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Clarify tool validation error messages (#1023)

Browse source 
- DefaultJsonSchemaValidator now emits a neutral message;
- Call sites in ToolInputValidator and the server output handlers
  prepend their own context prefix,
- Integration test assertions strengthened accordingly.

Closes #986

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

* address review comments

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

* address review comments

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

---------

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>
This commit is contained in:
Christian Tzolov 2026-06-11 09:45:28 +02:00 • committed by GitHub
commit 99faac65f7
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8 changed files with 30 additions and 25 deletions
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7
mcp-core/src/main/java/io/modelcontextprotocol/server/McpAsyncServer.java
View file

@ -26,7 +26,6 @@ import io.modelcontextprotocol.spec.McpSchema;
import io.modelcontextprotocol.spec.McpSchema.CallToolResult;
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;
@ -433,9 +432,11 @@ public class McpAsyncServer {
var validation = this.jsonSchemaValidator.validate(outputSchema, result.structuredContent());
if (!validation.valid()) {
logger.warn("Tool call result validation failed: {}", validation.errorMessage());
String message = "Tool (" + request.name() + ") output validation failed: "
+ validation.errorMessage();
logger.warn(message);
return CallToolResult.builder()
.content(List.of(McpSchema.TextContent.builder(validation.errorMessage()).build()))
.content(List.of(McpSchema.TextContent.builder(message).build()))
.isError(true)
.build();
}

30
mcp-core/src/main/java/io/modelcontextprotocol/server/McpStatelessAsyncServer.java
View file

@ -4,9 +4,19 @@
package io.modelcontextprotocol.server;
import io.modelcontextprotocol.json.TypeRef;
import io.modelcontextprotocol.json.McpJsonMapper;
import java.time.Duration;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.function.BiFunction;
import io.modelcontextprotocol.common.McpTransportContext;
import io.modelcontextprotocol.json.McpJsonMapper;
import io.modelcontextprotocol.json.TypeRef;
import io.modelcontextprotocol.json.schema.JsonSchemaValidator;
import io.modelcontextprotocol.server.McpStatelessServerFeatures.AsyncResourceTemplateSpecification;
import io.modelcontextprotocol.spec.McpError;
@ -28,16 +38,6 @@ import org.slf4j.LoggerFactory;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;
import java.time.Duration;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.function.BiFunction;
import static io.modelcontextprotocol.spec.McpError.RESOURCE_NOT_FOUND;
/**
@ -293,9 +293,11 @@ public class McpStatelessAsyncServer {
var validation = this.jsonSchemaValidator.validate(outputSchema, result.structuredContent());
if (!validation.valid()) {
logger.warn("Tool call result validation failed: {}", validation.errorMessage());
String message = "Tool (" + request.name() + ") output validation failed: "
+ validation.errorMessage();
logger.warn(message);
return CallToolResult.builder()
.content(List.of(McpSchema.TextContent.builder(validation.errorMessage()).build()))
.content(List.of(McpSchema.TextContent.builder(message).build()))
.isError(true)
.build();
}

5
mcp-core/src/main/java/io/modelcontextprotocol/util/ToolInputValidator.java
View file

@ -42,9 +42,10 @@ public final class ToolInputValidator {
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());
String message = "Tool (" + tool.name() + ") input validation failed: " + validation.errorMessage();
logger.warn(message);
return CallToolResult.builder()
.content(List.of(McpSchema.TextContent.builder(validation.errorMessage()).build()))
.content(List.of(McpSchema.TextContent.builder(message).build()))
.isError(true)
.build();
}

3
mcp-json-jackson2/src/main/java/io/modelcontextprotocol/json/schema/jackson2/DefaultJsonSchemaValidator.java
View file

@ -76,8 +76,7 @@ public class DefaultJsonSchemaValidator implements JsonSchemaValidator {
// Check if validation passed
if (!validationResult.isEmpty()) {
return ValidationResponse
.asInvalid("Validation failed: structuredContent does not match tool outputSchema. "
+ "Validation errors: " + validationResult);
.asInvalid("Validation failed: JSON schema validation errors: " + validationResult);
}
return ValidationResponse.asValid(jsonStructuredOutput.toString());

2
mcp-json-jackson2/src/test/java/io/modelcontextprotocol/json/jackson2/DefaultJsonSchemaValidatorTests.java
View file

@ -308,7 +308,7 @@ class DefaultJsonSchemaValidatorTests {
assertFalse(response.valid());
assertNotNull(response.errorMessage());
assertTrue(response.errorMessage().contains("Validation failed"));
assertTrue(response.errorMessage().contains("structuredContent does not match tool outputSchema"));
assertTrue(response.errorMessage().contains("JSON schema validation errors"));
}
@Test

3
mcp-json-jackson3/src/main/java/io/modelcontextprotocol/json/schema/jackson3/DefaultJsonSchemaValidator.java
View file

@ -75,8 +75,7 @@ public class DefaultJsonSchemaValidator implements JsonSchemaValidator {
// Check if validation passed
if (!validationResult.isEmpty()) {
return ValidationResponse
.asInvalid("Validation failed: structuredContent does not match tool outputSchema. "
+ "Validation errors: " + validationResult);
.asInvalid("Validation failed: JSON schema validation errors: " + validationResult);
}
return ValidationResponse.asValid(jsonStructuredOutput.toString());

2
mcp-json-jackson3/src/test/java/io/modelcontextprotocol/json/DefaultJsonSchemaValidatorTests.java
View file

@ -308,7 +308,7 @@ class DefaultJsonSchemaValidatorTests {
assertFalse(response.valid());
assertNotNull(response.errorMessage());
assertTrue(response.errorMessage().contains("Validation failed"));
assertTrue(response.errorMessage().contains("structuredContent does not match tool outputSchema"));
assertTrue(response.errorMessage().contains("JSON schema validation errors"));
}
@Test

3
mcp-test/src/test/java/io/modelcontextprotocol/server/ToolInputValidationIntegrationTests.java
View file

@ -182,6 +182,9 @@ class ToolInputValidationIntegrationTests {
assertThat(result.isError()).isTrue();
String errorMessage = ((TextContent) result.content().get(0)).text();
assertThat(errorMessage).startsWith("Tool (test-tool) input validation failed:");
assertThat(errorMessage).containsIgnoringCase("Validation failed");
assertThat(errorMessage).containsIgnoringCase("JSON schema validation errors");
assertThat(errorMessage).containsIgnoringCase(expectedErrorSubstring);
}
finally {