Fix: Handle logging/setLevel request for server (#358)

Fix #293 

Also fixed a bug with initialization: properties must be declared before
the init block; otherwise, the initialization order is violated and an
NPE will occur in the init block.

## Motivation and Context
After initialization, MCP clients send a `logging/setLevel` request if
the logging capability is enabled. This request was previously not
handled, leading to an error that blocked the use of the MCP server in
the inspector and other clients.


## How Has This Been Tested?
Added integration test

## Breaking Changes
None

## Types of changes
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] 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-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/ServerSession.kt

@@ -16,6 +16,7 @@ import io.modelcontextprotocol.kotlin.sdk.InitializedNotification
 import io.modelcontextprotocol.kotlin.sdk.LATEST_PROTOCOL_VERSION
 import io.modelcontextprotocol.kotlin.sdk.ListRootsRequest
 import io.modelcontextprotocol.kotlin.sdk.ListRootsResult
+import io.modelcontextprotocol.kotlin.sdk.LoggingLevel
 import io.modelcontextprotocol.kotlin.sdk.LoggingMessageNotification
 import io.modelcontextprotocol.kotlin.sdk.Method
 import io.modelcontextprotocol.kotlin.sdk.Method.Defined
@@ -27,6 +28,8 @@ import io.modelcontextprotocol.kotlin.sdk.SUPPORTED_PROTOCOL_VERSIONS
 import io.modelcontextprotocol.kotlin.sdk.ToolListChangedNotification
 import io.modelcontextprotocol.kotlin.sdk.shared.Protocol
 import io.modelcontextprotocol.kotlin.sdk.shared.RequestOptions
+import kotlinx.atomicfu.AtomicRef
+import kotlinx.atomicfu.atomic
 import kotlinx.coroutines.CompletableDeferred
 import kotlinx.serialization.json.JsonObject
 
@@ -43,22 +46,6 @@ public open class ServerSession(
     @Suppress("ktlint:standard:backing-property-naming")
     private var _onClose: () -> Unit = {}
 
-    init {
-        // Core protocol handlers
-        setRequestHandler<InitializeRequest>(Method.Defined.Initialize) { request, _ ->
-            handleInitialize(request)
-        }
-        setNotificationHandler<InitializedNotification>(Method.Defined.NotificationsInitialized) {
-            _onInitialized()
-            CompletableDeferred(Unit)
-        }
-    }
-
-    /**
-     * The capabilities supported by the server, related to the session.
-     */
-    private val serverCapabilities = options.capabilities
-
     /**
      * The client's reported capabilities after initialization.
      */
@@ -71,6 +58,37 @@ public open class ServerSession(
     public var clientVersion: Implementation? = null
         private set
 
+    /**
+     * The capabilities supported by the server, related to the session.
+     */
+    private val serverCapabilities = options.capabilities
+
+    /**
+     * The current logging level set by the client.
+     * When null, all messages are sent (no filtering).
+     */
+    private val currentLoggingLevel: AtomicRef<LoggingLevel?> = atomic(null)
+
+    init {
+        // Core protocol handlers
+        setRequestHandler<InitializeRequest>(Defined.Initialize) { request, _ ->
+            handleInitialize(request)
+        }
+        setNotificationHandler<InitializedNotification>(Defined.NotificationsInitialized) {
+            _onInitialized()
+            CompletableDeferred(Unit)
+        }
+
+        // Logging level handler
+        if (options.capabilities.logging != null) {
+            setRequestHandler<LoggingMessageNotification.SetLevelRequest>(Defined.LoggingSetLevel) { request, _ ->
+                currentLoggingLevel.value = request.level
+                logger.debug { "Logging level set to: ${request.level}" }
+                EmptyRequestResult()
+            }
+        }
+    }
+
     /**
      * Registers a callback to be invoked when the server has completed initialization.
      */
@@ -160,12 +178,20 @@ public open class ServerSession(
 
     /**
      * Sends a logging message notification to the client.
+     * Messages are filtered based on the current logging level set by the client.
+     * If no logging level is set, all messages are sent.
      *
      * @param notification The logging message notification.
      */
     public suspend fun sendLoggingMessage(notification: LoggingMessageNotification) {
-        logger.trace { "Sending logging message: ${notification.params.data}" }
-        notification(notification)
+        if (serverCapabilities.logging != null) {
+            if (isMessageAccepted(notification.params.level)) {
+                logger.trace { "Sending logging message: ${notification.params.data}" }
+                notification(notification)
+            } else {
+                logger.trace { "Filtering out logging message with level ${notification.params.level}" }
+            }
+        }
     }
 
     /**
@@ -318,6 +344,7 @@ public open class ServerSession(
 
             Defined.LoggingSetLevel -> {
                 if (serverCapabilities.logging == null) {
+                    logger.error { "Server does not support logging (required for $method)" }
                     throw IllegalStateException("Server does not support logging (required for $method)")
                 }
             }
@@ -381,4 +408,24 @@ public open class ServerSession(
             instructions = instructions,
         )
     }
+
+    /**
+     * Checks if a message with the given level should be ignored based on the current logging level.
+     *
+     * @param level The level of the message to check.
+     * @return true if the message should be ignored (filtered out), false otherwise.
+     */
+    private fun isMessageIgnored(level: LoggingLevel): Boolean {
+        val current = currentLoggingLevel.value ?: return false // If no level is set, don't filter
+
+        return level.ordinal < current.ordinal
+    }
+
+    /**
+     * Checks if a message with the given level should be accepted based on the current logging level.
+     *
+     * @param level The level of the message to check.
+     * @return true if the message should be accepted (not filtered out), false otherwise.
+     */
+    private fun isMessageAccepted(level: LoggingLevel): Boolean = !isMessageIgnored(level)
 }