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Clarify tool validation error messages #1023

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Merged
tzolov merged 3 commits into from
Jun 11, 2026
Merged

Clarify tool validation error messages #1023

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d1f62a5
Clarify tool validation error messages
tzolov Jun 11, 2026
f89bb99
address review comments
tzolov Jun 11, 2026
2291c7b
address review comments
tzolov Jun 11, 2026
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7 changes: 4 additions & 3 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 @@ -26,7 +26,6 @@
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;
Expand Down Expand Up @@ -433,9 +432,11 @@ public Mono<CallToolResult> apply(McpAsyncServerExchange exchange, McpSchema.Cal
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();
}
Expand Down
30 changes: 16 additions & 14 deletions mcp-core/src/main/java/io/modelcontextprotocol/server/McpStatelessAsyncServer.java
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Expand Up @@ -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;
Expand All @@ -28,16 +38,6 @@
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;

/**
Expand Down Expand Up @@ -293,9 +293,11 @@ public Mono<CallToolResult> apply(McpTransportContext transportContext, McpSchem
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();
}
Expand Down
5 changes: 3 additions & 2 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
Expand Up @@ -42,9 +42,10 @@ public static CallToolResult validate(McpSchema.Tool tool, Map<String, Object> a
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();
}
Expand Down
3 changes: 1 addition & 2 deletions ...rc/main/java/io/modelcontextprotocol/json/schema/jackson2/DefaultJsonSchemaValidator.java
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Expand Up @@ -76,8 +76,7 @@ public ValidationResponse validate(Map<String, Object> schema, Object structured
// 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());
Expand Down
2 changes: 1 addition & 1 deletion .../src/test/java/io/modelcontextprotocol/json/jackson2/DefaultJsonSchemaValidatorTests.java
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Expand Up @@ -308,7 +308,7 @@ void testValidateWithInvalidTypeSchema() {
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
Expand Down
3 changes: 1 addition & 2 deletions ...rc/main/java/io/modelcontextprotocol/json/schema/jackson3/DefaultJsonSchemaValidator.java
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Expand Up @@ -75,8 +75,7 @@ public ValidationResponse validate(Map<String, Object> schema, Object structured
// 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());
Expand Down
2 changes: 1 addition & 1 deletion ...-jackson3/src/test/java/io/modelcontextprotocol/json/DefaultJsonSchemaValidatorTests.java
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Expand Up @@ -308,7 +308,7 @@ void testValidateWithInvalidTypeSchema() {
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
Expand Down
3 changes: 3 additions & 0 deletions ...est/src/test/java/io/modelcontextprotocol/server/ToolInputValidationIntegrationTests.java
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Expand Up @@ -182,6 +182,9 @@ void invalidInput_withDefaultValidation_shouldReturnToolError(String serverType,

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 {
Expand Down
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