fix: migrate web server serialization from Gson to kotlinx-serialization

gradle/libs.versions.toml

@@ -93,6 +93,7 @@ kotlinx-coroutines-test = { module = "org.jetbrains.kotlinx:kotlinx-coroutines-t
 kotlinx-datetime = { module = "org.jetbrains.kotlinx:kotlinx-datetime", version.ref = "kotlinx-datetime" }
 kotlinx-serialization = { module = "org.jetbrains.kotlinx:kotlinx-serialization-json", version.ref = "kotlinx-serialization" }
 ktor-serialization-gson = { module = "io.ktor:ktor-serialization-gson", version.ref = "ktor" }
+ktor-serialization-kotlinx-json = { module = "io.ktor:ktor-serialization-kotlinx-json", version.ref = "ktor" }
 ktor-server-call-logging = { module = "io.ktor:ktor-server-call-logging", version.ref = "ktor" }
 ktor-server-content-negotiation = { module = "io.ktor:ktor-server-content-negotiation", version.ref = "ktor" }
 ktor-server-core = { module = "io.ktor:ktor-server-core", version.ref = "ktor" }

webserver/build.gradle.kts

@@ -16,6 +16,7 @@
 
 plugins {
   kotlin("jvm")
+  kotlin("plugin.serialization")
   id("application")
   id("java-library")
   id("maven-publish")
@@ -44,13 +45,14 @@ sourceSets {
 dependencies {
   implementation(project(":google-adk-kotlin-core"))
   implementation(libs.kotlinx.datetime)
+  implementation(libs.kotlinx.serialization)
 
   implementation(libs.graphviz.java)
 
   implementation(libs.opentelemetry.api)
   implementation(libs.opentelemetry.sdk)
 
-  implementation(libs.ktor.serialization.gson)
+  implementation(libs.ktor.serialization.kotlinx.json)
   implementation(libs.ktor.server.call.logging)
   implementation(libs.ktor.server.content.negotiation)
   implementation(libs.ktor.server.core)

webserver/src/jvmMain/kotlin/com/google/adk/kt/webserver/AdkWebServer.kt

@@ -16,8 +16,10 @@
 
 package com.google.adk.kt.webserver
 
+import com.google.adk.kt.annotations.FrameworkInternalApi
 import com.google.adk.kt.artifacts.ArtifactService
 import com.google.adk.kt.runners.Runner
+import com.google.adk.kt.serialization.adkJson
 import com.google.adk.kt.sessions.SessionService
 import com.google.adk.kt.telemetry.TelemetryConfig
 import com.google.adk.kt.webserver.AdkWebServer.StatusAwareLogger
@@ -32,10 +34,7 @@ import com.google.adk.kt.webserver.routes.sessionRoutes
 import com.google.adk.kt.webserver.routes.staticRoutes
 import com.google.adk.kt.webserver.telemetry.ApiServerSpanExporter
 import com.google.adk.kt.webserver.telemetry.OpenTelemetryConfig
-import com.google.gson.TypeAdapter
-import com.google.gson.stream.JsonReader
-import com.google.gson.stream.JsonWriter
-import io.ktor.serialization.gson.gson
+import io.ktor.serialization.kotlinx.json.json
 import io.ktor.server.application.Application
 import io.ktor.server.application.call
 import io.ktor.server.application.install
@@ -49,7 +48,6 @@ import io.ktor.server.request.uri
 import io.ktor.server.response.respondText
 import io.ktor.server.routing.get
 import io.ktor.server.routing.routing
-import kotlinx.datetime.Instant
 import org.slf4j.Logger
 import org.slf4j.LoggerFactory
 import org.slf4j.event.Level
@@ -123,24 +121,6 @@ class AdkWebServer(
     logger.info("Ktor server stopped")
   }
 
-  class InstantTypeAdapter : TypeAdapter<Instant>() {
-    override fun write(out: JsonWriter, value: Instant?) {
-      if (value == null) {
-        out.nullValue()
-      } else {
-        out.value(value.toEpochMilliseconds())
-      }
-    }
-
-    override fun read(reader: JsonReader): Instant? {
-      if (reader.peek() == com.google.gson.stream.JsonToken.NULL) {
-        reader.nextNull()
-        return null
-      }
-      return Instant.fromEpochMilliseconds(reader.nextLong())
-    }
-  }
-
   public class StatusAwareLogger(private val delegate: Logger) : Logger by delegate {
     override fun info(msg: String?) {
       if (msg != null && msg.contains("Status: 5")) {
@@ -152,6 +132,7 @@ class AdkWebServer(
   }
 }
 
+@OptIn(FrameworkInternalApi::class)
 fun Application.adkModule(
   sessionService: SessionService,
   artifactService: ArtifactService,
@@ -169,12 +150,7 @@ fun Application.adkModule(
       "Status: $status, HTTP method: $httpMethod, URI: $uri"
     }
   }
-  install(ContentNegotiation) {
-    gson {
-      setPrettyPrinting()
-      registerTypeAdapter(Instant::class.java, AdkWebServer.InstantTypeAdapter())
-    }
-  }
+  install(ContentNegotiation) { json(adkJson) }
 
   val otelConfig = OpenTelemetryConfig(apiServerSpanExporter)
   val sdkTracerProvider = otelConfig.sdkTracerProvider()

webserver/src/jvmMain/kotlin/com/google/adk/kt/webserver/models/ApiModels.kt

@@ -17,25 +17,30 @@
 package com.google.adk.kt.webserver.models
 
 import com.google.adk.kt.types.Content
+import kotlinx.serialization.Contextual
+import kotlinx.serialization.Serializable
 
+@Serializable
 data class AgentRunRequest(
   val appName: String,
   val userId: String,
   val sessionId: String? = null,
   val newMessage: Content? = null,
   val streaming: Boolean = false,
-  val stateDelta: Map<String, Any>? = null,
+  val stateDelta: Map<String, @Contextual Any>? = null,
   val invocationId: String? = null,
 )
 
+@Serializable
 data class RunRequest(val agentId: String, val input: String, val sessionId: String? = null)
 
-data class RunResponse(val output: String, val sessionId: String)
+@Serializable data class RunResponse(val output: String, val sessionId: String)
 
-data class TurnModel(val role: String, val content: String)
+@Serializable data class TurnModel(val role: String, val content: String)
 
-data class SessionModel(val sessionId: String, val turnHistory: List<TurnModel>)
+@Serializable data class SessionModel(val sessionId: String, val turnHistory: List<TurnModel>)
 
+@Serializable
 data class ErrorResponse(val error: String, val message: String, val details: String? = null)
 
 /**
@@ -45,13 +50,14 @@ data class ErrorResponse(val error: String, val message: String, val details: St
  * @property content The JSON string or text content of the event.
  * @property timestamp The ISO-8601 timestamp of the event.
  */
-data class SseModel(val type: String, val content: String, val timestamp: String)
+@Serializable data class SseModel(val type: String, val content: String, val timestamp: String)
 
+@Serializable
 data class SessionDto(
   val id: String?,
   val appName: String,
   val userId: String,
-  val state: Map<String, Any>?,
+  val state: Map<String, @Contextual Any>?,
   val events: List<com.google.adk.kt.events.Event>?,
   val lastUpdateTime: Long?,
 )

webserver/src/jvmMain/kotlin/com/google/adk/kt/webserver/routes/RunRoutes.kt

@@ -18,12 +18,13 @@ package com.google.adk.kt.webserver.routes
 
 import com.google.adk.kt.agents.RunConfig
 import com.google.adk.kt.agents.StreamingMode
+import com.google.adk.kt.annotations.FrameworkInternalApi
 import com.google.adk.kt.artifacts.ArtifactService
 import com.google.adk.kt.runners.InMemoryRunner
+import com.google.adk.kt.serialization.adkJson
 import com.google.adk.kt.sessions.SessionService
 import com.google.adk.kt.webserver.loaders.AgentLoader
 import com.google.adk.kt.webserver.models.AgentRunRequest
-import com.google.gson.Gson
 import io.ktor.http.ContentType
 import io.ktor.http.HttpStatusCode
 import io.ktor.server.application.call
@@ -37,7 +38,9 @@ import io.ktor.utils.io.writeStringUtf8
 import java.util.UUID
 import kotlinx.coroutines.flow.collect
 import kotlinx.coroutines.flow.toList
+import kotlinx.serialization.encodeToString
 
+@OptIn(FrameworkInternalApi::class)
 fun Route.runRoutes(
   agentLoader: AgentLoader,
   sessionService: SessionService,
@@ -111,7 +114,7 @@ fun Route.runRoutes(
           runConfig,
         )
         .collect { event ->
-          val data = Gson().toJson(event)
+          val data = adkJson.encodeToString(event)
           writeStringUtf8("data: $data\n\n")
           flush()
         }

webserver/src/jvmTest/kotlin/com/google/adk/kt/webserver/AdkWebServerTest.kt

@@ -179,8 +179,8 @@ class AdkWebServerTest {
     val response = client.get("/api/test-serialize")
     assertThat(response.status).isEqualTo(HttpStatusCode.OK)
     val body = response.bodyAsText()
-    assertThat(body).contains("\"output\": \"Ok output\"")
-    assertThat(body).contains("\"sessionId\": \"test-session\"")
+    assertThat(body).contains("\"output\":\"Ok output\"")
+    assertThat(body).contains("\"sessionId\":\"test-session\"")
   }
 
   @Test
@@ -196,8 +196,10 @@ class AdkWebServerTest {
       }
     assertThat(response.status).isEqualTo(HttpStatusCode.OK)
     val body = response.bodyAsText()
-    println("RESPONSE BODY: $body")
-    assertThat(body).isNotEmpty()
+    // adkJson has encodeDefaults=false, so default-false partial/interrupted are omitted.
+    assertThat(body).contains("\"turnComplete\":true")
+    assertThat(body).doesNotContain("\"partial\"")
+    assertThat(body).doesNotContain("\"interrupted\"")
   }
 
   @Test
@@ -213,5 +215,11 @@ class AdkWebServerTest {
       }
     assertThat(response.status).isEqualTo(HttpStatusCode.OK)
     assertThat(response.headers["Content-Type"]).contains("text/event-stream")
+    // The SSE stream serializes events with adkJson too, so the shape matches /run.
+    val body = response.bodyAsText()
+    assertThat(body).contains("data: ")
+    assertThat(body).contains("\"turnComplete\":true")
+    assertThat(body).doesNotContain("\"partial\"")
+    assertThat(body).doesNotContain("\"interrupted\"")
   }
 }

webserver/src/jvmTest/kotlin/com/google/adk/kt/webserver/routes/AppRoutesTest.kt

@@ -17,12 +17,14 @@
 package com.google.adk.kt.webserver.routes
 
 import com.google.adk.kt.agents.BaseAgent
+import com.google.adk.kt.annotations.FrameworkInternalApi
+import com.google.adk.kt.serialization.adkJson
 import com.google.adk.kt.webserver.loaders.AgentLoader
 import com.google.common.truth.Truth.assertThat
 import io.ktor.client.request.get
 import io.ktor.client.statement.bodyAsText
 import io.ktor.http.HttpStatusCode
-import io.ktor.serialization.gson.gson
+import io.ktor.serialization.kotlinx.json.json
 import io.ktor.server.application.install
 import io.ktor.server.plugins.contentnegotiation.ContentNegotiation
 import io.ktor.server.routing.routing
@@ -31,6 +33,7 @@ import org.junit.Test
 import org.junit.runner.RunWith
 import org.junit.runners.JUnit4
 
+@OptIn(FrameworkInternalApi::class)
 @RunWith(JUnit4::class)
 class AppRoutesTest {
 
@@ -44,7 +47,7 @@ class AppRoutesTest {
   fun listApps_returnsAppList() = testApplication {
     val fakeLoader = FakeAgentLoader(agentList = listOf("app1", "app2"))
     application {
-      install(ContentNegotiation) { gson { setPrettyPrinting() } }
+      install(ContentNegotiation) { json(adkJson) }
       routing { appRoutes(fakeLoader) }
     }
 
@@ -60,7 +63,7 @@ class AppRoutesTest {
   fun listApps_empty_returnsEmptyList() = testApplication {
     val fakeLoader = FakeAgentLoader(agentList = emptyList())
     application {
-      install(ContentNegotiation) { gson { setPrettyPrinting() } }
+      install(ContentNegotiation) { json(adkJson) }
       routing { appRoutes(fakeLoader) }
     }