diff --git a/core/build.gradle.kts b/core/build.gradle.kts
index 844671c5..f145bf05 100644
--- a/core/build.gradle.kts
+++ b/core/build.gradle.kts
@@ -16,6 +16,7 @@
 
 plugins {
   kotlin("multiplatform")
+  kotlin("plugin.serialization")
   id("com.android.library")
   alias(libs.plugins.ksp)
   id("maven-publish")
@@ -30,6 +31,7 @@ kotlin {
       dependencies {
         implementation(libs.kotlinx.coroutines.core)
         implementation(libs.kotlinx.datetime)
+        implementation(libs.kotlinx.serialization)
       }
     }
     val commonTest by getting {
diff --git a/core/src/androidUnitTest/kotlin/com/google/adk/kt/JsonSerializationFallbackAndroidTest.kt b/core/src/androidUnitTest/kotlin/com/google/adk/kt/JsonSerializationFallbackAndroidTest.kt
new file mode 100644
index 00000000..aeae1dab
--- /dev/null
+++ b/core/src/androidUnitTest/kotlin/com/google/adk/kt/JsonSerializationFallbackAndroidTest.kt
@@ -0,0 +1,40 @@
+/*
+ * 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
+
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import com.google.adk.kt.serialization.Json
+import kotlin.test.assertEquals
+import kotlinx.serialization.json.JsonPrimitive
+import org.junit.Test
+import org.junit.runner.RunWith
+
+/**
+ * Android behavior of the non-`@Serializable` fallback in `Json.serializeToJsonElement` (org.json).
+ */
+@RunWith(AndroidJUnit4::class)
+class JsonSerializationFallbackAndroidTest {
+
+  // Not `@Serializable`, so the platform fallback (org.json on Android) is used.
+  private data class Unannotated(val v: Int)
+
+  @Test
+  fun nonSerializableType_stringifiesViaOrgJson() {
+    // org.json cannot reflect, so the value is stringified via `toString()`.
+    assertEquals(JsonPrimitive("Unannotated(v=7)"), Json.serializeToJsonElement(Unannotated(7)))
+  }
+}
diff --git a/core/src/commonMain/kotlin/com/google/adk/kt/serialization/Json.kt b/core/src/commonMain/kotlin/com/google/adk/kt/serialization/Json.kt
index 15968696..36690c3b 100644
--- a/core/src/commonMain/kotlin/com/google/adk/kt/serialization/Json.kt
+++ b/core/src/commonMain/kotlin/com/google/adk/kt/serialization/Json.kt
@@ -16,6 +16,11 @@
 
 package com.google.adk.kt.serialization
 
+import kotlin.reflect.KClass
+import kotlinx.serialization.InternalSerializationApi
+import kotlinx.serialization.json.JsonElement
+import kotlinx.serialization.serializerOrNull
+
 /** Platform-independent utility for JSON serialization. */
 interface Json {
   /** Serializes an object to a JSON string. */
@@ -24,7 +29,23 @@ interface Json {
   /** Parses a JSON string to a map. */
   fun fromJsonToMap(json: String): Map<String, Any?>
 
-  companion object : Json by getJson()
+  companion object : Json by getJson() {
+    private val KMP_JSON = kotlinx.serialization.json.Json { explicitNulls = false }
+
+    /**
+     * Converts [value] to a [JsonElement]: kotlinx.serialization for `@Serializable` types,
+     * otherwise the platform [Json] fallback (best-effort, not round-trippable).
+     */
+    @OptIn(InternalSerializationApi::class)
+    internal fun serializeToJsonElement(value: Any): JsonElement {
+      @Suppress("UNCHECKED_CAST") val serializer = (value::class as KClass<Any>).serializerOrNull()
+      return if (serializer != null) {
+        KMP_JSON.encodeToJsonElement(serializer, value)
+      } else {
+        KMP_JSON.parseToJsonElement(toJsonString(value))
+      }
+    }
+  }
 }
 
 internal expect fun getJson(): Json
diff --git a/core/src/commonTest/kotlin/com/google/adk/kt/serialization/SerializeToJsonElementTest.kt b/core/src/commonTest/kotlin/com/google/adk/kt/serialization/SerializeToJsonElementTest.kt
new file mode 100644
index 00000000..d6244b89
--- /dev/null
+++ b/core/src/commonTest/kotlin/com/google/adk/kt/serialization/SerializeToJsonElementTest.kt
@@ -0,0 +1,45 @@
+/*
+ * 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.serialization
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.Json as KotlinxJson
+
+class SerializeToJsonElementTest {
+
+  @Serializable private data class Point(val x: Int, val y: Int, val label: String? = null)
+
+  @Test
+  fun serializableType_emitsJsonObject() {
+    // `@Serializable` -> structured JSON object via kotlinx.serialization (JVM and Android).
+    assertEquals(
+      KotlinxJson.parseToJsonElement("""{"x":1,"y":2}"""),
+      Json.serializeToJsonElement(Point(1, 2)),
+    )
+  }
+
+  @Test
+  fun serializableType_omitsNullProperties() {
+    // `explicitNulls = false`: a null property (`label`) is dropped from the encoded object.
+    assertEquals(
+      KotlinxJson.parseToJsonElement("""{"x":1,"y":2}"""),
+      Json.serializeToJsonElement(Point(1, 2, label = null)),
+    )
+  }
+}
diff --git a/core/src/jvmTest/kotlin/com/google/adk/kt/JsonSerializationFallbackJvmTest.kt b/core/src/jvmTest/kotlin/com/google/adk/kt/JsonSerializationFallbackJvmTest.kt
new file mode 100644
index 00000000..9bf424e2
--- /dev/null
+++ b/core/src/jvmTest/kotlin/com/google/adk/kt/JsonSerializationFallbackJvmTest.kt
@@ -0,0 +1,38 @@
+/*
+ * 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
+
+import com.google.adk.kt.serialization.Json
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlinx.serialization.json.Json as KotlinxJson
+
+/** JVM behavior of the non-`@Serializable` fallback in `Json.serializeToJsonElement` (Gson). */
+class JsonSerializationFallbackJvmTest {
+
+  // Not `@Serializable`, so the platform fallback (Gson on JVM) is used.
+  private data class Unannotated(val v: Int)
+
+  @Test
+  fun nonSerializableType_reflectsToJsonObjectViaGson() {
+    // On JVM the Gson-backed fallback reflects the data class into a structured JSON object.
+    assertEquals(
+      KotlinxJson.parseToJsonElement("""{"v":7}"""),
+      Json.serializeToJsonElement(Unannotated(7)),
+    )
+  }
+}