feat: pause a workflow for user input

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

@@ -323,9 +323,9 @@ public final void routes(List<? extends Route> routes) {
   }
 
   /**
-   * Returns the user's answers to the long-running calls that this workflow node or its descendants
-   * made in an earlier turn, keyed by interrupt ID. A node that reruns on resume reads these
-   * answers. A null answer has a null value in the map.
+   * Returns the user's answers to the requests for input and long-running calls that this workflow
+   * node or its descendants made in an earlier turn, keyed by interrupt ID. A node that reruns on
+   * resume reads these answers. A null answer has a null value in the map.
    *
    * @throws IllegalStateException if this is not the context of a workflow node.
    */

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

@@ -119,21 +119,27 @@ public Optional<Schema> stateSchema() {
 
   /**
    * Runs the node and emits its events. Each emission of {@link #runNode} that is an {@link Event}
-   * passes through, and any other value becomes the output of a new event.
+   * passes through, a {@link RequestInput} becomes the event that asks for it, and any other value
+   * becomes the output of a new event.
    */
   final Flowable<Event> run(Context context, @Nullable Object nodeInput) {
     InvocationContext invocationContext = checkNotNull(context).invocationContext();
     // The node runner fills in the author, as only it knows the node's place in the graph.
     return Flowable.<Object>defer(() -> runNode(context, nodeInput))
         .map(
-            item ->
-                item instanceof Event event
-                    ? event
-                    : Event.builder()
-                        .id(invocationContext.newUuid())
-                        .timestamp(invocationContext.now().toEpochMilli())
-                        .author("")
-                        .output(item)
-                        .build());
+            item -> {
+              if (item instanceof Event event) {
+                return event;
+              }
+              if (item instanceof RequestInput request) {
+                return request.toEvent(invocationContext);
+              }
+              return Event.builder()
+                  .id(invocationContext.newUuid())
+                  .timestamp(invocationContext.now().toEpochMilli())
+                  .author("")
+                  .output(item)
+                  .build();
+            });
   }
 }

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

@@ -98,9 +98,10 @@ default boolean requiresAllPredecessors() {
   /**
    * Runs the node on {@code nodeInput}, the output it receives from its predecessors.
    *
-   * <p>An emitted {@link Event} passes through, and any other emitted value becomes the node's
-   * output (at most one per run); emit nothing for no output, since RxJava forbids null. The
-   * wildcard lets an implementation return a {@code Flowable} of its own type.
+   * <p>An emitted {@link Event} passes through, an emitted {@link RequestInput} pauses the workflow
+   * until the user answers it, and any other emitted value becomes the node's output (at most one
+   * per run); emit nothing for no output, since RxJava forbids null. The wildcard lets an
+   * implementation return a {@code Flowable} of its own type.
    */
   @Experimental
   Flowable<?> runNode(Context context, @Nullable Object nodeInput);

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

@@ -196,9 +196,9 @@ public ImmutableSet<String> interruptIds() {
   }
 
   /**
-   * Returns the user's answers to the long-running calls that this node or its descendants raised
-   * in an earlier turn, keyed by interrupt ID. A null answer has a null value in the map. The map
-   * is empty unless the node reruns on resume.
+   * Returns the user's answers to the requests for input and long-running calls that this node or
+   * its descendants raised in an earlier turn, keyed by interrupt ID. A null answer has a null
+   * value in the map. The map is empty unless the node reruns on resume.
    */
   public Map<String, @Nullable Object> resumeInputs() {
     return resumeInputs;

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

@@ -0,0 +1,127 @@
+/*
+ * 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.workflow;
+
+import com.google.adk.JsonBaseModel;
+import com.google.adk.agents.InvocationContext;
+import com.google.adk.annotations.Experimental;
+import com.google.adk.events.Event;
+import com.google.auto.value.AutoValue;
+import com.google.common.collect.ImmutableSet;
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
+import com.google.genai.types.Content;
+import com.google.genai.types.FunctionCall;
+import com.google.genai.types.Part;
+import com.google.genai.types.Schema;
+import java.util.LinkedHashMap;
+import java.util.Map;
+import java.util.Optional;
+import java.util.UUID;
+import org.jspecify.annotations.Nullable;
+
+/**
+ * A node's request for user input, which pauses the workflow until the user answers it. A node
+ * emits it from {@link Node#runNode}, and the runtime records it as a long-running {@value
+ * #FUNCTION_CALL_NAME} function call whose ID is {@link #interruptId}. The user answers with a
+ * function response that carries the same ID.
+ */
+@Experimental
+@AutoValue
+public abstract class RequestInput {
+
+  /** The name of the function call that carries a request for input. */
+  public static final String FUNCTION_CALL_NAME = "adk_request_input";
+
+  static final String INTERRUPT_ID_KEY = "interruptId";
+  static final String MESSAGE_KEY = "message";
+  static final String PAYLOAD_KEY = "payload";
+  static final String RESPONSE_SCHEMA_KEY = "response_schema";
+
+  /** Returns the ID of the request, which the answering function response carries. */
+  public abstract String interruptId();
+
+  /** Returns the message to show the user, if any. */
+  public abstract Optional<String> message();
+
+  /** Returns additional data for rendering the request, if any. */
+  public abstract Optional<Object> payload();
+
+  /**
+   * Returns the schema the answer must match, if any. An answer of the form {@code {"result":
+   * value}} is unwrapped first, and a string value is read as JSON unless the schema accepts a
+   * string.
+   */
+  public abstract Optional<Schema> responseSchema();
+
+  /** Returns a builder whose interrupt ID is a new random UUID. */
+  public static Builder builder() {
+    return new AutoValue_RequestInput.Builder().interruptId(UUID.randomUUID().toString());
+  }
+
+  public abstract Builder toBuilder();
+
+  /**
+   * Returns the long-running function-call event that records this request. It has the model role,
+   * so history rewriting keeps it as the call that the answer responds to.
+   *
+   * @throws IllegalArgumentException if the payload has no JSON form
+   */
+  Event toEvent(InvocationContext invocationContext) {
+    Map<String, @Nullable Object> args = new LinkedHashMap<>();
+    args.put(INTERRUPT_ID_KEY, interruptId());
+    args.put(MESSAGE_KEY, message().orElse(null));
+    // Converted here so a payload with no JSON form fails the asking node, not the session service.
+    args.put(PAYLOAD_KEY, payload().map(RequestInput::toJsonValue).orElse(null));
+    args.put(RESPONSE_SCHEMA_KEY, responseSchema().map(RequestInput::toJsonValue).orElse(null));
+    FunctionCall call =
+        FunctionCall.builder().name(FUNCTION_CALL_NAME).id(interruptId()).args(args).build();
+    return Event.builder()
+        .id(invocationContext.newUuid())
+        .timestamp(invocationContext.now().toEpochMilli())
+        .author("")
+        .content(
+            Content.builder()
+                .role("model")
+                .parts(Part.builder().functionCall(call).build())
+                .build())
+        .longRunningToolIds(ImmutableSet.of(interruptId()))
+        .build();
+  }
+
+  private static @Nullable Object toJsonValue(Object value) {
+    return JsonBaseModel.getMapper().convertValue(value, Object.class);
+  }
+
+  /** Builder for {@link RequestInput}. */
+  @AutoValue.Builder
+  public abstract static class Builder {
+
+    @CanIgnoreReturnValue
+    public abstract Builder interruptId(String interruptId);
+
+    @CanIgnoreReturnValue
+    public abstract Builder message(@Nullable String message);
+
+    @CanIgnoreReturnValue
+    public abstract Builder payload(@Nullable Object payload);
+
+    @CanIgnoreReturnValue
+    public abstract Builder responseSchema(@Nullable Schema responseSchema);
+
+    public abstract RequestInput build();
+  }
+}