Fix deserialization

Author: nerzhulart

kotlin/kotlin-acp/src/commonMain/kotlin/io/agentclientprotocol/rpc/JsonRpc.kt

@@ -4,9 +4,7 @@ package io.agentclientprotocol.rpc
 
 import kotlinx.serialization.ExperimentalSerializationApi
 import kotlinx.serialization.Serializable
-import kotlinx.serialization.json.ClassDiscriminatorMode
-import kotlinx.serialization.json.Json
-import kotlinx.serialization.json.JsonElement
+import kotlinx.serialization.json.*
 import kotlin.jvm.JvmInline
 
 /**
@@ -85,7 +83,30 @@ public val ACPJson: Json by lazy {
         ignoreUnknownKeys = true
         encodeDefaults = true
         isLenient = true
-        classDiscriminatorMode = ClassDiscriminatorMode.POLYMORPHIC
         explicitNulls = false
     }
+}
+
+/**
+ * Helper function to decode JSON-RPC messages based on field presence.
+ * JSON-RPC 2.0 spec distinguishes message types by which fields are present:
+ * - Response: has "id" and ("result" or "error")
+ * - Request: has "id" and "method"
+ * - Notification: has "method" but no "id"
+ */
+public fun decodeJsonRpcMessage(jsonString: String): JsonRpcMessage {
+    val element = ACPJson.parseToJsonElement(jsonString)
+    require(element is JsonObject) { "Expected JSON object" }
+
+    val hasId = element.containsKey("id")
+    val hasMethod = element.containsKey("method")
+    val hasResult = element.containsKey("result")
+    val hasError = element.containsKey("error")
+
+    return when {
+        hasId && (hasResult || hasError) -> ACPJson.decodeFromJsonElement(JsonRpcResponse.serializer(), element)
+        hasId && hasMethod -> ACPJson.decodeFromJsonElement(JsonRpcRequest.serializer(), element)
+        hasMethod -> ACPJson.decodeFromJsonElement(JsonRpcNotification.serializer(), element)
+        else -> error("Unable to determine JsonRpcMessage type from JSON structure")
+    }
 }

kotlin/kotlin-acp/src/commonMain/kotlin/io/agentclientprotocol/transport/StdioTransport.kt

@@ -2,6 +2,7 @@ package io.agentclientprotocol.transport
 
 import io.agentclientprotocol.rpc.ACPJson
 import io.agentclientprotocol.rpc.JsonRpcMessage
+import io.agentclientprotocol.rpc.decodeJsonRpcMessage
 import io.agentclientprotocol.util.DispatcherIO
 import io.github.oshai.kotlinlogging.KotlinLogging
 import kotlinx.atomicfu.atomic
@@ -57,7 +58,7 @@ public class StdioTransport(
                     }
 
                     val jsonRpcMessage = try {
-                        ACPJson.decodeFromString<JsonRpcMessage>(line)
+                        decodeJsonRpcMessage(line)
                     } catch (t: Throwable) {
                         logger.error(t) { "Failed to decode JSON message: $line" }
                         continue

kotlin/kotlin-acp/src/commonTest/kotlin/io/agentclientprotocol/mock/TestTransport.kt

@@ -1,7 +1,7 @@
 package io.agentclientprotocol.mock
 
-import io.agentclientprotocol.rpc.ACPJson
 import io.agentclientprotocol.rpc.JsonRpcMessage
+import io.agentclientprotocol.rpc.decodeJsonRpcMessage
 import io.agentclientprotocol.transport.Transport
 import kotlinx.coroutines.channels.Channel
 import kotlinx.coroutines.channels.ReceiveChannel
@@ -80,7 +80,7 @@ class TestTransport : Transport {
      * Parses the JSON string and determines the message type based on the presence of fields.
      */
     public fun receiveMessage(jsonMessage: String) {
-        val message = ACPJson.decodeFromString<JsonRpcMessage>(jsonMessage)
+        val message = decodeJsonRpcMessage(jsonMessage)
         receiveMessage(message)
     }
 }