInvocationContext(), null);
+    Node child = TestNode.of("child");
+    RunNodeConfig config = RunNodeConfig.builder().useAsOutput(true).build();
+
+    IllegalArgumentException thrown =
+        assertThrows(IllegalArgumentException.class, () -> context.runNode(child, config));
+
+    assertThat(thrown).hasMessageThat().contains("third argument");
+  }
+
   @Test
   public void searchMemory_onCallbackContext_findsRememberedEvents() {
     InvocationContext invocationContext = newInvocationContext();