Fix Kotlin nullable fields in MCP tool input schema

mcp/mcp-annotations/pom.xml

@@ -67,6 +67,18 @@
 			<scope>test</scope>
 		</dependency>
 
+		<dependency>
+			<groupId>org.jetbrains.kotlin</groupId>
+			<artifactId>kotlin-stdlib</artifactId>
+			<scope>test</scope>
+		</dependency>
+
+		<dependency>
+			<groupId>org.jetbrains.kotlin</groupId>
+			<artifactId>kotlin-reflect</artifactId>
+			<scope>test</scope>
+		</dependency>
+
 	</dependencies>
 
 

mcp/mcp-annotations/src/main/java/org/springframework/ai/mcp/annotation/method/tool/utils/McpJsonSchemaGenerator.java

@@ -51,8 +51,10 @@
 import org.springframework.ai.mcp.annotation.McpToolParam;
 import org.springframework.ai.mcp.annotation.context.McpAsyncRequestContext;
 import org.springframework.ai.mcp.annotation.context.McpSyncRequestContext;
+import org.springframework.ai.model.KotlinModule;
 import org.springframework.ai.util.json.JsonParser;
 import org.springframework.ai.util.json.schema.JsonSchemaGenerator.SchemaOption;
+import org.springframework.core.KotlinDetector;
 import org.springframework.core.Nullness;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ConcurrentReferenceHashMap;
@@ -85,16 +87,21 @@ public final class McpJsonSchemaGenerator {
 		Module springAiSchemaModule = PROPERTY_REQUIRED_BY_DEFAULT ? new McpSpringAiSchemaModule()
 				: new McpSpringAiSchemaModule(McpSpringAiSchemaModule.Option.PROPERTY_REQUIRED_FALSE_BY_DEFAULT);
 
-		SchemaGeneratorConfig subtypeConfig = new SchemaGeneratorConfigBuilder(SchemaVersion.DRAFT_2020_12,
-				OptionPreset.PLAIN_JSON)
+		SchemaGeneratorConfigBuilder subtypeConfigBuilder = new SchemaGeneratorConfigBuilder(
+				SchemaVersion.DRAFT_2020_12, OptionPreset.PLAIN_JSON)
 			.with(jacksonModule)
 			.with(openApiModule)
 			.with(springAiSchemaModule)
 			.with(Option.EXTRA_OPEN_API_FORMAT_VALUES)
 			.with(Option.STANDARD_FORMATS)
 			.with(Option.PLAIN_DEFINITION_KEYS)
-			.without(Option.SCHEMA_VERSION_INDICATOR)
-			.build();
+			.without(Option.SCHEMA_VERSION_INDICATOR);
+
+		if (KotlinDetector.isKotlinReflectPresent()) {
+			subtypeConfigBuilder.with(new KotlinModule());
+		}
+
+		SchemaGeneratorConfig subtypeConfig = subtypeConfigBuilder.build();
 
 		SUBTYPE_SCHEMA_GENERATOR = new SchemaGenerator(subtypeConfig);
 	}

mcp/mcp-annotations/src/test/kotlin/org/springframework/ai/mcp/annotation/method/tool/utils/McpJsonSchemaGeneratorKotlinTests.kt

@@ -0,0 +1,82 @@
+/*
+ * Copyright 2023-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.ai.mcp.annotation.method.tool.utils
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import org.springframework.ai.mcp.annotation.McpTool
+import org.springframework.ai.mcp.annotation.McpToolParam
+import tools.jackson.databind.JsonNode
+import tools.jackson.databind.json.JsonMapper
+
+class McpJsonSchemaGeneratorKotlinTests {
+
+	private val jsonMapper = JsonMapper()
+
+	@Test
+	fun `nullable Kotlin properties are not required in MCP tool input schemas`() {
+		val method = SearchTools::class.java.getMethod("search", Filter::class.java)
+
+		val schema = McpJsonSchemaGenerator.generateForMethodInput(method)
+		val schemaNode = jsonMapper.readTree(schema)
+		val filterNode = schemaNode["properties"]["filter"]
+		val topLevelRequired = schemaNode["required"]
+		val filterRequired = filterNode["required"]
+
+		assertThat(requiredNames(topLevelRequired)).doesNotContain("filter")
+		assertThat(requiredNames(filterRequired)).doesNotContain("name", "ids")
+	}
+
+	@Test
+	fun `non-null Kotlin constructor properties without defaults remain required`() {
+		val method = SearchTools::class.java.getMethod("mixed", SearchRequest::class.java)
+
+		val schema = McpJsonSchemaGenerator.generateForMethodInput(method)
+		val schemaNode = jsonMapper.readTree(schema)
+		val requestNode = schemaNode["properties"]["request"]
+		val requestRequired = requiredNames(requestNode["required"])
+
+		assertThat(requestRequired).contains("query")
+		assertThat(requestRequired).doesNotContain("filter")
+	}
+
+	private fun requiredNames(required: JsonNode?): List<String> {
+		if (required == null || required.isNull) {
+			return emptyList()
+		}
+		return required.iterator().asSequence().map { it.asString() }.toList()
+	}
+
+	private data class Filter(val name: String? = null, val ids: List<String>? = null)
+
+	private data class SearchRequest(val query: String, val filter: String? = null)
+
+	private class SearchTools {
+
+		@McpTool(description = "Search")
+		fun search(@McpToolParam(required = false) filter: Filter? = null): String {
+			return "ok"
+		}
+
+		@McpTool(description = "Mixed")
+		fun mixed(@McpToolParam(required = false) request: SearchRequest? = null): String {
+			return "ok"
+		}
+
+	}
+
+}