feat: add one-way ADK Java to Kotlin engine interop module

pom.xml

@@ -36,6 +36,7 @@
     <module>tutorials/city-time-weather</module>
     <module>tutorials/live-audio-single-agent</module>
     <module>a2a</module>
+    <module>tokt</module>
   </modules>
 
   <properties>
@@ -67,6 +68,10 @@
     <truth.version>1.4.5</truth.version>
     <jspecify.version>1.0.0</jspecify.version>
     <rxjava.version>3.1.12</rxjava.version>
+    <!-- ADK Kotlin engine interop (google-adk-tokt module). -->
+    <kotlin.version>2.1.20</kotlin.version>
+    <kotlinx-coroutines.version>1.10.2</kotlinx-coroutines.version>
+    <adk-kotlin.version>0.6.0</adk-kotlin.version>
     <reactor-core.version>3.7.0</reactor-core.version>
     <wiremock.version>2.35.1</wiremock.version>
     <assertj.version>3.27.7</assertj.version>

tokt/pom.xml

@@ -0,0 +1,142 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+ 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.
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <parent>
+    <groupId>com.google.adk</groupId>
+    <artifactId>google-adk-parent</artifactId>
+    <version>1.7.1-SNAPSHOT</version><!-- {x-version-update:google-adk:current} -->
+  </parent>
+
+  <artifactId>google-adk-tokt</artifactId>
+  <name>Agent Development Kit - Kotlin engine interop</name>
+  <description>One-way interop that adapts ADK Java agents, tools, toolsets, plugins, services, and models so they can run on the ADK Kotlin engine.</description>
+
+  <dependencies>
+    <!-- ADK Java facade types being adapted. -->
+    <dependency>
+      <groupId>com.google.adk</groupId>
+      <artifactId>google-adk</artifactId>
+      <version>${project.version}</version>
+    </dependency>
+    <!-- ADK Kotlin engine the Java types are adapted to. -->
+    <dependency>
+      <groupId>com.google.adk</groupId>
+      <artifactId>google-adk-kotlin-core-jvm</artifactId>
+      <version>${adk-kotlin.version}</version>
+    </dependency>
+    <dependency>
+      <groupId>com.google.genai</groupId>
+      <artifactId>google-genai</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>io.reactivex.rxjava3</groupId>
+      <artifactId>rxjava</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>com.google.guava</groupId>
+      <artifactId>guava</artifactId>
+      <version>33.0.0-jre</version>
+    </dependency>
+    <dependency>
+      <groupId>org.jspecify</groupId>
+      <artifactId>jspecify</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.jetbrains.kotlin</groupId>
+      <artifactId>kotlin-stdlib</artifactId>
+      <version>${kotlin.version}</version>
+    </dependency>
+    <dependency>
+      <groupId>org.jetbrains.kotlinx</groupId>
+      <artifactId>kotlinx-coroutines-core-jvm</artifactId>
+      <version>${kotlinx-coroutines.version}</version>
+    </dependency>
+    <dependency>
+      <groupId>org.jetbrains.kotlinx</groupId>
+      <artifactId>kotlinx-coroutines-rx3</artifactId>
+      <version>${kotlinx-coroutines.version}</version>
+    </dependency>
+    <dependency>
+      <groupId>org.jetbrains.kotlinx</groupId>
+      <artifactId>kotlinx-coroutines-reactive</artifactId>
+      <version>${kotlinx-coroutines.version}</version>
+    </dependency>
+
+    <!-- Test. -->
+    <dependency>
+      <groupId>org.jetbrains.kotlin</groupId>
+      <artifactId>kotlin-test-junit</artifactId>
+      <version>${kotlin.version}</version>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <version>4.13.2</version>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+
+  <build>
+    <plugins>
+      <!-- Kotlin sources live under src/main/java (mirroring the Blaze layout), so point the
+           Kotlin compiler at those directories explicitly. -->
+      <plugin>
+        <groupId>org.jetbrains.kotlin</groupId>
+        <artifactId>kotlin-maven-plugin</artifactId>
+        <version>${kotlin.version}</version>
+        <configuration>
+          <jvmTarget>${java.version}</jvmTarget>
+          <args>
+            <arg>-opt-in=kotlin.time.ExperimentalTime</arg>
+          </args>
+        </configuration>
+        <executions>
+          <execution>
+            <id>compile</id>
+            <phase>compile</phase>
+            <goals>
+              <goal>compile</goal>
+            </goals>
+            <configuration>
+              <sourceDirs>
+                <sourceDir>${project.basedir}/src/main/java</sourceDir>
+              </sourceDirs>
+            </configuration>
+          </execution>
+          <execution>
+            <id>test-compile</id>
+            <phase>test-compile</phase>
+            <goals>
+              <goal>test-compile</goal>
+            </goals>
+            <configuration>
+              <sourceDirs>
+                <sourceDir>${project.basedir}/src/test/java</sourceDir>
+              </sourceDirs>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+      <plugin>
+        <artifactId>maven-surefire-plugin</artifactId>
+      </plugin>
+    </plugins>
+  </build>
+</project>

tokt/src/main/java/com/google/adk/tokt/JavaAdkToKt.kt

@@ -0,0 +1,102 @@
+/*
+ * 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.tokt
+
+import com.google.adk.artifacts.BaseArtifactService as JavaArtifactService
+import com.google.adk.kt.artifacts.ArtifactService as KtArtifactService
+import com.google.adk.kt.memory.MemoryService as KtMemoryService
+import com.google.adk.kt.models.Model as KtModel
+import com.google.adk.kt.plugins.Plugin as KtPlugin
+import com.google.adk.kt.sessions.SessionService as KtSessionService
+import com.google.adk.kt.tools.BaseTool as KtBaseTool
+import com.google.adk.kt.tools.Toolset as KtToolset
+import com.google.adk.memory.BaseMemoryService as JavaMemoryService
+import com.google.adk.models.BaseLlm as JavaBaseLlm
+import com.google.adk.plugins.Plugin as JavaPlugin
+import com.google.adk.sessions.BaseSessionService as JavaSessionService
+import com.google.adk.tokt.adapters.JavaModelToKt
+import com.google.adk.tokt.adapters.JavaPluginToKt
+import com.google.adk.tokt.adapters.JavaToolToKt
+import com.google.adk.tokt.adapters.JavaToolsetToKt
+import com.google.adk.tokt.services.javaArtifactServiceAsKt
+import com.google.adk.tokt.services.javaMemoryServiceAsKt
+import com.google.adk.tokt.services.javaSessionServiceAsKt
+import com.google.adk.tools.BaseTool as JavaBaseTool
+import com.google.adk.tools.BaseToolset as JavaBaseToolset
+
+/**
+ * Forward interop entry point: adapt ADK Java tools, toolsets, plugins, services, and models so
+ * they can run on the ADK Kotlin engine. Whole-agent conversion is intentionally omitted (running a
+ * Java agent's own multi-step flow on the Kotlin runner is not yet supported); wrap the Java SPI
+ * pieces into a Kotlin `LlmAgent` instead.
+ *
+ * **Session is re-projected, not a live handle.** An adapted Java tool / toolset / plugin sees the
+ * session through `context.session()`, which is re-projected on each call, so a *fresh* read is
+ * always current. The returned `Session` is a snapshot though: unlike native ADK Java its
+ * `events()` list does not grow in place and its state is a copy. Re-read `context.session()` each
+ * time; do not cache a `Session` (or its `events()`) across tool calls or turns. State *writes* via
+ * `toolContext.state()` / `callbackContext.state()` still propagate to the Kotlin.
+ */
+object JavaAdkToKt {
+
+  /**
+   * Adapts an ADK Java tool. Its `ToolContext.session()` is a per-call snapshot; see [JavaAdkToKt].
+   */
+  @JvmStatic fun asKtTool(javaTool: JavaBaseTool): KtBaseTool = JavaToolToKt(javaTool)
+
+  /** Adapts a whole collection of ADK Java tools (e.g. an `LlmAgent`'s `tools`). */
+  @JvmStatic
+  fun asKtTools(javaTools: List<JavaBaseTool>): List<KtBaseTool> = javaTools.map { asKtTool(it) }
+
+  /**
+   * Adapts an ADK Java toolset. Its `ReadonlyContext` session is a per-call snapshot; see
+   * [JavaAdkToKt].
+   */
+  @JvmStatic fun asKtToolset(javaToolset: JavaBaseToolset): KtToolset = JavaToolsetToKt(javaToolset)
+
+  /** Adapts a whole collection of ADK Java toolsets. */
+  @JvmStatic
+  fun asKtToolsets(javaToolsets: List<JavaBaseToolset>): List<KtToolset> = javaToolsets.map {
+    asKtToolset(it)
+  }
+
+  /**
+   * Adapts an ADK Java plugin. Its callback contexts expose a per-call session snapshot; see
+   * [JavaAdkToKt].
+   */
+  @JvmStatic fun asKtPlugin(javaPlugin: JavaPlugin): KtPlugin = JavaPluginToKt(javaPlugin)
+
+  /** Adapts a whole collection of ADK Java plugins (e.g. a `Runner`'s `plugins`). */
+  @JvmStatic
+  fun asKtPlugins(javaPlugins: List<JavaPlugin>): List<KtPlugin> = javaPlugins.map {
+    asKtPlugin(it)
+  }
+
+  @JvmStatic fun asKtModel(javaLlm: JavaBaseLlm): KtModel = JavaModelToKt(javaLlm)
+
+  @JvmStatic
+  fun asKtSessionService(service: JavaSessionService): KtSessionService =
+    javaSessionServiceAsKt(service)
+
+  @JvmStatic
+  fun asKtArtifactService(service: JavaArtifactService): KtArtifactService =
+    javaArtifactServiceAsKt(service)
+
+  @JvmStatic
+  fun asKtMemoryService(service: JavaMemoryService): KtMemoryService =
+    javaMemoryServiceAsKt(service)
+}

tokt/src/main/java/com/google/adk/tokt/adapters/JavaModelToKt.kt

@@ -0,0 +1,58 @@
+/*
+ * 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.tokt.adapters
+
+import com.google.adk.kt.models.LlmRequest
+import com.google.adk.kt.models.LlmResponse
+import com.google.adk.kt.models.Model
+import com.google.adk.models.BaseLlm as JavaBaseLlm
+import com.google.adk.tokt.codecs.LlmRequestCodec
+import com.google.adk.tokt.codecs.LlmResponseCodec
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.emitAll
+import kotlinx.coroutines.flow.flow
+import kotlinx.coroutines.flow.flowOn
+import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.reactive.asFlow
+
+/**
+ * Java -> Kotlin adapter: presents a user's ADK Java [JavaBaseLlm] as the Kotlin's [Model] so the
+ * ADK Kotlin agent loop can call it. The model seam for routing ADK Java's `LlmAgent` onto the
+ * Kotlin.
+ *
+ * The Kotlin `LlmRequest` is converted to a Java `LlmRequest` ([LlmRequestCodec]), the Java model's
+ * RxJava `Flowable<LlmResponse>` is consumed as a coroutine [Flow] (via
+ * kotlinx-coroutines-reactive) and each response is converted back ([LlmResponseCodec]).
+ */
+internal class JavaModelToKt(private val javaLlm: JavaBaseLlm) : Model {
+
+  override val name: String = javaLlm.model()
+
+  // Deferred into `flow {}` and dispatched on IO: the Java model's request build + generation run
+  // off the engine dispatcher (RxJava is synchronous by default), and a synchronous throw is routed
+  // through the Flow's error channel rather than escaping at collection time.
+  override fun generateContent(request: LlmRequest, stream: Boolean): Flow<LlmResponse> =
+    flow {
+        emitAll(
+          javaLlm.generateContent(LlmRequestCodec.toJava(request), stream).asFlow().map {
+            LlmResponseCodec.fromJava(it)
+          }
+        )
+      }
+      .flowOn(Dispatchers.IO)
+}