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feat!: #697 added tool input arguments validation #873

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18c626b
feat!: #697 added tool input arguments validation causes tool executi…
ashakirin Mar 18, 2026
219bc3c
Merge branch 'main' into feature/697-tools-input-validation
Kehrlann Apr 1, 2026
27f3a38
Update mcp-core/src/main/java/io/modelcontextprotocol/util/ToolInputV…
ashakirin Apr 3, 2026
7610e99
Update mcp-test/src/test/java/io/modelcontextprotocol/server/ToolInpu…
ashakirin Apr 3, 2026
1cabd8f
chore: #697 fixed author name
ashakirin Apr 3, 2026
ec41a40
feat: #697 removed system property option to change default validatio…
ashakirin Apr 7, 2026
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57 changes: 24 additions & 33 deletions mcp-core/src/main/java/io/modelcontextprotocol/server/McpAsyncServer.java
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Original file line number Diff line number Diff line change
Expand Up @@ -4,46 +4,24 @@

package io.modelcontextprotocol.server;

import java.time.Duration;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.Set;
import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.function.BiFunction;

import io.modelcontextprotocol.json.McpJsonMapper;
import io.modelcontextprotocol.json.TypeRef;
import io.modelcontextprotocol.json.schema.JsonSchemaValidator;
import io.modelcontextprotocol.spec.DefaultMcpStreamableServerSessionFactory;
import io.modelcontextprotocol.spec.McpClientSession;
import io.modelcontextprotocol.spec.McpError;
import io.modelcontextprotocol.spec.McpSchema;
import io.modelcontextprotocol.spec.McpSchema.CallToolResult;
import io.modelcontextprotocol.spec.*;
import io.modelcontextprotocol.spec.McpSchema.*;
import io.modelcontextprotocol.spec.McpSchema.CompleteResult.CompleteCompletion;
import io.modelcontextprotocol.spec.McpSchema.ErrorCodes;
import io.modelcontextprotocol.spec.McpSchema.LoggingLevel;
import io.modelcontextprotocol.spec.McpSchema.PromptReference;
import io.modelcontextprotocol.spec.McpSchema.ResourceReference;
import io.modelcontextprotocol.spec.McpSchema.SetLevelRequest;
import io.modelcontextprotocol.spec.McpSchema.Tool;
import io.modelcontextprotocol.spec.McpServerSession;
import io.modelcontextprotocol.spec.McpServerTransportProvider;
import io.modelcontextprotocol.spec.McpServerTransportProviderBase;
import io.modelcontextprotocol.spec.McpStreamableServerTransportProvider;
import io.modelcontextprotocol.util.Assert;
import io.modelcontextprotocol.util.DefaultMcpUriTemplateManagerFactory;
import io.modelcontextprotocol.util.McpUriTemplateManagerFactory;
import io.modelcontextprotocol.util.Utils;
import io.modelcontextprotocol.util.*;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;

import java.time.Duration;
import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.function.BiFunction;

import static io.modelcontextprotocol.spec.McpError.RESOURCE_NOT_FOUND;

/**
Expand Down Expand Up @@ -98,6 +76,8 @@ public class McpAsyncServer {

private final JsonSchemaValidator jsonSchemaValidator;

private final boolean validateToolInputs;

private final McpSchema.ServerCapabilities serverCapabilities;

private final McpSchema.Implementation serverInfo;
Expand Down Expand Up @@ -129,7 +109,8 @@ 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();
Expand All @@ -142,6 +123,7 @@ 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);
Expand All @@ -157,7 +139,8 @@ 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();
Expand All @@ -170,6 +153,7 @@ 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);
Expand Down Expand Up @@ -543,6 +527,13 @@ private McpRequestHandler<CallToolResult> toolsCallRequestHandler() {
.build());
}

McpSchema.Tool tool = toolSpecification.get().tool();
CallToolResult validationError = ToolInputValidator.validate(tool, callToolRequest.arguments(),
this.validateToolInputs, this.jsonMapper, this.jsonSchemaValidator);
if (validationError != null) {
return Mono.just(validationError);
}

return toolSpecification.get().callHandler().apply(exchange, callToolRequest);
};
}
Expand Down
71 changes: 65 additions & 6 deletions mcp-core/src/main/java/io/modelcontextprotocol/server/McpServer.java
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Expand Up @@ -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);
}

}
Expand All @@ -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);
}

}
Expand All @@ -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
Expand Down Expand Up @@ -421,6 +423,18 @@ public AsyncSpecification<S> strictToolNameValidation(boolean strict) {
return this;
}

/**
* Sets whether to validate tool inputs against the tool's input schema. When set,
* this takes priority over the system property
* {@code io.modelcontextprotocol.validateToolInputs}.
* @param validate true to validate inputs and return error on validation failure
* @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
Expand Down Expand Up @@ -818,7 +832,8 @@ 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);
}

Expand Down Expand Up @@ -849,7 +864,7 @@ 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);
}

Expand All @@ -872,6 +887,8 @@ 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
Expand Down Expand Up @@ -1004,6 +1021,18 @@ public SyncSpecification<S> strictToolNameValidation(boolean strict) {
return this;
}

/**
* Sets whether to validate tool inputs against the tool's input schema. When set,
* this takes priority over the system property
* {@code io.modelcontextprotocol.validateToolInputs}.
* @param validate true to validate inputs and return error on validation failure
* @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
Expand Down Expand Up @@ -1401,6 +1430,8 @@ 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
Expand Down Expand Up @@ -1530,6 +1561,18 @@ public StatelessAsyncSpecification strictToolNameValidation(boolean strict) {
return this;
}

/**
* Sets whether to validate tool inputs against the tool's input schema. When set,
* this takes priority over the system property
* {@code io.modelcontextprotocol.validateToolInputs}.
* @param validate true to validate inputs and return error on validation failure
* @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
Expand Down Expand Up @@ -1859,7 +1902,8 @@ 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);
}

}
Expand All @@ -1884,6 +1928,8 @@ 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
Expand Down Expand Up @@ -2013,6 +2059,18 @@ public StatelessSyncSpecification strictToolNameValidation(boolean strict) {
return this;
}

/**
* Sets whether to validate tool inputs against the tool's input schema. When set,
* this takes priority over the system property
* {@code io.modelcontextprotocol.validateToolInputs}.
* @param validate true to validate inputs and return error on validation failure
* @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
Expand Down Expand Up @@ -2360,7 +2418,8 @@ 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);
}

Expand Down
14 changes: 13 additions & 1 deletion mcp-core/src/main/java/io/modelcontextprotocol/server/McpStatelessAsyncServer.java
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand Down Expand Up @@ -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();
Expand All @@ -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<>();

Expand Down Expand Up @@ -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.jsonMapper, this.jsonSchemaValidator);
if (validationError != null) {
return Mono.just(validationError);
}

return toolSpecification.get().callHandler().apply(ctx, callToolRequest);
};
}
Expand Down
60 changes: 60 additions & 0 deletions mcp-core/src/main/java/io/modelcontextprotocol/util/ToolInputValidator.java
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Original file line number Diff line number Diff line change
@@ -0,0 +1,60 @@
/*
* Copyright 2026-2026 the original author or authors.
*/

package io.modelcontextprotocol.util;

import java.util.List;
import java.util.Map;

import io.modelcontextprotocol.json.McpJsonMapper;
import io.modelcontextprotocol.json.TypeRef;
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 jsonMapper the JSON mapper for schema conversion
* @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, McpJsonMapper jsonMapper, JsonSchemaValidator validator) {
if (!validateToolInputs || tool.inputSchema() == null || validator == null) {
return null;
}
Map<String, Object> inputSchema = jsonMapper.convertValue(tool.inputSchema(),
new TypeRef<Map<String, Object>>() {
});
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@Kehrlann Kehrlann Apr 1, 2026

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I wonder if we should leave this marshalling on the hot path. Look at what we've done in DefaultJsonSchemaValidator, there's a schema cache to avoid specifically doing to much of these operations.

Maybe we should have something similar here? Or maybe we should update the JsonSchemaValidator to also validate JsonSchema objects?

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@Kehrlann Kehrlann Apr 2, 2026 •
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#886 points out similar issues ; there are open PRs about it #749

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@ashakirin ashakirin Apr 3, 2026

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thanks for pointing it, I will wait until #749 is merged

Map<String, Object> args = arguments != null ? arguments : Map.of();
var validation = validator.validate(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;
}

}
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