feat!: #697 added tool input arguments validation

mcp-core/src/main/java/io/modelcontextprotocol/server/McpAsyncServer.java

@@ -38,6 +38,7 @@
 import io.modelcontextprotocol.util.Assert;
 import io.modelcontextprotocol.util.DefaultMcpUriTemplateManagerFactory;
 import io.modelcontextprotocol.util.McpUriTemplateManagerFactory;
+import io.modelcontextprotocol.util.ToolInputValidator;
 import io.modelcontextprotocol.util.Utils;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -98,6 +99,8 @@ public class McpAsyncServer {
 
 	private final JsonSchemaValidator jsonSchemaValidator;
 
+	private final boolean validateToolInputs;
+
 	private final McpSchema.ServerCapabilities serverCapabilities;
 
 	private final McpSchema.Implementation serverInfo;
@@ -129,7 +132,8 @@ public class McpAsyncServer {
 	 */
 	McpAsyncServer(McpServerTransportProvider mcpTransportProvider, McpJsonMapper jsonMapper,
 			McpServerFeatures.Async features, Duration requestTimeout,
-			McpUriTemplateManagerFactory uriTemplateManagerFactory, JsonSchemaValidator jsonSchemaValidator) {
+			McpUriTemplateManagerFactory uriTemplateManagerFactory, JsonSchemaValidator jsonSchemaValidator,
+			boolean validateToolInputs) {
 		this.mcpTransportProvider = mcpTransportProvider;
 		this.jsonMapper = jsonMapper;
 		this.serverInfo = features.serverInfo();
@@ -142,6 +146,7 @@ public class McpAsyncServer {
 		this.completions.putAll(features.completions());
 		this.uriTemplateManagerFactory = uriTemplateManagerFactory;
 		this.jsonSchemaValidator = jsonSchemaValidator;
+		this.validateToolInputs = validateToolInputs;
 
 		Map<String, McpRequestHandler<?>> requestHandlers = prepareRequestHandlers();
 		Map<String, McpNotificationHandler> notificationHandlers = prepareNotificationHandlers(features);
@@ -157,7 +162,8 @@ public class McpAsyncServer {
 
 	McpAsyncServer(McpStreamableServerTransportProvider mcpTransportProvider, McpJsonMapper jsonMapper,
 			McpServerFeatures.Async features, Duration requestTimeout,
-			McpUriTemplateManagerFactory uriTemplateManagerFactory, JsonSchemaValidator jsonSchemaValidator) {
+			McpUriTemplateManagerFactory uriTemplateManagerFactory, JsonSchemaValidator jsonSchemaValidator,
+			boolean validateToolInputs) {
 		this.mcpTransportProvider = mcpTransportProvider;
 		this.jsonMapper = jsonMapper;
 		this.serverInfo = features.serverInfo();
@@ -170,6 +176,7 @@ public class McpAsyncServer {
 		this.completions.putAll(features.completions());
 		this.uriTemplateManagerFactory = uriTemplateManagerFactory;
 		this.jsonSchemaValidator = jsonSchemaValidator;
+		this.validateToolInputs = validateToolInputs;
 
 		Map<String, McpRequestHandler<?>> requestHandlers = prepareRequestHandlers();
 		Map<String, McpNotificationHandler> notificationHandlers = prepareNotificationHandlers(features);
@@ -543,6 +550,13 @@ private McpRequestHandler<CallToolResult> toolsCallRequestHandler() {
 					.build());
 			}
 
+			McpSchema.Tool tool = toolSpecification.get().tool();
+			CallToolResult validationError = ToolInputValidator.validate(tool, callToolRequest.arguments(),
+					this.validateToolInputs, this.jsonSchemaValidator);
+			if (validationError != null) {
+				return Mono.just(validationError);
+			}
+
 			return toolSpecification.get().callHandler().apply(exchange, callToolRequest);
 		};
 	}

mcp-core/src/main/java/io/modelcontextprotocol/server/McpServer.java

@@ -243,7 +243,7 @@ public McpAsyncServer build() {
 					: McpJsonDefaults.getSchemaValidator();
 
 			return new McpAsyncServer(transportProvider, jsonMapper == null ? McpJsonDefaults.getMapper() : jsonMapper,
-					features, requestTimeout, uriTemplateManagerFactory, jsonSchemaValidator);
+					features, requestTimeout, uriTemplateManagerFactory, jsonSchemaValidator, validateToolInputs);
 		}
 
 	}
@@ -269,7 +269,7 @@ public McpAsyncServer build() {
 			var jsonSchemaValidator = this.jsonSchemaValidator != null ? this.jsonSchemaValidator
 					: McpJsonDefaults.getSchemaValidator();
 			return new McpAsyncServer(transportProvider, jsonMapper == null ? McpJsonDefaults.getMapper() : jsonMapper,
-					features, requestTimeout, uriTemplateManagerFactory, jsonSchemaValidator);
+					features, requestTimeout, uriTemplateManagerFactory, jsonSchemaValidator, validateToolInputs);
 		}
 
 	}
@@ -293,6 +293,8 @@ abstract class AsyncSpecification<S extends AsyncSpecification<S>> {
 
 		boolean strictToolNameValidation = ToolNameValidator.isStrictByDefault();
 
+		boolean validateToolInputs = true;
+
 		/**
 		 * The Model Context Protocol (MCP) allows servers to expose tools that can be
 		 * invoked by language models. Tools enable models to interact with external
@@ -421,6 +423,17 @@ public AsyncSpecification<S> strictToolNameValidation(boolean strict) {
 			return this;
 		}
 
+		/**
+		 * Sets whether to validate tool inputs against the tool's input schema.
+		 * @param validate true to validate inputs and return error on validation failure,
+		 * false to skip validation. Defaults to true.
+		 * @return This builder instance for method chaining
+		 */
+		public AsyncSpecification<S> validateToolInputs(boolean validate) {
+			this.validateToolInputs = validate;
+			return this;
+		}
+
 		/**
 		 * Sets the server capabilities that will be advertised to clients during
 		 * connection initialization. Capabilities define what features the server
@@ -818,7 +831,8 @@ public McpSyncServer build() {
 			var asyncServer = new McpAsyncServer(transportProvider,
 					jsonMapper == null ? McpJsonDefaults.getMapper() : jsonMapper, asyncFeatures, requestTimeout,
 					uriTemplateManagerFactory,
-					jsonSchemaValidator != null ? jsonSchemaValidator : McpJsonDefaults.getSchemaValidator());
+					jsonSchemaValidator != null ? jsonSchemaValidator : McpJsonDefaults.getSchemaValidator(),
+					validateToolInputs);
 			return new McpSyncServer(asyncServer, this.immediateExecution);
 		}
 
@@ -849,7 +863,7 @@ public McpSyncServer build() {
 					: McpJsonDefaults.getSchemaValidator();
 			var asyncServer = new McpAsyncServer(transportProvider,
 					jsonMapper == null ? McpJsonDefaults.getMapper() : jsonMapper, asyncFeatures, this.requestTimeout,
-					this.uriTemplateManagerFactory, jsonSchemaValidator);
+					this.uriTemplateManagerFactory, jsonSchemaValidator, validateToolInputs);
 			return new McpSyncServer(asyncServer, this.immediateExecution);
 		}
 
@@ -872,6 +886,8 @@ abstract class SyncSpecification<S extends SyncSpecification<S>> {
 
 		boolean strictToolNameValidation = ToolNameValidator.isStrictByDefault();
 
+		boolean validateToolInputs = true;
+
 		/**
 		 * The Model Context Protocol (MCP) allows servers to expose tools that can be
 		 * invoked by language models. Tools enable models to interact with external
@@ -1004,6 +1020,17 @@ public SyncSpecification<S> strictToolNameValidation(boolean strict) {
 			return this;
 		}
 
+		/**
+		 * Sets whether to validate tool inputs against the tool's input schema.
+		 * @param validate true to validate inputs and return error on validation failure,
+		 * false to skip validation. Defaults to true.
+		 * @return This builder instance for method chaining
+		 */
+		public SyncSpecification<S> validateToolInputs(boolean validate) {
+			this.validateToolInputs = validate;
+			return this;
+		}
+
 		/**
 		 * Sets the server capabilities that will be advertised to clients during
 		 * connection initialization. Capabilities define what features the server
@@ -1401,6 +1428,8 @@ class StatelessAsyncSpecification {
 
 		boolean strictToolNameValidation = ToolNameValidator.isStrictByDefault();
 
+		boolean validateToolInputs = true;
+
 		/**
 		 * The Model Context Protocol (MCP) allows servers to expose tools that can be
 		 * invoked by language models. Tools enable models to interact with external
@@ -1530,6 +1559,17 @@ public StatelessAsyncSpecification strictToolNameValidation(boolean strict) {
 			return this;
 		}
 
+		/**
+		 * Sets whether to validate tool inputs against the tool's input schema.
+		 * @param validate true to validate inputs and return error on validation failure,
+		 * false to skip validation. Defaults to true.
+		 * @return This builder instance for method chaining
+		 */
+		public StatelessAsyncSpecification validateToolInputs(boolean validate) {
+			this.validateToolInputs = validate;
+			return this;
+		}
+
 		/**
 		 * Sets the server capabilities that will be advertised to clients during
 		 * connection initialization. Capabilities define what features the server
@@ -1859,7 +1899,8 @@ public McpStatelessAsyncServer build() {
 					this.resources, this.resourceTemplates, this.prompts, this.completions, this.instructions);
 			return new McpStatelessAsyncServer(transport, jsonMapper == null ? McpJsonDefaults.getMapper() : jsonMapper,
 					features, requestTimeout, uriTemplateManagerFactory,
-					jsonSchemaValidator != null ? jsonSchemaValidator : McpJsonDefaults.getSchemaValidator());
+					jsonSchemaValidator != null ? jsonSchemaValidator : McpJsonDefaults.getSchemaValidator(),
+					validateToolInputs);
 		}
 
 	}
@@ -1884,6 +1925,8 @@ class StatelessSyncSpecification {
 
 		boolean strictToolNameValidation = ToolNameValidator.isStrictByDefault();
 
+		boolean validateToolInputs = true;
+
 		/**
 		 * The Model Context Protocol (MCP) allows servers to expose tools that can be
 		 * invoked by language models. Tools enable models to interact with external
@@ -2013,6 +2056,17 @@ public StatelessSyncSpecification strictToolNameValidation(boolean strict) {
 			return this;
 		}
 
+		/**
+		 * Sets whether to validate tool inputs against the tool's input schema.
+		 * @param validate true to validate inputs and return error on validation failure,
+		 * false to skip validation. Defaults to true.
+		 * @return This builder instance for method chaining
+		 */
+		public StatelessSyncSpecification validateToolInputs(boolean validate) {
+			this.validateToolInputs = validate;
+			return this;
+		}
+
 		/**
 		 * Sets the server capabilities that will be advertised to clients during
 		 * connection initialization. Capabilities define what features the server
@@ -2360,7 +2414,8 @@ public McpStatelessSyncServer build() {
 			var asyncServer = new McpStatelessAsyncServer(transport,
 					jsonMapper == null ? McpJsonDefaults.getMapper() : jsonMapper, asyncFeatures, requestTimeout,
 					uriTemplateManagerFactory,
-					this.jsonSchemaValidator != null ? this.jsonSchemaValidator : McpJsonDefaults.getSchemaValidator());
+					this.jsonSchemaValidator != null ? this.jsonSchemaValidator : McpJsonDefaults.getSchemaValidator(),
+					validateToolInputs);
 			return new McpStatelessSyncServer(asyncServer, this.immediateExecution);
 		}
 

mcp-core/src/main/java/io/modelcontextprotocol/server/McpStatelessAsyncServer.java

@@ -21,6 +21,7 @@
 import io.modelcontextprotocol.util.Assert;
 import io.modelcontextprotocol.util.DefaultMcpUriTemplateManagerFactory;
 import io.modelcontextprotocol.util.McpUriTemplateManagerFactory;
+import io.modelcontextprotocol.util.ToolInputValidator;
 import io.modelcontextprotocol.util.Utils;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -77,9 +78,12 @@ public class McpStatelessAsyncServer {
 
 	private final JsonSchemaValidator jsonSchemaValidator;
 
+	private final boolean validateToolInputs;
+
 	McpStatelessAsyncServer(McpStatelessServerTransport mcpTransport, McpJsonMapper jsonMapper,
 			McpStatelessServerFeatures.Async features, Duration requestTimeout,
-			McpUriTemplateManagerFactory uriTemplateManagerFactory, JsonSchemaValidator jsonSchemaValidator) {
+			McpUriTemplateManagerFactory uriTemplateManagerFactory, JsonSchemaValidator jsonSchemaValidator,
+			boolean validateToolInputs) {
 		this.mcpTransportProvider = mcpTransport;
 		this.jsonMapper = jsonMapper;
 		this.serverInfo = features.serverInfo();
@@ -92,6 +96,7 @@ public class McpStatelessAsyncServer {
 		this.completions.putAll(features.completions());
 		this.uriTemplateManagerFactory = uriTemplateManagerFactory;
 		this.jsonSchemaValidator = jsonSchemaValidator;
+		this.validateToolInputs = validateToolInputs;
 
 		Map<String, McpStatelessRequestHandler<?>> requestHandlers = new HashMap<>();
 
@@ -409,6 +414,13 @@ private McpStatelessRequestHandler<CallToolResult> toolsCallRequestHandler() {
 					.build());
 			}
 
+			McpSchema.Tool tool = toolSpecification.get().tool();
+			CallToolResult validationError = ToolInputValidator.validate(tool, callToolRequest.arguments(),
+					this.validateToolInputs, this.jsonSchemaValidator);
+			if (validationError != null) {
+				return Mono.just(validationError);
+			}
+
 			return toolSpecification.get().callHandler().apply(ctx, callToolRequest);
 		};
 	}

mcp-core/src/main/java/io/modelcontextprotocol/util/ToolInputValidator.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2026-2026 the original author or authors.
+ */
+
+package io.modelcontextprotocol.util;
+
+import java.util.List;
+import java.util.Map;
+
+import io.modelcontextprotocol.json.schema.JsonSchemaValidator;
+import io.modelcontextprotocol.spec.McpSchema;
+import io.modelcontextprotocol.spec.McpSchema.CallToolResult;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Validates tool input arguments against JSON schema.
+ *
+ * @author Andrei Shakirin
+ */
+public final class ToolInputValidator {
+
+	private static final Logger logger = LoggerFactory.getLogger(ToolInputValidator.class);
+
+	private ToolInputValidator() {
+	}
+
+	/**
+	 * Validates tool arguments against the tool's input schema.
+	 * @param tool the tool definition containing the input schema
+	 * @param arguments the arguments to validate
+	 * @param validateToolInputs whether validation is enabled
+	 * @param validator the JSON schema validator (may be null)
+	 * @return CallToolResult with isError=true if validation fails, null if valid or
+	 * validation skipped
+	 */
+	public static CallToolResult validate(McpSchema.Tool tool, Map<String, Object> arguments,
+			boolean validateToolInputs, JsonSchemaValidator validator) {
+		if (!validateToolInputs || tool.inputSchema() == null || validator == null) {
+			return null;
+		}
+		Map<String, Object> args = arguments != null ? arguments : Map.of();
+		var validation = validator.validate(tool.inputSchema(), args);
+		if (!validation.valid()) {
+			logger.warn("Tool '{}' input validation failed: {}", tool.name(), validation.errorMessage());
+			return CallToolResult.builder()
+				.content(List.of(new McpSchema.TextContent(validation.errorMessage())))
+				.isError(true)
+				.build();
+		}
+		return null;
+	}
+
+}