feat: add a Vertex AI-backed SessionService

core/build.gradle.kts

@@ -78,6 +78,7 @@ kotlin {
         implementation(libs.google.gson)
         implementation(libs.google.cloud.storage)
         implementation(libs.mcp)
+        implementation(libs.okhttp)
         implementation(libs.kotlinx.coroutines.reactor)
         implementation(libs.slf4j.api)
         implementation(libs.google.flogger.extensions)

core/src/commonMain/kotlin/com/google/adk/kt/serialization/Base64ByteArraySerializer.kt

@@ -0,0 +1,50 @@
+/*
+ * 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
+ *
+ *     https://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.kt.serialization
+
+import kotlin.io.encoding.Base64
+import kotlin.io.encoding.ExperimentalEncodingApi
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.descriptors.PrimitiveKind
+import kotlinx.serialization.descriptors.PrimitiveSerialDescriptor
+import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
+
+/**
+ * Serializes a [ByteArray] as a base64 string.
+ *
+ * This matches the proto3-JSON representation of `bytes` fields, which is what the Vertex AI APIs
+ * (including the managed Session Service) expect. The kotlinx default encodes a `ByteArray` as a
+ * JSON array of numbers, which those APIs reject (`"Proto field is not repeating, cannot start
+ * list."`). Apply it with `@Serializable(with = Base64ByteArraySerializer::class)` on `ByteArray`
+ * fields that are persisted or sent over such wires (e.g. `Part.thoughtSignature`, `Blob.data`).
+ */
+@OptIn(ExperimentalEncodingApi::class)
+object Base64ByteArraySerializer : KSerializer<ByteArray> {
+  override val descriptor: SerialDescriptor =
+    PrimitiveSerialDescriptor(
+      "com.google.adk.kt.serialization.Base64ByteArray",
+      PrimitiveKind.STRING,
+    )
+
+  override fun serialize(encoder: Encoder, value: ByteArray) {
+    encoder.encodeString(Base64.encode(value))
+  }
+
+  override fun deserialize(decoder: Decoder): ByteArray = Base64.decode(decoder.decodeString())
+}

core/src/commonMain/kotlin/com/google/adk/kt/types/Blob.kt

@@ -16,14 +16,15 @@
 
 package com.google.adk.kt.types
 
+import com.google.adk.kt.serialization.Base64ByteArraySerializer
 import kotlinx.serialization.Serializable
 
 /** Represents binary data. */
 @Serializable
 data class Blob(
   val mimeType: String? = null,
   val displayName: String? = null,
-  val data: ByteArray? = null,
+  @Serializable(with = Base64ByteArraySerializer::class) val data: ByteArray? = null,
 ) {
   override fun equals(other: Any?): Boolean {
     if (this === other) return true

core/src/commonMain/kotlin/com/google/adk/kt/types/Part.kt

@@ -16,6 +16,7 @@
 package com.google.adk.kt.types
 
 import com.google.adk.kt.annotations.FrameworkInternalApi
+import com.google.adk.kt.serialization.Base64ByteArraySerializer
 import kotlinx.serialization.Contextual
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.Transient
@@ -49,7 +50,7 @@ constructor(
   /** Indicates whether the part represents the model's thought process. */
   val thought: Boolean? = null,
   /** An opaque signature for the thought. */
-  val thoughtSignature: ByteArray? = null,
+  @Serializable(with = Base64ByteArraySerializer::class) val thoughtSignature: ByteArray? = null,
   /** Metadata for a video part (segment offsets and frame rate). */
   val videoMetadata: VideoMetadata? = null,
   /** Arbitrary key-value metadata associated with this part. The map must be JSON serializable. */

core/src/jvmMain/kotlin/com/google/adk/kt/sessions/HttpApiClient.kt

@@ -0,0 +1,94 @@
+/*
+ * 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.kt.sessions
+
+import com.google.auth.oauth2.GoogleCredentials
+import okhttp3.MediaType.Companion.toMediaType
+import okhttp3.OkHttpClient
+import okhttp3.Request
+import okhttp3.RequestBody.Companion.toRequestBody
+import okhttp3.Response
+
+/**
+ * A simplified HTTP client for interacting with the Vertex AI Session Service.
+ *
+ * @property project The Google Cloud Project ID.
+ * @property location The Google Cloud Location (region) where the service resides. The special
+ *   value `"global"` selects the global, non-region-prefixed endpoint.
+ * @property httpClient The underlying [OkHttpClient] instance to use.
+ * @property credentials The Google credentials used for authentication.
+ * @property baseUrlOverride Optional override for the API base URL (useful for testing). When set,
+ *   it must already include the API version segment (e.g. `http://localhost:8080/v1beta1`).
+ */
+class HttpApiClient(
+  private val project: String,
+  private val location: String,
+  private val httpClient: OkHttpClient = OkHttpClient(),
+  private val credentials: GoogleCredentials = defaultCredentials(),
+  private val baseUrlOverride: String? = null,
+) {
+  private val baseUrl: String =
+    baseUrlOverride
+      ?: if (location == "global") {
+        "https://aiplatform.googleapis.com/$API_VERSION"
+      } else {
+        "https://$location-aiplatform.googleapis.com/$API_VERSION"
+      }
+
+  /**
+   * Sends an authenticated HTTP request to the Vertex AI Session Service.
+   *
+   * This method refreshes the credentials if they are expired, injects the necessary
+   * `Authorization` and `Content-Type` headers (plus `x-goog-user-project` when the credentials
+   * declare a quota project), and executes the request synchronously.
+   *
+   * @param httpMethod The HTTP method to use (`GET`, `POST`, or `DELETE`).
+   * @param path The relative API path (e.g. `reasoningEngines/engine-id/sessions`).
+   * @param requestJson The JSON payload for `POST` requests. Ignored for `GET` and `DELETE`.
+   * @return The [Response] object from OkHttp. The caller is responsible for closing it.
+   * @throws IllegalArgumentException if the provided [httpMethod] is not supported.
+   */
+  fun request(httpMethod: String, path: String, requestJson: String): Response {
+    val url = "$baseUrl/projects/$project/locations/$location/$path"
+    val requestBuilder = Request.Builder().url(url)
+
+    credentials.refreshIfExpired()
+    requestBuilder.header("Authorization", "Bearer ${credentials.accessToken.tokenValue}")
+    requestBuilder.header("Content-Type", "application/json")
+    credentials.quotaProjectId?.let { requestBuilder.header("x-goog-user-project", it) }
+
+    when (httpMethod.uppercase()) {
+      "POST" ->
+        requestBuilder.post(
+          requestJson.toRequestBody("application/json; charset=utf-8".toMediaType())
+        )
+      "GET" -> requestBuilder.get()
+      "DELETE" -> requestBuilder.delete()
+      else -> throw IllegalArgumentException("Unsupported HTTP method: $httpMethod")
+    }
+
+    return httpClient.newCall(requestBuilder.build()).execute()
+  }
+
+  companion object {
+    private const val API_VERSION = "v1beta1"
+
+    private fun defaultCredentials(): GoogleCredentials =
+      GoogleCredentials.getApplicationDefault()
+        .createScoped("https://www.googleapis.com/auth/cloud-platform")
+  }
+}