Adds optional _meta field to Tool

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

@@ -1242,7 +1242,12 @@ public data class Tool(
      * Optional additional tool information.
      */
     val annotations: ToolAnnotations?,
-) {
+
+    /**
+     * Optional metadata for the tool.
+     */
+    override val _meta: JsonObject = EmptyJsonObject,
+) : WithMeta {
     @Serializable
     public data class Input(val properties: JsonObject = EmptyJsonObject, val required: List<String>? = null) {
         @OptIn(ExperimentalSerializationApi::class)

kotlin-sdk-core/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/ToolSerializationTest.kt

@@ -3,6 +3,7 @@ package io.modelcontextprotocol.kotlin.sdk
 import io.kotest.assertions.json.shouldEqualJson
 import io.modelcontextprotocol.kotlin.sdk.shared.McpJson
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.JsonPrimitive
 import kotlinx.serialization.json.buildJsonObject
 import kotlin.test.Test
@@ -44,6 +45,9 @@ class ToolSerializationTest {
               }
             },
             "required": ["temperature", "conditions", "humidity"]
+          },
+          "_meta": {
+            "_for_test_only": true
           }
         }
     """.trimIndent()
@@ -91,6 +95,9 @@ class ToolSerializationTest {
             },
             required = listOf("temperature", "conditions", "humidity"),
         ),
+        _meta = buildJsonObject {
+            put("_for_test_only", JsonPrimitive(true))
+        }
     )
 
     //region Serialize
@@ -411,6 +418,7 @@ class ToolSerializationTest {
         name: String = "get_weather",
         title: String? = null,
         outputSchema: String? = null,
+        _meta: String? = null
     ): String {
         val stringBuilder = StringBuilder()
 
@@ -453,6 +461,14 @@ class ToolSerializationTest {
                 )
         }
 
+        stringBuilder
+            .appendLine(",")
+            .append(
+                """
+                "_meta": ${_meta ?: "{}"}
+                """.trimIndent(),
+            )
+
         stringBuilder
             .appendLine()
             .appendLine("}")
@@ -464,6 +480,7 @@ class ToolSerializationTest {
         name: String = "get_weather",
         title: String? = null,
         outputSchema: Tool.Output? = null,
+        @Suppress("LocalVariableName") _meta: JsonObject = EmptyJsonObject,
     ): Tool = Tool(
         name = name,
         title = title,
@@ -482,6 +499,7 @@ class ToolSerializationTest {
             required = listOf("location"),
         ),
         outputSchema = outputSchema,
+        _meta = _meta
     )
 
     //endregion Private Methods

kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/Server.kt

@@ -3,6 +3,7 @@ package io.modelcontextprotocol.kotlin.sdk.server
 import io.github.oshai.kotlinlogging.KotlinLogging
 import io.modelcontextprotocol.kotlin.sdk.CallToolRequest
 import io.modelcontextprotocol.kotlin.sdk.CallToolResult
+import io.modelcontextprotocol.kotlin.sdk.EmptyJsonObject
 import io.modelcontextprotocol.kotlin.sdk.GetPromptRequest
 import io.modelcontextprotocol.kotlin.sdk.GetPromptResult
 import io.modelcontextprotocol.kotlin.sdk.Implementation
@@ -32,6 +33,7 @@ import kotlinx.collections.immutable.minus
 import kotlinx.collections.immutable.persistentListOf
 import kotlinx.collections.immutable.persistentMapOf
 import kotlinx.collections.immutable.toPersistentSet
+import kotlinx.serialization.json.JsonObject
 
 private val logger = KotlinLogging.logger {}
 
@@ -217,6 +219,7 @@ public open class Server(
      * @param inputSchema The expected input schema for the tool.
      * @param outputSchema The optional expected output schema for the tool.
      * @param toolAnnotations Optional additional tool information.
+     * @param _meta Optional metadata as a [JsonObject].
      * @param handler A suspend function that handles executing the tool when called by the client.
      * @throws IllegalStateException If the server does not support tools.
      */
@@ -227,9 +230,10 @@ public open class Server(
         title: String? = null,
         outputSchema: Tool.Output? = null,
         toolAnnotations: ToolAnnotations? = null,
+        @Suppress("LocalVariableName") _meta: JsonObject = EmptyJsonObject,
         handler: suspend (CallToolRequest) -> CallToolResult,
     ) {
-        val tool = Tool(name, title, description, inputSchema, outputSchema, toolAnnotations)
+        val tool = Tool(name, title, description, inputSchema, outputSchema, toolAnnotations, _meta)
         addTool(tool, handler)
     }