feat: honor the plain-text continuation flag and allow only one resumability flag

`ResumabilityConfig.plainTextContinuationAutoResume` previously had no effect. It
now selects the same resumption behavior as `resumable`, and building a config
with both flags set throws `IllegalArgumentException`.

PiperOrigin-RevId: 985095038

Author: damianmomotgoogle

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

@@ -306,10 +306,14 @@ public Optional<ContextCacheConfig> contextCacheConfig() {
 
   /**
    * Returns whether the current invocation is resumable. Mirrors Python ADK v1's {@code
-   * InvocationContext.is_resumable}.
+   * InvocationContext.is_resumable}. The deprecated plain-text continuation shim selects the same
+   * resumption behavior, so it reports resumable too; the two are mutually exclusive.
    */
+  @SuppressWarnings("deprecation") // The shim it reads is deprecated by design.
   public boolean isResumable() {
-    return resumabilityConfig != null && resumabilityConfig.isResumable();
+    return resumabilityConfig != null
+        && (resumabilityConfig.isResumable()
+            || resumabilityConfig.isPlainTextContinuationAutoResume());
   }
 
   private static class InvocationCostManager {

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

@@ -16,13 +16,18 @@
 
 package com.google.adk.apps;
 
+import static com.google.common.base.Preconditions.checkArgument;
+
 import com.google.auto.value.AutoValue;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 
 /**
  * App resumability config, mirroring Python ADK v1's {@code ResumabilityConfig}: pause on a
  * long-running call and resume from the last event. Applies to all agents in the app.
  *
+ * <p>The two flags select the same resumption behavior and are mutually exclusive: set {@link
+ * #isResumable()}, or the deprecated shim, but not both.
+ *
  * @deprecated Partial feature: only event-reconstruction-based pause/resume for {@code
  *     SequentialAgent} is implemented. Full session resumability (persisted agent state, durable
  *     resume, other workflow agents) is not yet available. Forward-compatible: the same config will
@@ -41,10 +46,13 @@ public abstract class ResumabilityConfig {
    * default, matching Python ADK, where a plain-text {@code runAsync} always starts a new
    * invocation and a paused invocation is resumed explicitly.
    *
+   * <p>Selects the same resumption behavior as {@link #isResumable()}, with which it is mutually
+   * exclusive; it differs only in also resuming on a plain-text continuation.
+   *
    * @deprecated Back-compat shim for callers that deliver a resume as a plain-text turn. Migrate to
    *     {@code Runner.runAsync(userId, sessionId, invocationId, message, runConfig, stateDelta)}
-   *     (or send a function response to the paused call) and stop setting this flag; it will be
-   *     removed.
+   *     (or send a function response to the paused call) and set {@link #isResumable()} instead;
+   *     this flag will be removed.
    */
   @Deprecated
   public abstract boolean isPlainTextContinuationAutoResume();
@@ -70,6 +78,22 @@ public abstract static class Builder {
     @CanIgnoreReturnValue
     public abstract Builder plainTextContinuationAutoResume(boolean value);
 
-    public abstract ResumabilityConfig build();
+    abstract ResumabilityConfig autoBuild();
+
+    /**
+     * Builds the config, rejecting a combination of flags that has no defined behavior.
+     *
+     * @throws IllegalArgumentException if both resumability and the deprecated shim are set; they
+     *     select the same behavior, so exactly one may be enabled.
+     */
+    @SuppressWarnings("deprecation") // Validating the deprecated shim against the supported flag.
+    public ResumabilityConfig build() {
+      ResumabilityConfig config = autoBuild();
+      checkArgument(
+          !(config.isResumable() && config.isPlainTextContinuationAutoResume()),
+          "resumable and plainTextContinuationAutoResume are mutually exclusive: set resumable for"
+              + " the supported flag, or the deprecated shim, but not both.");
+      return config;
+    }
   }
 }

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

@@ -863,9 +863,15 @@ private boolean isTransferableAcrossAgentTree(BaseAgent agentToRun) {
     return true;
   }
 
-  /** Returns whether resumability is enabled for this runner's app. */
+  /**
+   * Returns whether resumability is enabled for this runner's app, by either the supported flag or
+   * the deprecated plain-text continuation shim, which selects the same behavior.
+   */
+  @SuppressWarnings("deprecation") // The shim it reads is deprecated by design.
   private boolean isResumable() {
-    return resumabilityConfig != null && resumabilityConfig.isResumable();
+    return resumabilityConfig != null
+        && (resumabilityConfig.isResumable()
+            || resumabilityConfig.isPlainTextContinuationAutoResume());
   }
 
   /** Returns the agent that should handle the next request based on session history. */

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

@@ -20,6 +20,7 @@
 import static org.junit.Assert.assertThrows;
 import static org.mockito.Mockito.mock;
 
+import com.google.adk.apps.ResumabilityConfig;
 import com.google.adk.artifacts.BaseArtifactService;
 import com.google.adk.events.Event;
 import com.google.adk.memory.BaseMemoryService;
@@ -73,6 +74,44 @@ public void setUp() {
     activeStreamingTools.put("test-tool", new ActiveStreamingTool(new LiveRequestQueue()));
   }
 
+  // The deprecated shim selects the same resumption behavior as resumable(true), so every
+  // resumability branch keyed on isResumable() must treat it identically.
+  @Test
+  public void isResumable_shimOnly_reportsResumable() {
+    InvocationContext shimContext = contextWith(resumabilityConfigWithShim());
+    InvocationContext resumableContext = contextWith(resumabilityConfigResumable());
+    InvocationContext neitherContext = contextWith(null);
+
+    assertThat(shimContext.isResumable()).isTrue();
+    assertThat(resumableContext.isResumable()).isTrue();
+    assertThat(neitherContext.isResumable()).isFalse();
+  }
+
+  @SuppressWarnings("deprecation") // Exercises the deprecated shim.
+  private static ResumabilityConfig resumabilityConfigWithShim() {
+    return ResumabilityConfig.builder().plainTextContinuationAutoResume(true).build();
+  }
+
+  @SuppressWarnings("deprecation") // ResumabilityConfig is deprecated until durable resumability.
+  private static ResumabilityConfig resumabilityConfigResumable() {
+    return ResumabilityConfig.builder().resumable(true).build();
+  }
+
+  @SuppressWarnings("deprecation") // ResumabilityConfig is deprecated until durable resumability.
+  private InvocationContext contextWith(ResumabilityConfig resumabilityConfig) {
+    return InvocationContext.builder()
+        .sessionService(mockSessionService)
+        .artifactService(mockArtifactService)
+        .pluginManager(pluginManager)
+        .invocationId(testInvocationId)
+        .agent(mockAgent)
+        .session(session)
+        .userContent(userContent)
+        .runConfig(runConfig)
+        .resumabilityConfig(resumabilityConfig)
+        .build();
+  }
+
   @Test
   public void testBuildWithUserContent() {
     InvocationContext context =

core/src/test/java/com/google/adk/apps/ResumabilityConfigTest.java

@@ -0,0 +1,65 @@
+/*
+ * 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.apps;
+
+import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.assertThrows;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Tests for {@link ResumabilityConfig}. */
+@RunWith(JUnit4.class)
+@SuppressWarnings("deprecation") // Exercises the deprecated plainTextContinuationAutoResume shim.
+public final class ResumabilityConfigTest {
+
+  @Test
+  public void build_defaults_selectNoResumption() {
+    ResumabilityConfig config = ResumabilityConfig.builder().build();
+
+    assertThat(config.isResumable()).isFalse();
+    assertThat(config.isPlainTextContinuationAutoResume()).isFalse();
+  }
+
+  @Test
+  public void build_resumableOnly_succeeds() {
+    ResumabilityConfig config = ResumabilityConfig.builder().resumable(true).build();
+
+    assertThat(config.isResumable()).isTrue();
+    assertThat(config.isPlainTextContinuationAutoResume()).isFalse();
+  }
+
+  @Test
+  public void build_legacyShimOnly_succeeds() {
+    ResumabilityConfig config =
+        ResumabilityConfig.builder().plainTextContinuationAutoResume(true).build();
+
+    assertThat(config.isResumable()).isFalse();
+    assertThat(config.isPlainTextContinuationAutoResume()).isTrue();
+  }
+
+  @Test
+  public void build_bothFlags_throws() {
+    ResumabilityConfig.Builder builder =
+        ResumabilityConfig.builder().resumable(true).plainTextContinuationAutoResume(true);
+
+    IllegalArgumentException thrown = assertThrows(IllegalArgumentException.class, builder::build);
+
+    assertThat(thrown).hasMessageThat().contains("mutually exclusive");
+  }
+}