fix(core): write null Task.ttl instead of omitting it

kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types/notification.kt

@@ -4,6 +4,7 @@ package io.modelcontextprotocol.kotlin.sdk.types
 
 import kotlinx.serialization.EncodeDefault
 import kotlinx.serialization.ExperimentalSerializationApi
+import kotlinx.serialization.KeepGeneratedSerializer
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.JsonElement
@@ -413,7 +414,8 @@ public data class TaskStatusNotification(override val params: TaskStatusNotifica
  * @property pollInterval Suggested polling interval in milliseconds.
  * @property meta Optional metadata for this notification.
  */
-@Serializable
+@KeepGeneratedSerializer
+@Serializable(with = TaskStatusNotificationParamsSerializer::class)
 public data class TaskStatusNotificationParams(
     override val taskId: String,
     override val status: TaskStatus,

kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types/serializers.kt

@@ -16,8 +16,10 @@ import kotlinx.serialization.json.JsonContentPolymorphicSerializer
 import kotlinx.serialization.json.JsonDecoder
 import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonEncoder
+import kotlinx.serialization.json.JsonNull
 import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.JsonPrimitive
+import kotlinx.serialization.json.JsonTransformingSerializer
 import kotlinx.serialization.json.contentOrNull
 import kotlinx.serialization.json.jsonObject
 import kotlinx.serialization.json.jsonPrimitive
@@ -235,6 +237,29 @@ internal object ResourceContentsPolymorphicSerializer :
     }
 }
 
+// ============================================================================
+// Task Serializers
+// ============================================================================
+
+/**
+ * Writes `"ttl": null` for a task with unlimited retention: the schema requires the key,
+ * but [McpJson] drops null properties before a property-level serializer could write it.
+ */
+internal open class TaskFieldsSerializer<T : TaskFields>(serializer: KSerializer<T>) :
+    JsonTransformingSerializer<T>(serializer) {
+    override fun transformSerialize(element: JsonElement): JsonElement {
+        val fields = element.jsonObject
+        return if ("ttl" in fields) fields else JsonObject(fields + ("ttl" to JsonNull))
+    }
+}
+
+internal object TaskSerializer : TaskFieldsSerializer<Task>(Task.generatedSerializer())
+
+internal object GetTaskResultSerializer : TaskFieldsSerializer<GetTaskResult>(GetTaskResult.generatedSerializer())
+
+internal object TaskStatusNotificationParamsSerializer :
+    TaskFieldsSerializer<TaskStatusNotificationParams>(TaskStatusNotificationParams.generatedSerializer())
+
 // ============================================================================
 // Request Serializers
 // ============================================================================

kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types/tasks.kt

@@ -4,6 +4,7 @@ package io.modelcontextprotocol.kotlin.sdk.types
 
 import kotlinx.serialization.EncodeDefault
 import kotlinx.serialization.ExperimentalSerializationApi
+import kotlinx.serialization.KeepGeneratedSerializer
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.JsonElement
@@ -54,7 +55,8 @@ public sealed interface TaskFields {
  * @property ttl Actual retention duration from creation in milliseconds, null for unlimited.
  * @property pollInterval Suggested polling interval in milliseconds.
  */
-@Serializable
+@KeepGeneratedSerializer
+@Serializable(with = TaskSerializer::class)
 public data class Task(
     override val taskId: String,
     override val status: TaskStatus,
@@ -176,7 +178,8 @@ public data class GetTaskRequestParams(
  * @property pollInterval Suggested polling interval in milliseconds.
  * @property meta Optional metadata for this response.
  */
-@Serializable
+@KeepGeneratedSerializer
+@Serializable(with = GetTaskResultSerializer::class)
 public data class GetTaskResult(
     override val taskId: String,
     override val status: TaskStatus,

kotlin-sdk-core/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/types/NotificationTest.kt

@@ -1,5 +1,6 @@
 package io.modelcontextprotocol.kotlin.sdk.types
 
+import io.kotest.assertions.json.shouldEqualJson
 import io.kotest.assertions.withClue
 import io.kotest.matchers.shouldBe
 import io.kotest.matchers.types.shouldBeInstanceOf
@@ -240,13 +241,41 @@ class NotificationTest {
                 "taskId": "task-2",
                 "status": "completed",
                 "createdAt": "2025-01-01T00:00:00Z",
-                "lastUpdatedAt": "2025-01-01T00:02:00Z"
+                "lastUpdatedAt": "2025-01-01T00:02:00Z",
+                "ttl": null
               }
             }
             """.trimIndent(),
         )
     }
 
+    @Test
+    fun `should keep null ttl when converting TaskStatusNotification to JSONRPCNotification`() {
+        val notification = TaskStatusNotification(
+            TaskStatusNotificationParams(
+                taskId = "task-3",
+                status = TaskStatus.Working,
+                createdAt = "2025-01-01T00:00:00Z",
+                lastUpdatedAt = "2025-01-01T00:00:00Z",
+                ttl = null,
+            ),
+        )
+
+        McpJson.encodeToString(notification.toJSON()) shouldEqualJson """
+            {
+              "jsonrpc": "2.0",
+              "method": "notifications/tasks/status",
+              "params": {
+                "taskId": "task-3",
+                "status": "working",
+                "createdAt": "2025-01-01T00:00:00Z",
+                "lastUpdatedAt": "2025-01-01T00:00:00Z",
+                "ttl": null
+              }
+            }
+        """.trimIndent()
+    }
+
     @Test
     fun `should deserialize TaskStatusNotification without params`() {
         val json = """

kotlin-sdk-core/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/types/TasksTest.kt

@@ -137,6 +137,45 @@ class TasksTest {
         verifySerialization<ServerResult>(result, McpJson, json)
     }
 
+    @Test
+    fun `should serialize GetTaskResult with minimal fields`() {
+        val result = GetTaskResult(
+            taskId = "task-21",
+            status = TaskStatus.Working,
+            createdAt = "2025-01-01T00:00:00Z",
+            lastUpdatedAt = "2025-01-01T00:00:00Z",
+            ttl = null,
+        )
+        val json = """
+            {
+              "taskId": "task-21",
+              "status": "working",
+              "createdAt": "2025-01-01T00:00:00Z",
+              "lastUpdatedAt": "2025-01-01T00:00:00Z",
+              "ttl": null
+            }
+        """.trimIndent()
+
+        verifySerialization<ClientResult>(result, McpJson, json)
+        verifySerialization<ServerResult>(result, McpJson, json)
+    }
+
+    @Test
+    fun `should decode Task without ttl as unlimited`() {
+        val task = McpJson.decodeFromString<Task>(
+            """
+            {
+              "taskId": "task-3",
+              "status": "working",
+              "createdAt": "2025-01-01T00:00:00Z",
+              "lastUpdatedAt": "2025-01-01T00:00:00Z"
+            }
+            """.trimIndent(),
+        )
+
+        task.ttl.shouldBeNull()
+    }
+
     @Test
     fun `should preserve arbitrary payload fields in GetTaskPayloadResult`() {
         val json = """
@@ -189,7 +228,8 @@ class TasksTest {
                   "taskId": "task-1",
                   "status": "working",
                   "createdAt": "2025-01-01T00:00:00Z",
-                  "lastUpdatedAt": "2025-01-01T00:00:00Z"
+                  "lastUpdatedAt": "2025-01-01T00:00:00Z",
+                  "ttl": null
                 },
                 {
                   "taskId": "task-2",