---
title: MCP Client
description: Learn how to use the Model Context Protocol (MCP) client to interact with MCP servers
---
# MCP Client
The MCP Client is a key component in the Model Context Protocol (MCP) architecture, responsible for establishing and managing connections with MCP servers. It implements the client-side of the protocol, handling:
- Protocol version negotiation to ensure compatibility with servers
- Capability negotiation to determine available features
- Message transport and JSON-RPC communication
- Tool discovery and execution with optional schema validation
- Resource access and management
- Prompt system interactions
- Optional features like roots management, sampling, and elicitation support
- Progress tracking for long-running operations
!!! tip
The core `io.modelcontextprotocol.sdk:mcp` module provides STDIO, SSE, and Streamable HTTP client transport implementations without requiring external web frameworks.
The Spring-specific WebFlux transport (`mcp-spring-webflux`) is now part of [Spring AI](https://docs.spring.io/spring-ai/reference/2.0-SNAPSHOT/api/mcp/mcp-overview.html) 2.0+ (group `org.springframework.ai`) and is no longer shipped by this SDK.
See the [MCP Client Boot Starter](https://docs.spring.io/spring-ai/reference/2.0-SNAPSHOT/api/mcp/mcp-client-boot-starter-docs.html) documentation for Spring-based client setup.
The client provides both synchronous and asynchronous APIs for flexibility in different application contexts.
=== "Sync API"
```java
// Create a sync client with custom configuration
McpSyncClient client = McpClient.sync(transport)
.requestTimeout(Duration.ofSeconds(10))
.capabilities(ClientCapabilities.builder()
.roots(true) // Enable roots capability
.sampling() // Enable sampling capability
.elicitation() // Enable elicitation capability
.build())
.sampling(request -> new CreateMessageResult(response))
.elicitation(request -> new ElicitResult(ElicitResult.Action.ACCEPT, content))
.build();
// Initialize connection
client.initialize();
// List available tools
ListToolsResult tools = client.listTools();
// Call a tool
CallToolResult result = client.callTool(
CallToolRequest.builder("calculator")
.arguments(Map.of("operation", "add", "a", 2, "b", 3))
.build()
);
// List and read resources
ListResourcesResult resources = client.listResources();
ReadResourceResult resource = client.readResource(
ReadResourceRequest.builder("resource://uri").build()
);
// List and use prompts
ListPromptsResult prompts = client.listPrompts();
GetPromptResult prompt = client.getPrompt(
GetPromptRequest.builder("greeting").arguments(Map.of("name", "Spring")).build()
);
// Add/remove roots
client.addRoot(new Root("file:///path", "description"));
client.removeRoot("file:///path");
// Close client
client.closeGracefully();
```
=== "Async API"
```java
// Create an async client with custom configuration
McpAsyncClient client = McpClient.async(transport)
.requestTimeout(Duration.ofSeconds(10))
.capabilities(ClientCapabilities.builder()
.roots(true) // Enable roots capability
.sampling() // Enable sampling capability
.elicitation() // Enable elicitation capability
.build())
.sampling(request -> Mono.just(new CreateMessageResult(response)))
.elicitation(request -> Mono.just(new ElicitResult(ElicitResult.Action.ACCEPT, content)))
.toolsChangeConsumer(tools -> Mono.fromRunnable(() -> {
logger.info("Tools updated: {}", tools);
}))
.resourcesChangeConsumer(resources -> Mono.fromRunnable(() -> {
logger.info("Resources updated: {}", resources);
}))
.promptsChangeConsumer(prompts -> Mono.fromRunnable(() -> {
logger.info("Prompts updated: {}", prompts);
}))
.progressConsumer(progress -> Mono.fromRunnable(() -> {
logger.info("Progress: {}", progress);
}))
.build();
// Initialize connection and use features
client.initialize()
.flatMap(initResult -> client.listTools())
.flatMap(tools -> {
return client.callTool(CallToolRequest.builder("calculator")
.arguments(Map.of("operation", "add", "a", 2, "b", 3))
.build());
})
.flatMap(result -> {
return client.listResources()
.flatMap(resources ->
client.readResource(ReadResourceRequest.builder("resource://uri").build())
);
})
.flatMap(resource -> {
return client.listPrompts()
.flatMap(prompts ->
client.getPrompt(GetPromptRequest.builder("greeting")
.arguments(Map.of("name", "Spring"))
.build())
);
})
.flatMap(prompt -> {
return client.addRoot(new Root("file:///path", "description"))
.then(client.removeRoot("file:///path"));
})
.doFinally(signalType -> {
client.closeGracefully().subscribe();
})
.subscribe();
```
## Client Transport
The transport layer handles the communication between MCP clients and servers, providing different implementations for various use cases. The client transport manages message serialization, connection establishment, and protocol-specific communication patterns.
### STDIO
Creates transport for process-based communication using stdin/stdout:
```java
ServerParameters params = ServerParameters.builder("npx")
.args("-y", "@modelcontextprotocol/server-everything", "dir")
.build();
McpTransport transport = new StdioClientTransport(params, McpJsonDefaults.getMapper());
```
### Streamable HTTP
=== "Streamable HttpClient"
Creates a Streamable HTTP client transport for efficient bidirectional communication. Included in the core `mcp` module:
```java
McpTransport transport = HttpClientStreamableHttpTransport
.builder("http://your-mcp-server")
.endpoint("/mcp")
.build();
```
The Streamable HTTP transport supports:
- Resumable streams for connection recovery
- Configurable connect timeout
- Custom HTTP request customization
- Multiple protocol version negotiation
=== "Streamable WebClient (external)"
Creates Streamable HTTP WebClient-based client transport. Requires the `mcp-spring-webflux` dependency from [Spring AI](https://docs.spring.io/spring-ai/reference/2.0-SNAPSHOT/api/mcp/mcp-overview.html) 2.0+ (group `org.springframework.ai`):
```java
McpTransport transport = WebFluxSseClientTransport
.builder(WebClient.builder().baseUrl("http://your-mcp-server"))
.build();
```
### SSE HTTP (Legacy)
=== "SSE HttpClient"
Creates a framework-agnostic (pure Java API) SSE client transport. Included in the core `mcp` module:
```java
McpTransport transport = HttpClientSseClientTransport.builder("http://your-mcp-server").build();
```
=== "SSE WebClient (external)"
Creates WebFlux-based SSE client transport. Requires the `mcp-spring-webflux` dependency from [Spring AI](https://docs.spring.io/spring-ai/reference/2.0-SNAPSHOT/api/mcp/mcp-overview.html) 2.0+ (group `org.springframework.ai`):
```java
WebClient.Builder webClientBuilder = WebClient.builder()
.baseUrl("http://your-mcp-server");
McpTransport transport = new WebFluxSseClientTransport(webClientBuilder);
```
## Protocol Version Negotiation
During `initialize()`, the client sends the list of protocol versions its transport supports (newest first) and the server picks one it also supports. The negotiated version is returned on the `InitializeResult`:
```java
McpSyncClient client = McpClient.sync(transport).build();
InitializeResult initResult = client.initialize();
String negotiatedVersion = initResult.protocolVersion();
```
Built-in transports (`StdioClientTransport`, `HttpClientStreamableHttpTransport`, `HttpClientSseClientTransport`) advertise every protocol version the SDK understands (`2024-11-05`, `2025-03-26`, `2025-06-18`, `2025-11-25`) via their default `protocolVersions()` implementation, so negotiation normally settles on the newest version both sides support. To restrict a custom transport to a specific subset of versions, override `protocolVersions()` on your `McpClientTransport` implementation:
```java
public class RestrictedTransport extends StdioClientTransport {
// ...
@Override
public List protocolVersions() {
return List.of("2025-06-18"); // only negotiate this version
}
}
```
If the server responds with a version the transport didn't advertise, `initialize()` fails with an `McpError`.
## Client Capabilities
The client can be configured with various capabilities:
```java
var capabilities = ClientCapabilities.builder()
.roots(true) // Enable filesystem roots support with list changes notifications
.sampling() // Enable LLM sampling support
.elicitation() // Enable elicitation support (form and URL modes)
.build();
```
You can also configure elicitation with specific mode support:
```java
var capabilities = ClientCapabilities.builder()
.elicitation(true, false) // Enable form-based elicitation, disable URL-based
.build();
```
### Roots Support
Roots define the boundaries of where servers can operate within the filesystem:
```java
// Add a root dynamically
client.addRoot(new Root("file:///path", "description"));
// Remove a root
client.removeRoot("file:///path");
// Notify server of roots changes
client.rootsListChangedNotification();
```
The roots capability allows servers to:
- Request the list of accessible filesystem roots
- Receive notifications when the roots list changes
- Understand which directories and files they have access to
### Sampling Support
Sampling enables servers to request LLM interactions ("completions" or "generations") through the client:
```java
// Configure sampling handler
Function samplingHandler = request -> {
// Sampling implementation that interfaces with LLM
return new CreateMessageResult(response);
};
// Create client with sampling support
var client = McpClient.sync(transport)
.capabilities(ClientCapabilities.builder()
.sampling()
.build())
.sampling(samplingHandler)
.build();
```
This capability allows:
- Servers to leverage AI capabilities without requiring API keys
- Clients to maintain control over model access and permissions
- Support for both text and image-based interactions
- Optional inclusion of MCP server context in prompts
### Elicitation Support
Elicitation enables servers to request additional information or user input through the client. This is useful when a server needs clarification or confirmation during an operation:
```java
// Configure form elicitation handler
Function formElicitationHandler = request -> {
// Present the request to the user and collect their response
// The request contains a message and a schema describing the expected input
Map userResponse = collectUserInput(request.message(), request.requestedSchema());
return new ElicitResult(ElicitResult.Action.ACCEPT, userResponse);
};
// Configure URL elicitation handler
Function urlElicitationHandler = request -> {
// Prompt the user to visit the URL
// e.g. openBrowser(request.url());
return new ElicitResult(ElicitResult.Action.ACCEPT, Map.of());
};
// Create client with elicitation support
var client = McpClient.sync(transport)
.capabilities(ClientCapabilities.builder()
.elicitation(true, true) // enables both form and URL elicitation
.build())
.elicitation(formElicitationHandler)
.urlElicitation(urlElicitationHandler)
.build();
```
The `ElicitResult` supports three actions:
- `ACCEPT` - The user accepted and provided the requested information
- `DECLINE` - The user declined to provide the information
- `CANCEL` - The operation was cancelled
You can optionally have the client fill in missing values from the schema's `default` declarations before returning an accepted result to the server:
```java
var client = McpClient.sync(transport)
.applyElicitationDefaults(true) // default is false
.elicitation(formElicitationHandler)
.build();
```
When enabled, any keys absent from an accepted `ElicitResult.content` are populated with the `default` values declared in the request's `requestedSchema`.
#### URL Elicitation Required Handling
When a server requires out-of-band URL elicitation but the client has not negotiated support for it (or the server strictly requires out-of-band handling), the server may return a `URL_ELICITATION_REQUIRED` error during tool execution or prompt retrieval.
```java
try {
mcpClient.callTool(new McpSchema.CallToolRequest("tool1", Map.of()));
} catch (McpError e) {
if (e.getJsonRpcError().code() == McpSchema.ErrorCodes.URL_ELICITATION_REQUIRED) {
// Extract elicitation requests from the error data
Map data = (Map) e.getJsonRpcError().data();
TypeRef typeRef = new TypeRef() {};
var requests = McpJsonDefaults.getMapper()
.convertValue(data.get("elicitations"), typeRef);
for (var req : requests) {
// handle elicitation requests
}
}
}
```
#### Elicitation Complete Notification (SEP-1036)
After a user finishes an out-of-band URL elicitation flow (for example, completing an OAuth authorization in a browser), the server sends a `notifications/elicitation/complete` message so the client knows it can stop waiting and re-check the outcome. Register a consumer to receive it:
```java
var client = McpClient.sync(transport)
.capabilities(ClientCapabilities.builder()
.elicitation(true, true)
.build())
.urlElicitation(urlElicitationHandler)
.elicitationCompleteConsumer(notification -> {
System.out.println("Elicitation " + notification.elicitationId() + " completed, re-checking outcome");
})
.build();
```
On the server side, send the notification once the out-of-band flow resolves (e.g. after the user completes the OAuth redirect), using the ID assigned to the original `ElicitUrlRequest` and the session that issued it:
```java
server.sendElicitationComplete(exchange.sessionId(),
new McpSchema.ElicitationCompleteNotification("oauth-123"));
```
### Logging Support
The client can register a logging consumer to receive log messages from the server and set the minimum logging level to filter messages:
```java
var mcpClient = McpClient.sync(transport)
.loggingConsumer(notification -> {
System.out.println("Received log message: " + notification.data());
})
.build();
mcpClient.initialize();
mcpClient.setLoggingLevel(McpSchema.LoggingLevel.INFO);
// Call the tool that sends logging notifications
CallToolResult result = mcpClient.callTool(CallToolRequest.builder("logging-test").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)
### Progress Notifications
The client can register a progress consumer to track the progress of long-running operations:
```java
var mcpClient = McpClient.sync(transport)
.progressConsumer(progress -> {
System.out.println("Progress: " + progress.progress() + "/" + progress.total());
})
.build();
```
### Pinging the Server
The client can send a `ping` request to check that the server is alive and responsive:
```java
McpSyncClient client = McpClient.sync(transport).build();
client.initialize();
Object result = client.ping(); // blocks until the server responds, or the request times out
```
The async equivalent, `McpAsyncClient.ping()`, returns a `Mono` that completes when the server responds.
### Request Timeouts and Cancellation
Every request the client sends (`callTool`, `readResource`, `ping`, etc.) is bounded by the `requestTimeout` configured on the client builder (default 20 seconds):
```java
McpSyncClient client = McpClient.sync(transport)
.requestTimeout(Duration.ofSeconds(10))
.build();
```
If a response doesn't arrive within that window, the pending call fails with a timeout error (an `McpError` on the sync API, or an error signal on the corresponding `Mono` for the async API) instead of blocking indefinitely. The server builder has an equivalent `requestTimeout(Duration)` option (default 10 hours) bounding requests the server sends to the client, such as sampling or elicitation.
The SDK does not currently send or process the MCP `notifications/cancelled` message, so timing out a request only stops the caller from waiting on it it does not notify the other side that the in-flight operation should stop executing.
## Using MCP Clients
### Tool Execution
Tools are server-side functions that clients can discover and execute. The MCP client provides methods to list available tools and execute them with specific parameters. Each tool has a unique name and accepts a map of parameters.
=== "Sync API"
```java
// List available tools
ListToolsResult tools = client.listTools();
// Call a tool with a CallToolRequest
CallToolResult result = client.callTool(
CallToolRequest.builder("calculator")
.arguments(Map.of(
"operation", "add",
"a", 1,
"b", 2
))
.build()
);
```
=== "Async API"
```java
// List available tools asynchronously
client.listTools()
.doOnNext(tools -> tools.tools().forEach(tool ->
System.out.println(tool.name())))
.subscribe();
// Call a tool asynchronously
client.callTool(CallToolRequest.builder("calculator")
.arguments(Map.of(
"operation", "add",
"a", 1,
"b", 2
))
.build())
.subscribe();
```
### Tool Schema Validation and Caching
The client supports optional JSON schema validation for tool call results and automatic schema caching:
```java
var client = McpClient.sync(transport)
.jsonSchemaValidator(myValidator) // Enable schema validation
.enableCallToolSchemaCaching(true) // Cache tool schemas
.build();
```
### Pagination
`listTools`, `listResources`, `listResourceTemplates`, and `listPrompts` all accept an optional opaque `cursor` string, and their results carry a `nextCursor` that is non-null while more pages remain. Loop until `nextCursor` is `null` to collect every page:
```java
List allTools = new ArrayList();
String cursor = null;
do {
ListToolsResult page = client.listTools(cursor);
allTools.addAll(page.tools());
cursor = page.nextCursor();
} while (cursor != null);
```
Each paginated method also accepts an optional `_meta` map alongside the cursor, e.g. `client.listTools(cursor, Map.of("key", "value"))`, for passing request metadata through to the server.
### Resource Access
Resources represent server-side data sources that clients can access using URI templates. The MCP client provides methods to discover available resources and retrieve their contents through a standardized interface.
=== "Sync API"
```java
// List available resources
ListResourcesResult resources = client.listResources();
// Read a resource
ReadResourceResult resource = client.readResource(
ReadResourceRequest.builder("resource://uri").build()
);
```
=== "Async API"
```java
// List available resources asynchronously
client.listResources()
.doOnNext(resources -> resources.resources().forEach(resource ->
System.out.println(resource.name())))
.subscribe();
// Read a resource asynchronously
client.readResource(ReadResourceRequest.builder("resource://uri").build())
.subscribe();
```
### Resource Subscriptions
When the server advertises `resources.subscribe` support, clients can subscribe to individual resources and receive a callback whenever the server pushes a `notifications/resources/updated` notification for that URI. The SDK automatically re-reads the resource on notification and delivers the updated contents to the registered consumer.
Register a consumer on the client builder, then subscribe/unsubscribe at any time:
=== "Sync API"
```java
McpSyncClient client = McpClient.sync(transport)
.resourcesUpdateConsumer(contents -> {
// called with the updated resource contents after each notification
System.out.println("Resource updated: " + contents);
})
.build();
client.initialize();
// Subscribe to a specific resource URI
client.subscribeResource(McpSchema.SubscribeRequest.builder("custom://resource").build());
// ... later, stop receiving updates
client.unsubscribeResource(McpSchema.UnsubscribeRequest.builder("custom://resource").build());
```
=== "Async API"
```java
McpAsyncClient client = McpClient.async(transport)
.resourcesUpdateConsumer(contents -> Mono.fromRunnable(() -> {
System.out.println("Resource updated: " + contents);
}))
.build();
client.initialize()
.then(client.subscribeResource(McpSchema.SubscribeRequest.builder("custom://resource").build()))
.subscribe();
// ... later, stop receiving updates
client.unsubscribeResource(McpSchema.UnsubscribeRequest.builder("custom://resource").build())
.subscribe();
```
### Prompt System
The prompt system enables interaction with server-side prompt templates. These templates can be discovered and executed with custom parameters, allowing for dynamic text generation based on predefined patterns.
=== "Sync API"
```java
// List available prompt templates
ListPromptsResult prompts = client.listPrompts();
// Get a prompt with parameters
GetPromptResult prompt = client.getPrompt(
GetPromptRequest.builder("greeting").arguments(Map.of("name", "World")).build()
);
```
=== "Async API"
```java
// List available prompt templates asynchronously
client.listPrompts()
.doOnNext(prompts -> prompts.prompts().forEach(prompt ->
System.out.println(prompt.name())))
.subscribe();
// Get a prompt asynchronously
client.getPrompt(GetPromptRequest.builder("greeting").arguments(Map.of("name", "World")).build())
.subscribe();
```