[WIP] StreamableHttpServerTransport implementation (#449)

# [WIP] StreamableHttpServerTransport implementation

- Implement StreamableHttpServerTransport for MCP spec
- Support both SSE streaming and direct HTTP JSON responses
- Add event store interface for resumability

> [!IMPORTANT]  
> The work is in progress. StreamableHttpServerTransport is not
production-ready yet!!!

Based on #235 

## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->

## Breaking Changes
None

## Types of changes
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed

kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/shared/Protocol.kt

@@ -4,6 +4,7 @@ import io.github.oshai.kotlinlogging.KotlinLogging
 import io.modelcontextprotocol.kotlin.sdk.types.CancelledNotification
 import io.modelcontextprotocol.kotlin.sdk.types.CancelledNotificationParams
 import io.modelcontextprotocol.kotlin.sdk.types.EmptyResult
+import io.modelcontextprotocol.kotlin.sdk.types.JSONRPCEmptyMessage
 import io.modelcontextprotocol.kotlin.sdk.types.JSONRPCError
 import io.modelcontextprotocol.kotlin.sdk.types.JSONRPCNotification
 import io.modelcontextprotocol.kotlin.sdk.types.JSONRPCRequest
@@ -249,6 +250,7 @@ public abstract class Protocol(@PublishedApi internal val options: ProtocolOptio
                 is JSONRPCRequest -> onRequest(message)
                 is JSONRPCNotification -> onNotification(message)
                 is JSONRPCError -> onResponse(null, message)
+                is JSONRPCEmptyMessage -> Unit
             }
         }
 

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

@@ -1,7 +1,5 @@
 package io.modelcontextprotocol.kotlin.sdk.types
 
-import io.modelcontextprotocol.kotlin.sdk.types.Icon.Theme.Dark
-import io.modelcontextprotocol.kotlin.sdk.types.Icon.Theme.Light
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.JsonObject
@@ -12,6 +10,8 @@ import kotlinx.serialization.json.JsonObject
 
 public const val LATEST_PROTOCOL_VERSION: String = "2025-06-18"
 
+public const val DEFAULT_NEGOTIATED_PROTOCOL_VERSION: String = "2025-03-26"
+
 public val SUPPORTED_PROTOCOL_VERSIONS: List<String> = listOf(
     LATEST_PROTOCOL_VERSION,
     "2025-03-26",

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

@@ -85,6 +85,11 @@ public sealed interface JSONRPCMessage {
     public val jsonrpc: String
 }
 
+@Serializable
+public data object JSONRPCEmptyMessage : JSONRPCMessage {
+    override val jsonrpc: String = JSONRPC_VERSION
+}
+
 // ============================================================================
 // JSONRPCRequest
 // ============================================================================
@@ -197,7 +202,7 @@ public data class JSONRPCResponse(val id: RequestId, val result: RequestResult =
  * @property error Details about the error that occurred, including error code and message.
  */
 @Serializable
-public data class JSONRPCError(val id: RequestId, val error: RPCError) : JSONRPCMessage {
+public data class JSONRPCError(val id: RequestId?, val error: RPCError) : JSONRPCMessage {
     @EncodeDefault
     override val jsonrpc: String = JSONRPC_VERSION
 }

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

@@ -384,6 +384,7 @@ internal object JSONRPCMessagePolymorphicSerializer :
             "result" in jsonObject -> JSONRPCResponse.serializer()
             "method" in jsonObject && "id" in jsonObject -> JSONRPCRequest.serializer()
             "method" in jsonObject -> JSONRPCNotification.serializer()
+            jsonObject.isEmpty() || jsonObject.keys == setOf("jsonrpc") -> JSONRPCEmptyMessage.serializer()
             else -> throw SerializationException("Invalid JSONRPCMessage type: ${jsonObject.keys}")
         }
     }

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

@@ -430,4 +430,16 @@ class JsonRpcTest {
         request.method shouldBe "notifications/log"
         request.params shouldBeSameInstanceAs params
     }
+
+    @Test
+    fun `should deserialize JSONRPCEmptyMessage`() {
+        val json = """
+            {
+              "jsonrpc": "2.0"
+            }
+        """.trimIndent()
+
+        val message = McpJson.decodeFromString<JSONRPCMessage>(json)
+        message shouldBeSameInstanceAs JSONRPCEmptyMessage
+    }
 }