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feat: Add Tool.outputSchema and CallToolResult.structuredContent #302

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7011142
feat: Add Tool.outputSchema and CallToolResult.structuredContent
pantanurag555 Jun 9, 2025
31f453e
Minor fixes/changes
pantanurag555 Jun 9, 2025
aaf7add
Store optional json schemas in cache
pantanurag555 Jun 9, 2025
f088e75
Merge remote-tracking branch 'upstream/main' into output-schema
pantanurag555 Jun 10, 2025
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1 change: 1 addition & 0 deletions mcp-test/src/main/java/io/modelcontextprotocol/server/.LCKAbstractMcpSyncServerTests.java~
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@@ -0,0 +1 @@
/local/home/pantanur/workspace/java-sdk/mcp-test/src/main/java/io/modelcontextprotocol/server/AbstractMcpSyncServerTests.java
6 changes: 4 additions & 2 deletions mcp-test/src/main/java/io/modelcontextprotocol/server/AbstractMcpSyncServerTests.java
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Expand Up @@ -4,6 +4,7 @@

package io.modelcontextprotocol.server;

import java.util.HashMap;
import java.util.List;

import io.modelcontextprotocol.spec.McpError;
Expand All @@ -17,6 +18,7 @@
import io.modelcontextprotocol.spec.McpSchema.ServerCapabilities;
import io.modelcontextprotocol.spec.McpSchema.Tool;
import io.modelcontextprotocol.spec.McpServerTransportProvider;

import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
Expand Down Expand Up @@ -125,7 +127,7 @@ void testAddTool() {

@Test
void testAddDuplicateTool() {
Tool duplicateTool = new McpSchema.Tool(TEST_TOOL_NAME, "Duplicate tool", emptyJsonSchema);
Tool duplicateTool = new McpSchema.Tool(TEST_TOOL_NAME, "Duplicate tool", emptyJsonSchema, emptyJsonSchema);

var mcpSyncServer = McpServer.sync(createMcpTransportProvider())
.serverInfo("test-server", "1.0.0")
Expand All @@ -134,7 +136,7 @@ void testAddDuplicateTool() {
.build();

assertThatThrownBy(() -> mcpSyncServer.addTool(new McpServerFeatures.SyncToolSpecification(duplicateTool,
(exchange, args) -> new CallToolResult(List.of(), false))))
(exchange, args) -> new CallToolResult(List.of(), false, new HashMap<String, Object>()))))
.isInstanceOf(McpError.class)
.hasMessage("Tool with name '" + TEST_TOOL_NAME + "' already exists");

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7 changes: 7 additions & 0 deletions mcp/pom.xml
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Expand Up @@ -202,6 +202,13 @@
<scope>test</scope>
</dependency>

<!-- Json validator dependency -->
<dependency>
<groupId>com.networknt</groupId>
<artifactId>json-schema-validator</artifactId>
<version>1.5.7</version>
</dependency>
Comment on lines +205 to +210
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Worth getting extra clarification from maintainers on if we want schema validation as part of this or a separate effort, along the lines of the discussion in #271.

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Discussed some of the issues offline with @LucaButBoring. I wanted to get clarity around the scope of the work on the issue that I had created. There are a few questions that we might want to answer regarding the output validation:

  1. Do we want validation to be part of this PR or do we want to separate it out and think more about how we can cache/validate the results?
  2. Should cache be in the client class or should we construct a separate cache class?
  3. Should there be refresh intervals for cache invalidation? How else can we deal with it and should it be configurable by the client?
  4. Is a Map the best way to represent cache or should we opt for an in-memory cache implementation like Guava Cache?

Would like inputs from the maintainers to decide on the best way to proceed.


</dependencies>


Expand Down
56 changes: 53 additions & 3 deletions mcp/src/main/java/io/modelcontextprotocol/client/McpAsyncClient.java
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Expand Up @@ -14,6 +14,7 @@
import java.util.function.Function;

import com.fasterxml.jackson.core.type.TypeReference;

import io.modelcontextprotocol.spec.McpClientSession;
import io.modelcontextprotocol.spec.McpClientSession.NotificationHandler;
import io.modelcontextprotocol.spec.McpClientSession.RequestHandler;
Expand All @@ -33,8 +34,11 @@
import io.modelcontextprotocol.spec.McpTransport;
import io.modelcontextprotocol.util.Assert;
import io.modelcontextprotocol.util.Utils;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import io.modelcontextprotocol.server.McpServerFeatures;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;
import reactor.core.publisher.Sinks;
Expand Down Expand Up @@ -123,6 +127,11 @@ public class McpAsyncClient {
*/
private McpSchema.Implementation serverInfo;

/**
* Cached tool output schemas.
*/
private final HashMap<String, McpSchema.JsonSchema> toolsOutputSchemaCache;

/**
* Roots define the boundaries of where servers can operate within the filesystem,
* allowing them to understand which directories and files they have access to.
Expand Down Expand Up @@ -171,6 +180,7 @@ public class McpAsyncClient {
this.transport = transport;
this.roots = new ConcurrentHashMap<>(features.roots());
this.initializationTimeout = initializationTimeout;
this.toolsOutputSchemaCache = new HashMap<>();

// Request Handlers
Map<String, RequestHandler<?>> requestHandlers = new HashMap<>();
Expand Down Expand Up @@ -287,6 +297,14 @@ public McpSchema.Implementation getClientInfo() {
return this.clientInfo;
}

/**
* Get the cached tool output schemas.
* @return The cached tool output schemas
*/
public HashMap<String, McpSchema.JsonSchema> getToolsOutputSchemaCache() {
return this.toolsOutputSchemaCache;
}

/**
* Closes the client connection immediately.
*/
Expand Down Expand Up @@ -525,7 +543,13 @@ public Mono<McpSchema.CallToolResult> callTool(McpSchema.CallToolRequest callToo
if (this.serverCapabilities.tools() == null) {
return Mono.error(new McpError("Server does not provide tools capability"));
}
return this.mcpSession.sendRequest(McpSchema.METHOD_TOOLS_CALL, callToolRequest, CALL_TOOL_RESULT_TYPE_REF);
// Refresh tool output schema cache, if necessary, prior to making tool call
Mono<Void> refreshCacheMono = Mono.empty();
if (!this.toolsOutputSchemaCache.containsKey(callToolRequest.name())) {
refreshCacheMono = refreshToolOutputSchemaCache();
}
return refreshCacheMono.then(this.mcpSession.sendRequest(McpSchema.METHOD_TOOLS_CALL, callToolRequest,
CALL_TOOL_RESULT_TYPE_REF));
});
}

Expand All @@ -547,8 +571,34 @@ public Mono<McpSchema.ListToolsResult> listTools(String cursor) {
if (this.serverCapabilities.tools() == null) {
return Mono.error(new McpError("Server does not provide tools capability"));
}
return this.mcpSession.sendRequest(McpSchema.METHOD_TOOLS_LIST, new McpSchema.PaginatedRequest(cursor),
LIST_TOOLS_RESULT_TYPE_REF);
return this.mcpSession
.sendRequest(McpSchema.METHOD_TOOLS_LIST, new McpSchema.PaginatedRequest(cursor),
LIST_TOOLS_RESULT_TYPE_REF)
.doOnNext(result -> {
// Cache tools output schema
if (result.tools() != null) {
// Cache tools output schema
result.tools()
.forEach(tool -> this.toolsOutputSchemaCache.put(tool.name(), tool.outputSchema()));
}
});
});
}

/**
* Refreshes the tool output schema cache by fetching all tools from the server.
* @return A Mono that completes when all tool output schemas have been cached
*/
private Mono<Void> refreshToolOutputSchemaCache() {
return this.withInitializationCheck("refreshing tool output schema cache", initializedResult -> {

// Use expand operator to handle pagination in a reactive way
return this.listTools(null).expand(result -> {
if (result.nextCursor() != null) {
return this.listTools(result.nextCursor());
}
return Mono.empty();
}).then();
});
}

Expand Down
74 changes: 70 additions & 4 deletions mcp/src/main/java/io/modelcontextprotocol/client/McpSyncClient.java
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Expand Up @@ -5,15 +5,28 @@
package io.modelcontextprotocol.client;

import java.time.Duration;
import java.util.HashMap;
import java.util.Set;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.node.ObjectNode;
import com.networknt.schema.JsonSchema;
import com.networknt.schema.JsonSchemaFactory;
import com.networknt.schema.SpecVersion;
import com.networknt.schema.ValidationMessage;

import io.modelcontextprotocol.spec.McpError;
import io.modelcontextprotocol.spec.McpSchema;
import io.modelcontextprotocol.spec.McpSchema.ClientCapabilities;
import io.modelcontextprotocol.spec.McpSchema.GetPromptRequest;
import io.modelcontextprotocol.spec.McpSchema.GetPromptResult;
import io.modelcontextprotocol.spec.McpSchema.ListPromptsResult;
import io.modelcontextprotocol.util.Assert;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
* A synchronous client implementation for the Model Context Protocol (MCP) that wraps an
Expand Down Expand Up @@ -55,6 +68,8 @@ public class McpSyncClient implements AutoCloseable {

private static final Logger logger = LoggerFactory.getLogger(McpSyncClient.class);

private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();

// TODO: Consider providing a client config to set this properly
// this is currently a concern only because AutoCloseable is used - perhaps it
// is not a requirement?
Expand Down Expand Up @@ -206,7 +221,8 @@ public Object ping() {
/**
* Calls a tool provided by the server. Tools enable servers to expose executable
* functionality that can interact with external systems, perform computations, and
* take actions in the real world.
* take actions in the real world. If tool contains an output schema, validates the
* tool result structured content against the output schema.
* @param callToolRequest The request containing: - name: The name of the tool to call
* (must match a tool name from tools/list) - arguments: Arguments that conform to the
* tool's input schema
Expand All @@ -215,7 +231,53 @@ public Object ping() {
* Boolean indicating if the execution failed (true) or succeeded (false/absent)
*/
public McpSchema.CallToolResult callTool(McpSchema.CallToolRequest callToolRequest) {
return this.delegate.callTool(callToolRequest).block();
McpSchema.CallToolResult result = this.delegate.callTool(callToolRequest).block();
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Note: Shift to sync instead.

HashMap<String, McpSchema.JsonSchema> toolsOutputSchemaCache = this.delegate.getToolsOutputSchemaCache();
// Should not be triggered but added for completeness
if (!toolsOutputSchemaCache.containsKey(callToolRequest.name())) {
throw new McpError("Tool with name '" + callToolRequest.name() + "' not found");
}
if (result != null && toolsOutputSchemaCache.get(callToolRequest.name()) != null) {
if (result.structuredContent() == null) {
throw new McpError("CallToolResult validation failed: structuredContent is null and "
+ "does not match tool outputSchema.");
}

McpSchema.JsonSchema outputSchema = toolsOutputSchemaCache.get(callToolRequest.name());

try {
// Convert outputSchema to string
String outputSchemaString = OBJECT_MAPPER.writeValueAsString(outputSchema);

// Create JsonSchema validator
ObjectNode schemaNode = (ObjectNode) OBJECT_MAPPER.readTree(outputSchemaString);
// Set additional properties to false if not specified in output schema
if (!schemaNode.has("additionalProperties")) {
schemaNode.put("additionalProperties", false);
}
JsonSchema schema = JsonSchemaFactory.getInstance(SpecVersion.VersionFlag.V202012)
.getSchema(schemaNode);

// Convert structured content in reult to JsonNode
JsonNode jsonNode = OBJECT_MAPPER.valueToTree(result.structuredContent());

// Validate outputSchema against structuredContent
Set<ValidationMessage> validationResult = schema.validate(jsonNode);

// Check if validation passed
if (!validationResult.isEmpty()) {
// Handle validation errors
throw new McpError(
"CallToolResult validation failed: structuredContent does not match tool outputSchema.");
}
}
catch (JsonProcessingException e) {
// Log error if output schema can't be parsed to prevent erroring out for
// successful call tool request
logger.error("Encountered exception when parsing outputSchema: {}", e);
}
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Worth getting other opinions on this, I think this should be rethrown (potentially wrapped in McpError to keep it unchecked).

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I agree. For now, changing this to log a warning. This leaves the option open for the client side implementation to proceed as it deems fit. I would like feedback from others about what would be the ideal behavior in this scenario.

}
return result;
}

/**
Expand Down Expand Up @@ -353,4 +415,8 @@ public McpSchema.CompleteResult completeCompletion(McpSchema.CompleteRequest com
return this.delegate.completeCompletion(completeRequest).block();
}

private void isStrict(boolean b) {
throw new UnsupportedOperationException("Not supported yet.");
}

}
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