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Overview

The MCP Server is a foundational component in the Model Context Protocol (MCP) architecture that provides tools, resources, and capabilities to clients. It implements the server-side of the protocol, responsible for:

  • Exposing tools that clients can discover and execute
  • Managing resources with URI-based access patterns
  • Providing prompt templates and handling prompt requests
  • Supporting capability negotiation with clients
  • Implementing server-side protocol operations
  • Managing concurrent client connections
  • Providing structured logging and notifications

The server supports both synchronous and asynchronous APIs, allowing for flexible integration in different application contexts.

// Create a server with custom configuration
McpSyncServer syncServer = McpServer.sync(transport)
    .serverInfo("my-server", "1.0.0")
    .capabilities(ServerCapabilities.builder()
        .resources(true)     // Enable resource support
        .tools(true)         // Enable tool support
        .prompts(true)       // Enable prompt support
        .logging()           // Enable logging support
        .build())
    .build();

// Initialize the server
syncServer.initialize();

// Register tools, resources, and prompts
syncServer.addTool(syncToolRegistration);
syncServer.addResource(syncResourceRegistration);
syncServer.addPrompt(syncPromptRegistration);

// Send logging notifications
syncServer.loggingNotification(LoggingMessageNotification.builder()
    .level(LoggingLevel.INFO)
    .logger("custom-logger")
    .data("Server initialized")
    .build());

// Close the server when done
syncServer.close();

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Server Transport

The transport layer in the MCP SDK is responsible for handling the communication between clients and servers. It provides different implementations to support various communication protocols and patterns. The SDK includes several built-in transport implementations:

Create in-process based transport:

StdioServerTransport transport = new StdioServerTransport(new ObjectMapper());

Provides bidirectional JSON-RPC message handling over standard input/output streams with non-blocking message processing, serialization/deserialization, and graceful shutdown support.

Key features:

  • Bidirectional communication through stdin/stdout
  • Process-based integration support
  • Simple setup and configuration
  • Lightweight implementation

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Server Capabilities

The server can be configured with various capabilities:

var capabilities = ServerCapabilities.builder()
    .resources(false, true)  // Resource support with list changes notifications
    .tools(true)            // Tool support with list changes notifications
    .prompts(true)          // Prompt support with list changes notifications
    .logging()              // Enable logging support (enabled by default with loging level INFO)
    .build();

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Logging Support

The server provides structured logging capabilities that allow sending log messages to clients with different severity levels:

// Send a log message to clients
server.loggingNotification(LoggingMessageNotification.builder()
    .level(LoggingLevel.INFO)
    .logger("custom-logger")
    .data("Custom log message")
    .build());

Clients can control the minimum logging level they receive through the mcpClient.setLoggingLevel(level) request. Messages below the set level will be filtered out. Supported logging levels (in order of increasing severity): DEBUG (0), INFO (1), NOTICE (2), WARNING (3), ERROR (4), CRITICAL (5), ALERT (6), EMERGENCY (7)

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Tool Registration

// Sync tool registration
var syncToolRegistration = new McpServerFeatures.SyncToolRegistration(
    new Tool("calculator", "Basic calculator", Map.of(
        "operation", "string",
        "a", "number",
        "b", "number"
    )),
    arguments -> {
        // Tool implementation
        return new CallToolResult(result, false);
    }
);

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Resource Registration

// Sync resource registration
var syncResourceRegistration = new McpServerFeatures.SyncResourceRegistration(
    new Resource("custom://resource", "name", "description", "mime-type", null),
    request -> {
        // Resource read implementation
        return new ReadResourceResult(contents);
    }
);

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Prompt Registration

// Sync prompt registration
var syncPromptRegistration = new McpServerFeatures.SyncPromptRegistration(
    new Prompt("greeting", "description", List.of(
        new PromptArgument("name", "description", true)
    )),
    request -> {
        // Prompt implementation
        return new GetPromptResult(description, messages);
    }
);

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Error Handling

The SDK provides comprehensive error handling through the McpError class, covering protocol compatibility, transport communication, JSON-RPC messaging, tool execution, resource management, prompt handling, timeouts, and connection issues. This unified error handling approach ensures consistent and reliable error management across both synchronous and asynchronous operations.

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