--- description: 'Instructions for building Model Context Protocol (MCP) servers in Java with the MCP Java SDK 2.x' applyTo: '**/*.java, **/pom.xml, **/build.gradle, **/build.gradle.kts' --- # Java MCP Server Development These instructions apply to the [MCP Java SDK](https://github.com/modelcontextprotocol/java-sdk) 2.x. SDK 2.0.x supports the MCP specification revision [2025-11-25](https://modelcontextprotocol.io/specification/2025-11-25) and the earlier revisions. SDK 3.x will add the revision [2026-07-28](https://modelcontextprotocol.io/specification/2026-07-28). ## Instructions - Import the BOM `io.modelcontextprotocol.sdk:mcp-bom:2.0.1` and add the `io.modelcontextprotocol.sdk:mcp` artifact. - In Gradle, import the BOM with `implementation(platform("io.modelcontextprotocol.sdk:mcp-bom:2.0.1"))`. Also add `testRuntimeOnly("org.junit.platform:junit-platform-launcher")`. Without it, the `test` task fails. - The `mcp` artifact contains the SDK core and the Jackson 3 JSON mapper. For Jackson 2, use `mcp-core` and `mcp-json-jackson2`. - Do not use code from SDK 0.x or 1.x. Read the [migration guide](https://github.com/modelcontextprotocol/java-sdk/blob/main/MIGRATION-2.0.md) when you update a server. - The packages are `io.modelcontextprotocol.server`, `io.modelcontextprotocol.spec`, and `io.modelcontextprotocol.json`. The protocol types are nested in `McpSchema`. - Make a server with `McpServer.sync(transport)` or `McpServer.async(transport)`. Set `serverInfo(name, version)` and `capabilities(...)`, and then call `build()`. - Use the sync server for most servers. Use the async server with Project Reactor (`Mono`) only when the tool code does not block a thread. - For stdio transport, use `new StdioServerTransportProvider(McpJsonDefaults.getMapper())`. - Enable only the capabilities that the server uses: `ServerCapabilities.builder().tools(false).resources(false, true).prompts(true).completions().build()`. `tools(listChanged)` and `prompts(listChanged)` enable list change notifications. `resources(subscribe, listChanged)` also enables resource subscriptions. - Register tools with `SyncToolSpecification.builder().tool(tool).callHandler((exchange, request) -> ...)`. The handler gets the arguments as a `Map` from `request.arguments()`. - Use the builders that take the required fields first: `Tool.builder(name, inputSchema)`, `Resource.builder(uri, name)`, `ResourceTemplate.builder(uriTemplate, name)`, `Prompt.builder(name)`, `TextContent.builder(text)`, and `ReadResourceResult.builder(contents)`. The builders without arguments are deprecated. - A required field must not be `null`. A `null` value causes an `IllegalArgumentException`. - Give `inputSchema` and `outputSchema` as `Map` values. The `JsonSchema` record is deprecated. - The server checks each schema against JSON Schema 2020-12 when you call `build()` or `addTool()`. An incorrect schema causes an `IllegalArgumentException`. - The SDK validates the arguments against `inputSchema` before it calls the handler. It also validates `structuredContent` against `outputSchema`. If the validation fails, the client gets a tool result with `isError` set to `true`. - If a tool has an `outputSchema`, return `structuredContent` in each successful result. Without it, the SDK returns a tool result with `isError` set to `true`. - For machine-readable output, set `outputSchema` on the tool and return `CallToolResult.builder().structuredContent(map)`. Also add the same data as text content for older clients. - Set tool hints with `ToolAnnotations.builder().readOnlyHint(true).build()`. Use `destructiveHint(true)` for a destructive tool. - Return `CallToolResult.builder().addTextContent(message).isError(true).build()` for an error that the model must read. - Do not throw an exception or an `McpError` for a tool failure. The client gets a JSON-RPC error, not a tool result. Many hosts do not show this error to the model. - Use `McpError.builder(McpSchema.ErrorCodes.INVALID_PARAMS).message(...).build()` only for a protocol error, for example an unknown resource URI. - Register resources with `SyncResourceSpecification`, resource templates with `SyncResourceTemplateSpecification`, prompts with `SyncPromptSpecification`, and completions with `SyncCompletionSpecification`. - Define resource templates with RFC 6570 URI templates, for example `users://{id}`. - Report progress with `exchange.progressNotification(...)` only when `request.progressToken()` is not `null`. - Get user input with `exchange.createElicitation(ElicitFormRequest.builder(message, requestedSchema).build())`. First, make sure that `exchange.getClientCapabilities().elicitation()` is not `null`. - Do not use sampling (`exchange.createMessage()`). The revision 2026-07-28 deprecates it. Call the LLM provider API directly. - Do not use protocol logging (`exchange.loggingNotification()`). The revision 2026-07-28 deprecates it. Log with SLF4J. - Do not use roots (`exchange.listRoots()`). The revision 2026-07-28 deprecates it. Get paths from tool arguments, resource URIs, or the server configuration. - Add or remove tools at runtime with `server.addTool(...)` and `server.removeTool(name)`. Then call `server.notifyToolsListChanged()`. - In a stdio server, stdout is the protocol channel. Do not call `System.out.println()`. Use `slf4j-simple` (it writes to stderr), or set the target of the Logback console appender to `System.err`. - For a remote server, use Streamable HTTP. The SSE transports are deprecated. - For a servlet container, use `HttpServletStreamableServerTransportProvider`. For a server without sessions, use `HttpServletStatelessServerTransport`. - The HTTP transports do not check the `Host` and `Origin` headers by default. Always set `securityValidator(DefaultServerTransportSecurityValidator.builder()...build())`. This also applies to the Spring AI starters. - For Spring Boot, use the MCP starters of [Spring AI](https://docs.spring.io/spring-ai/reference/api/mcp/mcp-overview.html) 2.0 or later, for example `spring-ai-starter-mcp-server-webmvc`. Do not use the `io.modelcontextprotocol.sdk:mcp-spring-*` artifacts. - In Spring AI, write tools as bean methods with `@McpTool` and `@McpToolParam` (from `org.springframework.ai.mcp.annotation`). - For browser clients, configure CORS. Allow the `Mcp-*` request headers. Expose the `Mcp-Session-Id` response header. - Package a stdio server as one executable JAR with the `maven-shade-plugin`. In Gradle, use the `application` plugin and the start script from `installDist`. Do not use `./gradlew run` as the client command. - Test a server with `McpClient.sync(new StdioClientTransport(params, McpJsonDefaults.getMapper()))`. The SDK has no in-memory transport. - Test servers manually with the MCP Inspector. ## Best Practices - Give each tool one responsibility. - Write clear tool descriptions. The description tells the model when to use the tool. - Add a `description` to each property in `inputSchema`. - Put each tool specification in its own method. This keeps the tools easy to test. - In the async server, do not block the event loop. Run code that blocks with `Mono.fromCallable(...).subscribeOn(Schedulers.boundedElastic())`. - Use environment variables for configuration. - Be careful when a tool gives access to the file system or the network. ## Common Patterns The snippets do not show all imports. The protocol types are nested in `io.modelcontextprotocol.spec.McpSchema`. `Mono` is in `reactor.core.publisher`, and `Schedulers` is in `reactor.core.scheduler`. ### Basic Server Setup (stdio) ```java import java.util.List; import java.util.Map; import io.modelcontextprotocol.json.McpJsonDefaults; import io.modelcontextprotocol.server.McpServer; import io.modelcontextprotocol.server.McpServerFeatures.SyncToolSpecification; import io.modelcontextprotocol.server.transport.StdioServerTransportProvider; import io.modelcontextprotocol.spec.McpSchema.CallToolResult; import io.modelcontextprotocol.spec.McpSchema.ServerCapabilities; import io.modelcontextprotocol.spec.McpSchema.Tool; public final class McpServerApp { static SyncToolSpecification addTool() { Tool tool = Tool.builder("add", Map.of( "type", "object", "properties", Map.of( "a", Map.of("type", "integer", "description", "First number"), "b", Map.of("type", "integer", "description", "Second number")), "required", List.of("a", "b"))) .description("Add two integers.") .build(); return SyncToolSpecification.builder() .tool(tool) .callHandler((exchange, request) -> { long a = ((Number) request.arguments().get("a")).longValue(); long b = ((Number) request.arguments().get("b")).longValue(); return CallToolResult.builder().addTextContent(String.valueOf(a + b)).build(); }) .build(); } public static void main(String[] args) { McpServer.sync(new StdioServerTransportProvider(McpJsonDefaults.getMapper())) .serverInfo("my-server", "0.1.0") .capabilities(ServerCapabilities.builder().tools(false).build()) .tools(addTool()) .build(); } } ``` ### Tool with Structured Output ```java static SyncToolSpecification wordCountTool() { Tool tool = Tool.builder("word_count", Map.of( "type", "object", "properties", Map.of("text", Map.of("type", "string")), "required", List.of("text"))) .description("Count the words in a text.") .outputSchema(Map.of( "type", "object", "properties", Map.of("words", Map.of("type", "integer")), "required", List.of("words"))) .annotations(ToolAnnotations.builder().readOnlyHint(true).build()) .build(); return SyncToolSpecification.builder() .tool(tool) .callHandler((exchange, request) -> { String text = (String) request.arguments().get("text"); int words = text.isBlank() ? 0 : text.trim().split("\\s+").length; return CallToolResult.builder() .addTextContent("{\"words\":" + words + "}") .structuredContent(Map.of("words", words)) .build(); }) .build(); } ``` ### Error Handling ```java static SyncToolSpecification divideTool() { Tool tool = Tool.builder("divide", Map.of( "type", "object", "properties", Map.of("a", Map.of("type", "number"), "b", Map.of("type", "number")), "required", List.of("a", "b"))) .description("Divide a by b.") .build(); return SyncToolSpecification.builder() .tool(tool) .callHandler((exchange, request) -> { double a = ((Number) request.arguments().get("a")).doubleValue(); double b = ((Number) request.arguments().get("b")).doubleValue(); if (b == 0) { return CallToolResult.builder().addTextContent("b must not be zero.").isError(true).build(); } return CallToolResult.builder().addTextContent(String.valueOf(a / b)).build(); }) .build(); } ``` ### Resources and Resource Templates ```java static SyncResourceSpecification configResource() { return new SyncResourceSpecification( Resource.builder("config://app", "app-config").mimeType("application/json").build(), (exchange, request) -> ReadResourceResult.builder(List.of( TextResourceContents.builder(request.uri(), "{\"debug\":false}") .mimeType("application/json") .build())) .build()); } static SyncResourceTemplateSpecification userResource() { return new SyncResourceTemplateSpecification( ResourceTemplate.builder("users://{id}", "user").build(), (exchange, request) -> { String id = request.uri().substring("users://".length()); return ReadResourceResult.builder(List.of( TextResourceContents.builder(request.uri(), "User " + id).build())) .build(); }); } ``` Register them with `.resources(configResource())` and `.resourceTemplates(userResource())`. ### Prompt with Completion ```java static SyncPromptSpecification reviewPrompt() { Prompt prompt = Prompt.builder("code-review") .description("Review code in a language.") .arguments(List.of(PromptArgument.builder("language").required(true).build())) .build(); return new SyncPromptSpecification(prompt, (exchange, request) -> GetPromptResult.builder(List.of( PromptMessage.builder(Role.USER, TextContent.builder("Review this " + request.arguments().get("language") + " code.").build()) .build())) .build()); } static SyncCompletionSpecification languageCompletion() { List languages = List.of("java", "kotlin", "scala"); return new SyncCompletionSpecification(new PromptReference("code-review"), (exchange, request) -> new CompleteResult(new CompleteResult.CompleteCompletion( languages.stream().filter(l -> l.startsWith(request.argument().value())).toList(), null, false))); } ``` Register them with `.prompts(reviewPrompt())` and `.completions(languageCompletion())`. ### Async Tool with Progress ```java static AsyncToolSpecification lookupTool() { Tool tool = Tool.builder("lookup", Map.of( "type", "object", "properties", Map.of("query", Map.of("type", "string")), "required", List.of("query"))) .description("Look up a query in a slow backend.") .build(); return AsyncToolSpecification.builder() .tool(tool) .callHandler((exchange, request) -> { Object token = request.progressToken(); Mono progress = token == null ? Mono.empty() : exchange.progressNotification( ProgressNotification.builder(token, 0.5).total(1.0).message("Halfway").build()); return progress.then(Mono.fromCallable(() -> slowLookup((String) request.arguments().get("query"))) .subscribeOn(Schedulers.boundedElastic()) .map(text -> CallToolResult.builder().addTextContent(text).build())); }) .build(); } ``` Register it on `McpServer.async(transport)`. ### Streamable HTTP Server (Servlet) ```java HttpServletStreamableServerTransportProvider transport = HttpServletStreamableServerTransportProvider.builder() .jsonMapper(McpJsonDefaults.getMapper()) .mcpEndpoint("/mcp") .securityValidator(DefaultServerTransportSecurityValidator.builder() .allowedHost("localhost:8080") .allowedOrigin("http://localhost:*") .build()) .build(); McpServer.sync(transport) .serverInfo("my-server", "0.1.0") .capabilities(ServerCapabilities.builder().tools(false).build()) .tools(addTool()) .build(); ``` The transport is a Jakarta Servlet. Register it in a servlet container at `/mcp` with async support. ### Spring AI Tool ```java @Component class CalculatorTools { @McpTool(name = "add", description = "Add two integers") public int add(@McpToolParam(description = "First number") int a, @McpToolParam(description = "Second number") int b) { return a + b; } } ``` ### Integration Test ```java class McpServerAppTest { private static McpSyncClient client; @BeforeAll static void startServer() { String java = Path.of(System.getProperty("java.home"), "bin", "java").toString(); ServerParameters server = ServerParameters.builder(java) .args("-cp", System.getProperty("java.class.path"), McpServerApp.class.getName()) .build(); client = McpClient.sync(new StdioClientTransport(server, McpJsonDefaults.getMapper())).build(); client.initialize(); } @AfterAll static void stopServer() { client.close(); } @Test void add() { CallToolResult result = client.callTool( CallToolRequest.builder("add").arguments(Map.of("a", 2, "b", 3)).build()); assertEquals("5", ((TextContent) result.content().get(0)).text()); } } ```