@@ -0,0 +1,368 @@
+/*
+ * 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 static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+import static com.google.common.base.Preconditions.checkState;
+import static com.google.common.collect.ImmutableList.toImmutableList;
+
+import com.google.adk.annotations.Experimental;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
+import com.google.common.collect.Lists;
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
+import java.util.function.Consumer;
+
+/**
+ * A builder for the edges of a workflow graph, declared as chains of calls that start at {@link
+ * #from} or {@link #fromStart}. Create one with {@code new EdgesBuilder()} and call {@link #build},
+ * or let {@link Workflow.Builder#edges(Consumer)} do both. A node is identified by its instance, so
+ * create each node once.
+ *
+ * <pre>{@code
+ * Workflow support =
+ *     Workflow.builder()
+ *         .name("support_triage")
+ *         .edges(
+ *             g -> {
+ *               g.fromStart()
+ *                   .chain(classify, category)
+ *                   .route(
+ *                       r -> {
+ *                         r.on("billing").then(billing);
+ *                         r.otherwise(summarize);
+ *                       });
+ *               g.from(billing).then(summarize);
+ *             })
+ *         .build();
+ * }</pre>
+ */
+@Experimental
+public final class EdgesBuilder {
+
+  private final List<Edge> edges = new ArrayList<>();
+
+  /** Starts a chain of edges at {@code node}: {@code g.from(a).then(b).then(c)}. */
+  public EdgeChain from(Node node) {
+    return new EdgeChain(this, checkNotNull(node));
+  }
+
+  /**
+   * Starts a chain at {@code nodes} (such as a computed list) so they can join without a fan-out,
+   * like {@link #from(Node, Node, Node...)}.
+   */
+  public NodeGroupChain from(Collection<? extends Node> nodes) {
+    return new NodeGroupChain(this, ImmutableList.copyOf(nodes));
+  }
+
+  /**
+   * Starts a chain at {@code first}, {@code second} and each node in {@code rest} so they can join
+   * without a fan-out: {@code g.from(a, b).joinTo(join)}.
+   */
+  public NodeGroupChain from(Node first, Node second, Node... rest) {
+    return from(Lists.asList(first, second, rest));
+  }
+
+  /** Starts a chain of edges at {@link Start}: {@code g.fromStart().then(a)}. */
+  public EdgeChain fromStart() {
+    return from(Start.INSTANCE);
+  }
+
+  /** Returns the edges added so far, in the order they were added. */
+  public ImmutableList<Edge> build() {
+    return ImmutableList.copyOf(edges);
+  }
+
+  void add(Edge edge) {
+    edges.add(edge);
+  }
+
+  void addAll(List<Edge> newEdges) {
+    edges.addAll(newEdges);
+  }
+
+  /** Returns an edge from {@code from} to each of {@code targets}, failing if a target is null. */
+  static ImmutableList<Edge> edgesTo(
+      Node from, Collection<? extends Node> targets, ImmutableList<Route> routes) {
+    return targets.stream().map(to -> new Edge(from, to, routes)).collect(toImmutableList());
+  }
+
+  /**
+   * A chain of edges positioned at a node, started by {@link EdgesBuilder#from(Node)}, {@link
+   * EdgesBuilder#fromStart} or {@link NodeGroupChain#joinTo}. {@link #then(Node)} and {@link
+   * #chain} add edges and continue from the last node they connect, a fan-out continues from its
+   * targets as a group, and a routing map ends the chain.
+   */
+  @Experimental
+  public static final class EdgeChain {
+
+    private final EdgesBuilder builder;
+    private final Node node;
+
+    EdgeChain(EdgesBuilder builder, Node node) {
+      this.builder = builder;
+      this.node = node;
+    }
+
+    /** Connects the current node to {@code next} and continues from {@code next}. */
+    @CanIgnoreReturnValue
+    public EdgeChain then(Node next) {
+      builder.add(new Edge(node, next));
+      return new EdgeChain(builder, next);
+    }
+
+    /**
+     * Fans out by connecting the current node to each of {@code targets} (such as a computed list)
+     * so they run concurrently, and continues from them as a group that {@link
+     * NodeGroupChain#joinTo} can join. An empty collection adds no edges.
+     */
+    @CanIgnoreReturnValue
+    public NodeGroupChain then(Collection<? extends Node> targets) {
+      ImmutableList<Node> nodes = ImmutableList.copyOf(targets);
+      builder.addAll(edgesTo(node, nodes, ImmutableList.of()));
+      return new NodeGroupChain(builder, nodes);
+    }
+
+    /**
+     * Fans out by connecting the current node to {@code first}, {@code second} and each node in
+     * {@code rest}, like {@link #then(Collection)}: {@code g.from(a).then(b, c).joinTo(join)}.
+     */
+    @CanIgnoreReturnValue
+    public NodeGroupChain then(Node first, Node second, Node... rest) {
+      return then(Lists.asList(first, second, rest));
+    }
+
+    /**
+     * Connects the current node to {@code first}, {@code second} and each node in {@code rest} in
+     * order, and continues from the last one: {@code g.fromStart().chain(a, b)} is short for {@code
+     * g.fromStart().then(a).then(b)}.
+     */
+    @CanIgnoreReturnValue
+    public EdgeChain chain(Node first, Node second, Node... rest) {
+      // Copying rejects a null node before any edge is added.
+      ImmutableList<Node> nodes = ImmutableList.copyOf(Lists.asList(first, second, rest));
+      EdgeChain chain = this;
+      for (Node next : nodes) {
+        chain = chain.then(next);
+      }
+      return chain;
+    }
+
+    /**
+     * Adds the current node's routing map, which {@code routeMapDefinition} fills, and ends the
+     * chain.
+     *
+     * @throws IllegalArgumentException if the routing map has no entries
+     * @throws IllegalStateException if an entry started by {@link RouteMap#on} has no target
+     */
+    public void route(Consumer<? super RouteMap> routeMapDefinition) {
+      RouteMap routeMap = new RouteMap(node);
+      routeMapDefinition.accept(routeMap);
+      ImmutableList<Edge> routeEdges = routeMap.build();
+      checkArgument(
+          !routeEdges.isEmpty(),
+          "The routing map of node '%s' needs at least one entry.",
+          node.name());
+      builder.addAll(routeEdges);
+    }
+  }
+
+  /**
+   * A chain of edges positioned at a node group from a fan-out, {@link EdgesBuilder#from(Node,
+   * Node, Node...)}, or {@link EdgesBuilder#from(Collection)}, which {@link #joinTo} connects to a
+   * {@link JoinNode}.
+   */
+  @Experimental
+  public static final class NodeGroupChain {
+
+    private final EdgesBuilder builder;
+    private final ImmutableList<Node> nodes;
+
+    NodeGroupChain(EdgesBuilder builder, ImmutableList<Node> nodes) {
+      this.builder = builder;
+      this.nodes = nodes;
+    }
+
+    /**
+     * Connects each node in the group to {@code join}, which runs once all of them complete, and
+     * continues from {@code join}.
+     */
+    @CanIgnoreReturnValue
+    public EdgeChain joinTo(JoinNode join) {
+      checkNotNull(join);
+      for (Node node : nodes) {
+        builder.add(new Edge(node, join));
+      }
+      return new EdgeChain(builder, join);
+    }
+  }
+
+  /**
+   * A routing map configured in {@link EdgeChain#route} that maps each route the source node may
+   * emit to the nodes it leads to, written {@code r.on("billing").then(billing)}. Mapping a route
+   * to several nodes fans out on that route.
+   */
+  @Experimental
+  public static final class RouteMap {
+
+    private final Node from;
+    private final ImmutableList.Builder<Edge> edges = ImmutableList.builder();
+    private final List<RouteMapEntry> pendingEntries = new ArrayList<>();
+    private boolean built = false;
+
+    RouteMap(Node from) {
+      this.from = from;
+    }
+
+    /**
+     * Starts an entry for {@code route} as a {@link Route.Tag}; {@link RouteMapEntry#then}
+     * completes it.
+     */
+    public RouteMapEntry on(String route) {
+      return on(new Route.Tag(route));
+    }
+
+    /**
+     * Starts an entry for {@code route} as a {@link Route.Flag}; {@link RouteMapEntry#then}
+     * completes it.
+     */
+    public RouteMapEntry on(boolean route) {
+      return on(new Route.Flag(route));
+    }
+
+    /**
+     * Starts an entry for {@code route}, such as a {@link Route.Num}; {@link RouteMapEntry#then}
+     * completes it.
+     */
+    public RouteMapEntry on(Route route) {
+      return start(ImmutableList.of(route));
+    }
+
+    /**
+     * Starts an entry whose edges fire once when the source node emits any of {@code routes}, built
+     * with {@link #anyOf}; {@link RouteMapEntry#then} completes it.
+     */
+    public RouteMapEntry on(RouteSet routes) {
+      return start(routes.routes());
+    }
+
+    /**
+     * Leads to each target when no other route of the source node matches, including when it emits
+     * none.
+     */
+    public void otherwise(Node first, Node... rest) {
+      add(ImmutableList.of(Route.Default.INSTANCE), Lists.asList(first, rest));
+    }
+
+    /** Groups string routes as {@link Route.Tag}s for {@link #on(RouteSet)}. */
+    public RouteSet anyOf(String first, String second, String... rest) {
+      return new RouteSet(Lists.transform(Lists.asList(first, second, rest), Route.Tag::new));
+    }
+
+    /** Groups routes, such as {@link Route.Num}s, for {@link #on(RouteSet)}. */
+    public RouteSet anyOf(Route first, Route second, Route... rest) {
+      return new RouteSet(Lists.asList(first, second, rest));
+    }
+
+    /**
+     * Returns the map's edges and closes the map, so an entry started or completed after {@link
+     * EdgeChain#route} returns fails instead of being lost.
+     *
+     * @throws IllegalStateException if an entry started by {@code on} has no target
+     */
+    ImmutableList<Edge> build() {
+      built = true;
+      checkState(
+          pendingEntries.isEmpty(),
+          "on(...) in the routing map of node '%s' has no target; complete it with then(node).",
+          from.name());
+      return edges.build();
+    }
+
+    /** Adds an edge for {@code routes} to each of {@code targets} and completes {@code entry}. */
+    void complete(RouteMapEntry entry, ImmutableList<Route> routes, List<Node> targets) {
+      add(routes, targets);
+      pendingEntries.remove(entry);
+    }
+
+    private RouteMapEntry start(ImmutableList<Route> routes) {
+      checkNotBuilt();
+      RouteMapEntry entry = new RouteMapEntry(this, routes);
+      pendingEntries.add(entry);
+      return entry;
+    }
+
+    private void add(ImmutableList<Route> routes, List<Node> targets) {
+      checkNotBuilt();
+      edges.addAll(edgesTo(from, targets, routes));
+    }
+
+    private void checkNotBuilt() {
+      checkState(
+          !built,
+          "The routing map of node '%s' was already built; add its entries inside route(...).",
+          from.name());
+    }
+  }
+
+  /**
+   * A routing-map entry started by {@link RouteMap#on} whose routes have no target yet; {@link
+   * #then} completes it.
+   */
+  @Experimental
+  public static final class RouteMapEntry {
+
+    private final RouteMap routeMap;
+    private final ImmutableList<Route> routes;
+
+    RouteMapEntry(RouteMap routeMap, ImmutableList<Route> routes) {
+      this.routeMap = routeMap;
+      this.routes = routes;
+    }
+
+    /**
+     * Leads to {@code first} and each node in {@code rest}, with one edge to each, when the source
+     * node emits any of this entry's routes.
+     */
+    public void then(Node first, Node... rest) {
+      routeMap.complete(this, routes, Lists.asList(first, rest));
+    }
+  }
+
+  /**
+   * A set of routes that select a single edge, which fires once when its source emits any of them;
+   * separate edges to the same target would fire once each. {@link RouteMap#anyOf} builds one and
+   * drops duplicate routes.
+   */
+  @Experimental
+  public static final class RouteSet {
+
+    private final ImmutableList<Route> routes;
+
+    RouteSet(List<? extends Route> routes) {
+      this.routes = ImmutableSet.<Route>copyOf(routes).asList();
+    }
+
+    ImmutableList<Route> routes() {
+      return routes;
+    }
+  }
+}
@@ -28,6 +28,7 @@
 import io.reactivex.rxjava3.core.Flowable;
 import java.util.List;
 import java.util.Optional;
+import java.util.function.Consumer;
 import org.jspecify.annotations.Nullable;
 
 /**
@@ -118,6 +119,18 @@ public Builder edges(Edge... edges) {
       return edges(ImmutableList.copyOf(edges));
     }
 
+    /**
+     * Sets the edges that {@code graphDefinition} adds to a new {@link EdgesBuilder}.
+     *
+     * <p>For example: {@code .edges(g -> g.fromStart().chain(a, b))}.
+     */
+    @CanIgnoreReturnValue
+    public Builder edges(Consumer<? super EdgesBuilder> graphDefinition) {
+      EdgesBuilder builder = new EdgesBuilder();
+      graphDefinition.accept(builder);
+      return edges(builder.build());
+    }
+
     @CanIgnoreReturnValue
     public Builder description(String description) {
       this.description = checkNotNull(description);