io.modelcontextprotocol.server

Interface McpServer

public interface McpServer

Factory class for creating Model Context Protocol (MCP) servers. MCP servers expose tools, resources, and prompts to AI models through a standardized interface.

This class serves as the main entry point for implementing the server-side of the MCP specification. The server's responsibilities include:

• Exposing tools that models can invoke to perform actions
• Providing access to resources that give models context
• Managing prompt templates for structured model interactions
• Handling client connections and requests
• Implementing capability negotiation

Thread Safety: Both synchronous and asynchronous server implementations are thread-safe. The synchronous server processes requests sequentially, while the asynchronous server can handle concurrent requests safely through its reactive programming model.

Error Handling: The server implementations provide robust error handling through the McpError class. Errors are properly propagated to clients while maintaining the server's stability. Server implementations should use appropriate error codes and provide meaningful error messages to help diagnose issues.

The class provides factory methods to create either:

• McpAsyncServer for non-blocking operations with reactive responses
• McpSyncServer for blocking operations with direct responses

Example of creating a basic synchronous server:

 McpServer.sync(transportProvider)
     .serverInfo("my-server", "1.0.0")
     .tool(new Tool("calculator", "Performs calculations", schema),
           (exchange, args) -> new CallToolResult("Result: " + calculate(args)))
     .build();
 

Example of creating a basic asynchronous server:

 McpServer.async(transportProvider)
     .serverInfo("my-server", "1.0.0")
     .tool(new Tool("calculator", "Performs calculations", schema),
           (exchange, args) -> Mono.fromSupplier(() -> calculate(args))
               .map(result -> new CallToolResult("Result: " + result)))
     .build();
 

Example with comprehensive asynchronous configuration:

 McpServer.async(transportProvider)
     .serverInfo("advanced-server", "2.0.0")
     .capabilities(new ServerCapabilities(...))
     // Register tools
     .tools(
         new McpServerFeatures.AsyncToolSpecification(calculatorTool,
             (exchange, args) -> Mono.fromSupplier(() -> calculate(args))
                 .map(result -> new CallToolResult("Result: " + result))),
         new McpServerFeatures.AsyncToolSpecification(weatherTool,
             (exchange, args) -> Mono.fromSupplier(() -> getWeather(args))
                 .map(result -> new CallToolResult("Weather: " + result)))
     )
     // Register resources
     .resources(
         new McpServerFeatures.AsyncResourceSpecification(fileResource,
             (exchange, req) -> Mono.fromSupplier(() -> readFile(req))
                 .map(ReadResourceResult::new)),
         new McpServerFeatures.AsyncResourceSpecification(dbResource,
             (exchange, req) -> Mono.fromSupplier(() -> queryDb(req))
                 .map(ReadResourceResult::new))
     )
     // Add resource templates
     .resourceTemplates(
         new ResourceTemplate("file://{path}", "Access files"),
         new ResourceTemplate("db://{table}", "Access database")
     )
     // Register prompts
     .prompts(
         new McpServerFeatures.AsyncPromptSpecification(analysisPrompt,
             (exchange, req) -> Mono.fromSupplier(() -> generateAnalysisPrompt(req))
                 .map(GetPromptResult::new)),
         new McpServerFeatures.AsyncPromptRegistration(summaryPrompt,
             (exchange, req) -> Mono.fromSupplier(() -> generateSummaryPrompt(req))
                 .map(GetPromptResult::new))
     )
     .build();
 

Author:

Christian Tzolov, Dariusz Jędrzejczyk

See Also:

McpAsyncServer, McpSyncServer, McpServerTransportProvider

Nested Class Summary

Nested Classes

Modifier and Type	Interface and Description
static class	McpServer.AsyncSpec Deprecated.
static class	McpServer.AsyncSpecification Asynchronous server specification.
static class	McpServer.SyncSpec Deprecated.
static class	McpServer.SyncSpecification Synchronous server specification.

Method Summary

Modifier and Type	Method and Description
static McpServer.AsyncSpecification	async(McpServerTransportProvider transportProvider) Starts building an asynchronous MCP server that provides non-blocking operations.
static McpServer.AsyncSpec	async(ServerMcpTransport transport) Deprecated. This method will be removed in 0.9.0. Use async(McpServerTransportProvider) instead.
static McpServer.SyncSpecification	sync(McpServerTransportProvider transportProvider) Starts building a synchronous MCP server that provides blocking operations.
static McpServer.SyncSpec	sync(ServerMcpTransport transport) Deprecated. This method will be removed in 0.9.0. Use sync(McpServerTransportProvider) instead.

Method Detail

sync

static McpServer.SyncSpecification sync(McpServerTransportProvider transportProvider)

Starts building a synchronous MCP server that provides blocking operations. Synchronous servers block the current Thread's execution upon each request before giving the control back to the caller, making them simpler to implement but potentially less scalable for concurrent operations.

Parameters:

transportProvider - The transport layer implementation for MCP communication.

Returns:

A new instance of McpServer.SyncSpecification for configuring the server.

sync

@Deprecated
static McpServer.SyncSpec sync(ServerMcpTransport transport)

Deprecated. This method will be removed in 0.9.0. Use sync(McpServerTransportProvider) instead.

Starts building a synchronous MCP server that provides blocking operations. Synchronous servers block the current Thread's execution upon each request before giving the control back to the caller, making them simpler to implement but potentially less scalable for concurrent operations.

Parameters:

transport - The transport layer implementation for MCP communication

Returns:

A new instance of McpServer.SyncSpec for configuring the server.

async

static McpServer.AsyncSpecification async(McpServerTransportProvider transportProvider)

Starts building an asynchronous MCP server that provides non-blocking operations. Asynchronous servers can handle multiple requests concurrently on a single Thread using a functional paradigm with non-blocking server transports, making them more scalable for high-concurrency scenarios but more complex to implement.

Parameters:

transportProvider - The transport layer implementation for MCP communication.

Returns:

A new instance of McpServer.AsyncSpecification for configuring the server.

async

@Deprecated
static McpServer.AsyncSpec async(ServerMcpTransport transport)

Deprecated. This method will be removed in 0.9.0. Use async(McpServerTransportProvider) instead.

Starts building an asynchronous MCP server that provides non-blocking operations. Asynchronous servers can handle multiple requests concurrently on a single Thread using a functional paradigm with non-blocking server transports, making them more scalable for high-concurrency scenarios but more complex to implement.

Parameters:

transport - The transport layer implementation for MCP communication

Returns:

A new instance of McpServer.AsyncSpec for configuring the server.